Do candidate genes mediating conspecific sperm precedence affect sperm competitive ability within species? A test case in Drosophila

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.

Seminal fluid gene expression and reproductive fitness in Drosophila melanogaster

Background

The rapid evolution of seminal fluid proteins (SFPs) has been suggested to be driven by adaptations to postcopulatory sexual selection (e.g. sperm competition). However, we have recently shown that most SFPs evolve rapidly under relaxed selective pressures. Given the role of SFPs in competition for fertilization phenotypes, like the ability to transfer and store sperm and the modulation of female receptivity and ovulation, the prevalence of selectively relaxed SFPs appears as a conundrum. One possible explanation is that selection on SFPs might be relaxed in terms of protein amino acid content, but adjustments of expression are essential for post-mating function. Interestingly, there is a general lack of systematic implementation of gene expression perturbation assays to monitor their effect on phenotypes related to sperm competition.

Results

We successfully manipulated the expression of 16 SFP encoding genes using tissue-specific knockdowns (KDs) and determined the effect of these genes’ perturbation on three important post-mating phenotypes: female refractoriness to remating, defensive (P1), and offensive (P2) sperm competitive abilities in Drosophila melanogaster. Our analyses show that KDs of tested SFP genes do not affect female refractoriness to remating and P2, however, most gene KDs significantly decreased P1. Moreover, KDs of SFP genes that are selectively constrained in terms of protein-coding sequence evolution have lower P1 than KDs of genes evolving under relaxed selection.

Conclusions

Our results suggest a more predominant role, than previously acknowledged, of variation in gene expression than coding sequence changes on sperm competitive ability in D. melanogaster.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01975-1.

A putative de novo evolved gene required for spermatid chromatin condensation in Drosophila melanogaster

Comparative genomics has enabled the identification of genes that potentially evolved de novo from non-coding sequences. Many such genes are expressed in male reproductive tissues, but their functions remain poorly understood. To address this, we conducted a functional genetic screen of over 40 putative de novo genes with testis-enriched expression in Drosophila melanogaster and identified one gene, atlas, required for male fertility. Detailed genetic and cytological analyses showed that atlas is required for proper chromatin condensation during the final stages of spermatogenesis. Atlas protein is expressed in spermatid nuclei and facilitates the transition from histone- to protamine-based chromatin packaging. Complementary evolutionary analyses revealed the complex evolutionary history of atlas. The protein-coding portion of the gene likely arose at the base of the Drosophila genus on the X chromosome but was unlikely to be essential, as it was then lost in several independent lineages. Within the last ~15 million years, however, the gene moved to an autosome, where it fused with a conserved non-coding RNA and evolved a non-redundant role in male fertility. Altogether, this study provides insight into the integration of novel genes into biological processes, the links between genomic innovation and functional evolution, and the genetic control of a fundamental developmental process, gametogenesis.

Expression of Mst89B and CG31287 is Needed for Effective Sperm Storage and Egg Fertilization in Drosophila

In Drosophila, male reproductive fitness can be affected by any number of processes, ranging from development of gametes, transfer to and storage of mature sperm within the female sperm storage organs, and utilization of sperm for fertilization. We have previously identified the 89B cytogenetic map position of D. melanogaster as a hub for genes that effect male paternity success when disturbed. Here, we used RNA interference to test 11 genes that are highly expressed in the testes and located within the 89B region for their role in sperm competition and male fecundity when their expression is perturbed. Testes-specific knockdown (KD) of bor and CSN5 resulted in complete sterility, whereas KD of CG31287, Manf and Mst89B, showed a breakdown in sperm competitive success when second to mate (P2 < 0.5) and reduced fecundity in single matings. The low fecundity of Manf KD is explained by a significant reduction in the amount of mature sperm produced. KD of Mst89B and CG31287 does not affect sperm production, sperm transfer into the female bursa or storage within 30 min after mating. Instead, a significant reduction of sperm in female storage is observed 24 h after mating. Egg hatchability 24 h after mating is also drastically reduced for females mated to Mst89B or CG31287 KD males, and this reduction parallels the decrease in fecundity. We show that normal germ-line expression of Mst89B and CG31287 is needed for effective sperm usage and egg fertilization.

