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.
(plesiomorphic) features of the sperm of Ascaphus, shared with those of
urodeles and basal amniotes and apparently basic to tetrapods, include
elongate conical acrosome, a subacrosomal cone (periperforatorial
slender rodlike perforatorium which continues posteriorly in an
canal, and elongation of the nucleus which tapers within the acrosome.
lissamphibian autapomorphies (distinctive advanced characters) which
therefore, be considered plesiomorphic for Ascaphus are location of the
mitochondria unilaterally relative to the flagellum and their
the paraxonemal rod. This rod is equivalent to the major and minor
the sperm tail of other anurans, and these structures appear to be
homologous with a coarse fiber of the amniote axoneme. In Ascaphus, the
undulating membrane, which connects the major fiber to the axoneme as
autapomorphy of caecilians, urodeles, and many frogs, is reduced to a
broad bridge between the inner and outer regions of the paraxonemal
rod carries a longitudinal groove, along this bridge, which houses the
mitochondria. Reduction of the undulating membrane is the major
feature (apomorphy) of the spermatozoon of Ascaphus. This reduction
ascribed simply to occurrence of internal fertilization in Ascaphus and
tentatively interpreted as spermatozoal paedomorphism, as the
membrane persists in internally fertilizing urodeles and in caecilians.
argued that internal fertilization is basic (pleisomorphic) in the
and that external fertilization has been secondarily acquired in the
of those urodeles and anurans that exhibit it.