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.