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. 2010 Sep;186(1):1–8. doi: 10.1534/genetics.110.121459

Ninety Years of Drosophila melanogaster Hybrids

Daniel A Barbash 1,1
PMCID: PMC2940278  PMID: 20855573

Abstract

Within 10 years of the beginning of experimental genetic research on Drosophila melanogaster, in 1919, A. H. Sturtevant discovered its sibling species, D. simulans. He hybridized the two species and made fundamental discoveries about the genetic basis of hybrid incompatibility. The complete sterility of surviving F1 hybrids frustrated Sturtevant and his vision of comprehensively exploring the genetics of interspecific differences. But over the next 90 years, a combination of clever genetic tricks and close observation of natural variation has led to a wealth of discovery using these and other hybrids of D. melanogaster and D. simulans, resulting in an advanced understanding of speciation and the evolution of morphology, gene regulation, and behavior.

“MANY attempts have been made to hybridize different species of Drosophila, but hitherto all but the combination to be described here have been unsuccessful” (Sturtevant 1920, p. 488). So begins Sturtevant's article on hybrids between Drosophila melanogaster and D. simulans. What did Sturtevant hope to learn from studying species hybrids? His goals can be inferred from the disappointment that he expresses in the article's second sentence, where he describes the study as being only a “partial success” due to the parental species being extremely similar yet still producing only sterile or lethal F1 hybrids. It is clear that, to study the genetic basis of species differences, Sturtevant was searching for species pairs with large phenotypic differences that were also amenable to backcrossing. Thus his disappointment. Sturtevant was too modest, because in his 1920 article he in fact had detailed important discoveries, particularly concerning the genetic basis of hybrid lethality. Here I review Sturtevant's findings and some of what has followed from studying these hybrids over the subsequent 90 years. Several themes recur:

  • Forced creativity: The limitation of being unable to get beyond the F1 hybrid generation has motivated clever alternatives to backcrossing.

  • Exploiting natural variation: Without the many genetic reagents available in D. melanogaster, mutagenesis screens in D. simulans are much less feasible. The sterility of F1 hybrids also precludes direct selection of mutants affecting F1 hybrid phenotypes. These limitations have motivated the screening of natural populations for genetic variation in hybrid incompatibility traits as a way of identifying the causal genes and of overriding some of the incompatibilities.

  • Searching for better species: The lack of backcrossing led to the search for alternative species models, including the search for closer siblings species of D. simulans.

  • Premature pessimism: Sturtevant showed great insight in his early studies, but was overly pessimistic in implying that D. melanogaster and D. simulans have few interesting differences. Many behavioral, ecological, population genetics, and gene expression differences between these species have since been discovered, some of which were reviewed in a special issue of Genetica (Capy and Gibert 2004).

Two sibling species of D. simulans, D. mauritiana and D. sechellia, were discovered on islands in the Indian ocean (Tsacas and David 1974; Tsacas and BäChli 1981). Both species are able to hybridize with D. melanogaster. In terms of hybrid lethality and sterility, the outcome of these crosses is largely the same as with D. simulans (Lachaise et al. 1986). I will therefore discuss D. melanogaster hybrids with all three species. Nineteen years ago in this journal, Provine (1991) recounted the history of the discovery of D. simulans and of early experiments on D. melanogaster/D. simulans hybrids, as well as an appreciative assessment of Sturtevant's contributions to evolutionary biology.

Comparative genomics ca. 1920:

Spontaneous visible mutations in D. melanogaster fueled the great series of fundamental discoveries in genetics made by the Columbia University fly group. By 1929 Sturtevant and colleagues had discovered 54 different visible mutants in D. simulans (Sturtevant 1929). Many of these mutations were similar to known D. melanogaster mutations and could be tested for allelism between species by asking whether they complemented in F1 hybrids. Numerous examples of noncomplementation were found. As noted by Provine (1991, p. 2), Sturtevant (1921, p. 206) concluded that he had made the first discovery of “parallel mutations in distinct species” and thus had “definite proof that related species do have many genes in common, and that identical mutations may occur in different species.” In modern terms, Sturtevant had demonstrated that the two species have orthologous genes and that the function of the genes in each species is the same. This discovery between species separated by only a few million years does not sound particularly surprising in light of contemporary knowledge, but it is difficult to imagine its reception during the 1920s. Little in the published record indicates whether Sturtevant thought his discovery was surprising.

Hybrid incompatibilities:

Sturtevant (1920) found that only one sex of F1 hybrids survives, and which sex survives depends on the direction of crossing. Crosses with D. melanogaster mothers produce only F1 daughters, while those with D. simulans mothers produce mostly F1 sons (F1 female lethality having a variable penetrance). But digging deeper, he realized that F1 lethality is not in fact sex-specific in either direction of crossing. Taking advantage of bridges' discovery (1916) that XXY females have an elevated rate of sex-chromosome nondisjunction, he crossed D. melanogaster XXY females to D. simulans males and obtained many exceptional F1 male hybrids carrying the D. simulans X (henceforth referred to as Xsim), in contrast to the complete lethality of regular F1 males carrying the D. melanogaster X (Xmel). He also noted the absence of any viable exceptional F1 females carrying two copies of Xmel.

In the reciprocal cross, Sturtevant obtained the reciprocal effect. While Xmel/Xsim daughters of D. simulans are normally lethal, XXY D. simulans females crossed to D. melanogaster males produced viable Xsim/Xsim exceptional daughters but no corresponding exceptional Xmel/O males. Sturtevant had discovered that F1 lethality in both directions of crossing is not sex-specific, but rather X chromosome-specific. I believe that this represents the first genetic mapping of a hybrid incompatibility effect. Furthermore, because the D. melanogaster X is associated with lethality and the D. simulans X with viability, Sturtevant demonstrated an asymmetric chromosome-specific effect. This asymmetry is an important prediction in subsequent models developed to explain the evolution of hybrid incompatibilities (Dobzhansky 1937; Muller 1940). As Sturtevant (1920, p. 206) concluded: “it appears that hybrids develop only if they carry a simulans X, but that in the presence of simulans cytoplasm a melanogaster X usually inhibits development even though a simulans X is also present.” This hypothesis implies (1) that Xsim contains genes necessary for hybrid survival in both directions of crossing and (2) that Xmel contains genes deleterious for hybrid survival but only when they interact with maternally inherited factors of D. simulans. [Later experiments refuted an alternative hypothesis raised by Sturtevant (1929) that the D. simulans Y might be causing lethality (Yamamoto 1992)].

