Hermaphroditism can compensate for the sex ratio in the Astyanax scabripinnis species complex (Teleostei: Characidae): expanding the B chromosome study model

Abstract

Fishes are among the vertebrates with the greatest plasticity with respect to sex determination and differentiation. Astyanax scabripinnis makes up a Neotropical species complex with isolated populations living in river sources and streams and has been used as a model in B chromosome studies. B chromosomes, by definition, are additional to the karyotype and not essential to the carrier organisms. The aim of the present investigation was to determine whether reproductive characteristics and deviation in the sex ratio (1:1) are related to B chromosomes in this species complex. Sample collections were taken from specimens, and gonads were classified based on histological development stages. Fluorescence in situ hybridization using microdissected B chromosome probes was employed for the quantification of individuals with B chromosomes. The sex ratio was estimated and gonadosomatic and hepatosomatic indices were calculated independently for males and females with and without B chromosomes. The occurrence of B chromosomes was related to possible distinct reproductive strategies in the population studied, especially in relation to energetic contribution and gonad development stages. The data suggest that this species complex exhibits functional hermaphroditism and possibly bidirectional sexual inversion. However, hermaphroditism seems to be related to environmental variables, such as temperature and population density, rather than the presence or absence of B chromosomes.