124. Jamieson, B.G.M. and Grier, H. J. 1993g. Influences
of phylogenetic postion and fetilization biology on spermatozoal ultrastructure
exemplified by exocoetoid and poeciliid fish. Hydrobiologia 271, 11-25.
In a cladistic analysis, poeciliids and zenarchopterids
homoplasically show elongation and flattening of the nucleus at right
angles to the plane of the central axonemal singlets; in both the tip of
the nucleus appears rounded in the plane of flattening but pointed in
the plane at right angles. The two families differ in the distribution of
mitochondria in the elongate midpiece: circumferential in poeciliids but
bilateral in zenarchopterids. In poeciliid sperm and independently in Zenarchopterus,
the individual mitochondria are considerably more extensive circumferentially
than longitudinally; they differ in poeciliids in being C-shaped. In Hemirhamphodon
they are moderately elongate. In Dermogenys and Nomorhamphus they
have been modified monophyletically as a pair of elongate mitochondrial
derivatives. A wide cytoplasmic periaxonemal sheath (not seen in poeciliids)
appears to have developed monophyletically in the ancestry of Hemirhamphodon,
Dermogenys and Nomorhamphus with acquisition of radial
rodlets only in Hemirhamphodon. A distinctive development in poeciliids
is the submitochondrial net. Poeciliids have greatly reduced the axonemal fins
which are a synapomorphy of the Actinopteri. Exocoetoids have retained
well developed fins in Arrhamphus, Dermogenys and Nomorhamphus
but reduction has occurred in Zenarchopterus, in which the fins are
small, and, apparently independently, in Hemirhamphodon in which fins
are absent. A posterior extension of the nucleus over the base of the axoneme
is C-shaped and embraces almost the entire circumference of the axoneme
in poeciliids but, independently developed, in zenarchopterids is a
'dorsal' plate. Its absence in Hemirhamphodon is computed as a loss.
These modifications relative to the aquasperm condition are deduced to have
been occasioned by the adoption of internal fertilization. To what extent
they are constrained by features of the genome peculiar to poeciliids,
zenarchopterids or atherinomorphs or are demanded by minute differences in
fertilization biology, or by a combination of the two, is not at present
determinable.