Reduced Reproductive Capacity in Diploid and Triploid Hybrid Sunfish

Abstract

Reproductive capacity of 2-year-old sibling green sunfish Lepomis cyanellus male × bluegill L. macrochirus female diploid and triploid hybrids was investigated using histology and flow cytometry. Gonads of triploid females were immature compared with diploids, and gonads possessed cells that primarily appeared to be oogonia or immature oocytes; most cells were triploid. Gonadal development in triploid males appeared to be more advanced (based on observed cell types) than development in triploid females, although male triploids failed to yield milt during stripping. Some spermatozoa in triploid testes were completely differentiated, having tails. No cells, however, were less than 3n and most were 6n, which indicates that meiosis is interrupted at the first reduction division in male triploids but that some cells did complete spermiogenesis. Diploid male hybrids possessed a small proportion of haploid cells, and spermatozoa in their testes appeared normal. The largest proportion (more then 60%) of diploid hybrid testicular cells were 4n, but most cells (over 95%) appeared to be spermatozoa or spermatids, which suggests that there were problems with meiotic pairing during spermatogenesis in diploid hybrids. These results demonstrate that the production of haploid gametes did not occur in triploid hybrids and was diminished in diploid hybrids; the latter is possibly the result of a species isolation mechanism in a genus showing substantial natural interspecific hybridization.