Reproductive and electrophoretic evidence for genetic maintenance of dimorphism in the ascidian Pyura stolonifera near Melbourne, Australia

Abstract

On the central coast of Victoria, Australia, the ascidian Pyura stolonifera assumes two morphs, a yellow morph that lives exclusively in bays, and a brown form that dwells mainly on outer coasts. Reproductive (gonad condition and larval rearing) studies and allozyme electrophoretic analyses were conducted as part of a multidisciplinary investigation designed to elucidate the environmental and genetic components of dimorphism in this species. Allopatrically, both morphs were reproductively mature in spring, summer, and autumn, but in winter, only yellow ascidians were ever ripe. Sympatrically, on one sampling date both morphs were mature, while on another occasion only the brown form was ripe. These data suggest that the breeding seasons of the two morphs differ somewhat. Fixed electrophoretic differences at two loci (MDH, GPI) in both allopatry and sympatry, demonstrate that these morphs are reproductively isolated and therefore separate species. These findings suggest that genetic factors play a role in maintaining the morphological differences that exist between morphs (species).