Cytonuclear evidence for hybridogenetic reproduction in natural populations

25 26 Although most vertebrates reproduce sexually, a small number of fishes, amphibians, 27 and reptiles are known in which reproduction is asexual i.e. without meiotic 28 recombination. In fishes, these so-called “unisexual” lineages usually comprise only 29 females, and utilize co-occurring males of a related sexual species to reproduce via 30 gynogenesis or hybridogenesis. Here we examine patterns of microsatellite and 31 mitochondrial DNA (mtDNA) variation in a widespread group of freshwater fishes 32 (carp gudgeons; Hypseleotris spp.) to investigate a long-standing proposal that this 33 group includes unisexual forms. We show that the mtDNA genome of most carp 34 gudgeons in tributaries of the Goulburn River belong to one of two deeply divided 35 clades (~10% cyt b divergence) and that nuclear variation divides the same 36 individuals into four distinct groups. Group 1 exhibits the genotypic proportions of a 37 random mating population and has a 1:1 sex ratio. Two other groups are extremely 38 sex-biased (98% male, 96% female), exhibit excess heterozygosity at most loci and 39 share at least one allele per locus with group 1. We propose that these two groups 40 represent “unisexual” hybridogenetic lineages, and that both utilize co-occurring 41 group 1 as sexual host. Interestingly, the fourth distinct group appears to represent 42 hybrid offspring of the two putative hybridogenetic lineages. The propagation of 43 clonal haploid genomes by both males and females and the ability of these clones to 44 unite and form sexually mature diploid hybrid offspring may represent a novel 45 mechanism that contributes to the dynamics of coexistence between hybridogenetic 46 lineages and their sexual hosts. 47