Male Copulatory Structures in Reproductively Functional Female Live‐Bearing Fish <i>Pseudopoecilia fria</i>

The presence of sexual characteristics typical of one sex in the opposite sex is more common than has been previously recognized. When changes in the environment or the genome alter sex-specific regulatory processes, individuals may develop novel reproductive phenotypes. We recently discovered a population of the live-bearing fish <i>Pseudopoecilia fria</i> (Cyprinodontiformes: Poeciliidae) in which females were found to possess gonopodia, reproductive organs normally exclusive to male fish that are used in the transfer of sperm in mating. While we are not yet able to identify the specific mechanisms underlying this phenotype, we tentatively attribute it to female masculinization. We document that this process has occurred in a significant proportion of the females sampled in the population. Geometric morphometric analyses showed that females were potentially divided into two discrete phenotypes, though with considerable intra-group variance. Additionally, all females with gonopodia were gravid, and one individual gave birth to live offspring. Therefore, these alternate female morphs appear reproductively functional. We discuss several potential explanations for this phenomenon, including exposure to masculinizing water-borne pollutants and the remote possibility of a fixed polymorphism. Additionally, we suggest a number of lines of research which could be motivated by this discovery.