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Variation and Heterozygosity in Sexually vs. Clonally Reproducing Populations of Poeciliopsis
Robert C. Vrijenhoek, Robert A. Angus and R. Jack Schultz
Vol. 31, No. 4 (Dec., 1977), pp. 767-781
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2407438
Page Count: 15
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Fishes of the genus Poeciliopsis comprise both sexual and unisexual forms that coexist in northwest Mexico. This study compares genetic variability and heterozygosity in P. occidentalis, a sexual species, and P. monacha-occidentalis, a diploid unisexual that lives and mates with P. occidentalis. The unisexuals are hybrids derived from the sexual species P. monacha and P. occidentalis. They reproduce through a mechanism termed hybridogenesis in which the P. monacha genome is transmitted clonally from mother to daughter while the paternal (occidentalis) genome is discarded each generation and replaced by mating with males of P. occidentalis. Electrophoretic variation was assessed at 25 protein loci for 202 P. occidentalis and 213 P. monacha-occidentalis from five sites: three in the Rio Mayo where P. monacha and P. occidentalis are sympatric and hybrid synthesis of unisexuals presumably still occurs, and two in northern rivers where P. monacha does not occur. P. occidentalis had low levels of polymorphism (P̄ averaged 10.7%) and heterozygosity (H̄ averaged 1.8%). Genetic similarities between sites ranged from .999 in the Rio Mayo to .800 between the northernmost river and a Rio Mayo site. P. monacha-occidentalis was more polymorphic than P. occidentalis as a result of combining variation from two species (P̄ averaged 19.4%); it was also highly heterozygous (H̄ averaged 42.5%). Unisexuals inherited occidentalis genes in the same frequencies as they occurred in local occidentalis populations. Apparently the monacha genomes of unisexuals are compatible with any occidentalis genes they encounter. The clonally inherited monacha genomes of unisexuals consisted of three different genotypes in the Rio Mayo and only one each in the other two rivers. Recurrent hybrid syntheses of unisexuals appears to be an important influence on the number of hemiclones at specific sites. The single monacha genotype in the northernmost P. monacha-occidentalis population contained unique alleles at two loci. This unisexual provides the first evidence that clonally reproducing vertebrates diverge genetically via mutation subsequent to their hybrid origins.
Evolution © 1977 Society for the Study of Evolution