In an important way, however, his conclusion was only half right. In the cross of D. simulans mothers to D. melanogaster fathers, Sturtevant's interpretation turned out be entirely correct, at least concerning the Xmel inhibitory effect: Xmel does directly prevent development in a D. simulans cytoplasm. Sawamura et al. (1993c) discovered a pericentric heterochromatic region of Xmel called Zygotic hybrid rescue that is responsible for this lethal effect because deleting it suppresses hybrid female lethality. Subsequent research has demonstrated that lethality is caused by missegregation of this heterochromatic region of Xmel during early embryonic mitoses (Ferree and Barbash 2009). Thus Xmel indeed directly “inhibits development” in the presence of D. simulans maternal cytoplasm.

But in the reciprocal cross of D. melanogaster mothers to D. simulans fathers, it is not true that hybrids develop only if they carry a copy of Xsim. Rather, they are fully viable only if they do not carry a deleterious allele on Xmel. Sturtevant's hypothesis was that viability requires Xsim but is not inhibited by Xmel in this direction of crossing. This reasoning was based in part on the fact that Xsim/Xmel hybrid daughters are usually viable in this cross. But as Sturtevant (1929) himself noted, regular Xmel/Xsim females are poorly viable at high temperatures. This temperature-dependent female lethality as well as the fully penetrant lethality of Xmel/Ysim hybrid sons are caused by the same X-linked D. melanogaster gene (Hmr; Barbash et al. 2000). When the Hmr gene is removed by mutation, then Xmel/Ysim hybrid sons survive quite well, demonstrating that Xsim is not required for viability. The essential distinction is that lethality is caused by the presence of the wild-type activity of this D. melanogaster gene, not by the absence of an essential D. simulans gene product (as implied in Sturtevant's interpretation). Xmel therefore inhibits development in both directions of crossing.

In all crosses, the surviving F1 hybrids remained sterile, precluding study of these hybrid lethality effects by conventional backcrossing approaches. Muller and Pontecorvo, however, invented an ingenious solution that allowed further mapping (Muller and Pontecorvo 1940; Pontecorvo 1943). Triploids of D. melanogaster were widely used by the Morgan school to investigate a range of questions. Triploids segregate one or two copies of each chromosome during meiosis. When mated to diploids, progeny will be aneuploid if the triploid parent transmits different numbers of different chromosomes. Muller and Pontecorvo realized that they could create “partial hybrids” by mating triploid D. melanogaster females to heavily irradiated D. simulans males such that the D. simulans fathers transmitted zero or one intact copy of each chromosome, depending on the extent of radiation damage. These diploid partial hybrids, if viable, would contain different combinations of chromosomes, including genotypes normally obtainable only from a backcross of F1 hybrids to D. melanogaster (albeit without the possibility of having recombinant chromosomes). They obtained several genotypes of female and male viable hybrids homozygous or hemizygous for Xmel. These combinations included 2mel/2mel; 3mel/3sim and 2mel/2sim; 3mel/3mel, but not 2mel/2sim; 3mel/3sim. Pontecorvo (1943) concluded that hybrid male lethality in normal diploid crosses of D. melanogaster females and D. simulans males is caused by a three-way lethal interaction among Xmel, 2sim, and 3sim. The D. melanogaster Hmr and D. simulans Lhr genes are at least partially responsible for the Xmel and 2sim effects, respectively (Brideau et al. 2006); the cause of the chromosome 3 effect remains unidentified.

In their initial experiment, Muller and Pontecorvo (1940) recovered a single fertile partial hybrid that turned out to be heterozygous for the D. simulans Y and 4th chromosomes in an otherwise D. melanogaster background. Mating of this animal to D. melanogaster allowed them to introgress 4sim into D. melanogaster. They found that 4sim causes male but not female sterility when homozygous (Muller and Pontecorvo 1941). Masly et al. (2006) recreated an introgression strain of 4sim into D. melanogaster and discovered that an essential male fertility gene, Jyalpha, has transposed from chromosome 4 in D. melanogaster and outgroup species to chromosome 3 in the ancestor of D. simulans. D. melanogaster flies homozygous for 4sim thus lack any copy of JYalpha and are sterile. Although F1 D. melanogaster/D. simulans hybrid males are sterile due to other causes, this process of gene transposition might cause incompatibility in other hybrids and also represents a mechanism distinct from divergence in gene function that can cause hybrid incompatibility.

After Pontecorvo's analysis of partial hybrids research, the genetics of D. melanogaster/D. simulans hybrid incompatibilities remained largely dormant until Watanabe's (1979) stunning discovery of the Lethal hybrid rescue (Lhr) mutation in D. simulans. This mutation, when crossed from D. simulans males to D. melanogaster females, dominantly suppresses the F1 lethality of the hybrid sons. Preliminary experiments suggested that the Lhr mutation maps to a single locus on the second chromosome, leading to the conclusion that “the isolation of melanogaster and simulans may depend on fewer genes than previously thought” (Watanabe 1979). There was hope that isolating such a suppressor mutation would lead to an understanding of the reason why hybrid males normally die, but that hope was tempered by the formidable obstacle of identifying such a suppressor from D. simulans during this pre-genomic era. This challenge motivated Hutter and Ashburner (1987) to search for an analogous suppressor in D. melanogaster. They found one, but it clearly was a different gene because it mapped to the X chromosome rather than to the 2nd. This suppressor, Hybrid male rescue (Hmr), was similar to Lhr in again mapping to a single region, confirming that F1 hybrid male lethality can be overridden by single-gene mutations. Molecular analysis of Hmr and Lhr later showed that both suppressor alleles are loss-of-function mutations, demonstrating that it is indeed the wild-type activity of each gene that kills hybrids (Barbash et al. 2003; Brideau et al. 2006).

