Low genetic differentiation between isolated populations of the colonial ascidian Symplegma rubra Monniot, C. 1972
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Pelagic larvae are highly important for maintaining the gene flow among populations of sessile marine invertebrates. Colonial ascidians consist, exclusively, of brooding species, with lecithotrophic larvae that have a limited dispersal. As a result, there is a marked differentiation among populations. In this work, we used allozyme electrophoresis to access the genetic variation in four populations of Symplegma rubra, a colonial ascidian frequently found in the intertidal zone of Southeastern Brazilian coast. High variability was found at three of the four sites sampled, the exception being Praia Grande in the State of Rio de Janeiro. At this site, there was a great preponderance of clones, which possibly reflected the enclosed nature of the location and its low water circulation that reduce the dispersal capabilities of these animals. S. rubra did not conform to expectations for random mating (Hardy–Weinberg equilibrium): there was a deficit of heterozygotes that was more related to the small population size than to inbreeding processes, since Fis analysis per locus revealed a deficiency of heterozygotes at only one locus—MDH*. The greatest variation in allele frequency was found for GPI-2*. Analyses of genetic variability revealed moderate differentiation among the populations (FST=0.051), which was unexpected for a species with a low dispersal capability. Rafting, a frequently underestimated means of dispersal, may be the main mode of gene flow in this species over large areas, since colonies of S. rubra are frequently seen growing on drift material and there is no evidence that the larva survive for a long time in the plankton.
KeywordsPolymorphic Locus Asexual Reproduction Larval Dispersal Pelagic Larva Colonial Species
We are indebted to Centro de Biologia Marinha da Universidade de São Paulo for providing logistical support. We offer thanks to CGM Delboni for help in the sampling and SCS Andrade, MT Grombone, MT Almeida, A Abreu and especially J José for help in the data analysis. We are also grateful to three anonymous referees whose comments greatly improved the manuscript. One of us (GMD) also offers thanks to CAPES for the grant that supported his studies for the master degree.
This work was supported by the State of São Paulo Research Foundation (FAPESP) within the BIOTA/FAPESP—The Biodiversity Virtual Institute Program (www.biotasp.org.br).
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