THE ULTRASTRUCTURE OF SPERMATOGENESIS AND SPERMATOZOA IN PSEUDODIPLORCHIS-AMERICANUS (MONOGENEA, POLYSTOMATIDAE)

The basic characteristics of spermatogenesis and the ultrastructure of the mature spermatozoa show uniformity throughout the parasitic platyhelminths, and the monogenean Pseudodiplorchis americanus follows this general pattern. Primary spermatogonia undergo mitosis to produce 8 primary spermatocytes which divide by meiosis to produce 16 secondary spermatocytes and eventually a syncytial rosette of 32 spermatids. Each spermatid differentiates into a mature spermatozoon bearing two axonemes, a nucleus, a mitochondrion, and cortical microtubules. Although transmission of P. americanus is restricted to only a few nights each year, the production of male gametes appears to be continuous throughout adult life and exhibits no seasonality. Characteristically, cortical microtubules run the entire length of polyopisthocotylean spermatozoa but they are absent along part of the spermatozoon of P. americanus and one other polystomatid species previously examined. This variation on the standard pattern separates the Polystomatidae from the rest of the Monogenea and emphasises the distinctive taxonomic position and the homogeneity of many structural features within this platyhelminth family.