Follicular cell differentiation in polytrophic ovaries of a moth midge, Tinearia alternata.

Dipteran ovaries consist of structural-functional units termed egg chambers. Each egg chamber is composed of a cluster of germ cells enveloped by a simple somatic follicular epithelium. With the progress of oogenesis, initially an almost uniform population of follicular cells (FCs) becomes diversified into a few subgroups, which significantly differ in their function and behaviour. From the extensive genetic and molecular studies on Drosophila it became evident that the mode of diversification of FCs and the interactions between distinct FC subpopulations and the germ-line cells are essential for a proper course of oogenesis and the generation of oocyte/embryo polarity. Recent comparative studies showed that major dipteran lineages may significantly differ in the mode of FC differentiation. The most essential difference occurs in the ability of the FCs to undertake migrations within the egg chamber. In contrast to long distance, invasive migrations characteristic of distinct FC subgroups in the egg chambers of the most derived flies (Brachycera), including Drosophila, the FCs in the ovaries of more ancestral Nematocera lack migratory activity and change their location only within the epithelial layer. Comparative analyses indicate that the FCs in the representatives of particular evolutionary lineages within Nematocera may differ in their behaviour during oogenesis. In this report we describe the FC differentiation pathway in the egg chambers of a moth midge, T. alternata (Psychodomorpha). Comparison with representatives of craneflies (Nematocera: Polyneura) showed that differences in the behaviour of FCs and in the number of FC subpopulations between Polyneura and Psychodomorpha, may depend on different oogenesis dynamics. In spite of the observed differences, some functional homologies between distinct subsets of the FCs in dipteran ovaries are postulated.