Female mediation of competitive fertilization success in Drosophila melanogaster

How females store and use sperm after remating can generate postcopulatory sexual selection on male ejaculate traits. Variation in ejaculate performance traits generally is thought to be intrinsic to males but is likely to interact with the environment in which sperm compete (e.g., the female reproductive tract). Our understanding of female contributions to competitive fertilization success is limited, however, in part because of the challenges involved in observing events within the reproductive tract of internally fertilizing species while discriminating among sperm from competing males. Here, we used females from crosses among isogenic lines of Drosophila melanogaster, each mated to two genetically standardized males (the first with green- and the second with red-tagged sperm heads) to demonstrate heritable variation in female remating interval, progeny production rate, sperm-storage organ morphology, and a number of sperm performance, storage, and handling traits. We then used multivariate analyses to examine relationships between this female-mediated variation and competitive paternity. In particular, the timing of female ejection of excess second-male and displaced first-male sperm was genetically variable and, by terminating the process of sperm displacement, significantly influenced the relative numbers of sperm from each male competing for fertilization, and consequently biased paternity. Our results demonstrate that females do not simply provide a static arena for sperm competition but rather play an active and pivotal role in postcopulatory processes. Resolving the adaptive significance of genetic variation in female-mediated mechanisms of sperm handling is critical for understanding sexual selection, sexual conflict, and the coevolution of male and female reproductive traits

Virgin.sim

Data on sperm transfer and storage after first mating for D. simulans. Variables: Treat – treatment, transfer or storage; Male – transgenic line by sperm label, GFP or RFP; Cop – copulation duration in minutes; Bursa – number of sperm in the bursa; Prox – number of sperm in the proximal half of the seminal receptacle (SR); Dist – number of sperm in the distal half of the SR; Caps – number of sperm in the paired spermathecal caps combined; Ducts – number of sperm in the paired spermathecal ducts combine

Data from: Rapid diversification of sperm precedence traits and processes among three sibling Drosophila species

Postcopulatory sexual selection is credited with driving rapid evolutionary diversification of reproductive traits and the formation of reproductive isolating barriers between species. This judgment, however, has largely been inferred rather than demonstrated due to general lack of knowledge about processes and traits underlying variation in competitive fertilization success. Here, we resolved processes determining sperm fate in twice-mated females, using transgenic Drosophila simulans and D. mauritiana populations with fluorescently-labeled sperm heads. Comparisons among these two species and D. melanogaster revealed a shared motif in the mechanisms of sperm precedence, with postcopulatory sexual selection potentially occurring during any of the three discrete stages: (1) insemination, (2) sperm storage, and (3) sperm use for fertilization, and involving four distinct phenomena: (1) sperm transfer, (2) sperm displacement, (3) sperm ejection, and (4) sperm selection for fertilizations. Yet, underlying the qualitative similarities were significant quantitative differences in nearly every relevant character and process. We evaluate these species differences in light of concurrent investigations of within-population variation in competitive fertilization success and postmating/prezygotic reproductive isolation in hybrid matings between species to forge an understanding of the relationship between microevolutionary processes and macroevolutionary patterns as pertains to postcopulatory sexual selection in this group

Rapid diversification of sperm precedence traits and processes among three sibling Drosophila species

Postcopulatory sexual selection is credited with driving rapid evolutionary diversification of reproductive traits and the formation of reproductive isolating barriers between species. This judgment, however, has largely been inferred rather than demonstrated due to general lack of knowledge about processes and traits underlying variation in competitive fertilization success. Here, we resolved processes determining sperm fate in twice-mated females, using transgenic Drosophila simulans and Drosophila mauritiana populations with fluorescently labeled sperm heads. Comparisons among these two species and Drosophila melanogaster revealed a shared motif in the mechanisms of sperm precedence, with postcopulatory sexual selection potentially occurring during any of the three discrete stages: (1) insemination; (2) sperm storage; and (3) sperm use for fertilization, and involving four distinct phenomena: (1) sperm transfer; (2) sperm displacement; (3) sperm ejection; and (4) sperm selection for fertilizations. Yet, underlying the qualitative similarities were significant quantitative differences in nearly every relevant character and process. We evaluate these species differences in light of concurrent investigations of within-population variation in competitive fertilization success and postmating/prezygotic reproductive isolation in hybrid matings between species to forge an understanding of the relationship between microevolutionary processes and macroevolutionary patterns as pertains to postcopulatory sexual selection in this group

How multivariate ejaculate traits determine competitive fertilization success in Drosophila melanogaster