What of the female F1 lethality found in the reciprocal cross of D. simulans females to D. melanogaster males? Crosses of D. simulans Lhr females to wild-type D. melanogaster males produced modest viability of F1 daughters (Watanabe 1979). Likewise, crosses of wild-type D. simulans females to D. melanogaster males carrying the rescue chromosome In(1)AB, Hmr2 also produced viable F1 daughters (Hutter et al. 1990). These observations led to a model in which the wild-type alleles of Hmr and Lhr cause lethality of both F1 hybrid sons of D. melanogaster mothers and F1 hybrid daughters of D. simulans mothers (Hutter et al. 1990). One complication, however, is Sturtevant's (1929) observation that F1 hybrid female lethality is variably penetrant, depending in part on variation in D. simulans (Sawamura et al. 1993a; Orr 1996). Another complication is that Sawamura et al. (1993c) found that the previously mentioned Zhr gene suppresses the F1 female lethality but not the F1 male lethality. They further found that some In(1)AB, Hmr2 chromosomes carry Zhr mutations. These complicating facts underscore the need to demonstrate by mapping experiments that phenotypes associated with different hybrid rescue strains are actually caused by the specific hybrid rescue mutation and by not other variants in the genetic background. A data reassessment led to a convincing argument that the hybrid lethalities in the reciprocal D. melanogaster/D. simulans crosses have distinct genetic causes (Sawamura et al. 1993b).

Considering the many insights gained from study of these hybrid rescuing mutations and the ease of performing mutagenesis screens in D. melanogaster, it is surprising that no one has systematically screened for more such suppressors of lethality. Two screens have been performed, however, using large collections of D. melanogaster deletion strains that searched for the opposite effect: genomic regions that when deleted cause rather than suppress hybrid lethality. Coyne et al. (1998) crossed females from >100 D. melanogaster deletion strains to D. simulans males in a search for chromosomal regions that when deleted cause F1 female lethality and found only a handful of regions causing partially penetrant lethality. Presgraves (2003) repeated such a screen but used Lhr mutant D. simulans males to look for chromosomal regions that when deleted suppress Lhr-dependent male rescue. This screen was clearly more sensitive, identifying 20 lethal regions, and has since led to the discovery of incompatibilities caused by components of the nuclear pore (Tang and Presgraves 2009). These incompatibilities appear not to normally affect F1 male viability, suggesting that if Sturtevant had achieved his goal of obtaining fertile F1 hybrids, additional incompatibilities would become apparent among the backcross progeny.

The realization of Sturtevant's dream?

In 1996, such backcross studies suddenly appeared possible when Davis et al. (1996) reported the discovery of fertile F1 female hybrids. They found that, with certain strain combinations, F1 hybrid daughters from D. simulans mothers and D. melanogaster fathers were partially fertile and could produce some backcross progeny. The genetic basis of sterility rescue was unclear, but this report suggested the exciting possibility of performing high-resolution genetic mapping between D. melanogaster and D. simulans through backcrossing—Sturtevant's dream realized. A subsequent study discovered fertility rescue in F1 female progeny of the reciprocal cross of D. melanogaster females to either D. simulans or D. mauritiana males (Barbash and Ashburner 2003). Rescue was also shown to depend in part on mutations in Hmr, demonstrating that Hmr+ is a dominant female sterility gene. Backcross progeny were again obtained, but with a low yield of less than one viable progeny per F1 hybrid parent.

This low yield has tempered some of the hope that genetic mapping could be applied across the genome between D. melanogaster and D. simulans. Nevertheless, at least four successful interspecific introgressions have been obtained and used to address different questions. Two separate regions of chromosome 2L were simultaneously introgressed from D. simulans into D. melanogaster and, when made homozygous, were inferred to contain several genes causing male sterility and at least one gene causing female sterility (Sawamura et al. 2000, 2004). These studies confirm that backcross hybrids do indeed have more incompatibilities than F1 hybrids. The bwD region on chromosome 2R of D. melanogaster was introgressed into D. simulans to examine its nuclear organization in a foreign-species background (Sage and Csink 2003). In a third case, fertility rescue in F1 hybrids was used to introgress mitochondria from D. simulans into D. melanogaster, allowing a detailed study of fitness effects in strains that are hybrids between their nuclear and mitochondrial genomes (Rand et al. 2006). Finally, in the fourth case, the recreation of the IVsim introgression into D. melanogaster mentioned previously was created using fertility rescue strains (Masly et al. 2006).

Morphological differences and hidden regulatory divergence:

D. melanogaster and D. simulans are morphologically very similar, with the key diagnostic differences being the size and shape of two cuticular structures that surround the male genitalia. Although first reported by Sturtevant (1919), he later gave credit for this discovery to Bridges (Sturtevant 1920). Closer examination discovered a difference between the species in the size of a region on the second leg devoid of tiny hairs (trichomes). By examining phenotypes in F1 hybrids of D. melanogaster and D. simulans Ultrabithorax (Ubx) mutations, Stern (1998) showed that this phenotypic difference is caused in part by divergence in the Ubx gene.

Similar studies identified the genetic basis of a naked cuticle phenotype in larvae that is distinct to D. sechellia. F1 D. melanogaster/D. sechellia hybrids were found to have the D. melanogaster phenotype, while F1 hybrids mutant for the D. melanogaster ovo/shavenbaby gene display the D. sechellia phenotype. Additional experiments showed that this morphological difference is caused by divergence in regulatory regions of ovo/svb between D. melanogaster and D. sechellia (Sucena and Stern 2000).

