Does male gonopodial morphology affect male-female mating positioning in the livebearing fish Xenophallus umbratilis?

Xenophallus umbratilis is a freshwater livebearing fish that exhibits unique antisymmetry in the male gonopodium, which terminates in either a dextral or sinistral twist. This asymmetry in the gonopodium suggests that males might exhibit side-biased behavior when interacting with females to mate. We conducted two assays to assess the laterality of male and female mating interactions based on gonopodial morphology. We observed lateralized mating behavior in one test where males with sinistral gonopodial morphology interacted with a single female. However, we did not find lateralized mating behavior in males with dextral gonopodial morphology. We also examined male and female positioning in trials that placed a single female with five males, all with the same morphology. These trials also showed no evidence of lateralized body positioning.

Alternative reproductive tactics in male freshwater fish influence the accuracy of species recognition

Sexual conflict can result in coercive mating. Because males bear low costs of heterospecific mating, coercive males may engage in misdirected mating attempts toward heterospecific females. In contrast, sexual selection through consensual mate choice can cause mate recognition cues among species to diverge, leading to more accurate species recognition. Some species show both coercive mating and mate choice-associated courtship behaviors as male alternative reproductive tactics. We hypothesized that if the selection pressures on each tactic differ, then the accuracy of species recognition would also change depending on the mating tactic adopted. We tested this hypothesis in the guppy (Poecilia reticulata) and mosquitofish (Gambusia affinis) by a series of choice experiments. Poecilia reticulata and G. affinis males both showed imperfect species recognition and directed all components of mating behavior toward heterospecific females. They tended to direct courtship displays more frequently toward conspecific than heterospecific females. With male P. reticulata, however, accurate species recognition disappeared when they attempted coercive copulation: they directed coercions more frequently toward heterospecific females. We also found that heterospecific sexual interaction had little effect on the fecundity of gravid females, which suggests that prepregnancy interactions likely underpin the exclusion of G. affinis by P. reticulata in our region.

Rapid evolution and plasticity of genitalia

Genital morphology exhibits tremendous variation and is intimately linked with fitness. Sexual selection, nonmating natural selection and neutral forces have been explored as potential drivers of genital divergence. Though less explored, genitalia may also be plastic in response to the developmental environment. In poeciliid fishes, the length of the male intromittent organ, the gonopodium, may be driven by sexual selection if longer gonopodia attract females or aid in forced copulation attempts or by nonmating natural selection if shorter gonopodia allow predator evasion. The rearing environment may also affect gonopodium development. Using an experimental introduction of Trinidadian guppies into four replicate streams with reduced predation risk, we tested whether this new environment caused the evolution of genitalia. We measured gonopodium length after rearing the source and introduced populations for two generations in the laboratory to remove maternal and other environmental effects. We split full-sibling brothers into different rearing treatments to additionally test for developmental plasticity of gonopodia in response to predator cues and food levels as well as the evolution of plasticity. The introduced populations had shorter gonopodia after accounting for body size, demonstrating rapid genital evolution in 2-3 years (8-12 generations). Brothers reared on low food levels had longer gonopodia relative to body size than those on high food, reflecting maintenance of gonopodium length despite a reduction in body size. In contrast, gonopodium length was not significantly different in response to the presence or absence of predator cues. Because the plastic response to low food was maintained between the source and introduced populations, there was no evidence that plasticity evolved. This study demonstrates the importance of both evolution and developmental plasticity in explaining genital variation.

Shifts in Reproductive Investment in Response to Competitors Lower Male Reproductive Success

AbstractIn many species, males exhibit phenotypic plasticity in sexually selected traits when exposed to social cues about the intensity of sexual competition. To date, however, few studies have tested how this plasticity affects male reproductive success. We initially tested whether male mosquitofish, Gambusia holbrooki (Poeciliidae), change their investment in traits under pre- and postcopulatory sexual selection depending on the social environment. For a full spermatogenesis cycle, focal males were exposed to visual and chemical cues of rivals that were either present (competitive treatment) or absent (control). Males from the competitive treatment had significantly slower-swimming sperm but did not differ in sperm count from control males. When two males competed for a female, competitive treatment males also made significantly fewer copulation attempts and courtship displays than control males. Further, paternity analysis of 708 offspring from 148 potential sires, testing whether these changes in reproductive traits affected male reproductive success, showed that males previously exposed to cues about the presence of rivals sired significantly fewer offspring when competing with a control male. We discuss several possible explanations for these unusual findings.

