Sperm ultrastructure of Macrodasys caudatus (Gastrotricha: Macrodasyida) and a sperm-based phylogenetic analysis of Gastrotricha

The spermatozoon of Macrodasys caudatus Remane, 1927 (Macrodasyida: Macrodasyidae) was studied and compared with both the spermatozoal patterns of M. sp. as well as with our unpublished observations on two species of Urodasys, the only other genus studied in the family. The spermatozoon of M. caudatus is similar to that of M. sp.: in both cells a mitochondrial helix surrounds nucleus and acrosome and a monolayered striated cylinder encloses a conventional axoneme. The differences between the spermatozoa of the two species concern their acrosome organization. Preliminary data on the spermatozoa of Urodasys anorektoxys and U. acanthostylis (Macrodasyidae) are reported. Their spermatozoa greatly differ from the basic sperm model of Macrodasys: they lack mitochondria, but have an unusual ‘finger print-like’ chromatin condensation and a peculiar acrosome morphology, suggesting a possible polyphyletism of Macrodasyidae. The spermatozoa of both Macrodasys species do not conform with the general plan described for macrodasyidans, mainly for the reverse position of the mitochondrion, which in the other macrodasyidans is enclosed by a spring-shaped nucleus. A parsimony analysis was performed on a great number of spermatological data on quante species of both orders. Its results are congruent with those of the traditional systematics. A new set of autapomorphies characterising the Gastrotricha sperm is suggested. Our analysis suggests that spermatozoal characters are useful in resolving monophyletic groups and to broaden the basis of evidence in phylogenetic analyses of gastrotrichs.