These studies demonstrate the existence of distinct morphological differences between D. melanogaster and its siblings, which are in some cases attributable to divergence in single genes. The simple genetic basis of these phenotypes made them solvable using F1 hybrids. F1 hybrids have also revealed that gene regulatory pathways can have functionally detectable divergence, even for seemingly stable phenotypes. Sturtevant (1920) noted that F1 hybrids are often missing large thoracic bristles (macrochaete), yet the number and location of these bristles are identical in both D. melanogaster and D. simulans. This observation of hybrid breakdown in the conserved bristle pattern was a key example in the concept of developmental systems drift (True and Haag 2001). This theory suggests that genetic regulatory mechanisms can evolve and diverge even as the phenotypes that they control remain under stabilizing selection.

The idea that apparently identical traits between species can arise from divergent genes was apparent to Muller and Pontecorvo (1941) when they reported that D. melanogaster flies homozygous for 4sim manifest a range of phenotypes in addition to male sterility, including “slight, variable” morphological defects and “semi-simulans male genitalia,” while heterozygotes show unusual complementation of two different 4mel mutations. Summarizing all the effects attributable to divergence of the tiny chromosome 4 between D. melanogaster and D. simulans (as well as other effects of Y chromosome divergence), Muller and Pontecorvo (1941, p. 157) concluded that “the fact that even these minor chromosomes exhibit so many gene differences indicates that the reaction systems producing the similar phenotypes of apparently closely related species may be highly divergent. Hybrid sterility is but one expression of this cryptic divergence, which need not in itself have had a selective value.” This inference of extensive genetic variation was certainly unanticipated by Sturtevant in his initial comparison of the species, and other important implications of Muller and Pontecorvo's conclusion have been discussed by Provine (1991).

Weisbrot (1963) also observed a wide range of morphological defects in four female partial hybrids that he generated using the method of Muller and Pontecorvo. Like them, he also concluded that closely related and apparently similar species such as D. melanogaster and D. simulans are likely to have different alleles of many of their genes, and thus speciation would result from changes at many loci. Interestingly, Weisbrot (1963) cites Sturtevant (1948) as the clearest proponent of the alternative view that only a small number of loci directly involved in speciation would be different, even between well-separated species.

Further analyses have strongly suggested that the hybrid bristle-loss phenotype is due to divergence in genes that regulate the bristle pattern, rather than to an indirect consequence of unrelated hybrid incompatibilities. Biddle (1932) performed an extensive study of variation in bristle loss involving a total of 52,784 hybrids. Among Biddle's conclusions was that the degree of bristle loss did not correlate with other morphological defects and that hybrid females generally showed less bristle loss than Xsim hybrid males despite being poorly viable at higher temperatures. Biddle also found that the X chromosome was primarily responsible for variation in hybrid bristle loss among different D. simulans strains. Takano later showed that Xsim F1 hybrid males have more severe bristle loss than Xmel F1 males and identified a QTL on the D. simulans X responsible for variation among D. simulans lines (Takano 1998; Takano-Shimizu 2000). The identity of these gene(s) remains unknown. The pro-neural genes achaete-scute or their regulators are good candidates because genetic studies suggest that achaete-scute misregulation is a primary cause of the hybrid bristle-loss phenotype (Skaer and Simpson 2000).

Another conserved regulatory pathway, X chromosome dosage compensation, also shows evidence of disruption in hybrids. The X has reduced transcription levels in lethal hybrids, and several components of the dosage compensation complex fail to localize to the X (Pal Bhadra et al. 2006; Chatterjee et al. 2007).

Genome-scale regulatory divergence:

The bristle-loss phenotype identified one example of a hybrid defect that is likely to reflect a failure in gene expression in hybrids. Gene misexpression now appears to be common in hybrids. Of 19 D. melanogaster lacZ enhancer trap lines, 13 showed aberrant expression in D. melanogaster/D. simulans or D. melanogaster/D. mauritiana hybrids compared with their expression in D. melanogaster (Hammerle and Ferrus 2003). A large-scale analysis of >4000 transcripts also found many changes in expression level in F1 hybrid females that were not simply additive between the D. melanogaster and D. simulans parents (Ranz et al. 2004).

What is the underlying cause of such misexpression, and what is it revealing about the divergence of gene regulation between species? Wittkopp et al. (2004) developed a clever method of distinguishing whether gene expression changes between D. melanogaster and D. simulans are caused by divergence in cis-regulatory sequences of individual genes vs. divergence in trans-acting regulators. They quantified gene expression levels in both species, as well as the species-specific expression of each allele in reciprocal F1 hybrids. Substantial trans-acting effects were found, but more strikingly, almost all genes demonstrating divergence in expression between the species showed evidence of divergence in their cis regulatory sequences. A recent larger-scale study has corroborated these conclusions (Graze et al. 2009).

Behavioral differences:

Studies of interspecific differences in behavior provide a powerful complement to mutagenesis screens for identifying the genetic basis of behavior. Interspecific behavioral differences are also an important contributor to reproductive isolation and speciation. Hybrids helped to identify one of the first genes involved in such a behavioral difference. D. melanogaster and D. simulans males differ substantially in the courtship songs that they generate by wing vibration. These differences include the length of the interval between pulses (the interpulse interval) and the fluctuations in the interpulse interval that change rhythmically over tens of seconds (the rhythm). By using attached X chromosomes or the Lhr mutation, Kyriacou and Hall (1986) generated F1 hybrid males from D. melanogaster mothers that carried either Xmel or Xsim. Their study found that the species difference for rhythm maps to the X, while the interpulse interval difference is autosomal. Subsequent transformation experiments demonstrated that the difference in song rhythm between the species is controlled by divergence in a small region of the X-linked period gene (Wheeler et al. 1991).

