Quantitative proteomics reveals rapid divergence in the postmating response of female reproductive tracts among sibling species

Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation

How barriers to gene flow arise and are maintained are key questions in evolutionary biology. Speciation research has mainly focused on barriers that occur either before mating or after zygote formation. In comparison, postmating prezygotic (PMPZ) isolation-a barrier that acts after gamete release but before zygote formation-is less frequently investigated but may hold a unique role in generating biodiversity. Here we discuss the distinctive features of PMPZ isolation, including the primary drivers and molecular mechanisms underpinning PMPZ isolation. We then present the first comprehensive survey of PMPZ isolation research, revealing that it is a widespread form of prezygotic isolation across eukaryotes. The survey also exposes obstacles in studying PMPZ isolation, in part attributable to the challenges involved in directly measuring PMPZ isolation and uncovering its causal mechanisms. Finally, we identify outstanding knowledge gaps and provide recommendations for improving future research on PMPZ isolation. This will allow us to better understand the nature of this often-neglected reproductive barrier and its contribution to speciation.

Current Status of Omics Studies Elucidating the Features of Reproductive Biology in Blood-Feeding Insects

Female insects belonging to the genera Anopheles, Aedes, Glossina, and Rhodnius account for the majority of global vector-borne disease mortality. In response to mating, these female insects undergo several molecular, physiological, and behavioral changes. Studying the dynamic post-mating molecular responses in these insects that transmit human diseases can lead to the identification of potential targets for the development of novel vector control methods. With the continued advancements in bioinformatics tools, we now have the capability to delve into various physiological processes in these insects. Here, we discuss the availability of multiple datasets describing the reproductive physiology of the common blood-feeding insects at the molecular level. Additionally, we compare the male-derived triggers transferred during mating to females, examining both shared and species-specific factors. These triggers initiate post-mating genetic responses in female vectors, affecting not only their reproductive success but also disease transmission.

Female factors modulate Sex Peptide's association with sperm in Drosophila melanogaster

BACKGROUND

Male-derived seminal fluid proteins (SFPs) that enter female fruitflies during mating induce a myriad of physiological and behavioral changes, optimizing fertility of the mating pair. Some post-mating changes in female Drosophila melanogaster persist for ~10-14 days. Their long-term persistence is because the seminal protein that induces these particular changes, the Sex Peptide (SP), is retained long term in females by binding to sperm, with gradual release of its active domain from sperm. Several other "long-term response SFPs" (LTR-SFPs) "prime" the binding of SP to sperm. Whether female factors play a role in this process is unknown, though it is important to study both sexes for a comprehensive physiological understanding of SFP/sperm interactions and for consideration in models of sexual conflict.

RESULTS

We report here that sperm in male ejaculates bind SP more weakly than sperm that have entered females. Moreover, we show that the amount of SP, and other SFPs, bound to sperm increases with time and transit of individual seminal proteins within the female reproductive tract (FRT). Thus, female contributions are needed for maximal and appropriate binding of SP, and other SFPs, to sperm. Towards understanding the source of female molecular contributions, we ablated spermathecal secretory cells (SSCs) and/or parovaria (female accessory glands), which contribute secretory proteins to the FRT. We found no dramatic change in the initial levels of SP bound to sperm stored in mated females with ablated or defective SSCs and/or parovaria, indicating that female molecules that facilitate the binding of SP to sperm are not uniquely derived from SSCs and parovaria. However, we observed higher levels of SP (and sperm) retention long term in females whose SSCs and parovaria had been ablated, indicating secretions from these female tissues are necessary for the gradual release of Sex Peptide's active region from stored sperm.

CONCLUSION

This study reveals that the SP-sperm binding pathway is not entirely male-derived and that female contributions are needed to regulate the levels of SP associated with sperm stored in their storage sites.

