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 Macrodasys sp. Ruppert 1978, and with preliminary observations of two species of Urodasys, the only other genus studied in the family. The spermatozoon of M. caudatus is similar to that of Macrodasys sp.: in both cells a mitochondrial helix surrounds the nucleus and the acrosome, and a monolayered striateti cylinder encloses a conventional axoneme; the differences between the spermatozoa of the two species concern their acrosome organization. preliminary data on the spermatozoa of Urodasijs anorcktoxys and U. acanthostylis (Macrodasyidae) are reported. Their spermatozoa greatly differ from the spermatozoon of Macrodasys, since they lack mitochondria and bave unusual "fingerprint-like" chromatin condensation and peculiar acrosome morphology, suggesting a possible polyphyly of the Macrodasyidae. The spermatozoa of both Macrodasys species do not conform to the generai pian described for Macrodasyida, mainly due to 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 of 28 species from both orders: its results are congruent with those of the traditional systematics. A new set of autapomorphies characterizing the Gastrotricha sperm is proposed. Our analysis suggests that spermatozoal characters are useful in resolving monophyletic groups and broadening the basis of evidente in phylogenetic analyses of gastrotrichs.