INFLUENCES OF PHYLOGENETIC POSITION AND FERTILIZATION BIOLOGY ON SPERMATOZOAL ULTRASTRUCTURE EXEMPLIFIED BY EXOCOETOID AND POECILIID FISH

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 a t right angles. The two families differ in the distribution of mitocho ndria in the elongate midpiece: circumferential in poeciliids but bila teral in zenarchopterids. In poeciliid sperm and independently in Zena rchopterus, the individual mitochondria are considerably more extensiv e circumferentially than longitudinally; they differ in poeciliids in being C-shaped. In Hemirhamphodon they are moderately elongate. In Der mogenys and Nomorhamphus they have been modified monophyletically as a pair of elongate mitochondrial derivatives. A wide cytoplasmic periax onemal sheath (not seen in poeciliids) appears to have developed monop hyletically in the ancestry of Hemirhamphodon, Dermogenys and Nomorham phus with acquisition of radial rodlets only in Hemirhamphodon. A dist inctive development in poeciliids is the submitochondrial net. Poecili ids have greatly reduced the axonemal fins which are a synapomorphy of the Actinopteri. Exocoetoids have retained well developed fins in Arr hamphus, Dermogenys and Nomorhamphus but reduction has occurred in Zen archopterus, in which the fins are small, and, apparently independentl y, in Hemirhamphodon in which fins are absent. A posterior extension o f the nucleus over the base of the axoneme is C-shaped and embraces al most the entire circumference of the axoneme in poeciliids but, indepe ndently 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 constrain ed by features of the genome peculiar to poeciliids, zenarchopterids o r atherinomorphs or are demanded by minute differences in fertilizatio n biology, or by a combination of the two, is not at present determina ble.