Divergent evolutionary trajectories shape the postmating transcriptional profiles of conspecifically and heterospecifically mated cactophilic Drosophila females

Postmating-prezygotic (PMPZ) reproductive isolation is hypothesized to result from divergent coevolutionary trajectories of sexual selection and/or sexual conflict in isolated populations. However, the genetic basis of PMPZ incompatibilities between species is poorly understood. Here, we use a comparative framework to compare global gene expression in con- and heterospecifically mated Drosophila mojavensis and D. arizonae female reproductive tracts. We find striking divergence between the species in the female postmating transcriptional response to conspecific mating, including differences in differential expression (DE), alternative splicing (AS), and intron retention (IR). As predicted, heterospecific matings produce disrupted transcriptional profiles, but the overall patterns of misregulation are different between the reciprocal crosses. Moreover, we find a positive correlation between postmating transcriptional divergence between species and levels of transcriptional disruption in heterospecific crosses. This result indicates that mating responsive genes that have diverged more in expression also have more disrupted transcriptional profiles in heterospecifically mated females. Overall, our results provide insights into the evolution of PMPZ isolation and lay the foundation for future studies aimed at identifying specific genes involved in PMPZ incompatibilities and the evolutionary forces that have contributed to their divergence in closely related species.

Comparison of global gene expression patterns in con- and heterospecifically mated Drosophila mojavensis and Drosophila arizonae suggest that mating-responsive genes with divergent expression also exhibit more disrupted transcriptional profiles in heterospecifically mated females, providing further insight into the evolution of postmating-prezygotic reproductive isolation.

Experimental sexual selection reveals rapid evolutionary divergence in sex-specific transcriptomes and their interactions following mating

Post copulatory interactions between the sexes in internally fertilizing species elicits both sexual conflict and sexual selection. Macroevolutionary and comparative studies have linked these processes to rapid transcriptomic evolution in sex‐specific tissues and substantial transcriptomic post mating responses in females, patterns of which are altered when mating between reproductively isolated species. Here, we tested multiple predictions arising from sexual selection and conflict theory about the evolution of sex‐specific and tissue‐specific gene expression and the post mating response at the microevolutionary level. Following over 150 generations of experimental evolution under either reduced (enforced monogamy) or elevated (polyandry) sexual selection in Drosophila pseudoobscura, we found a substantial effect of sexual selection treatment on transcriptomic divergence in virgin male and female reproductive tissues (testes, male accessory glands, the female reproductive tract and ovaries). Sexual selection treatment also had a dominant effect on the post mating response, particularly in the female reproductive tract – the main arena for sexual conflict – compared to ovaries. This effect was asymmetric with monandry females typically showing more post mating responses than polyandry females, with enriched gene functions varying across treatments. The evolutionary history of the male partner had a larger effect on the post mating response of monandry females, but females from both sexual selection treatments showed unique patterns of gene expression and gene function when mating with males from the alternate treatment. Our microevolutionary results mostly confirm comparative macroevolutionary predictions on the role of sexual selection on transcriptomic divergence and altered gene regulation arising from divergent coevolutionary trajectories between sexual selection treatments.

The life history of Drosophila sperm involves molecular continuity between male and female reproductive tracts

In species with internal fertilization, sperm spend an important part of their lives within the female. To examine the life history of the sperm during this time, we used semiquantitative proteomics and sex-specific isotopic labeling in fruit flies to determine the extent of molecular continuity between male and female reproductive tracts and provide a global catalog of sperm-associated proteins. Multiple seminal fluid proteins and female proteins associate with sperm immediately after mating. Few seminal fluid proteins remain after long-term sperm storage, whereas female-derived proteins constitute one-fifth of the postmating sperm proteome by then. Our data reveal a molecular “hand-off” from males to females, which we postulate to be an important component of sperm–female interactions.