D. melanogaster and D. simulans females differ in their cuticular hydrocarbons, and these differences contribute to mating isolation between the species (Coyne and Oyama 1995). To map the causal genes, Coyne (1996) used a method of creating partial hybrids. This method was originally developed by Grell (1976) for mapping an interspecific difference in an isozyme. Mapping is done using D. melanogaster stocks that contain a compound chromosome for one autosomal arm along with two free copies of the other arm. The D. simulans stocks used carry a translocation of the Y to one autosome arm, along with a freely segregating copy of the other arm and a wild-type version of the same autosome. When, for example, C(3L) D. melanogaster females are mated with T(Y;3) D. simulans males, F1 female hybrids that are homozygous for either 3Lmel or 3Lsim and heterozygous for the remainder of the genome can potentially be produced. In crosses with all four possible D. melanogaster compound stocks, not all possible hybrid genotypes survive (Grell 1976). Nevertheless, the interspecific difference in female hydrocarbons was shown to map largely to chromosome 3, with a particularly large contribution from recessive genes on 3Lsim (Coyne 1996).

Closer in:

D. melanogaster and D. simulans show complete reproductive isolation, and it is thus difficult to distinguish which among the many hybrid incompatibilities may have been most relevant during early stages of speciation. The divergence of D. simulans from its sibling species D. sechellia and D. mauritiana occurred more recently (Kliman et al. 2000). One consequence is that crosses between pairs of these species produce fertile F1 female hybrids (Lachaise et al. 1986), allowing for regions to be introgressed between them. Their similarity to D. melanogaster facilitated the use of genetic (Coyne 1983) and molecular (Perez et al. 1993) markers so that backcross progeny can be easily tracked and introgression lengths determined. Two of the most important results from these studies showed that hybrid male sterility genes are much more common among these species than either hybrid female sterility or hybrid inviability genes and that more hybrid male sterility genes have accumulated on the X compared to the autosomes (Hollocher and Wu 1996; True et al. 1996; Tao et al. 2003; Masly and Presgraves 2007). Fine-scale mapping of one introgression led to the identification of Odysseus, the first known hybrid sterility gene (Ting et al. 2000).

And farther out:

The archives of unpublished results undoubtedly contain records of failures of D. melanogaster to mate to species more distant than D. simulans. The goal of making such hybrids has obvious appeal for studying morphological and behavioral traits that have diverged among Drosophila species. Several researchers have attempted to circumvent mating isolation by introducing the pole cells (germline precursor cells) from foreign species into D. melanogaster to create chimeras with a D. melanogaster soma and a foreign species germline. Such chimeras can then be mated to D. melanogaster to create hybrids essentially equivalent to the product of a mating between foreign species females and D. melanogaster males. Attempts using species from two non-melanogaster subgroups of the melanogaster group produced hybrids, but they died as embryos or larvae (Lawrence et al. 1993). The species used were too distant from D. melanogaster, which inspired Sánchez and Santamaria (1997) to try again using more closely related species from within the melanogaster subgroup. Remarkably, viable hybrids were obtained using pole cells transplanted from either D. yakuba or D. teissieri, demonstrating that no dominant hybrid lethality genes have evolved between these species and D. melanogaster. Even more striking was that at least some of these hybrids were male, and the D. melanogaster/D. yakuba hybrid males were almost as viable as hybrid females. Thus X-linked recessive hybrid lethality genes such as Hmr have also not evolved between these species, at least not on the D. yakuba X.

Matute et al. (2009) have recently discovered that D. melanogaster females will mate to males of D. santomea, a sibling species of D. yakuba, and produce viable F1 female hybrids but no males. This cross is essentially the reciprocal of the above hybridization using D. melanogaster chimeras and suggests that there are X-linked recessive gene(s) on the D. melanogaster X that cause hybrid lethality with D. santomea. An intriguing question is whether Hmr is one of these genes.

D. santomea lacks abdominal pigmentation, a phenotype unique among melanogaster subgroup species. Divergence of the pigmentation locus tan was shown to contribute to this phenotypic difference between D. santomea and D. yakuba (Jeong et al. 2008). Matute et al. (2009) have used D. melanogaster/D. santomea hybrids to argue against this claim for the role of tan in pigmentation divergence, but Rebeiz et al. (2009) have persuasively countered that using hybrids between more distant species can lead to mis-inference about evolutionary divergence that has occurred between more closely related hybrids. For example, the D. melanogaster/D. santomea hybrids may have aberrations in gene expression that interact in unpredictable ways to affect expression of tan and other pigmentation genes. If so, then the goal of using D. melanogaster hybrids with distant species to infer the genetic basis of interspecific differences may be prone to artifactual results.

The past 20 years, and the next 90:

Since Provine's (1991) review of D. melanogaster/D. simulans hybrids almost 20 years ago, major progress has been achieved in identifying some of the genes that cause the hybrid incompatibility phenotypes discovered by Sturtevant (1920) and by Muller and Pontecorvo (1940). Phenotypic differences in behavioral and morphological traits have been discovered, and in some cases the causal genes have been mapped and identified using hybrids. The discovery of partial fertility rescue has perhaps not been the dramatic breakthrough that was hoped for, but several studies have achieved interspecific introgressions and used them to address important questions.

Undoubtedly the major advance over the past 20 years of research into these two species has been the publication of their genome sequences (Adams et al. 2000; Begun et al. 2007). Looking ahead, interspecific differences and hybrid incompatibilities between D. melanogaster and D. simulans can now be mapped much more rapidly. These genome sequences will also accelerate studies to address whether phenotypic differences and incompatibilities are direct consequences of adaptive evolution.

Hybrids will also offer many opportunities to dissect the genetic causes of behavioral differences beyond what has been discovered for the period locus. D. melanogaster and D. simulans differ markedly in mating behavior (Manning 1959), and these species-specific differences are an under-studied contributor to reproductive isolation in Drosophila.

