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

Abstract

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 closely related species to trigger parthenogenetic development of the embryo. Therefore 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 of the sperm fuses with the diploid egg nucleus – or in siblings with microchromosomes – if only subgenomic amounts of paternal DNA are included. In one of our diploid, microchromosome-carrying laboratory stocks we observed eight 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 comparison revealed that P. formosa males are functional males with reduced effective fertility. They show a deviation from the typical 23 bivalents in the synaptonemal complexes as well as in diakinesis due to the triploid state. They produced offspring but only with gynogenetic Amazon molly females. This shows that the probably aneuploid sperm from P. formosa males can trigger parthenogenetic development of unreduced eggs.

Extra Microchromosomes Play Male Determination Role in Polyploid Gibel Carp

“…In Chilomycterus spinosus (Noleto et al 2012) and one clone of P. formosa (Lamatsch et al 2000;Lamatsch et al 2010), they were revealed to be associated with male determination; whereas in Sphoeroides spengleri, they were found to be related to female determination (Noleto et al 2012). Intriguingly, the extra microchromosomes in male gibel carp are dispensable for a normal life cycle, and their number and morphology varies between individuals (Figure 2 and Figure S1).…”

Fertile triploid males—an uncommon case among hybrid vertebrates

“…The sterility of triploid male fishes has been documented for both artificially induced triploids [e.g., Rhodeus ocellatus ocellatus (Kawamura et al,'99), Carassius auratus (Gui et al,'92), Verasper moseri (Mori et al, 2006), Salvelinus fontinalis (Allen and Stanley, '78) and Oncorhynchus mykiss (Thorgaard and Gall,'79)] and for naturally occurring individuals, whose triploidy resulted from the incorporation of additional genomic material by an unreduced egg [e.g., Misgurnus anguillicaudatus (Matsubara et al, '95;Zhang and Arai, '99;Oshima et al, 2005), Poecilia formosa (Lamatsch et al, 2000b) and Tinca tinca (Flajshans et al,'93)]. …”

Sex Determination Diversity and Sex Chromosome Evolution in Poeciliid Fish

“…As a trigger for this form of reproduction, sexual contact with a male of another molly species living in the same habitat is necessary: gynogenesis is sperm-dependent, even if the male partner generally does not contribute to the genome of the progeny. However, paternal contribution can occur occasionally and leads to stable triploid clones or clones with microchromosomes [Schlupp et al, 1998;Lamatsch et al, 2000]. The evolutionary significance of these phenomena might reside in their participation in genetic variability in an otherwise clonal species.…”