Heritable Melanism and Parasitic Infection Both Result in Black-Spotted Mosquitofish

Abstract

Male Gambusia holbrooki (Eastern Mosquitofish) express a heritable pigmentation polymorphism: ≈99% of males are silver, and only ≈1% have a melanic, black-spotted pattern. Sex-linkage, an autosomal modifier, and temperature control the expression of this heritable melanism. In many teleosts, melanin also accumulates around the site of parasitic invasion. We have identified black-spot disease in wild mosquitofish from their native habitat. Here, we demonstrate convergence upon the black-pigmented phenotype through two means: 1) heritable melanism, and 2) melanic spotting on the silver genotype that results from infection with immature encysted trematodes. Females are silver and express greater avoidance of melanic males during mating attempts. The resemblance of the black-spotted pattern of the melanic genotype to that of silver genotype infected with trematodes may affect the fitness of melanic males if females perceive them as diseased. Alternatively, females may shun parasitized silver fish because they resemble the melanic genotype, which is larger and has a larger mating organ.