Ovary Organization and a Putative Germ Plasm in Tubuliferan and Terebrantian Thrips (Thysanoptera)

Comparison of the detailed morphology of ovaries allows for an evolutionary conclusion in insects. The current study characterized the ovary/ovariole organization in both thysanopteran subtaxa (Terebrantia and Tubulifera) and confirmed the presence of a (putative) germ plasm in the oocytes. Using transmission and scanning electron microscopy as well as histological methods, we show that the morphology of ovaries in the studied terebrantians and tubuliferans differs in the organization of the anterior ovary/ovariole region, that is, the germaria and terminal filaments. Whereas in tubuliferan, Haplothrips leucanthemi, the terminal filaments and germaria of all four ovarioles constituting a given ovary form single common structures (a common terminal filament and a germarial chamber, respectively), in the analyzed terebrantians (Frankliniella occidentalis and Thrips sp.), they remain individual, as in all other insect orders. Thus, the fusion of the anterior ovariole parts is interpreted as an apomorphy characteristic of tubuliferan thrips only. In contrast, the course of oogenesis in terebrantians and tubuliferans is rather similar and reveals one prominent feature: the presence of a distinct nuage layer encompassing oocyte nuclei. We also show that after reinitiation of the first meiotic division in representatives of both thysanopteran subgroups, a small irregular region of cytoplasm arises at the posterior oocyte extremity. We suggest that this region is homologous to the germ plasm of holometabolous insects.