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Comparative spermatology based on electron microscopy (EM) has been used extensively in several invertebrate phyla as an aid to phylogenetic resolution and clarification. In the Platyhelminthes a considerable number of parasitic species have been examined, particularly monogeneans and cestodes, and the findings applied to develop phylogenies. However, sperm of far fewer turbellarians have been studied and the results are mostly too fragmentary to be used in cladistic analyses.

Our preliminary EM investigations of sperm and spermiogenesis of eight schizorhynchs and two eukalyptorhynchs revealed a wide variety of characters and character states. These include (1) characters associated with spermiogenesis such as presence/absence and arrangement of dense bodies, manner of condensation of nuclear chromatin, appearance and fate of the structures in the zone of differentiation, and (2) characteristics of mature sperm including number and length of axonemes, presence/absence, nature and arrangement of dense bodies, appearance of nuclear chromatin, number, nature and arrangement of mitochondria, number and arangement of cortical microubules, presence/absence of lateral flanges and presence/absence and nature of anterior accessory structures.

Such rich variation promises to provide a useful tool for elucidating phylogenetic relationships within Kalyptorhynchia. From further detailed EM studies of a large number of kalyptorhynchs, we are optimistic that it will be possible to determine the plesiomorphic and apomorphic sperm and spermiogenetic character states and to use these in cladistic analyses.

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