The lek mating system of the worm pipefish (Nerophis lumbriciformis): a molecular maternity analysis and test of the phenotype-linked fertility hypothesis

High rates of male courtship in a female-ornamented pipefish

In species with sex-specific signalling traits that appear to be ornamental (i.e. are conspicuous and with no obvious natural selection benefit), the ornamented sex typically initiates courtship and is most active in courtship. Here, we report for the first time courtship displays in the extremely sexually dimorphic, female-ornamented wide-bodied pipefish (Stigmatopora nigra), revealing unexpected behaviours. Females use their sex-specific ornament during courtship displays, as expected, but rarely in female-female interactions. Surprisingly, males initiated 61% of reciprocated courtship bouts and chased females in 17% of the bouts. This chasing behaviour could be a form of male harassment or be indicative of female disinterest in ardent males, either of which was unexpected to be found in this female-ornamented species. Our results highlight the need to study the details of species' behaviours in considering the potential roles of sexual selection and sexual conflict in shaping sexual dimorphism.

Sperm of more colourful males are better adapted to ovarian fluids in lake char (Salmonidae)

Fish often spawn eggs with ovarian fluids that have been hypothesized to support the sperm of some males over others (cryptic female choice). Alternatively, sperm reactions to ovarian fluids could reveal male strategies. We used wild-caught lake char (Salvelinus umbla) to experimentally test whether sperm react differently to the presence of ovarian fluid, and whether any differential sperm reaction could be predicted by male breeding coloration, male inbreeding coefficients (based of 4150 SNPs) or the kinship coefficients between males and females. Male coloration was positively linked to body size and current health (based on lymphocytosis and thrombocytosis) but was a poor predictor of inbreeding or kinship coefficients. We found that sperm of more colourful males were faster in diluted ovarian fluids than in water only, while sperm of paler males were faster in water than in ovarian fluids. We then let equal numbers of sperm compete for fertilizations in the presence or absence of ovarian fluids and genetically assigned 1464 embryos (from 70 experimental trials) to their fathers. The presence of ovarian fluids significantly increased the success of the more colourful competitors. Sperm of less inbred competitors were more successful when tested in water only than in diluted ovarian fluids. The kinship coefficients had no significant effects on sperm traits or fertilization success in the presence of ovarian fluids, although parallel stress tests on embryos had revealed that females would profit more from mating with least related males rather than most coloured ones. We conclude that sperm of more colourful males are best adapted to ovarian fluids, and that the observed reaction norms suggest male strategies rather than cryptic female choice.

Strong Sexual Selection Does Not Induce Population Differentiation in a Fish Species with High Dispersal Potential: The Curious Case of the Worm Pipefish Nerophis lumbriciformis (Teleostei: Syngnathidae)

High levels of population differentiation are a common demographic pattern in syngnathids, even at small geographical scales. This is probably the end result of the common life history traits observed within the family, involving limited dispersal capabilities and strong habitat dependency. The worm pipefish, Nerophis lumbriciformis, which displays all these characteristics, also presents an additional variable potentially able to promote population differentiation: high sexual selection intensity, especially at the extremes of its distribution. Nevertheless, an early life pelagic stage, which presumably allows for admixture, could prevent population structuring. Here, we assessed the phylogeography of N. lumbriciformis through the amplification of the cytochrome b, 12S, and 16S rDNA mitochondrial markers as well as the rhodopsin nuclear marker, performed upon 119 individuals. We observed a genetically homogeneous population with indications of extensive gene flow. We tentatively attribute this finding to the dispersal potential of the species' pelagic larvae, supported by marine currents acting as major dispersal vectors. We also detected a signal of expansion towards the poles, consistent with the current climate change scenario. Despite the marked latitudinal differences in the phenotype of reproducing worm pipefish, the absence of clear population structuring suggests that phenotypic plasticity can have a significant role in the expression of sexual selection-related traits.

Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females

Within a species' distribution, populations are often exposed to diverse environments and may thus experience different sources of both natural and sexual selection. These differences are likely to impact the balance between costs and benefits to individuals seeking reproduction, thus entailing evolutionary repercussions. Here, we look into an unusual population (Baltic Sea) of the broadnosed pipefish, Syngnathus typhle, where males do not seem to select females based on size and hypothesize that this pattern may derive from a reduction in direct benefits to the male. We further hypothesize that if larger females do not persistently secure a higher reproductive success, either through pre- or postcopulatory sexual selection, a decrease in sexual size dimorphism in the Baltic population should be apparent, especially when contrasted with a well-studied population, inhabiting similar latitudes (Swedish west coast), where males prefer larger females. We found that, in the Baltic population, variation in female quality is low. We were unable to find differences in abortion rates or protein concentration in oocytes produced by females of contrasting sizes. Direct benefits from mating with large partners seem, thus, reduced in the Baltic population. We also found no evidence of any postcopulatory mechanism that could favor larger mothers as embryo development was unrelated to female size. While female size can still be selected through intrasexual competition or fecundity selection, the pressure for large female body size seems to be lower in the Baltic. Accordingly, we found a noticeable decrease in sexual size dimorphism in the Baltic population. We conclude that, although far from negating the significance of other selective processes, sexual selection seems to have a decisive role in supporting pipefish sexual size asymmetries.

Performing Parentage Analysis in the Presence of Inbreeding and Null Alleles

Parentage analysis is an important method that is used widely in zoological and ecological studies. Current mathematical models of parentage analyses usually assume that a population has a uniform genetic structure and that mating is panmictic. In a natural population, the geographic or social structure of a population, and/or nonrandom mating, usually leads to a genetic structure and results in genotypic frequencies deviating from those expected under the Hardy-Weinberg equilibrium (HWE). In addition, in the presence of null alleles, an observed genotype represents one of several possible true genotypes. The true father of a given offspring may thus be erroneously excluded in parentage analyses, or may have a low or negative LOD score. Here, we present a new mathematical model to estimate parentage that includes simultaneously the effects of inbreeding, null alleles, and negative amplification. The influences of these three factors on previous model are evaluated by Monte-Carlo simulations and empirical data, and the performance of our new model is compared under controlled conditions. We found that, for both simulated and empirical data, our new model outperformed other methods in many situations. We make available our methods in a new, free software package entitled parentage. This can be downloaded via http://github.com/huangkang1987/parentage.

Parabolic variation in sexual selection intensity across the range of a cold-water pipefish: implications for susceptibility to climate change

While an understanding of evolutionary processes in shifting environments is vital in the context of rapid ecological change, one of the most potent selective forces, sexual selection, remains curiously unexplored. Variation in sexual selection across a species range, especially across a gradient of temperature regimes, has the potential to provide a window into the possible impacts of climate change on the evolution of mating patterns. Here, we investigated some of the links between temperature and indicators of sexual selection, using a cold-water pipefish as model. We found that populations differed with respect to body size, length of the breeding season, fecundity, and sexual dimorphism across a wide latitudinal gradient. We encountered two types of latitudinal patterns, either linear, when related to body size, or parabolic in shape when considering variables related to sexual selection intensity, such as sexual dimorphism and reproductive investment. Our results suggest that sexual selection intensity increases toward both edges of the distribution and that the large differences in temperature likely play a significant role. Shorter breeding seasons in the north and reduced periods for gamete production in the south certainly have the potential to alter mating systems, breeding synchrony, and mate monopolization rates. As latitude and water temperature are tightly coupled across the European coasts, the observed patterns in traits related to sexual selection can lead to predictions regarding how sexual selection should change in response to climate change. Based on data from extant populations, we can predict that as the worm pipefish moves northward, a wave of decreasing selection intensity will likely replace the strong sexual selection at the northern range margin. In contrast, the southern populations will be followed by heightened sexual selection, which may exacerbate the problem of local extinction at this retreating boundary.