Success in sperm competition, occurring whenever females mate with multiple males, is predicted to be influenced by variation in ejaculate quality and interactions among competing sperm. Yet, apart from sperm number, relevant ejaculate characteristics and sperm-sperm interactions are poorly understood, particularly within a multivariate framework and the natural selective environment of the female reproductive tract. Here, we used isogenic lines of Drosophila melanogaster with distinguishable sperm to demonstrate and partition genetic variation in multiple sperm quality and performance traits. Next, by competing males from different lines, we show how rival sperm significantly influence each other's velocity and reveal that males with relatively slow and/or long sperm better displace rival sperm and resist displacement, thus avoiding ejection by the female from her reproductive tract. Finally, we establish fitness consequences of genetic variation in sperm quality and its role in securing a numerical advantage in storage by showing that offspring paternity is determined strictly by the representation of stored, competing sperm. These results provide novel insight into complex postcopulatory processes, illustrate that different ejaculate traits are critical at different biologically relevant time-points, and provide a critical foundation for elucidating the role of postcopulatory sexual selection in trait diversification and speciation

Velocity

Sperm velocity and sperm numbers in the SR. Variables: Treat - sxm indicates single mating between D. simulans female and D. mauritiana male, NR indicates "no remate" treatment, other treatments are rematings defined by the species identity of first and second male to mate with a D. simulans female, where "s" indicates D. simulans and "m" indicates D. mauritiana, such that "ss" indicates that both males were D. simulans, "ms" indicates that the first male was D. mauritiana and the second male was D. simulans, etc. Region – region of the female reproductive tract, bursa or SR. Male – species and color tag of male whose sperm velocity is recorded. Order - first or second male to mate. Remate - for remating treatments, day after first mating (day 0) on which female remated. Velmps – instantaneous linear velocity in microns per second, averaged for each sperm and for all sperm recorded in a movie. N – number of sperm tracked per movie. SR.G – number of GFP sperm in the SR. SR.R – number of RFP sperm in the SR

Data from: Postcopulatory sexual selection generates speciation phenotypes in Drosophila

Background: Identifying traits that reproductively isolate species, and the selective forces underlying their divergence, is a central goal of evolutionary biology and speciation research. There is growing recognition that postcopulatory sexual selection, which can drive rapid diversification of interacting ejaculate and female reproductive tract traits that mediate sperm competition, may be an engine of speciation. Conspecific sperm precedence (CSP) is a taxonomically widespread form of reproductive isolation, but the selective causes and divergent traits responsible for CSP are poorly understood. Results: To test the hypothesis that postcopulatory sexual selection can generate reproductive isolation, we expressed green (GFP) or red fluorescent protein (RFP) in sperm heads of recently diverged sister species, Drosophila simulans and D. mauritiana, to enable detailed resolution of species-specific sperm precedence mechanisms. Between-species divergence in sperm competition traits and mechanisms prompted six a priori predictions regarding mechanisms of CSP and degree of cross asymmetry in reproductive isolation. We resolved four distinct mechanisms of CSP that were highly consistent with predictions. These comprise interactions between multiple sex-specific traits, including two independent mechanisms by which females exert sophisticated control over sperm fate to favor the conspecific male. Conclusions: Our results confirm that reproductive isolation can quickly arise from diversifying (allopatric) postcopulatory sexual selection. This experimental approach to "speciation phenotypes" illustrates how knowledge of sperm precedence mechanisms can be used to predict the mechanisms and extent of reproductive isolation between populations and species

P2.mel

Paternity and sperm count data for D. melanogaster. Variables: Treat – time after mating for which paternity and sperm numbers in the female reproductive tract were quantified; P2 – P2 over entire time period; SR1 – number of first-male sperm in the entire SR; SR2 – number of second-male sperm in the entire SR; Sp1 – number of first-male sperm in both spermathecae combined; Sp2 – number of second-male sperm in both spermathecae combine

Remate.mel

Sperm numbers in space and time after second mating for D. melanogaster. Variables: Block - different experimental blocks, A and B; MinASM – time after the start of mating at which female was frozen; FirstM – transgenic line of first male by sperm label, GFP or RFP. Second male is automatically the reciprocal label; Remate – day after first mating (day 0) on which female remated; Cop2 – duration in minutes of the second copulation; Bursa1 – number of first-male sperm in the bursa; Bursa2 – number of second-male sperm in the bursa; Prox1 – number of first-male sperm in the proximal half of the SR; Prox2 – number of second-male sperm in the proximal half of the SR; Dist1 – number of first-male sperm in the distal half of the SR; Dist1 – number of first-male sperm in the distal half of the SR; Dist2 – number of second-male sperm in the distal half of the SR; Caps1 – number of first-male sperm in the two spermathecal caps combined; Caps2 – number of second-male sperm in the two spermathecal caps combined; Ducts1 – number of first-male sperm in the two spermathecal ducts combined; Ducts2 – number of second-male sperm in the two spermathecal ducts combine