Jamieson, B.G.M., Lee, M.S.Y. and Long, K. 1993d. Ultrastructure of the spermatozoon of the internally fertilizing frog Ascaphus truei (Ascaphidae: Anura: Amphibia) with phylogenetic considerations. Herpetologica 49, 52-65.
Spermatozoal ultrastructure endorses other evidence that Ascaphus is the sister-group of all other anurans. Primitive (plesiomorphic) features of the sperm of Ascaphus, shared with those of urodeles and basal amniotes and apparently basic to tetrapods, include the elongate conical acrosome, a subacrosomal cone (periperforatorial material), a slender rodlike perforatorium which continues posteriorly in an endonuclear canal, and elongation of the nucleus which tapers within the acrosome. Probable lissamphibian autapomorphies (distinctive advanced characters) which must, therefore, be considered plesiomorphic for Ascaphus are location of the mitochondria unilaterally relative to the flagellum and their association with the paraxonemal rod. This rod is equivalent to the major and minor fibers in the sperm tail of other anurans, and these structures appear to be jointly homologous with a coarse fiber of the amniote axoneme. In Ascaphus, the undulating membrane, which connects the major fiber to the axoneme as an autapomorphy of caecilians, urodeles, and many frogs, is reduced to a short, broad bridge between the inner and outer regions of the paraxonemal rod. The rod carries a longitudinal groove, along this bridge, which houses the mitochondria. Reduction of the undulating membrane is the major advanced feature (apomorphy) of the spermatozoon of Ascaphus. This reduction cannot be ascribed simply to occurrence of internal fertilization in Ascaphus and is tentatively interpreted as spermatozoal paedomorphism, as the undulating membrane persists in internally fertilizing urodeles and in caecilians. It is argued that internal fertilization is basic (pleisomorphic) in the Lissamphibia and that external fertilization has been secondarily acquired in the ancestry of those urodeles and anurans that exhibit it.