Evolution des cellules germinales et mesodermiques des testicules larvaires du doryphore, Leptinotarsa decemlineata say (Coleoptera: Chrysomelidae)

Abstract

Larval testes of Leptinotarsa decemlineata consist of 3 types of germinal cells: primary spermatogonia, secondary spermatogonia and spermatocytes. Meiotic prophase begins and continues until pachytene stage; its continuation and spermiogenesis occur during larval development. A mesodermal cell, the cystic cell, always separates the hemocoele and germinal cells. Secondary spermatogonia are connected by cytoplasmic bridges that contain only rough vesicular endoplasmic cisternae. The primary spermatocytes have more numerous bridges in which the rough vesicular endoplasmic cisternae become pronounced; likewise, the other cytoplasmic organelles increase in number and size during the prophase. The nucleus contains 2 divided nucleoli in the gonia; they are more compact in the spermatocytes. All germ cells show numerous extrusions of RNP particles. The cytoplasmic discharge of these particles through nuclear pores is progressively reduced in the secondary spermatogonia and in the pachytene spermatocytes. In these latter cells, the “nuages” are frequently associated with some mitochondria. Gonia and spermatocytes have cytoplasmic bulges, extruding small vesicles in the cystic cells. These extrusions probably avoid later degeneration. Nevertheless, the gonial necroses are frequent; they occur in 2 patterns: (1) during gonial destruction, death of the cystic cell precedes one of the gonia; it thus seems that the cystic cell is necessary for the maintenance of germinal cell integrity; (2) during the spermatocyte necrosis, this same cell phagocytoses the spermatocytes, and protects the gonad against a greater degradation. The cystic cells, which remain undamaged, from an interstitial tissue that communicates with the basal part of the testicular lobes. Certain testicular somatic cells, situated both in the basal part of the testes, and the spermduct, are secretory. Their cytolytic secretions cause a hollowing out of the intra- and extra-testicular vasa deferentia. We have shown, for the first time, that larval testes are bordered by basal lamina secreted by certain hemocytes, the plasmatocytes. They insert between the cysts and divide the gonads into testicular follicles. Plasmatocytes thus constitute the perifollicular tissue packing of the gonad.