Seminal fluid protein divergence among populations exhibiting postmating prezygotic reproductive isolation

ABSTRACTDespite holding a central role for fertilisation success, reproductive traits often show elevated rates of evolution and diversification. The rapid evolution of seminal fluid proteins (Sfps) within populations is predicted to cause mis-signalling between the male ejaculate and female reproductive tract between populations resulting in postmating prezygotic (PMPZ) isolation. Crosses between populations of Drosophila montana show PMPZ isolation in the form of reduced fertilisation success in both noncompetitive and competitive contexts. Here we test whether male ejaculate proteins deriving from either the accessory glands or the ejaculatory bulb differ between populations using liquid chromatography tandem mass spectrometry. We find more than 150 differentially abundant proteins between populations which may contribute to PMPZ isolation. These proteins include a number of proteases and peptidases, and several orthologs of D. melanogaster Sfps, all known to mediate fertilisation success and which mimic PMPZ isolation phenotypes. Males of one population typically produced greater quantities of Sfps and the strongest PMPZ isolation occurs in this direction. The accessory glands and ejaculatory bulb have different functions and the ejaculatory bulb contributes more to population differences than the accessory glands. Proteins with a secretory signal, but not Sfps, evolve faster than non-secretory proteins although the conservative criteria used to define Sfps may have impaired the ability to identify rapidly evolving proteins. We take advantage of quantitative proteomics data from three Drosophila species to determine shared and unique functional enrichments of Sfps that could be subject to selection between taxa and subsequently mediate PMPZ isolation. Our study provides the first high throughput quantitative proteomic evidence showing divergence of reproductive proteins implicated in the emergence of PMPZ isolation between populations.IMPACT SUMMARYIdentifying traits that prevent successful interbreeding is key to understanding early stages of the formation of new species, or speciation. Reproductive isolation arising prior to and during fertilisation frequently involves differences in how the sexes interact. In internally fertilising taxa, such interactions are mediated between the female reproductive tract where fertilisation occurs and the receipt of the ejaculate necessary for fertilisation. Because ejaculate proteins are at least partially responsible for these interactions, differences in male ejaculate protein composition could negatively impact fertilisation success, generating reproductive isolation. While the biological classes of ejaculate proteins are shared across all animal taxa, proteins that are secreted by males tend to show rapid evolution in gene expression and genetic sequence. Thus, reproductive proteins are suggested as prime targets facilitating reproductive isolation that arises after mating but before fertilisation (PostMating PreZygotic or PMPZ isolation). Most research on PMPZ isolation has focussed on differences between species for which it is not possible to determine the causative and temporal order of early speciation processes. Here, we test whether populations that exhibit few genetic differences but show strong PMPZ isolation also exhibit variation in ejaculate composition using quantitative high throughput proteomic analyses. We find a number of proteins are differentially abundant between populations including several known to impact fertilisation success in other species. We show that secreted proteins are evolving at an elevated rate, implicating their potential role in PMPZ isolation. We test divergence in ejaculate composition between species, finding a core set of functions that were conserved across species which last shared a common ancestor more than 40 million years ago along with species-specific investment. This work highlights the divergent evolution of reproductive proteins which may contribute to barriers between populations within a species early during speciation, extendable to similar analyses in other taxa in the future.

Within-population sperm competition intensity does not predict asymmetry in conpopulation sperm precedence

ABSTRACTPostcopulatory sexual selection can generate coevolutionary arms races between the sexes resulting in the rapid coevolution of reproductive phenotypes. As traits affecting fertilisation success diverge between populations postmating prezygotic barriers to gene flow may evolve. Conspecific sperm precedence is a form of such isolation thought to evolve early during speciation yet has mostly been studied between species. Here we show conpopulation sperm precedence between Drosophila montana populations. Using genomic data to estimate divergence times and patterns of gene flow between populations, we show gene flow has played a considerable role during divergence. We find conpopulation sperm precedence is asymmetric and is concordant with asymmetry in non-competitive postmating prezygotic reproductive isolation. These results suggest these phenomena have a shared mechanism, but we show that this asymmetry is unrelated to the strength of postcopulatory sexual selection acting within populations. We tested whether overlapping foreign and coevolved ejaculates within the female reproductive tract altered fertilisation success but found no effect. Our results show that neither time since divergence nor sperm competitiveness predicts the strength of postmating prezygotic reproductive isolation. We suggest that divergence of postcopulatory phenotypes resulting in postmating prezygotic isolation is potentially driven by cryptic female choice, or mutation order divergence.

Enrichment Increases Aggression in Zebrafish

Environmental enrichment, or the enhancement of an animal’s surroundings when in captivity to maximise its wellbeing, has been increasingly applied to fish species, particularly those used commercially. Laboratory species could also benefit from enrichment, but it is not always clear what constitutes an enriched environment. The zebrafish, Danio rerio, is used widely in research and is one of the most commonly-used laboratory animals. We investigated whether changing the structural complexity of housing tanks altered the behaviour of one strain of zebrafish. Fish were kept in three treatments: (1) very enhanced (VE); (2) mildly enhanced (ME); and (3) control (CT). Level of aggression, fertilisation success, and growth were measured at regular intervals in a subset of fish in each treatment group. The VE fish were more aggressive over time than either ME or CT fish, both in the number of attacks they made against a mirror image and in their tendency to stay close to their reflection rather than avoid it. Furthermore, VE fish were shorter than CT fish by the end of the experiment, though mass was not significantly affected. There was no significant effect of treatment on fertilisation success. These findings suggest that the way in which fish are housed in the laboratory can significantly affect their behaviour, and potentially, their growth. The zebrafish is a shoaling species with a dominance hierarchy, and so may become territorial over objects placed in the tank. The enrichment of laboratory tanks should consider aspects of the species’ behaviour.