Gametogenesis in amphimictic and parthenogenetic populations of Aphelenchus avenae.

The female reproductive system of Aphelenchus avenae, studied in orcein-stained material, showed a peculiar structural pattern not yet reported in other nematodes. Chromosome morphology and behavior during gametogenesis could be studied in more detail than in other tylenchid or aphelenchid species investigated to date. In a bisexual population from Australia, gametogenesis was by normal meiosis and reproduction by amphimixis. The haploid chromosome number was n=8 in both males and females, and no sex chromosomes were detected. Three monosexual populations from Australia, California, and North Carolina underwent oogenesis by meiosis but reproduced hy parthenogenesis. The haploid chromosome number was n=8 in the Australia and the North Carolina populations, but n=9 in the California population. Spermatogenesis in temperature-induced males of the California population was by normal meiosis, and sperm had n=9 chromosomes. Most chromosomes consisted of a central euchromatic section and two characteristic heterochromatic ends. No centromere was observed in any chromosome. The relationship between the California population with n=9 and all the other populations with n=8 chromosomes is not well understood.