GERM-CELL CLUSTER FORMATION IN INSECT OVARIES

Three different ovariole types exist in insects: panoistic, polytrophic- and telotrophic-meroistic. Their ontogenetic development is comparable to all insect orders. Each ovariole is composed of somatic tissues and germ cells. Panoistic ovarioles can be developed: (1) by totally blocking germ cell cluster division (e.g. in ''primitive' insect orders; and (2) after germ cell cluster formation by final cleavage of cystocytes, which develop as oocytes (e.g. in stoneflies or thrips). Polytrophic-meroistic ovaries, showing a set of identical characters, are found among hemimetabolous and holometabolous insects, indicating a ''basic type'' of common origin. One characteristic feature is the differentiation of only one oocyte, which is derived from one central cell of the cluster, whereas all other siblings are transformed into nurse cells. Telotrophic ovaries differ from polytrophic ovaries by retention of all nurse cells in the anterior trophic chamber. In addition, oocyte-nurse cell determination can be shifted towards more oocytes in a cluster, and clusters or subclusters can fuse by cell membrane reduction among nurse cells. This type of ovary developed independently 3 times from polytrophic ancestors and once in mayflies directly from panoistic ancestors.