Mixed-species shoals and the maintenance of a sexual–asexual mating system in mollies

Abstract

The mating system of the sexual–asexual complex of sailfin mollies, Poecilia latipinna, and Amazon mollies, P. formosa, depends on the close proximity of individuals in aggregations or shoals because P. formosais gynogenetic (that is, sperm only initiate embryogenesis but do not contribute genetic material) and must be inseminated by male P. latipinna. Furthermore, female P. latipinnacopy the mate choice of female P. formosawith the result that male P. latipinnabecome more attractive to conspecific females when they mate with P. formosa. Mate copying of heterospecifics has not been reported in other systems, and in mollies it could result from the inability of females to discriminate species. In experiments both P. formosaand P. latipinnafemales preferred to associate with a conspecific than a heterospecific, but preferred a larger heterospecific group to a smaller conspecific one. Females can thus discriminate between conspecific and heterospecific females, so that heterospecific mate copying is not a result of mistaken identity. Females preferred to associate with conspecifics, but this preference was overridden by preference for larger groups although the threshold differed between the two species.