Sexual and unisexual Australian carp gudgeons (Hypseleotris, Gobioidei: Eleotridae): insight from cytogenetics

The genus Hypseleotris, the most abundant and the most enigmatic group of native fish in Australia, represents a model system where interspecific hybridization among sympatric produces unisexual lineages behaving reproductively as so-called ‘sexual parasites’. The perpetuation of such lineages requires mating with a sexual congener, despite not passing any of its genes onto progeny. Based on the previous research, there are several puzzling evolutionary outcomes providing a significant challenge to the standard paradigm underpinning sexual parasitism. Here, we present cytogenetic analysis in five sexual species and two hybrid biotypes of Hypseleotris using standard and molecular cytogenetic protocols. We describe significant differences among and inside lineages (diploid chromosome number, chromosome morphologies and variability on the sub-chromosomal level). Moreover we observed unexceptional interindividual variability in males from different lineages. Differences were observed in chromosomal numbers and morphology between metaphases obtained from testes and somatic tissues suggesting unusual system of meiotic division. Our cytogenetic description of this unique unisexual fish species complex helps to disentangle of processes leading to origin and maintenance of uni/sexual lineages and contributes to general understanding of sex evolution.