Post-insemination selection dominates pre-insemination selection in driving rapid evolution of male competitive ability

Expansion of the split hygromycin toolkit for transgene insertion in Caenorhabditis elegans

Engineered sites for genetic transformation have simplified transgene insertion in Caenorhabditis elegans . These strategies include our split hygromycin system ​(Stevenson et al. 2020)​ which allows for integration-specific selection of transgenes. Here we have expanded the split hygromycin selection system to include two additional chromosomal locations, both of which are permissive for germline expression, as well as engineered landing pads in three additional natural isolates. Corresponding guide and empty repair template plasmids are also available for each of these sites.

Post-insemination sexual selection in males indirectly masculinizes the female transcriptome

Sex-specific regulation of gene expression is the most plausible way for generating sexually differentiated phenotypes from an essentially shared genome. However, since genetic material is shared, sex-specific selection in one sex can have an indirect response in the other sex. From a gene expression perspective, this tethered response can move one sex away from their wildtype expression state and impact potentially many gene regulatory networks. Here, using experimental evolution in the model nematode Caenorhabditis elegans , we explore the coupling of direct sexual selection on males with the transcriptomic response in females over microevolutionary timescales to uncover the extent to which post-insemination reproductive traits share a genetic basis between the sexes. We find that differential gene expression is driven by female ancestral or evolved generation alone and that male generation has no impact on changes in gene expression. Almost all differentially expressed genes were downregulated in evolved females. Moreover, 80% of these gene were located on the X chromosome and have wildtype female-biased expression profiles. Changes in gene expression profiles were likely driven through trans -acting pathways that are shared between the sexes. We found no evidence that the core dosage compensation machinery was impacted by experimental evolution. Together these data suggest masculinization of the female transcriptome driven by direct selection on male sperm competitive ability. Our results indicate that on short evolutionary timescales sexual selection can generate sexual conflict in expression space.

LAY SUMMARY

Sexual selection drives the evolution of some of the most dramatic phenotypic differences between the sexes. Such sexual dimorphism is so common across multicellular organisms that we often overlook how remarkable it is for shared genetic material to create numerous and complex sex differences. At an evolutionary level, sexual dimorphism furthers the opportunity for sex-specific selection to optimize the fitness of a given sex. As a consequence, sex-specific selection, such as sexual selection, can have an indirect evolutionary response in the other sex due to genetic associations created by the sexes sharing the same genome. This correlated evolutionary response can create sexual conflict by shifting a sex away from their fitness optimum. At the functional level, sexual dimorphism is generated is through sex-specific regulation of gene expression. Bridging the evolutionary response to sexual selection with the evolution of sex-specific gene regulation during post-mating interactions has proved challenging. We previously used experimental evolution to increase male fertility by directly selecting for increased sperm competitive ability. In this study, we examined the effect of this direct selection on males on gene expression patterns in females. Differential gene expression was determined by whether a female was ancestral or evolved generation, indicating that gene expression changes were an evolved response due to indirect selection on females. Significantly differentially expressed genes were downregulated in evolved females. These genes tended to be female-biased in wildtype individuals and located on the X chromosome. The downregulation of X-linked genes suggests expression levels in females equal to or lower than that in males. Together these results indicate a less female-like transcriptome after experimental evolution. This supports a sexual conflict scenario by which direct sexual selection on males indirectly masculinizes the female transcriptome over short evolutionary timescales.