Interactions between sperm and the female reproductive tract (FRT) are critical to reproductive success and yet are poorly understood. Because sperm complete their functional maturation within the FRT, the life history of sperm is likely to include a molecular “hand-off” from males to females. Although such intersexual molecular continuity is likely to be widespread among all internally fertilizing species, the identity and extent of female contributions are largely unknown. We combined semiquantitative proteomics with sex-specific isotopic labeling to catalog the posttesticular life history of the sperm proteome and determine the extent of molecular continuity between male and FRTs. We show that the Drosophila melanogaster sperm proteome undergoes substantial compositional changes after being transferred to the FRT. Multiple seminal fluid proteins initially associate with sperm, but most become undetectable after sperm are stored. Female-derived proteins also begin to associate with sperm immediately after mating, and they comprise nearly 20% of the postmating sperm proteome following 4 d of storage in the FRT. Female-derived proteins that associate with sperm are enriched for processes associated with energy metabolism, suggesting that female contributions support sperm viability during the prolonged period between copulation and fertilization. Our research provides a comprehensive characterization of sperm proteome dynamics and expands our understanding of the critical process of sperm–FRT interactions.

Drosophila female reproductive glands contribute to mating plug composition and the timing of sperm ejection

Reproductive traits that influence female remating and competitive fertilization rapidly evolve in response to sexual selection and sexual conflict. One such trait, observed across diverse animal taxa, is the formation of a structural plug inside the female reproductive tract (FRT), either during or shortly after mating. In Drosophila melanogaster, male seminal fluid forms a mating plug inside the female bursa, which has been demonstrated to influence sperm entry into storage and latency of female remating. Processing of the plug, including its eventual ejection from the female's reproductive tract, influences the competitive fertilization success of her mates and is mediated by female × male genotypic interactions. However, female contributions to plug formation and processing have received limited attention. Using developmental mutants that lack glandular FRT tissues, we reveal that these tissues are essential for mating plug ejection. We further use proteomics to demonstrate that female glandular proteins, and especially proteolytic enzymes, contribute to mating plug composition and have a widespread impact on plug formation and composition. Together, these phenotypic and molecular data identify female contributions to intersexual interactions that are a potential mechanism of post-copulatory sexual selection.

Male and female developmental temperature modulate post-copulatory interactions in a beetle

Sexual selection theory has proven to be fundamental to our understanding of the male-female (sperm-egg) interactions that characterise fertilisation. However, sexual selection does not operate in a void and abiotic environmental factors have been shown to modulate the outcome of pre-copularory sexual interactions. Environmental modulation of post-copulatory interactions are particularly likely because the form and function of primary reproductive traits appears to be acutely sensitive to temperature stress. Here we report the effects of developmental temperature on female reproductive architecture and the interaction between male and female developmental temperature on the outcome of sperm competition in the bruchid beetle Callosobruchus maculatus. When females were reared at developmental temperatures above and below typical temperatures the bursa copulatrix (site of spermatophore deposition) were smaller and, were either shorter and broader (high temperatures) or longer and thinner (low temperatures) than those reared at intermediate temperatures. Males and females reared at low developmental temperatures were less likely to mate than those reared at higher temperatures. Where copulation occurred, females reared at the highest temperature copulated for longest, whilst males reared at the lowest temperature spent longer in copula. Male developmental temperature had a significant impact on the outcome of sperm competition: males reared at 17 °C were largely unsuccessful in sperm competition against control (27 °C) males, although some of the variation in the outcome of sperm competition was a product of the interaction between male and female developmental temperature. Our results demonstrate that male-female interactions that characterise pre- and post-copulatory outcomes are sensitive to developmental temperature and that plasticity in cryptic female preferences could lead to heterogeneous selection on the male reproductive phenotype.

Pronounced Postmating Response in the Drosophila Female Reproductive Tract Fluid Proteome

Fertility depends on the progression of complex and coordinated postmating processes within the extracellular environment of the female reproductive tract (FRT). Molecular interactions between ejaculate and FRT proteins regulate many of these processes, including sperm motility, migration, storage, and modification, along with concurrent changes in the female. Although extensive progress has been made in the proteomic characterization of the male-derived components of sperm and seminal fluid, investigations into the FRT have remained more limited. To achieve a comparable level of knowledge regarding female-derived proteins that comprise the reproductive environment, we utilized semiquantitative MS-based proteomics to study the composition of the FRT tissue and, separately, the luminal fluid, before and after mating in Drosophila melanogaster. Our approach leveraged whole-fly isotopic labeling to delineate female proteins from transferred male ejaculate proteins. Our results revealed several characteristics that distinguish the FRT fluid proteome from the FRT tissue proteome: (1) the fluid proteome is encoded by genes with higher overall levels of FRT gene expression and tissue specificity, including many genes with enriched expression in the fat body, (2) fluid-biased proteins are enriched for metabolic functions, and (3) the fluid exhibits pronounced postmating compositional changes. The dynamic mating-induced proteomic changes in the FRT fluid inform our understanding of secretory mechanisms of the FRT, serve as a foundation for establishing female contributions to the ejaculate-female interactions that regulate fertility, and highlight the importance of applying proteomic approaches to characterize the composition and dynamics of the FRT environment.

