Scott Pitnick

EVOLUTION OF MULTIPLE KINDS OF FEMALE SPERM-STORAGE ORGANS IN DROSOPHILA

Females of all species belonging to the family Drosophilidae have two kinds of sperm‐storage organs: paired spherical spermathecae and a single elongate tubular seminal receptacle. We examined 113 species belonging to the genus Drosophila and closely allied genera and describe variation in female sperm‐storage organ use and morphology. The macroevolutionary pattern of organ dysfunction and morphological divergence suggests that ancestrally both kinds of organs stored sperm. Loss of use of the spermathecae has evolved at least 13 times; evolutionary regain of spermathecal function has rarely if ever occurred. Loss of use of the seminal receptacle has likely occurred only once; in this case, all descendant species possess unusually elaborate spermathecae. Data further indicate that the seminal receptacle is the primary sperm‐storage organ in Drosophila. This organ exhibits a pattern of strong correlated evolution with the length of sperm. The evolution of multiple kinds of female sperm‐storage organs and the rapidly divergent and correlated evolution of sperm and female reproductive tract morphology are discussed.

Male Gametic Strategies: Sperm Size, Testes Size, and the Allocation of Ejaculate Among Successive Mates by the Sperm-Limited Fly Drosophila pachea and Its Relatives

The gametic strategy of males comprises the amount of energy invested per sperm, the total amount invested in sperm production, and the pattern of sperm allocation among successive reproductive bouts. All of these variables were measured for each of the four species constituting the nannoptera species group of the Drosophilidae. Extreme interspecific variation was identified for all variables and enigmatic male reproductive strategies, including submaximal insemination of females, partitioning of ejaculate among successive mates, and production of few large sperm, were observed. Variation among species in female remating behavior was found to occur concomitantly with male remating behavior, probably because of female fertility demands. Relationships among testes size, sperm size, sperm numbers, and mating systems in these fruit flies are examined. These relationships are not consistent with patterns identified in studies of vertebrate taxa and suggest fundamental differences between vertebrates and invertebrates with respect to these traits. Hypotheses to explain the maintenance of male ejaculate delivery patterns that are consistent with sperm competition and bet-hedging theory are examined, as are potential selection pressures responsible for sperm-size evolution.

INVESTMENT IN TESTES AND THE COST OF MAKING LONG SPERM IN DROSOPHILA

Relationships among body mass, testis mass, sperm length, and the number of sperm produced were examined among 11 Drosophila species, after controlling for phylogenetic effects. This is the first study to examine many of these relationships comparatively in an invertebrate taxon; patterns observed among these variables were fundamentally different from those consistently reported in studies of vertebrates. In regression analyses, testis mass increased with body mass with an exponent greater than one, which indicates that males of larger-bodied Drosophila species make a proportionately greater energetic investment in testes than do males of smaller-bodied species. The positive allometry of testis mass is hypothesized to be a combined consequence of the unusual positive relationship between body mass and sperm length and the positive relationship between sperm length and testis mass. Interspecific variation in testis mass was found to be a function of variation in sperm length rather than variation in the number of sperm produced. Significant trade-offs were identified between sperm length and the number of sperm produced and transferred per copulation. Results are discussed in terms of the costs of producing longer sperm, the correlated evolution of sperm length and body size, the relationship between breeding system and sperm production patterns, and the nature of differences between vertebrates and invertebrates in sperm production and the size of testes.

Resolving Mechanisms of Competitive Fertilization Success in Drosophila melanogaster

Battle of the Sperm In insects, sperm from multiple matings are stored and retained, and are thought to compete for ova within a females reproductive tract. Manier et al. (p. 354, published online 18 March) visualized sperm from fruit flies transgenically tagged with green or red fluorescent protein within the reproductive tracts of female flies. Sperm showed more mobility within the female storage organs than expected, with those from the most recent copulation displacing sperm from previous males; however, sperm viability remained consistent over long-term storage and each males sperm was equally competitive in fertilizing the females eggs. Fluorescently labeled sperm allow direct visualization of their activity within the female reproductive tract of flies. Our understanding of postcopulatory sexual selection has been constrained by an inability to discriminate competing sperm of different males, coupled with challenges of directly observing live sperm inside the female reproductive tract. Real-time and spatiotemporal analyses of sperm movement, storage, and use within female Drosophila melanogaster inseminated by two transgenic males with, respectively, green and red sperm heads allowed us to unambiguously discriminate among hypothesized mechanisms underlying sperm precedence, including physical displacement and incapacitation of “resident” sperm by second males, female ejection of sperm, and biased use of competing sperm for fertilization. We find that competitive male fertilization success derives from a multivariate process involving ejaculate-female and ejaculate-ejaculate interactions, as well as complex sperm behavior in vivo.

Sperm morphological diversity

Publisher Summary This chapter reviews the current knowledge of variation in sperm morphology over several levels of biological organization: variation within males (both within and across ejaculates), among males, among populations, and among species, along with prevailing hypotheses addressing the adaptive significance of such variation. With regard to developmental mechanisms, three aspects of the physiology of sperm production serve to limit within-ejaculate variation in sperm phenotypes. First, the location of the testes and numerous aspects of testicular physiology of some taxa are clearly adaptations to maintain a homeostatic developmental environment for sperm. Second, developing spermatids may share cytoplasm. Third, sperm phenotypes are predominantly determined by testicular gene expression and hence the diploid genome of the male. Variation across ejaculates but within males can involve several traits including sperm numbers, overall semen quality and individual sperm quality. A special case of intramale variation in sperm form is found in species with sperm heteromorphism, in which different sperm forms are regularly produced by individuals. Differences among males in sperm morphology may derive from both genetic and environmental influences. Theories of condition-dependence basically posit that fitness-related traits are to a large extent dependent on an organisms underlying condition. Conclusions drawn from studies of sperm diversification between natural populations are reinforced by experimental evolution studies of sperm morphology in laboratory populations, as these studies address the evolvability of sperm traits and the nature of selection underlying sperm diversification. Furthermore, a discussion of evolutionary causes and consequences of sperm diversification, along with suggestions of fruitful areas for future exploration is presented.

