Genetic subdivision in the subtidal, clonal sea anemone Anthothoe albocincta

Abstract

Anthothoe albocincta, a common subtidal anemone along south-eastern Australia, reproduces both sexually through broadcast spawning and clonally through fission. Clones may be distinguished both by their electrophoretic genotypes and the colour of their tentacles and oral discs. Local populations typically consist of many, dense clonal aggregations. However, some clones appear to have locally extensive distributions, forming a series of separate aggregations. The capacity of clones to disperse among local populations is unknown. In this study we used an electrophoretic survey at six allozyme loci to quantify levels of variation among samples from each of 13 local populations and four geographic regions sampled between April 1992 and June 1993. These data revealed that populations of A. albocincta, separated by up to 930 km, were at least moderately subdivided. Levels of geographic variation were high and the average F _ST value (standardised genetic variation) was 0.27 based on genotypes of all individual polyps. This value reflects substantial variation both within (F _PR =0.13) and among (F _RT =0.16) regions. Estimates of gene flow among both neighbouring populations and regions are therefore low (N _e m=1.7 and 1.3, respectively). UPGMA (unweighted pair-group method using arithmetic averages) dendrograms suggest that a genetic discontinuity occurs at the very south-east corner of Australia, paralleling reports for two other south-eastern Australian marine invertebrates. In addition, our analyses and theoretical predictions imply that localised proliferation of clonal genotypes may have caused us to underestimate the potential importance of gene flow via larval dispersal. Moreover, the abundance and vast geographic range of this species suggests that widespread dispersal does occur. Collections from three populations covering the peroid December 1992 to June 1993 were examined by crude dissection along with histological sectioning, and showed A. albocincta to be dioecious, with unisexual clones. Eggs within the ovaries of six females sampled over a 3 mo period were small (96±4 μm) and similar to those of related species that produce planktotrophic larvae. In contrast, we found no evidence that clones were dispersed (shared) among neighbouring local populations. An average of only 6% of six-locus genotypes were common to pairs of local populations separated by up to 125 km, this being equal to the percentage expected through sexual reproduction alone. In addition, the percentage of shared genotypes did not decline with increasing geographic separation. These data imply that although asexual reproduction may be used to maintain local populations, the sexual production of genotypically diverse larvae is the primary source of widely dispersed colonists and hence of new clones.