Oogenesis in Tenebrio molitor: histological and autoradiographical observations on pupal and adult ovaries.

(i) Nurse cells and oocytes in the telotiophic ovary of Tenebrio molitor L. (Insecta, Coleoptera-Polyphaga) are differentiated in the larval stadium. (ii) During pupation DNA synthesis occurs in the nurse cells and is probably associated with polyploidy; some of them become multinucleate. The functional significance of these events is interpreted as preparation for subsequent massive RNA synthesis. (iii) The oviducts incorporate [H]thymidine and undergo elongation due to mitoses. This ceases at eclosion. (iv) RNA synthesis in the pupal ovary is low, but increases in the nurse cells and follicle cells just prior to eclosion. (v) In the adult ovary, once the growth phase has been initiated, a primary oocyte takes about six days to reach maturity. (vi) The nurse cells, though apparently lacking nucleoli, synthesize much stable RNA which reaches each oocyte via a trophic cord. (vii) The follicle cells undergo continuous DNA synthesis: some nuclei contain over 64 times the haploid amount of DNA. Replication is probably asynchronous within a nucleus: this may account for the phenomenon of simultaneous DNA and RNA synthesis in the follicle cells, which also lack nucleoli. (viii) Oogenesis has been divided into nine developmental stages, three of which are vitellogenic. (ix) The oocyte chromosomes are capable of RNA synthesis both when despiralized during early previtellogenesis and after karyosome formation, which occurs at stage 6. (x) The protein content of the oocyte appears to have a dual origin: at least part of the ooplasmic proteins form in situ; while yolk proteins are derived from the haemolymph. The extra-ovarian protein reaches the oocyte via spaces which develop between the follicle cells. (xi) The nucleoplasm becomes more heavily labelled with proteins than the ooplasm. With [H]leucine, methionine and phenylalanine but not with tryptophane or arginine, there is an increased incorporation into the karyosome. It is suggested that this karyosomeassociated protein may function in gene masking. (xii) The significance of these findings is discussed with reference to the literature.