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SPERM‐MEDIATED GENE FLOW AND THE GENETIC STRUCTURE OF A POPULATION OF THE COLONIAL ASCIDIAN BOTRYLLUS SCHLOSSERI

The genetic structure of populations of sessile and sedentary organisms is often characterized by microgeographic differentiation in gene frequencies and deviations from panmixia. In many terrestrial botanical systems, restricted gene flow via seed and pollen dispersal may have an important role in promoting such genetic patterns. Until recently, however, limited dispersal of the sexual propagules of benthic invertebrates has not been considered to play a comparable role in aquatic systems. Based on paternity analyses in the field using rare allozyme markers, it appears that concentrations of sibling sperm of the sessile, colonial ascidian Botryllus schlosseri decline rapidly within 50 cm of a source colony. In combination with spatially restricted dispersal of brooded larvae, limited dispersal of sperm should enhance the potential for genetic diversification and inbreeding. However, analysis of allelic and genotypic frequencies at three independent, polymorphic allozyme loci using F‐statistics provides little evidence for microgeographic variation in gene frequencies. This lack of differentiation can be explained in terms of the absolute number (rather than concentration) of gametes and larvae dispersing from a point source, which—depending on diffusion and geometric assumptions—may actually increase with distance. In contrast to the absence of differentiation, levels of inbreeding are high, even within the confines of 25 times 25–cm quadrats. The absence of genetic diversification and presence of inbreeding caution against inferring levels and causes of gene flow from indirect analysis of genetic structure and, conversely, making predictions about genetic and breeding structure based solely on direct analysis of gene flow.