Reductions and new inventions dominate oogenesis of Strepsiptera (Insecta)

Abstract

The endoparasitic life of strepsipterans (Insecta), especially neotenic females, reduces to a great extent external and internal organs. Light and electron microscopic investigation of ovaries of Elenchus tenuicornis (Kirby) confirms the following: (1) somatic tissues of ovaries are totally reduced, with the exception of some cells surrounding germ cell clusters; (2) a previtellogenic growth phase of oocytes is reduced; (3) nurse cells remain diploid and their membranes degenerate at the onset of vitellogenesis; (4) vitellogenesis is reduced, vitellin and fat vacuoles contribute only 50% to the final egg volume; and (5) chorionogenesis is reduced to a vitellin membrane. However, some features of normal development remain, allowing classification of the ovary type as polytrophic meroistic: (1) germ cells undergo synchronized, incomplete divisions, following the 2 n rule, where all former intercellular bridges become localized in one cystocyte, while the other has none; and (2) only one cell is determined as the oocyte, all other cystocytes serve as nurse cells and the surrounding somatic cells transform into follicular cells. Novel events in oogenesis of strepsipterans include fission of clusters during the phase of cluster mitoses, and protection of oocyte nuclei, while nurse cell nuclei degenerate in the same cytoplasm.