Onychophoran‐euclitellate relationships: evidence from spermatozoa l ultrastructure

Abstract

Onychophoran sperm share striking apomorphies with sperm of oligcchaetes: the elongate, cylindrical, strongly condensed nucleus with (enchytraeid oligochaetes only) spiral keel(s); interpolation of mitochondria, which are spiral as in some microdriles, between nucleus and axoneme (autapomorphy); presence of postmitochondrial annulus; occurrence of a basal cylinder at the anterior end of the two central singlets (autapomorphy). Onychophoran sperm differ notably from oligochaete sperm: in possessing periacrosomal material; in the helical coiling, reduction, or absence of the acrosome vesicle (though acrosomal coiling occurs in leeches); in the conical subacrosomal structure contrasting with the characteristic acrosome tube of euclitellates; in the absence of a perforatorium (though these two features could be consequent on the reduction of the acrosome); in lacking a distinct padlike thickening of the anterior nuclear envelope; in possessing (external to the mutual 9 + 2 axoneme) 9 peripheral singlets and a subplasmalemmal microtubular manchette (also present in questid annelids); and in the scattered, rather than regularly arranged, peripheral glycogen of the axoneme; they also lack two additional fibres which give the characteristic tetragon of the oligochaete axoneme; and, although the centriolar triplets are disrupted, having a more complete distal centriole than in euclitellates. The remarkable similarities to euclitellate sperm, coupled with embryological similarities between Onychophora and Euclitellata, suggest that the onychophoran‐euclitellate assemblage is a monophyletic group and the sister‐group of the myriapod‐hexapod assemblage. Wider phylogenetic implications of these findings are discussed.