Large-scale genome evolution is another research area for which hybrids may be profitably utilized. D. melanogaster and D. simulans are significantly diverged in heterochromatic satellite DNAs and transposable elements (Lohe and Roberts 1988; Vieira and Biémont 2004). It remains unclear, however, how much of such divergence contributes to differences in fitness and phenotype. One hybrid incompatibility has already been shown to be a direct consequence of heterochromatin divergence (Ferree and Barbash 2009). Because heterochromatic sequences cannot be easily manipulated for genetic analysis within species, examination of chromatin states and nuclear structure in hybrids can provide insight into the functional consequences of large-scale differences in DNA content.

Hybrids fascinate the biologist because they are the product of two separately evolved genomes combined in a single individual. Hybrid plants are of tremendous importance to agriculture, and hybrid animals have long served humans. Sturtevant's main contribution (1920) was to recognize the power in applying Drosophila genetics to a hybrid system. The greatest testimony to the impact of Sturtevant's 1920 article is that one can assert with confidence that it will continue to motivate discoveries beyond the imagination of its author.

Acknowledgments

I thank Adam Wilkins for his guidance and helpful suggestions. Our research on hybrids is supported by National Institutes of Health grant 2R01GM074737.

References

  1. Adams, M. D., S. E. Celniker, R. A. Holt, C. A. Evans, J. D. Gocayne et al., 2000. The genome sequence of Drosophila melanogaster. Science 287 2185–2195. [DOI] [PubMed] [Google Scholar]
  2. Barbash, D. A., and M. Ashburner, 2003. A novel system of fertility rescue in Drosophila hybrids reveals a link between hybrid lethality and female sterility. Genetics 163 217–226. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Barbash, D. A., J. Roote and M. Ashburner, 2000. The Drosophila melanogaster Hybrid male rescue gene causes inviability in male and female species hybrids. Genetics 154 1747–1771. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Barbash, D. A., D. F. Siino, A. M. Tarone and J. Roote, 2003. A rapidly evolving MYB-related protein causes species isolation in Drosophila. Proc. Natl. Acad. Sci. USA 100 5302–5307. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Begun, D. J., A. K. Holloway, K. Stevens, L. W. Hillier, Y. P. Poh et al., 2007. Population genomics: whole-genome analysis of polymorphism and divergence in Drosophila simulans. PLoS Biol. 5 e310. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Biddle, R. L., 1932. The bristles of hybrids between Drosophila melanogaster and Drosophila simulans. Genetics 17 153–174. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Brideau, N. J., H. A. Flores, J. Wang, S. Maheshwari, X. Wang et al., 2006. Two Dobzhansky-Muller genes interact to cause hybrid lethality in Drosophila. Science 314 1292–1295. [DOI] [PubMed] [Google Scholar]
  8. Bridges, C. B., 1916. Non-disjunction as proof of the chromosome theory of heredity. Genetics 1 1–52, 107–163. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Capy, P., and P. Gibert, 2004. Drosophila melanogaster, Drosophila simulans: so similar yet so different. Genetica 120 5–16. [DOI] [PubMed] [Google Scholar]
  10. Chatterjee, R. N., P. Chatterjee, A. Pal and M. Pal-Bhadra, 2007. Drosophila simulans Lethal hybrid rescue mutation (Lhr) rescues inviable hybrids by restoring X chromosomal dosage compensation and causes fluctuating asymmetry of development. J. Genet. 86 203–215. [DOI] [PubMed] [Google Scholar]
  11. Coyne, J. A., 1983. Genetic basis of differences in genital morphology among three sibling species of Drosophila. Evolution 37 1101–1118. [DOI] [PubMed] [Google Scholar]
  12. Coyne, J. A., 1996. Genetics of differences in pheromonal hydrocarbons between Drosophila melanogaster and D. simulans. Genetics 143 353–364. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Coyne, J. A., and R. Oyama, 1995. Localization of pheromonal sexual dimorphism in Drosophila melanogaster and its effect on sexual isolation. Proc. Natl. Acad. Sci. USA. 92 9505–9509. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Coyne, J. A., S. Simeonidis and P. Rooney, 1998. Relative paucity of genes causing inviability in hybrids between Drosophila melanogaster and D. simulans. Genetics 150 1091–1103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Davis, A. W., J. Roote, T. Morley, K. Sawamura, S. Herrmann et al., 1996. Rescue of hybrid sterility in crosses between D. melanogaster and D. simulans. Nature 380 157–159. [DOI] [PubMed] [Google Scholar]
  16. Dobzhansky, T., 1937. Genetics and the Origin of Species. Columbia University Press, New York.
  17. Ferree, P. M., and D. A. Barbash, 2009. Species-specific heterochromatin prevents mitotic chromosome segregation to cause hybrid lethality in Drosophila. PLoS Biol. 7 e1000234. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Graze, R. M., L. M. McIntyre, B. J. Main, M. L. Wayne and S. V. Nuzhdin, 2009. Regulatory divergence in Drosophila melanogaster and D. simulans, a genomewide analysis of allele-specific expression. Genetics 183 547–561. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Grell, E. H., 1976. Genetic analysis of aspartate aminotransferase isozymes from hybrids between Drosophila melanogaster and Drosophila simulans and mutagen-induced isozyme variants. Genetics 83 753–764. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Hammerle, B., and A. Ferrus, 2003. Expression of enhancers is altered in Drosophila melanogaster hybrids. Evol. Dev. 5 221–230. [DOI] [PubMed] [Google Scholar]
