1

570 that extensive pinocytosis occurs in the oocyte plasma membrane, resulting in the accu mulation of yolk protein in membrane-bound vacuoles. Vitellogenins are large (molecular weight 200,000-650,000), conjugated proteins containing both lipid and carbohydrate components. The latter are oligosaccharides and probably important as the agents by which the oocyte plasmalemma recognizes the vitellogenins prior to pinocytosis. Though most yolk protein is produced in the fat body, small contributions may be made by follicular epithelial cells, nurse cells where present, or the terminal oocyte. Studies have shown, for example, that in some species active RNA synthesis occurs in the follicular epithelium during vitello-genesis, especially the early stages, and that isotopically labeled amino acids are first taken up by follicle cells to appear later in protein spheres within the oocytes. In telotrophic and polytrophic ovarioles some protein may be transferred during early vitellogenesis to the oocyte from the nurse cells. However, the latter appear to be more important as suppliers of nucleic acids to the developing oocyte, and several autoradiographic and electron microscopic studies have shown the movement of labeled RNA or ribosomes down the trophic cord in telotrophic ovarioles or across adjacent nurse cells into the oocyte in polytrophic ovarioles. It is presumed that this RNA is then associated with protein synthesis within the oocyte. In addition to the RNA derived from nurse cells, RNA may also be produced by the oocyte nucleus for use in protein synthesis. In several species, active incorporation of labeled RNA precursors into the oocyte nucleus has been observed to occur early in vitellogenesis, concomitant with the accumulation of protein (non-membrane-bound) adjacent to the nuclear envelope.

Studies on the accumulation of lipid, carbohydrate, and other components of yolk are few. Though lipid may make up a considerable proportion of the yolk, its source remains doubtful in most species. An apparent association between the Golgi apparatus and the accumulation of lipid led early authors to suggest that the lipid was synthesized by the oocyte per se, though this has not been confirmed. Another suggestion that requires further study is that the follicle cells contribute lipid to the oocyte. In the polytrophic ovariole of Drosophila, the nurse cells supply lipids to the oocyte, though this apparently is not the case in Culex. In telotrophic ovarioles, lipid may be derived both from the nurse cells (early in vitellogenesis) and from the follicle cells.

Glycogen usually can be detected only in small amounts and, in meroistic ovarioles, after degeneration of the nurse cells. In Apis and Musca, labeled glucose injected into the hemolymph is rapidly accumulated by oocytes in late vitellogenesis and apparently converted to glycogen (Engelmann, 1970).

In the German cockroach, Blattella germanica, the terminal oocytes accumulate large amounts of hydrocarbons during vitellogenesis. The hydrocarbons apparently are synthesized by the epidermis of the abdominal sternites, then transported to the ovaries bound to a hemolymph lipophorin. Later, they become incorporated into the ootheca (Fan et al., 2002).

3.1.2. Vitelline Membrane and Chorion Formation

When vitellogenesis is completed, the vitelline membrane and, later, the chorion (eggshell) are formed. Though some early observations suggested that the vitelline membrane was produced by the oocyte itself, perhaps as a modification of the existing plasmalemma, recent studies covering a range of insect orders have confirmed that the vitelline membrane is secreted by the follicle cells. The nature of the membrane varies both among species (correlated with the egg's environment) and regionally over the egg surface. For example, the vitelline membrane of Drosophila melanogaster is perforated in the "collar"

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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