Drosophila oocyte proteome composition covaries with female mating status

Oocyte composition can directly influence offspring fitness, particularly in oviparous species such as most insects, where it is the primary form of parental investment. Oocyte production is also energetically costly, dependent on female condition and responsive to external cues. Here, we investigated whether mating influences mature oocyte composition in Drosophila melanogaster using a quantitative proteomic approach. Our analyses robustly identified 4,485 oocyte proteins and revealed that stage-14 oocytes from mated females differed significantly in protein composition relative to oocytes from unmated females. Proteins forming a highly interconnected network enriched for translational machinery and transmembrane proteins were increased in oocytes from mated females, including calcium binding and transport proteins. This mating-induced modulation of oocyte maturation was also significantly associated with proteome changes that are known to be triggered by egg activation. We propose that these compositional changes are likely to have fitness consequences and adaptive implications given the importance of oocyte protein composition, rather than active gene expression, to the maternal-to-zygotic transition and early embryogenesis.

The role of female reproductive fluid in sperm competition

The role of non-gametic components of the ejaculate (seminal fluid) in fertility and sperm competitiveness is now well established. Surprisingly, however, we know far less about female reproductive fluid (FRF) in the context of sexual selection, and insights into male-FRF interactions in the context of sperm competition have only recently emerged. Despite this limited knowledge, evidence from taxonomically diverse species has revealed insights into the effects of FRF on sperm traits that have previously been implicated in studies of sperm competition. Specifically, through the differential effects of FRF on a range of sperm traits, including chemoattraction and alterations in sperm velocity, FRF has been shown to exert positive phenotypic effects on the sperm of males that are preferred as mating partners, or those from the most compatible or genetically diverse males. Despite these tantalizing insights into the putative sexually selected functions of FRF, we largely lack a mechanistic understanding of these processes. Taken together, the evidence presented here highlights the likely ubiquity of FRF-regulated biases in fertilization success across a diverse range of taxa, thus potentially elevating the importance of FRF to other non-gametic components that have so far been studied largely in males. This article is part of the theme issue 'Fifty years of sperm competition'.

Quantitative proteomics reveals rapid divergence in the postmating response of female reproductive tracts among sibling species

Fertility depends, in part, on interactions between male and female reproductive proteins inside the female reproductive tract (FRT) that mediate postmating changes in female behaviour, morphology, and physiology. Coevolution between interacting proteins within species may drive reproductive incompatibilities between species, yet the mechanisms underlying postmating-prezygotic (PMPZ) isolating barriers remain poorly resolved. Here, we used quantitative proteomics in sibling Drosophila species to investigate the molecular composition of the FRT environment and its role in mediating species-specific postmating responses. We found that (i) FRT proteomes in D. simulans and D. mauritiana virgin females express unique combinations of secreted proteins and are enriched for distinct functional categories, (ii) mating induces substantial changes to the FRT proteome in D. mauritiana but not in D. simulans, and (iii) the D. simulans FRT proteome exhibits limited postmating changes irrespective of whether females mate with conspecific or heterospecific males, suggesting an active female role in mediating reproductive interactions. Comparisons with similar data in the closely related outgroup species D. melanogaster suggest that divergence is concentrated on the D. simulans lineage. Our study suggests that divergence in the FRT extracellular environment and postmating response contribute to previously described patterns of PMPZ isolation and the maintenance of species boundaries.