Cytophotometric evidence for triploidy in hybrids of the gynogenetic fish, <i>Poecilia formosa</i>

Abstract

Abstract Amounts of DNA per nucleus were estimated by microphotometry in Feulgen‐stained tissue sections from several species of poeciliid fish and their hybrid offspring. In terms of average nuclear volumes as well as relative DNA levels, the all‐female gynogenetic species P. formosa was indistinguishable from the related bisexual species P. sphenops , confirming the essentially diploid nature of the amazon molly, P. formosa . Although DNA values from the less closely related species P. vittata were about 15% higher than those from P. sphenops , both species showed expected patterns of DNA behavior in several somatic tissues and in developing male sex cells. Somatic nuclei from two types of interspecific hybrids contained more DNA than corresponding types of nuclei from typical maternal P. formosa , the additional amount being equivalent to the haploid DNA value characteristic of the participating paternal species. The results strongly suggest that these hybrids are genetic triploids, probably containing diploid maternal and haploid paternal chromosome contributions. The high incidence of chromosomal stickness, bridging, and fragmentation observed during metaphase I of spermatogenesis, and the occurrence of both “giant” and “micro” spermatids in testis of the hybrid P. formosa X P. vittata may well account for the apparent sterility of amazon molly offspring which show transmission of paternal characteristics. Production of triploid interspecific hybrids in poeciliid fish and corollary implications for interpreting previous tissue transplantation studies are discussed.