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Genetic variability in a temperate intertidal Phoronid, Phoronopsis viridis


The Phoronida are a coelomate phylum consisting of only two genera and about 12-15 described species. Phoronids probably represent the common ancestral stock of all lophophorates, and may be the most primitive living deuterostomes. Using the techniques of starch gel electrophoresis, we have studied genetic variation at 39 loci in 120 individuals of Phoronopsis viridis collected in Bodega Harbor, Bodega Bay, California. Allelic variation was found at 27 (69.2%) loci. If a locus is considered polymorphic when the frequency of the most common allele is no greater than 0.99, the proportion of polymorphic loci in the total sample is 48.7%. The average number of alleles per locus is 2.23. The expected frequency of heterozygous loci per individual on the assumption of Hardy-Weinberg equilibrium is 9.4%. There is evidence of inbreeding; the mean value of F, Wright's fixation index, is 0.21±0.02. Genetic variability in P. viridis is intermediate among marine invertebrates. The tropical clam, Tridacna maxima, has on the average 20.2% heterozygous loci per individual. At the other extreme, a brachiopod from Antarctica, Liothyrella notorcadensis, has an average of 3.9% heterozygous loci per individual. Among marine invertebrates, there seems to be a gradient of decreasing genetic variability from low to high latitudes, which may reflect their different adaptive strategies. © 1974 Plenum Publishing Corporation.

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