Male genital system and spermiogenesis of Nanorchestes amphibius (Acari: Endeostigmata: Nanorchestidae): anatomy, histology, and evolutionary implications.
In the present article the anatomy and histology of the male genital system of an endeostigmatid mite are described for the first time. The Endeostigmata probably are a paraphyletic group supposed to include the most primitive actinotrichid mites. In Nanorchestes amphibius, the testis comprises a paired germinal region connected with an unpaired glandular region. In the germinal region, spermiogenesis takes place in cysts of a somatic cell containing germ cells representing the same developmental stage. In the lumen of the glandular region, the spermatozoa are stored together with secretions of the glandular epithelium. These secretions are probably involved in the formation of spermatophores. From the glandular region, spermatozoa and secretions are released into the vasa deferentia that histologically can be divided into three sections, beginning with a short paired region with strong circular muscles serving as a sphincter, continuing with a paired proximal zone, followed by a short unpaired distal section. The distal vas deferens leads into the chitinous, unpaired ductus ejaculatorius which is followed by the progenital chamber. The ductus ejaculatorius is composed of a proximal section and a proximal, central, and anterior chamber. It is accompanied by a complex system of muscles and sclerites probably involved in the formation and ejaculation of the spermatophore. A similar organization can also be found in Prostigmata, but not in Oribatida. Anterior to the progenital chamber is located a paired accessory gland that probably produces a lipid secretion. Spermiogenesis is characterized by disintegration of the nuclear envelope, condensation of chromatin, and extensive reduction of the amount of sperm cell cytoplasm. The mature aflagellate, U-shaped spermatozoa are simple in structure and lack mitochondria and an acrosome complex. The results do not support the current view that Nanorchestidae are more closely related to Sarcoptiformes, i.e., Oribatida and Astigmata, than to Prostigmata.