Male Hybrid Sterility in the Mule Duck is Associated with Meiotic Arrest in Primary Spermatocytes

Hybrid sterility is a postzygotic reproductive isolation mechanism that prevents successful interbreeding between different species. The mule duck, an intergeneric F-1 hybrid between the domestic duck (Anas platyrhynchos) and Muscovy duck (Cairina moschata), displays sterility with gametogenesis failure in both sexes. Although the F-1 hybrid male is known to exhibit large-sized testes that produce no sperm, the spermatogenic phenotype has not been well described. In this study, we revealed the abnormal meiotic phenotype of the F-1 hybrid spermatocytes and dissimilarity in the karyotypes between the two parental species. Histological examination of the F-1 hybrid testis showed the accumulation of primary spermatocytes with irregular highly condensed chromosomes in the seminiferous epithelium, whereas secondary spermatocytes and postmeiotic cells were absent and many testicular cells undergoing apoptosis were present. Cytogenetic analyses of spermatogenic cells from the F-1 hybrid male revealed that meiosis succeeded in entering pachytene, but failed to progress beyond diakinesis-metaphase Tin primary spermatocytes, and that a number of degenerated spermatocytes were present at pachytene. Karyological observations showed morphological differences in chromosome 1 and the Z chromosome between the parental species. These results collectively suggest that the main cause of abnormal spermatogenesis in the F-1 hybrid is pachytene and/or metaphase I arrest, which possibly resulted from the failure of homologous chromosome pairing, recombination, and subsequent chromosome segregation due to chromosomal incompatibility between the parental species.