A molecular phylogenetic analysis of reproductive trait evolution in the soft coral genus Alcyonium

The soft coral genus Alcyonium is among the most reproductively diverse inv ertebrate taxa known: The genus includes species that vary both in mode of reproduction (including broadcast spawners, internal brooders, and external brooders) and sexual expression (gonochores, hermaphrodites, and a unisexu al parthenogen). Such diversity offers a unique opportunity to examine asso ciations between reproductive and morphological traits in a phylogenetic co ntext. We used an approximately 900-bp sequence of the nuclear ribosomal ge ne complex spanning the internal transcribed spacer (ITS) regions to constr uct a molecular phylogeny for 14 European and North American species of Alc yonium onto which we mapped the known distribution of reproductive and morp hological traits. The phylogeny suggests that hermaphroditism or parthenoge nesis has evolved independently at least twice in this genus, and always in internally brooding species. Broadcast spawning and external brooding only occur in species with large colony size, whereas all species with small co lony size brood their larvae internally. Internal brooding and small size a ppear to be ancestral in this genus; if this is the case, an association be tween broadcast spawning and large colony size has evolved independently in at least two clades. This tendency of small adults to brood their larvae w hile large adults broadcast spawn them into the plankton has been observed in a variety of solitary invertebrate taxa, but to date has not been docume nted in any other colonial invertebrates. Moreoever, it has been suggested that organisms with a colonial growth form should not experience the allome tric constraints on brood space that have been proposed to explain the asso ciation between adult size and mode of reproduction in solitary organisms. Unlike many other colonial groups, however, module (polyp) Size is strongly correlated with colony size in Alcyonium, and constraints on brooding may be imposed by module, rather than colony, allometry. The very close genetic relationship (< 1% sequence divergence) and shared polymorphisms among A. digitatum (a large, gonochoric broadcast spawner), A. siderium, and A. sp. A (intermediate-sized and small hermaphroditic, internal brooders) suggest that evolutionary transitions between broadcast spawning and brooding and b etween gonochorism and hermaphroditism can occur easily and rapidly in this group.