Egg formation in lepidoptera.

Reproductive biology in the Twentieth Century produced comprehensive descriptions of the mechanisms of egg formation in most of the major orders of insects. While many general principles of ovarian development and physiology emerged, every order turned out to have a set of its own special motifs. Discovery of the lepidopteran motifs is summarized in this essay. The emphasis is on developmental mechanisms, beginning with the early growth and differentiation of female germ cells and ending, after many turns in morphogenesis, physiology and biosynthesis, with eggs that are filled with yolk and encased in chorions. Examples of uniquely lepidopteran traits include the cellular composition of ovarian follicles, the number of tubular ovarioles in which they mature, the functions of cell-to-cell junctional complexes in their maturation, their use of glycosaminoglycans to maintain intercellular patency during vitellogenesis, the role of proton and calcium pumps in their ion physiology, a separate postvitellogenic period of water and inorganic ion uptake, and the fine structure and protein composition of their chorions. Discovery of this combination of idiosyncracies was based on advances in the general concepts and techniques of cell and molecular biology and on insights borrowed from studies on other insects. The lepidopteran ovary in turn has contributed much to the understanding of egg formation in insects generally.

Storage hexamer utilization in Manduca sexta.

In preparing for metamorphosis insects store in their hemolymph and fat bodies a major nutrient reserve of 500-kDa hexamerins. At least three hexamerins serve this function in Lepidoptera, including arylphorin (ArH) and two high methionine proteins (M-MtH and V-MtH). Six day-old adults of Manduca sexta are shown here to have consumed over 99% of their pupal reserves of ArH and in the case of males, 99.8% of M- and V-MtH. In support of egg formation, however, females at this stage retain over 25% of their pupal reserves of the high methionine proteins. Demonstrated here are three factors contributing to the methionine protein reserves in day-6 adult females. (1) Pupal stores of the methionine proteins average 1.67 times larger in females than in males. (2) A fraction of this pupal store remains undiminished during pharate adult development: centrifugation of homogenates partitions the hexamerins into a fraction that is soluble in PBS and a smaller, particle-associated fraction that is not. Pharate adults consume most of the soluble fraction and relatively little of the particulate fraction, which then constitutes over half of the methionine protein reserves of post-eclosion females. (3) Both soluble and particle-associated reserves double in the week following eclosion and this suggests that adult females may resume the synthesis of V- and M-MtH. Though differing in amino acid sequence and antigenic properties, V-MtH and M-MtH showed no significant differences in their storage and utilization profiles.

Ion physiology of vitellogenic follicles.

The ion physiology of vitellogenic follicles from a lepidopteran (Hyalophora cecropia) and a hemipteran (Rhodnius prolixus) are compared. Similarities that can be expected to occur in vitellogenic follicles of many other insects include: (1) gap junctions, which unite the cells of a follicle into an integrated electrical system, (2) transmembrane K(+) and H(+) gradients that account for over 60% of follicular membrane potentials, (3) absence of a Cl(-) potential, (but the opening of channels to this anion when vitellogenesis terminates in H. cecropia), (4) an electrogenic proton pump that supplements follicular membrane potentials, (5) Ca(2+) action potentials evoked by injecting depolarizing currents into oocytes, and (6) the use of osmotic pressure to control epithelial patency. Differences include: a Na(+)/K(+)-ATPase that accounts for about 20% of the follicular resting potential in R. prolixus but is absent from H. cecropia, and an intrafollicular Ca(2+) current that moves from oocyte to nurse cells through cytoplasmic bridges in H. cecropia. Evidence is also summarized for two promising mechanisms that require further substantiation: (1) transmission via gap junctions of a follicle cell product that promotes endocytosis in the oocyte; and (2) transport of the proton pump back and forth between cell surface and endosomes as the membrane that carries it recycles through successive rounds of vitellogenin uptake.

Sulfate and glucosamine labelling of the intercellular matrix in vitellogenic follicels of a moth.

InHyalophora cecropia the intercellular spaces of the follicles contain during vitellogenesis a matrix that can be labelled eitherin situ or in culture with35S-sulfate and3H-glucosamine. The matrix was demonstrated by autoradiography and also by treating follicles for 15 min with pronase, which released TCA-soluble matrix fragments with molecular weights of up to 2×106 daltons. Testicular hyaluronidase degraded the high molecular weight fragments, and thus it is probable that they are chondroitin sulfate-like mucopolysaccharides. With the termination of vitellogenesis new matrix is no longer deposited, and the pre-existing material is disassembled. The sulfated matrix may account for the patency of the intercellular diffusion channels essential for blood protein uptake and also for the low level, extracellular binding of blood proteins that characterizes the vitellogenic follicle inHyalophora.