Antagonistic selection on body size and sword length in a wild population of the swordtail fish, Xiphophorus multilineatus: Potential for intralocus tactical conflict.

Alternative reproductive tactics (ARTs) have provided valuable insights into how sexual selection and life history trade-offs can lead to variation within a sex. However, the possibility that tactics may constrain evolution through intralocus tactical conflict (IATC) is rarely considered. In addition, when IATC has been considered, the focus has often been on the genetic correlations between the ARTs, while evidence that the ARTs have different optima for associated traits and that at least one of the tactics is not at its optimum is often missing. Here, we investigate selection on three traits associated with the ARTs in the swordtail fish Xiphophorus multilineatus; body size, body shape, and the sexually selected trait for which these fishes were named, sword length (elongation of the caudal fin). All three traits are tactically dimorphic, with courter males being larger, deeper bodied and having longer swords, and the sneaker males being smaller, more fusiform and having shorter swords. Using measures of reproductive success in a wild population we calculated selection differentials, as well as linear and quadratic gradients. We demonstrated that the tactics have different optima and at least one of the tactics is not at its optimum for body size and sword length. Our results provide the first evidence of selection in the wild on the sword, an iconic trait for sexual selection. In addition, given the high probability that these traits are genetically correlated to some extent between the two tactics, our study suggests that IATC is constraining both body size and the sword from reaching their phenotypic optima. We discuss the importance of considering the role of IATC in the evolution of tactical dimorphism, how this conflict can be present despite tactical dimorphism, and how it is important to consider this conflict when explaining not only variation within a species but differences across species as well.

Feeding Rates in the Swordtail Fish Xiphophorus multilineatus: A Model System for Genetic Variation in Nutritional Programming.

A better understanding of the role of appetite regulation in obesity and metabolic disorders requires consideration of both genetic and developmental influences on appetite. Previously we detected genetic differences in responses to nutritional programming (e.g., the permanent influence that nutrition in early life has on the physiological and metabolic states in adults) on a presumed measure of appetite (feeding rate) in the swordtail fish Xiphophorus multilineatus. In this study we validate that feeding rate is a good measure of appetite, by first demonstrating that it is repeatable and correlated with food consumed when controlling for body size. Second, we detected a significant positive correlation between juvenile growth rates and feeding rates measured in adult males, when growth has ceased. In addition, feeding rates explained significant variation in the size of the nuchal hump, a fat deposit that develops after sexual maturity. Finally, we show that the feeding rates of "courter" males were significantly greater than "sneaker" males, alternative reproductive tactics that are influenced by variation in the Mc4r gene on the Y-chromosome. Our results suggest that examining feeding rate in X. multilineatus could provide valuable insights into how nutritional programming influences appetite independently from food intake, as well as insights into the mechanisms that produce the correlation between delayed maturation and faster growth rates of the courter males as compared with the sneaker males that mature early and grow slower in this species.