Isotocin increases female avoidance of males in a coercive mating system: Assessing the social salience hypothesis of oxytocin in a fish species

The nonapeptide oxytocin (and its fish homolog isotocin (IT)) is an evolutionarily-conserved hormone associated with social behaviors across most vertebrate taxa. Oxytocin has traditionally been regarded as a prosocial hormone, but studies on social cognition in mammalian models suggest it may play a more nuanced role in modulating social discrimination based on social salience and stimulus valence. Here we test IT and its role in regulating female social decision-making and anxiety behaviors in a live-bearing fish with a male coercive mating system. Gambusia affinis males engage in a forced mating strategy, with frequent harassment and attempted copulatory thrusts directed towards unwilling females. Exogenous IT produced anxiolytic responses in female G. affinis that altered exploration (time in center of tank) but not time in dark vs. light regions of the tank. Exogenous IT also produced context-specific changes in social tendency: IT-treated G. affinis females spent less time associating with males while association time with conspecific females was not altered. Further, while overall activity levels were not changed by IT treatment, the amount of social behaviors IT-treated females directed towards males, but not females, was reduced. Our results support the social salience hypothesis of oxytocin action in a teleost and suggest that oxytocin's critical input into social cognitive processing is conserved across vertebrate taxa.

Reproductive interference in live-bearing fish: the male guppy is a potential biological agent for eradicating invasive mosquitofish

The eradication of invasive exotic species is desirable but often infeasible. Here, we show that male guppies are a potential biological agent for eradicating invasive mosquitofish through the mechanism of reproductive interference, which is defined as any sexual behavior erratically directed at a different species that damages female and/or male fitness. Together with decades of data on species distribution, our field surveys suggest that mosquitofish initially became established on Okinawa Island before being replaced by the more recently introduced guppies. More importantly, our laboratory experiments suggest that reproductive interference was one of the mechanisms underlying this species exclusion, and that in this case, the negative effects were asymmetric, i.e., they only impacted mosquitofish. Reproductive interference may offer a safer and more convenient method of biological control than the traditional sterile male release method because radiation is not necessary.

Mating and aggressive behaviour of Brachyrhaphis olomina (Cyprinodontiformes: Poeciliidae)

Despite the increasing interest in the use of intromittent male genitalia and coercive mating behaviour in poeciliids, detailed studies of the mating behaviour of most species in this family are lacking. We describe here the mating and aggressive behaviours of Brachyrhaphis olomina, and correlate them with the condition of the female’s ovum and embryos (immature, mature and pregnant). B. olomina performed a wide range of aggressive (sidle spread, tail beating, coordinate) and mating behaviours (approximation, touch, lateral display, touch-lateral display). Some behaviours (e.g. tail beating) are shared with other poeciliids, but two sexual behaviours (touch and lateral display) and one aggressive (coordinate) behaviour may be unique to B. olomina and were not reported in a previous study. Differences in male behaviour when paired with a female with mature ovum (more mating displays, no agonistic movements) suggest that males detect the female’s reproductive condition from some distance. The distinctive nature of mating behaviour in B. olomina highlights the importance of studying different species to have a better understanding of the evolution of mating and aggressive behaviours in poeciliids. Digital video images related to the article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo170720bo01a, http://www.momo-p.com/showdetail-e.php?movieid=momo170720bo02a and http://www.momo-p.com/showdetail-e.php?movieid=momo170720bo03a.

Aggressive behaviours, food deprivation and the foraging gene

A pleiotropic gene governs multiple traits, which might constrain the evolution of complexity due to conflicting selection on these traits. However, if the pleiotropic effect is modular, then this can facilitate synergistic responses to selection on functionally related traits, thereby leveraging the evolution of complexity. To understand the evolutionary consequence of pleiotropy, the relation among functionally different traits governed by the same gene is key. We examined a pleiotropic function of the foraging (for) gene with its rover and sitter allelic variants in fruit fly, Drosophila melanogaster. We measured for's effect on adult male aggressive behaviours and whether this effect was shaped by for's known role in food-related traits. Rover exhibited higher levels of offensive behaviour than sitters and s2, a sitter-like mutant on rover genetic background. With a Markov chain model, we estimated the rate of aggression escalation, and found that the rover pattern of aggressive escalation more rapidly intensified fights. Subsequent analysis revealed that this was not caused by for's effect on food-related traits, suggesting that for might directly regulate aggressive behaviours. Food deprivation did not elevate aggression, but reduced intermediate-level aggressive behaviours. Aggression and other foraging-related behaviour might comprise a synergistic trait module underlaid by this pleiotropic gene.