Spermatogenesis in the earthworm Microchaetus pentheri (Oligochaeta, Microchaetidae)

Spermatogenesis in Microchaetus pentheri (Microchaetidae) follows the familiar pattern known for other oligochaetes. Spermatogenic stages develop around an anucleate cytophore from which they separate as mature spermatozoa. During spermiogenesis the nucleus elongates and becomes surmounted by a complex, elongate acrosome; the flagellar axoneme develops from the distal centriole. The centriole is positioned posterior to the mid-piece, which consists of six mitochondria radially adpressed to form a cylinder about 2 μm long. Microchaetus shows many plesiomorphic features in the structure of its acrosome, which are also seen in two other taxa of the Diplotesticulata, Haplotaxis (Haplotaxidae) and Sparganophilus (Sparganophilidae, Aquamegadrili), each of which has the greatest number of plesiomorphies in spermatozoal characters in its group. The Aquamegadrili constitute the sister-group of the Terrimegadrili which contain the earthworm families including the Microchaetidae. The numerous symplesiomorphies in spermatozoal characters do not, however, establish monophyly of microchaetids with haplotaxids and sparganophilids. An apomorphy in the acrosome of Microchaetus is its greater length (3.8 μm vs less than 1 μm in Haplotaxis and 1.5 μm in Sparganophilus), in this respect resembling other investigated terrimegadriles, the lumbricids, hormogastrids and megascolecids. The axial rod of the acrosome of Microchaetus appears apomorphic relative to that of Haplotaxis, Sparganophilus, lumbricids and megascolecids in lacking an anterior expansion, the capitulum. It ends posteriorly in a cylindrical body, somewhat resembling the node diagnostic of the axial rod of megascolecid earthworms