Variation and Clonal Structure in a Unisexual Fish

A genetic analysis of population structure in the unisexual fish, Poeciliopsis monacha-lucida, was conducted using protein phenotypes encoded by 22 loci. The all-female populations arose by hybridization between P. monacha and P. lucida in narrow zones where their ranges overlap. The unisexual hybrids are sustained by mating with males of P. lucida and are phenotypically intermediate, expressing both maternal and paternal traits. However, only the maternal (P. monacha) set of chromosomes is transmitted through oogenesis in these hybrids; recombination does not occur. By mating laboratory strains of P. monacha-lucida, derived independently from 37 wild females, with males of P. lucida of known genotype, the haploid genotype (haplotype) of the clonally inherited monacha genome was determined. Eight distinct haplotypes were identified among the clonally inherited genomes of the 37 strains. This variation indicates that a minimum of eight clonally inherited genomes (hemiclones) occur naturally, although each of these eight haplotypes may include additional variation at other loci that we failed to observe. Two of the eight haplotypes were found at three distant localities in the Rio Fuerte and may represent well-adapted, generalized clones that have spread to inhabit both stable and diverse habitats as well as fluctuating and depauperate habitats in this river. Additional, unique haplotypes were found only at localities where hybrid syntheses could occur. These haplotypes may represent newly arisen hemiclones that have not yet dispersed. When compared with sexual species, the unisexual fish maintain significantly higher levels of heterozygosity ranging from a low of 36.3% to a high of 50.0% of the tested gene loci in an individual. The high levels of heterozygosity, attributable to their hybrid origin, along with their reproductive potential and vigor apparently explain the success of these unisexual forms.