Genetic architecture of key social trait differs significantly between primitive and advanced eusocial species

The one trait that differentiates social and eusocial insects from solitary insects is reproductive division of labor (i.e., one or a few individuals of a group reproduce and others work). Currently, the general consensus of the molecular and behavioral underpinnings of social evolution is based on ideas in evolutionary developmental biology in which novel traits are linked to changes in regulatory mechanisms rather than novel genes (1–4). This is in particular true for a trait such as reproductive division of labor, because during the early stages of social evolution all individuals had the same reproductive potential (i.e., the trait was phenotypically plastic). It was only later in social evolution when the determination of queen versus worker was shifting from behavioral interactions between omnipotent individuals to developmental mechanisms that reacted to environmental conditions provided by the colony or environment [e.g., differential feeding (5)] that phenotypic plasticity of adult individuals was reduced. For example, in some highly derived ant species, workers emerge with no or nonfunctional ovaries and thus are true “somatic cells” of a superorganism (6). In PNAS, Patalano et al. (7) sequenced the genomes, brain transcriptomes, and methylomes of two eusocial species in which caste determination is still highly plastic (Polistes) or has reversed from a caste determination with a dedicated reproductive caste (queens) to the ancestral state with omnipotent individuals competing for reproductive rights by evolutionarily “losing” the queen phenotype (Dinoponera). Hence, this study gives us a rare glimpse into the genome organization and molecular mechanisms that represent or resemble the earliest stages in social evolution where reproductive castes were not yet fixed or morphologically differentiated, like in most species of the highly advanced eusocial species of ants, social bees, and termites.

The paper wasp genus Polistes was long overdue for a sociogenomic analysis because it represents the last major lineage that independently evolved eusociality (Vespidae) that has not been sequenced. Additionally, it developed eusociality relatively late compared with other eusocial groups such as ants and termites [approximately 65 Mya (8)] and has retained many traits from its solitary ancestors such as little morphological differentiation between queens and workers. Hence, its genome represents a sociogenome in the early stage of social evolution. Polistes have an annual lifecycle, that is, colonies disband after a few generations and only the newly mated queens/individuals of the last brood of

Patalano et al. show that the changes in the genetic architecture during the transition from solitary to eusocial lifestyle were mostly subtle.

a colony will survive and found new colonies. This means that future queens will start their careers by performing the same tasks as workers and only later, after the first brood is hatched, can they retire and concentrate on laying eggs and aggressively defend their status as dominant reproductive individuals of a colony. Hence, every new generation relives the transition from the ancestral solitary state to the derived eusocial state. Bumble bees and many halictid bees are similarly primitively eusocial. In contrast, Dinoponera is a genus of ponerine ants that, like several other ponerine ant genera, has lost a morphologically distinct queen caste. It has small perennial colonies [average colony size of Dinoponera quadriceps is 80 workers (9)] and young workers can reproduce and become the dominant egg layer (queen). In general, ponerine queens show very little morphological differentiation from their workers, with the exception of wings that they shed after the mating flight. Additionally, in many species, workers are still able to mate and reproduce when the queen is absent. Hence, in this whole ant subfamily, reproductive plasticity is still very high [e.g., in another sequenced ponerine ant, Harpegnathos saltator, queens are still present but once a queen dies or is lost mated workers take over her role (10)].

The genomes of both species do not differ significantly from those of other eusocial hymenopteran genomes in terms of size and composition. There is, however, one crucial exception.

Polistes is Missing Crucial Gene for DNA Methylation

Polistes canadensis is missing an ortholog for the DNMT3 (DNA methyltransferase) gene that is normally necessary for de novo DNA methylation. In vertebrates expression of DNMT3 is essential for a successful embryonic development (11), and in honey bees it has been shown that partial knockdown of DNMT3 in larvae causes most larvae to become reproductive “queens” (12). Partial knockdowns of DNMT3 in adult honey bees decreased DNA methylation in gene bodies, leading to significant effects on alternative splicing in fat tissues (13). Ever since these discoveries, DNA methylation has been the prime suspect for regulating division of labor and caste determination in eusocial insects. Subsequent genome sequences of bees, ants, and termites have always found a complete set of the molecular machinery to maintain and de novo generate methylated DNA (DNMT1-3). However, the effect of DNA methylation outside of honey bees has been less clear (e.g., refs. 14–16). This new discovery by Patalano et al. (7) that a major eusocial insect lineage (Vespidae) is missing a crucial component of the molecular machinery for de novo DNA methylation puts an end to the view that DNMT3-mediated DNA methylation is a general mechanism underlying caste determination in eusocial insects or even hymenoptera. However, the observation that P. canadensis has a level of DNA methylation comparable to that of other eusocial hymenoptera suggests other genes may be responsible for DNA methylation in these clades.

Polistes “shows more equitable levels of protein sharing with ants and bees” (7) than Dinoponera, which shares the majority of its predicted proteins with other ants. This is in part because there are no other sequenced wasp species but ∼10 other ant genomes (17), but it might also support a recent and very surprising finding that ants and bees share a more recent common ancestor than ants and wasps (18). Compared with other studies, Patalano et al. (7) found relatively fewer differentially expressed genes and no difference in the brain methylome or miRNA expression in a comparison of reproductive and nonreproductive individuals of both species.

Queens and Workers Do Not Differ Significantly in Their Brain Transcriptome

Despite the low number of differentially expressed genes, there were a few that were differentially expressed in both species. One of those genes, vitellogenin, is known to play a key role in caste determination and division of labor in honey bees (4) and in the more derived ant species, the red imported fire ant (Solenopsis invicta) (19).

This study supports the idea that many of the regulatory mechanisms of caste determination observed in highly derived eusocial species [e.g., DNA methylation in honey bees and ants (12–14) and miRNA differentiation in worker castes (10)] have evolved later in social evolution. Plausibly, during the early stages of social evolution, more subtle changes in gene regulatory networks [e.g., changes in transcription factor binding sites (20)] were sufficient to generate societies that collaborated successfully over millions of years, as in the vespid wasps or ponerine ants. A lack of these molecular mechanisms generating distinct reproductive and nonreproductive castes during early development might also constrain the further social evolution of these species toward a superorganism sensu Reeve and Hölldobler (6) because this allows for a high level of intracolonial competition for reproduction.

One limitation of this study is the focus on brain expression and methylome, because the ovaries and fat bodies are the main organs concerned with reproduction. Excluding these organs from the analysis likely strongly reduced the number of differentially expressed genes between reproductive and nonreproductive castes.

Nevertheless, this study clearly shows that the genetic architecture underlying social evolution and regulation of social traits is as variable as the life histories of eusocial insects, where societies can range from few individuals that have the same reproductive potential to superorganisms with a single queen and thousands of sterile workers. Patalano et al. (7) show that the changes in the genetic architecture during the transition from solitary to eusocial lifestyle were mostly subtle and did not require the recruitment of massive genetic or epigenetic regulatory mechanisms that characterize advanced eusocial societies.