Gonads and gametogenesis in Chaetodactylus osmiae (Acariformes: Astigmata: Chaetodactylidae) a parasite of solitary bees

Abstract

Chaetodactylus osmiae (Dufour, 1839) is a mite parasitizing the solitary bee - Osmia rufa L.- used as a commercial pollinator. In this study we present the anatomy of female and male reproductive systems of this species as well as its gonadal structure and gametogenesis at the ultrastructural level. The reproductive systems are similar to those of other Astigmata. The ovaries are paired and each contains germ-line cells – a giant nutritive ovarian cell connected via funnel-type intercellular bridges to oogonia and previtellogenic oocytes. Germinal cells are embedded in several large somatic stroma cells. Remarkable numerous protrusions of the nutritive ovarian cell penetrate into the stroma cell cytoplasm. Conspicuous ER cisterns run close and parallel to the surface of the germinal cells. Oocytes entering vitellogenesis disassociate with the nutritive cell and a vitelline envelope composed of heterogeneous material appears on their surface. When vitellogenesis is completed, the oocytes are full of lipid droplets and two types of yolk spheres; the vitelline envelope transforms into a thin and homogeneous chorion.Paired testes are located on one side of the body, whereas the opposite side is filled by a male accessory gland. In testis, germinal cells are embedded in a few somatic stroma cells. The earliest spermatogonia form a compact germarium, whereas later stages are dispersed randomly within the testis. Spermatocytes are characterized by a superficial spongy layer, formation of mitochondrial derivatives, loss of nuclear envelope and condensation of chromatin in threads. A single electron-dense lamella appears during the spermatid stage, separating chromatin threads from a large spongy body surrounded by arcuate, double-membrane bounded cisterns. In spermatids, the superficial spongy layer is absent. The testicular central cell in the germarium and structures related to meiotic division were not observed in the testes. Spermatozoa are multiform cells (approx. 4x11µm) containing electron-dense lamella (ca. 45 nm thick) surrounded by mitochondrial derivatives which separate chromatin threads 45-50 nm thick from remnants of the spongy body i.e. arcuate cistern profiles. Spermatozoa deposited in female spermatheca are more electron dense; the electron-dense lamella is deeply folded several times, whereas chromatin threads are present in the center of the spermatozoon and are either flanked by lamella folds or located more peripherally under the plasmalemma. Remnants of the spongy body are not discernible.