  21. Hollocher, H., and C.-I. Wu, 1996. Genetics of reproductive isolation in the Drosophila simulans clade: X vs. autosomal effects and male vs. female effects. Genetics 143 1243–1255. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Hutter, P., and M. Ashburner, 1987. Genetic rescue of inviable hybrids between Drosophila melanogaster and its sibling species. Nature 327 331–333. [DOI] [PubMed] [Google Scholar]
  23. Hutter, P., J. Roote and M. Ashburner, 1990. A genetic basis for the inviability of hybrids between sibling species of Drosophila. Genetics 124 909–920. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Jeong, S., M. Rebeiz, P. Andolfatto, T. Werner, J. True et al., 2008. The evolution of gene regulation underlies a morphological difference between two Drosophila sister species. Cell 132 783–793. [DOI] [PubMed] [Google Scholar]
  25. Kliman, R. M., P. Andolfatto, J. A. Coyne, F. Depaulis, M. Kreitman et al., 2000. The population genetics of the origin and divergence of the Drosophila simulans complex species. Genetics 156 1913–1931. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Kyriacou, C. P., and J. C. Hall, 1986. Interspecific genetic control of courtship song production and reception in Drosophila. Science 232 494–497. [DOI] [PubMed] [Google Scholar]
  27. Lachaise, D., J. R. David, F. Lemeunier, L. Tsacas and M. Ashburner, 1986. The reproductive relationships of Drosophila sechellia with Drosophila mauritiana, Drosophila simulans and Drosophila melanogaster from the Afrotropical region. Evolution 40 262–271. [DOI] [PubMed] [Google Scholar]
  28. Lawrence, P. A., M. Ashburner and P. Johnston, 1993. An attempt to hybridize Drosophila species using pole cell transplantation. Genetics 134 1145–1148. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Lohe, A. R., and P. A. Roberts, 1988. Evolution of satellite DNA sequences in Drosophila in Heterochromatin: Molecular and Structural Aspects, edited by R. S. Verma. Cambridge University Press, Cambridge, UK.
  30. Manning, A., 1959. The sexual behaviour of two sibling Drosophila species. Behaviour 15 123–145. [Google Scholar]
  31. Masly, J. P., and D. C. Presgraves, 2007. High-resolution genome-wide dissection of the two rules of speciation in Drosophila. PLoS Biol. 5 e243. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Masly, J. P., C. D. Jones, M. A. Noor, J. Locke and H. A. Orr, 2006. Gene transposition as a cause of hybrid sterility in Drosophila. Science 313 1448–1450. [DOI] [PubMed] [Google Scholar]
  33. Matute, D. R., I. A. Butler and J. A. Coyne, 2009. Little effect of the tan locus on pigmentation in female hybrids between Drosophila santomea and D. melanogaster. Cell 139 1180–1188. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Muller, H. J., 1940. Bearings of the ‘Drosophila’ work on systematics, pp. 185–268 in The New Systematics, edited by J. Huxley. Clarendon Press, Oxford.
  35. Muller, H. J., and G. Pontecorvo, 1940. Recombinants between Drosophila species, the F1 hybrids of which are sterile. Nature 146 199–200. [Google Scholar]
  36. Muller, H. J., and G. Pontecorvo, 1941. Recessive genes causing interspecific sterility and other disharmonies between Drosophila melanogaster and Drosophila simulans. Genetics 10 157. [Google Scholar]
  37. Orr, H. A., 1996. The unexpected recovery of hybrids in a Drosophila species cross: a genetic analysis. Genet. Res. 67 11–18. [DOI] [PubMed] [Google Scholar]
  38. Pal Bhadra, M., U. Bhadra and J. A. Birchler, 2006. Misregulation of sex-lethal and disruption of male-specific lethal complex localization in Drosophila species hybrids. Genetics 174 1151–1159. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Perez, D. E., C. I. Wu, N. A. Johnson and M. L. Wu, 1993. Genetics of reproductive isolation in the Drosophila simulans clade: DNA marker-assisted mapping and characterization of a hybrid-male sterility gene, Odysseus (Ods). Genetics 134 261–275. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Pontecorvo, G., 1943. Viability interactions between chromosomes of Drosophila melanogaster and Drosophila simulans. J. Genet. 45 51–66. [Google Scholar]
  41. Presgraves, D. C., 2003. A fine-scale genetic analysis of hybrid incompatibilities in Drosophila. Genetics 163 955–972. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Provine, W. B., 1991. Alfred Henry Sturtevant and crosses between Drosophila melanogaster and Drosophila simulans. Genetics 129 1–5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Rand, D. M., A. Fry and L. Sheldahl, 2006. Nuclear-mitochondrial epistasis and Drosophila aging: introgression of Drosophila simulans mtDNA modifies longevity in D. melanogaster nuclear backgrounds. Genetics 172 329–341. [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Ranz, J. M., K. Namgyal, G. Gibson and D. L. Hartl, 2004. Anomalies in the expression profile of interspecific hybrids of Drosophila melanogaster and Drosophila simulans. Genome Res. 14 373–379. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Rebeiz, M., M. Ramos-Womack, S. Jeong, P. Andolfatto, T. Werner et al., 2009. Evolution of the tan locus contributed to pigment loss in Drosophila santomea: a response to Matute et al. Cell 139 1189–1196. [DOI] [PubMed] [Google Scholar]
  46. Sage, B. T., and A. K. Csink, 2003. Heterochromatic self-association, a determinant of nuclear organization, does not require sequence homology in Drosophila. Genetics 165 1183–1193. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Sánchez, L., and P. Santamaria, 1997. Reproductive isolation and morphogenetic evolution in Drosophila analyzed by breakage of ethological barriers. Genetics 147 231–242. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Sawamura, K., T. Taira and T. K. Watanabe, 1993. a Hybrid lethal systems in the Drosophila melanogaster species complex. I. The maternal hybrid rescue (mhr) gene of Drosophila simulans. Genetics 133 299–305. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Sawamura, K., T. K. Watanabe and M.-T. Yamamoto, 1993. b Hybrid lethal systems in the Drosophila melanogaster species complex. Genetica 88 175–185. [DOI] [PubMed] [Google Scholar]
