Mate availability determines use of alternative reproductive phenotypes in hermaphrodites

In many species, individuals can employ alternative reproductive phenotypes, with profound consequences for individual fitness and population dynamics. This is particularly relevant for self-compatible hermaphrodites, which have exceptionally many reproductive options. Here we investigated the occurrence of reproductive phenotypes in the simultaneously hermaphroditic freshwater snail Radix balthica under experimentally simulated conditions of low versus moderate population density. We captured all mating behavior on camera and measured individual female lifetime reproductive success. We found every possible reproductive phenotype: (1) both male and female (i.e., truly hermaphroditic) reproduction, (2) purely female and (3) purely male reproduction, (4) male reproduction combined with self-fertilization and (5) female mating activity, (6) pure self-fertilization without mating and (7–8) two types of reproductive failure. Variation in alternative reproductive phenotypes was explained by mate availability (10.8%) and individual condition, approximated by a snail’s mean daily growth rate (17.5%). Increased mate availability resulted in a lower diversity of reproductive phenotypes, in particular increasing the frequency of true hermaphrodites. However, it lowered phenotype-specific fecundities and hence reduced the population growth rate. Snails in better condition were more likely to reproduce as true hermaphrodites or pure females, whereas low-condition snails tended to suffer reproductive failure. Overall, we show substantial variation in alternative reproductive phenotypes in a hermaphrodite, which is possibly in part maintained by fluctuations in population density and thus mate availability, and by variation in individual condition. We also provide evidence of an almost 2-fold increase in clutch size that can be ascribed specifically to mating as a female.

The influence of hybridization with domesticated conspecifics on alternative reproductive phenotypes in male Atlantic salmon in multiple temperature regimes

Alternative reproductive phenotypes represent adaptive life-history responses to local environments. Hybridization with domesticated conspecifics exposed to selection against one of the phenotypes could affect the plasticity and incidence of alternative reproductive phenotypes within wild populations, potentially influencing individual fitness and population viability. We addressed this hypothesis by undertaking a common-garden experiment on Atlantic salmon (Salmo salar), a species in which males mature either as large, migratory anadromous individuals or as small, generally nonmigratory parr. Comparing one wild population and two domesticated–wild hybrids (F1, wild backcrosses), we evaluated the incidence of parr maturity at three different temperatures. Parr maturation probability exhibited a significant quadratic relationship with body mass. Early maturation was absent in the coldest temperature treatment. Body-size maturation thresholds were higher in the warmest temperature treatment relative to the intermediate temperature treatment, resulting in a similar incidence of maturation in both treatments despite increased growth in the warmest temperature treatment. Although body-size thresholds for parr maturity did not differ between crosses, F1 hybrids and backcrosses exhibited a lower incidence of maturity relative to wild fish (4.8%, 9.3%, and 30.1%, respectively). Changes in the incidence of alternative maturation phenotypes resulting from temperature and domesticated–wild hybridization could have negative fitness consequences for wild populations.

Raffles, roles, and the outcome of sperm competition in sockeye salmon

In species with male alternative reproductive phenotypes, one phenotype is usually disadvantaged in mating competition. In salmonid fishes, large late-maturing males pair with nesting females and maintain close contact before and during spawning. Small early-maturing males have little contact with nesting females and, during spawning, begin to release sperm after the paired male. The effects of male phenotype and timing of ejaculation on success in sperm competition are not known. In this study, we determined paternity of offspring resulting from in vitro competitive fertilizations to examine these two aspects of sperm competition in sockeye salmon, Oncorhynchus nerka (Walbaum, 1792). When we fertilized eggs with mixtures of equal numbers of sperm from each of two male age classes, we found that success in sperm competition did not depend on male age. However, success in these competitive fertilizations did not conform to the fair raffle model of sperm competition, since paternity in most of the clutches was biased in favour of one male. When we added milt from two males sequentially to a batch of eggs, we found that sperm from the second male fertilized fewer eggs than sperm from the first male, but the difference was less than expected. In addition, a male's success when his milt was added first was not correlated with his success when his milt was added second.