Micro- and macrogeographic genetic structure in bryozoans with different larval strategies

Genetic structuring was examined in two marine bryozoans with different modes of reproduction and dispersal of sexually derived larvae: Alcyonidium gelatinosum=A. polyoum, which produces brooded, short-lived lecithotrophic larvae, and A. mytili, an egg broadcaster with pelagic larvae. Randomly Amplified Polymorphic DNA (RAPD) data from these species were further compared with a population of the freshwater bryozoan, Cristatella mucedo, which reproduces mainly by colony fission and production of asexual statoblasts. Microgeographic comparisons of colonies from single sites revealed higher levels of genetic divergence between A. mytili colonies (similarity coefficient, 1−S=0.60) than between those of A. gelatinosum (1−S=0.33). A macrogeographic survey of population structure was undertaken in A. gelatinosum and A. mytili populations from several geographically separated areas. Analysis of Molecular Variance (AMOVA) results showed that for A. gelatinosum, 8% of the variance was found between populations and 92% within populations, and for A. mytili, 10% of the variance was found between populations and 90% within. These values were similar between the two species, despite the differences in larval strategy, and indicate that there are high levels of population genetic subdivision in both species. Results show a general correspondence between patterns of microgeographic genetic structuring and predictions based on reproductive mode and dispersal capacity, although the species with a pelagic larva showed less genetic variation between populations than would have been predicted. Analysis of the C. mucedo population showed a highly clonal structure, as predicted and in agreement with previous studies. Such data represent the first case of a DNA-based population survey of bryozoans with differing reproductive strategies.