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. 2007 Sep 18;104(39):15412–15417. doi: 10.1073/pnas.0707445104

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© 2007 by The National Academy of Sciences of the USA

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Fig. 2. — Positive selection of the MSL complex and X chromosomal MSL-binding sites might result in hybrid incompatibility. (A) Two-locus D–M model for hybrid incompatibility between closely related species. Loci A and B interact in the ancestral species. During (reproductive or recombinational) isolation, there is a neutral fixation of the a and b alleles in the two populations, which is tolerated because the new alleles (a and b) are still compatible with the old alleles (B and A, respectively). However, this fixation results in hybrid incompatibility because of negative epistatic interactions between the new a and b alleles. This model can explain the onset of incompatibilities even under neutral evolution (56). (B) In the case of positive selection (bold arrows) driving the interaction of the A and B loci, only one lineage may evolve to the new a and b alleles, resulting in incompatibility with the other lineage, which still preserves the ancestral A and B alleles. (C) MSL1 and MSL2 could represent the A and B loci in the D–M model, with the positive selection (bold arrows) at their interaction interface resulting in hybrid inviability. Under this model, male hybrids containing either the D. melanogaster or D. simulans X chromosomes would be inviable because the protein composition is expected to be the same in both cases. (D) Model for hybrid incompatibility with MSL1–MSL2 and the X chromosomal MSL-binding sites, as A and B loci, respectively. Positive selection (bold arrows) in D. melanogaster has resulted in rapid evolution of the MSL1–MSL2 genes and (we infer) the X chromosomal MSL-binding sites. In male hybrids, D. simulans MSL1 and MSL2 are unable to recognize “newly evolved” MSL-binding sites on the D. melanogaster X chromosome resulting in mislocalization of the MSL complex in hybrids with a D. melanogaster X chromosome (44). However, hybrids with a D. simulans X chromosome localize the MSL complex normally (45) because the D. melanogaster MSL1 and MSL2 proteins retain an ancestral DNA-binding ability. (E) Known male hybrid incompatibility in D. melanogaster crosses to D. simulans. Male inviability occurs when a D. melanogaster X chromosome is combined with a hybrid autosomal background (43).