FORMATION OF GERM-CELL CLUSTER IN TUBULIFERAN THRIPS (THYSANOPTERA)

The ovarian structure and oogenesis in the larval stages of 2 tubulife ran species, Bactrothrips brevitubus (Idolothripinae) and Holothrips y uasai (Phlaeothripinae) of the Thysanoptera were examined using ultrat hin serial sections, with special reference to the cluster formation o f germ cells. No cells identifiable as stem cells were found in the ov arian rudiments of the Ist and 2nd-instar larvae. The clusters of oogo nial cells were observed frequently in the Ist-instar, but scarcely in the 2nd-instar larvae: all the oogonial clusters observed were compos ed of 2 cells. In the 2nd-instar larvae, the ovarian region posterior to the germarium, or the vitellarium, contained both solitary and clus tered oocytes. The oocyte clusters were composed of less than 5 cells. The oocytes, located in the posterior region of the vitellarium, were ail solitary and at the previtellogenic stages. A protuberance was fo und in some solitary germ cells. The structure may represent a remnant of the intercellular bridge, previously formed between the germ cells . The number of oocytes composing a cluster is small but does not alwa ys fit the 2(n)-rule. One possible explanation is the accelerated deta chment process of oocytes from a cluster. The cluster formation of ger m cells has been confirmed in the Tubulifera as well as in the Terebra ntia, and this phenomenon can be recognized as a general feature of th e panoistic ovaries of the Thysanoptera.