Genetic Factors Influencing Sperm Competition

Females of many different species often mate with multiple males, creating opportunities for competition among their sperm. Although originally unappreciated, sperm competition is now considered a central form of post-copulatory male–male competition that biases fertilization. Assays of differences in sperm competitive ability between males, and interactions between females and males, have made it possible to infer some of the main mechanisms of sperm competition. Nevertheless, classical genetic approaches have encountered difficulties in identifying loci influencing sperm competitiveness while functional and comparative genomic methodologies, as well as genetic variant association studies, have uncovered some interesting candidate genes. We highlight how the systematic implementation of approaches that incorporate gene perturbation assays in experimental competitive settings, together with the monitoring of progeny output or sperm features and behavior, has allowed the identification of genes unambiguously linked to sperm competitiveness. The emerging portrait from 45 genes (33 from fruit flies, 8 from rodents, 2 from nematodes, and 2 from ants) is their remarkable breadth of biological roles exerted through males and females, the non-preponderance of sperm genes, and their overall pleiotropic nature.

The Molecular Mechanisms of Gametic Incompatibility in Invertebrates

Fertilization (gamete fusion followed by zygote formation) is a multistage process. Each stage is mediated by ligand-receptor recognition of gamete interaction molecules. This recognition includes the movement of sperm in the gradient of egg chemoattractants, destruction of the egg envelope by acrosomal proteins, etc. Gametic incompatibility is one of the mechanisms of reproductive isolation. It is based on species-specific molecular interactions that prevent heterospecific fertilization. Although gametic incompatibility may occur in any sexually reproducing organism, it has been studied only in a few model species. Gamete interactions in different taxa involve generally similar processes, but they often employ non-homologous molecules. Gamete recognition proteins evolve rapidly, like immunity proteins, and include many taxon-specific families. In fact, recently appeared proteins particularly contribute to reproductive isolation via gametic incompatibility. Thus, we can assume a multiple, independent origin of this type of reproductive isolation throughout animal evolution. Gametic incompatibility can be achieved at any fertilization stage and entails different consequences at different taxonomic levels and ranges, from complete incompatibility between closely related species to partial incompatibility between distantly related taxa.

Conspecific sperm precedence is reinforced, but postcopulatory sexual selection weakened, in sympatric populations of Drosophila

Sexual selection can accelerate speciation by driving the evolution of reproductive isolation, but forces driving speciation could also reciprocally feedback on sexual selection. This might be particularly important in the context of 'reinforcement', where selection acts directly to increase prezygotic barriers to reduce the cost of heterospecific matings. Using assays of sperm competition within and between two sister species, we show a signature of reinforcement where these species interact: populations of Drosophila pseudoobscura that co-occur with sister species D. persimilis have an elevated ability to outcompete heterospecific sperm, consistent with selection for increased postcopulatory isolation. We also find these D. pseudoobscura populations have decreased sperm competitive ability against conspecifics, reducing the opportunity for sexual selection within these populations. Our findings demonstrate that direct selection to increase reproductive isolation against other species can compromise the efficacy of sexual selection within species, a collateral effect of reproductive traits responding to heterospecific interactions.

Moving Speciation Genetics Forward: Modern Techniques Build on Foundational Studies in Drosophila

The question of how new species evolve has been examined at every level, from macroevolutionary patterns of diversification to molecular population genetic analyses of specific genomic regions between species pairs. Drosophila has been at the center of many of these research efforts. Though our understanding of the speciation process has grown considerably over the past few decades, very few genes have been identified that contribute to barriers to reproduction. The development of advanced molecular genetic and genomic methods provides promising avenues for the rapid discovery of more genes that contribute to speciation, particularly those involving prezygotic isolation. The continued expansion of tools and resources, especially for species other than Drosophila melanogaster, will be most effective when coupled with comparative approaches that reveal the genetic basis of reproductive isolation across a range of divergence times. Future research programs in Drosophila have high potential to answer long-standing questions in speciation. These include identifying the selective forces that contribute to divergence between populations and the genetic basis of traits that cause reproductive isolation. The latter can be expanded upon to understand how the genetic basis of reproductive isolation changes over time and whether certain pathways and genes are more commonly involved.

COMP-1 promotes competitive advantage of nematode sperm

Competition among sperm to fertilize oocytes is a ubiquitous feature of sexual reproduction as well as a profoundly important aspect of sexual selection. However, little is known about the cellular mechanisms sperm use to gain competitive advantage or how these mechanisms are regulated genetically. In this study, we utilize a forward genetic screen in Caenorhabditis elegans to identify a gene, comp-1, whose function is specifically required in competitive contexts. We show that comp-1 functions in sperm to modulate their migration through and localization within the reproductive tract, thereby promoting their access to oocytes. Contrary to previously described models, comp-1 mutant sperm show no defects in size or velocity, thereby defining a novel pathway for preferential usage. Our results indicate not only that sperm functional traits can influence the outcome of sperm competition, but also that these traits can be modulated in a context-dependent manner depending on the presence of competing sperm.