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.