Unusual triploid males in a microchromosome-carrying clone of the Amazon molly, Poecilia formosa

The Amazon molly, Poecilia formosa, is an all-female fish of hybrid origin which reproduces by gynogenesis, i.e. it depends on sperm of males of close ly related species to trigger parthenogenetic development of the embryo. Th erefore: the offspring is clonal and identical to the mother. In rare cases the exclusion mechanism fails and paternal introgression occurs. This may result either in triploid offspring - if the whole haploid chromosome set o f the sperm fuses with the diploid egg nucleus - or in siblings with microc hromosomes - if only subgenomic amounts of paternal DNA are included. In on e of our diploid microchromosome-carrying laboratory stocks we observed eig ht triploid individuals which all developed into males. We investigated the mitotic and meiotic chromosomes, the synaptonemal complex (SC), and sperm production of these males, and compared them to males of the gonochoristic parental species (P. latipinna and P. mexicana) and their hybrids. This com parison revealed that P. formosa males are functional males with reduced ef fective fertility. They show a deviation from the typical 23 bivalents in t he synaptonemal complexes as well as in diakinesis due to the triploid stat e. They produced offspring but only with gynogenetic Amazon molly females. This shows that the probably aneuploid sperm from P. formosa males can trig ger parthenogenetic development of unreduced eggs. Copyright (C) 2001 S. Ka rger AG, Basel.