Possible effects of sperm limitation on the reproductive success of the Amazon molly, Poecilia formosa (Poeciliidae)

Using artifi cial insemination we studied the role of sperm number on relative reproductive success in the gynogenetic Amazon molly (Poecilia formosa). This species is unisexual, but depends on mating with males for sperm of closely related species (Poecilia mexicana and Poecilia latipinna) to trigger embryogenesis. The male genome is not incorporated into the female genome; inheritance is strictly maternal. The correlation of sperm number and relative reproductive success is logarithmic and a minimum of about 50,000 sperm was determined. Below this number, the number of offspring drops off. A comparison with another study, which showed that males of the Atlantic molly (Poecilia mexicana) provide fewer sperm for Amazon mollies as compared to conspecifi cs, shows nonetheless that Amazon mollies typically receive far more sperm from males of Poecilia mexicana and Poecilia latipinna than the minimum determined here.