Males' evolutionary responses to experimental removal of sexual selection.

We evaluated the influence of pre– and post–copulatory sexual selection upon male reproductive traits in a naturally promiscuous species, Drosophila melanogaster. Sexual selection was removed in two replicate populations through enforced monogamous mating with random mate assignment or retained in polyandrous controls. Monogamous mating eliminates all opportunities for mate competition, mate discrimination, sperm competition, cryptic female choice and, hence, sexual conflict. Levels of divergence between lines in sperm production and male fitness traits were quantified after 38–81 generations of selection. Three a priori predictions were tested: (i) male investment in spermatogenesis will be lower in monogamy–line males due to the absence of sperm competition selection, (ii) due to the evolution of increased male benevolence, the fitness of females paired with monogamy–line males will be higher than that of females paired with control–line males, and (iii) monogamy–line males will exhibit decreased competitive reproductive success relative to control–line males. The first two predictions were supported, whereas the third prediction was not. Monogamy males evolved a smaller body size and the size of their testes and the number of sperm within the testes were disproportionately further reduced. In contrast, the fitness of monogamous males (and their mates) was greater when reproducing in a non–competitive context: females mated once with monogamous males produced offspring at a faster rate and produced a greater total number of surviving progeny than did females mated to control males. The results indicate that sexual selection favours the production of increased numbers of sperm in D. melanogaster and that sexual selection favours some male traits conferring a direct cost to the fecundity of females.

Proximate Causes of Rensch’s Rule: Does Sexual Size Dimorphism in Arthropods Result from Sex Differences in Development Time?

A prominent interspecific pattern of sexual size dimorphism (SSD) is Rensch’s rule, according to which male body size is more variable or evolutionarily divergent than female body size. Assuming equal growth rates of males and females, SSD would be entirely mediated, and Rensch’s rule proximately caused, by sexual differences in development times, or sexual bimaturism (SBM), with the larger sex developing for a proportionately longer time. Only a subset of the seven arthropod groups investigated in this study exhibits Rensch’s rule. Furthermore, we found only a weak positive relationship between SSD and SBM overall, suggesting that growth rate differences between the sexes are more important than development time differences in proximately mediating SSD in a wide but by no means comprehensive range of arthropod taxa. Except when protandry is of selective advantage (as in many butterflies, Hymenoptera, and spiders), male development time was equal to (in water striders and beetles) or even longer than (in drosophilid and sepsid flies) that of females. Because all taxa show female‐biased SSD, this implies faster growth of females in general, a pattern markedly different from that of primates and birds (analyzed here for comparison). We discuss three potential explanations for this pattern based on life‐history trade‐offs and sexual selection.

Harm to females increases with male body size in Drosophila melanogaster

Previous studies indicate that female Drosophila melanogaster are harmed by their mates through copulation. Here, we demonstrate that the harm that males inflict upon females increases with male size. Specifically, both the lifespan and egg–production rate of females decreased significantly as an increasing function of the body size of their mates. Consequently, females mating with larger males had lower lifetime fitness. The detrimental effect of male size on female longevity was not mediated by male effects on female fecundity, egg–production rate or female–remating behaviour. Similarly, the influence of male size on female lifetime fecundity was independent of the male–size effect on female longevity. There was no relationship between female size and female resistance to male harm. Thus, although increasing male body size is known to enhance male mating success, it has a detrimental effect on the direct fitness of their mates. Our results indicate that this harm is a pleiotropic effect of some other selected function and not an adaptation. To the extent that females prefer to mate with larger males, this choice is harmful, a pattern that is consistent with the theory of sexually antagonistic coevolution.

Male size influences mate fecundity and remating interval in Drosophila melanogaster

Male size influences female remating behaviour in Drosophila melanogaster. Females remated rapidly when courted by large males, independent of the size of the previous mate. Size of first mate had no general influence upon female remating interval. Possible fitness consequences for females of the observed remating pattern were examined by comparing fecundity and fertility of females singly mated to males of varying size. Females mated to small males had higher fitness than females mated to large males, due to increased fecundity on the day of mating. The significance of these apparently conflicting observations is discussed with regard to male and female reproductive patterns.

Ejaculate-female coevolution in Drosophila mojavensis

Interspecific studies indicate that sperm morphology and other ejaculatory traits diverge more rapidly than other types of character in Drosophila and other taxa. This pattern has largely been attributed to postcopulatory sexual selection involving interaction between the sexes. Such divergence has been suggested to lead rapidly to reproductive isolation among populations and thus to be an ‘engine of speciation.’ Here, we test two critical predictions of this hypothesis: (i) there is significant variation in reproductive traits among incipient species; and (ii) divergence in interacting sex–specific traits exhibits a coevolutionary pattern among populations within a species, by examining geographical variation in Drosophila mojavensis, a species in the early stages of speciation. Significant among–population variation was identified in sperm length and female sperm–storage organ length, and a strong pattern of correlated evolution between these interacting traits was observed. In addition, crosses among populations revealed coevolution of male and female contributions to egg size. Support for these two important predictions confirms that coevolving internal characters that mediate successful reproduction may play an important part in speciation. The next step is to determine exactly what that role is.