Evolution of an all female fish Menidia-Clarkhubbsi Atherinidae.




Echelle, A.A.
Echelle, A.F.
Croier, C.D.

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Menidia clarkhubbsi is a recently discovered, all-female antherinid fish known primarily from 2 pools 280 km apart on the Texas coast of the Gulf of Mexico where it occurs with the bisexual species M. beryllina and M. peninsulae. Specimens identified as M. clarkshubbsi are characterized by 1 predominant genotype across 34 presumptive gene loci including 6 diallelic loci which are heterozygous. Unisexuality and virtual absence of evidence for genetic recombination indicate clonal population structure. Allelic composition, heightened heterozygosity and intermediate appearance of the gas bladder indicate that, like most other unisexual vertebrates, M. clarkshubbsi arose as an interspecific hybrid. Electrophoretic data for all known bisexual species of Menidia over large portions of the geographic ranges show that no hybrid conbination of populations surveyed so far would account for the entire allelic composition of M. clarkhubbsi. M. berylina seems likely to have been one of the parental species, but the second parent population is a M. peninsula-like form that has not been examined electrophoretically. Except at two loci the presumed missing ancestor would have typical M. peninsulae alleles; at Pgm-2 and Idh-1, respectively, it would have an allele known only in M. clarkshubbsi and an allele typical of M. colei, a close relative of M. peninsulae from the Yucatan penisula. Compared with other unisexual vertebrates, M. clarkshubbsi shows low heterozygosity (H=.18); correspondingly, genetic similarity between the hypothetical parental species (Nei's genitic identity .approx. .90) is markedly higher than that between the parents of other unisexual forms. Potential to produce a new unisexual lineage may exist even when the parents are genetically similar.


p. 772-784.


brackishwater fish, genetics, hybrids, hybridization, Menidia clarkhubbsi, Menidia beryllina, Menidia peninsulae