  50. Sawamura, K., M.-T. Yamamoto and T. K. Watanabe, 1993. c Hybrid lethal systems in the Drosophila melanogaster species complex. II. The Zygotic hybrid rescue (Zhr) gene of Drosophila melanogaster. Genetics 133 307–313. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Sawamura, K., A. W. Davis and C.-I. Wu, 2000. Genetic analysis of speciation by means of introgression into Drosophila melanogaster. Proc. Natl. Acad. Sci. USA 97 2652–2655. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Sawamura, K., T. L. Karr and M.-T. Yamamoto, 2004. Genetics of hybrid inviability and sterility in Drosophila: dissection of introgression of D. simulans genes in D. melanogaster genome. Genetica 120 253–260. [DOI] [PubMed] [Google Scholar]
  53. Skaer, N., and P. Simpson, 2000. Genetic analysis of bristle loss in hybrids between Drosophila melanogaster and D. simulans provides evidence for divergence of cis-regulatory sequences in the achaete-scute gene complex. Dev. Biol. 221 148–167. [DOI] [PubMed] [Google Scholar]
  54. Stern, D. L., 1998. A role of Ultrabithorax in morphological differences between Drosophila species. Nature 396 463–466. [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. Sturtevant, A. H., 1919. A new species closely resembling Drosophila melanogaster. Psyche 26 153–155. [Google Scholar]
  56. Sturtevant, A. H., 1920. Genetic studies of Drosophila simulans. I. Introduction. Hybrids with Drosophila melanogaster. Genetics 5 488–500. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Sturtevant, A. H., 1921. Genetic studies of Drosophila simulans. III. Autosomal genes. General discussion. Genetics 6 179–207. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Sturtevant, A. H., 1929. The genetics of Drosophila simulans. Carnegie Inst. Wash. Publ. 399 1–62. [Google Scholar]
  59. Sturtevant, A. H., 1948. The evolution and function of genes. Am. Sci. 36 225–236. [PubMed] [Google Scholar]
  60. Sucena, E., and D. L. Stern, 2000. Divergence of larval morphology between Drosophila sechellia and its sibling species caused by cis-regulatory evolution of ovo/shaven-baby. Proc. Natl. Acad. Sci. USA 97 4530–4534. [DOI] [PMC free article] [PubMed] [Google Scholar]
  61. Takano, T. S., 1998. Loss of notum macrochaetae as an interspecific hybrid anomaly between Drosophila melanogaster and D. simulans. Genetics 149 1435–1450. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Takano-Shimizu, T., 2000. Genetic screens for factors involved in the notum bristle loss of interspecific hybrids between Drosophila melanogaster and D. simulans. Genetics 156 269–282. [DOI] [PMC free article] [PubMed] [Google Scholar]
  63. Tang, S., and D. C. Presgraves, 2009. Evolution of the Drosophila nuclear pore complex results in multiple hybrid incompatibilities. Science 323 779–782. [DOI] [PMC free article] [PubMed] [Google Scholar]
  64. Tao, Y., S. Chen, D. L. Hartl and C. C. Laurie, 2003. Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. I. Differential accumulation of hybrid male sterility effects on the X and autosomes. Genetics 164 1383–1397. [DOI] [PMC free article] [PubMed] [Google Scholar]
  65. Ting, C.-T., S.-C. Tsaur and C.-I. Wu, 2000. The phylogeny of closely related species as revealed by the genealogy of a speciation gene, Odysseus. Proc. Natl. Acad. Sci. USA 97 5313–5316. [DOI] [PMC free article] [PubMed] [Google Scholar]
  66. True, J. R., and E. S. Haag, 2001. Developmental system drift and flexibility in evolutionary trajectories. Evol. Dev. 3 109–119. [DOI] [PubMed] [Google Scholar]
  67. True, J. R., B. S. Weir and C. C. Laurie, 1996. A genome-wide survey of hybrid incompatibility factors by the introgression of marked segments of Drosophila mauritiana chromosomes into Drosophila simulans. Genetics 142 819–837. [DOI] [PMC free article] [PubMed] [Google Scholar]
  68. Tsacas, L., and G. BäChli, 1981. Drosophila sechellia, n.sp., the eighth species from the melanogaster subgroup, from the Seychelles Islands (Diptera, Drosophilidae). Revue Fr. Ent. 3 146–150 (in French). [Google Scholar]
  69. Tsacas, L., and J. R. David, 1974 Drosophila mauritiana n.sp. of the melanogaster group from the Island Mauritius (Diptera, Drosophilidae). Bull. Soc. Entomol. Fr. 79 42–46 (in French).
  70. Vieira, C., and C. Biémont, 2004. Transposable element dynamics in two sibling species: Drosophila melanogaster and Drosophila simulans. Genetica 120 115–123. [DOI] [PubMed] [Google Scholar]
  71. Watanabe, T. K., 1979. A gene that rescues the lethal hybrids between Drosophila melanogaster and Drosophila simulans. Jpn. J. Genet. 54 325–331. [Google Scholar]
  72. Weisbrot, D. R., 1963. Studies of differences in the genetic architecture of related species of Drosophila. Genetics 48 1121–1139. [DOI] [PMC free article] [PubMed] [Google Scholar]
  73. Wheeler, D. A., C. P. Kyriacou, M. L. Greenacre, Q. Yu, J. E. Rutila et al., 1991. Molecular transfer of a species-specific behavior from Drosophila simulans to Drosophila melanogaster. Science 251 1082–1085. [DOI] [PubMed] [Google Scholar]
  74. Wittkopp, P. J., B. K. Haerum and A. G. Clark, 2004. Evolutionary changes in cis and trans gene regulation. Nature 430 85–88. [DOI] [PubMed] [Google Scholar]
  75. Yamamoto, M.-T., 1992. Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome. Genetica 87 151–158. [DOI] [PubMed] [Google Scholar]

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