Reproduction

3.1.1. Vitellogenesis

As noted above, vitellogenesis occurs, by and large, only in the terminal oocyte within an ovariole, yet in many species the process is highly synchronized among ovarioles and between ovaries; that is, the eggs are produced in batches. Why vitellogenesis does not occur to any great extent in the more distal oocytes is unclear, though various suggestions have been made. One suggestion is that the terminal oocyte, as the first to mature, that is, to become capable of vitellogenesis, simply outruns the competition. In other words, once the oocyte begins vitellogenesis and growth, its increasing surface area enables it to capture virtually all of the available nutrients. This, however, cannot be the complete answer because in many female insects vitellogenesis in the penultimate oocyte appears to be inhibited even after the terminal oocyte has completed its yolk deposition and become chorionated, provided that the mature egg is not laid. Two explanations have been proposed. Adams and co-workers (see Adams, 1970,1981) showed, in Musca domestica at least, an ovary containing mature eggs produces an oostatic hormone that prevents release of the ovarian ecdysiotropic hormone necessary for vitellogenesis (Section 3.1.3). In contrast, in Rhodnius prolixus and Locusta migratoria an antigonadotropic hormone is produced by the abdominal perisympathetic organs and thoracic ganglia, respectively, when the ovariole contains a mature egg. The function of this hormone, it is proposed, is to block the action of juvenile hormone on the follicle cells (Section 3.1.3), again preventing vitellogenesis (Huebner and Davey, 1973; Davey et al., 1993). Remarkably, the metacestode stage of the rat tapeworm, Hymenolepis diminuta, which infects female mealworm beetles (Tenebrio molitor), also produces an antigonadotropin. This acts similarly to that of Rhodnius, thus allowing the parasite to make use of resources originally intended for egg production (Hurd, 1998).

As yolk appears, it is seen to be made up almost entirely of roundish granules or vacuoles known as yolk spheres. Within the yolk spheres, protein, lipid, or carbohydrate can be detected. The membrane-bound protein yolk spheres are most abundant, followed by lipid droplets that are not membrane-bound. Relatively few glycogen-containing yolk spheres are usually present. Small amounts of nucleic acids are normally detectable, but these are not within the yolk spheres. The source of some of these materials is different in the various types of ovarioles.

In all ovarioles, however, almost all yolk protein is extraovarian in origin. In most insects the proteins are accumulated from the hemolymph. The source of these proteins, as was noted in Chapter 16, Section 5.4, is the fat body, which, during vitellogenesis, synthesizes* and releases large quantities of a few specific proteins (vitellogenins or female-specific proteins) that are selectively accumulated by the terminal oocytes. The higher Diptera are a notable exception in that the follicular epithelium is also a major source of yolk proteins; indeed, in the stable fly, Stomoxys calcitrans, all yolk protein production occurs here (Kelly, 1994).

Shortly before vitellogenesis, a space appears between the follicle cells and the terminal oocytes, and intercellular spaces develop in the follicular epithelium (patency), so that the oocytes become bathed in hemolymph. The tunica propria appears to be freely permeable to all solutes within the hemolymph. Electron microscopic and other studies have shown

* In insects that have fully developed eggs at eclosion, the proteins are synthesized (and stored) by the fat body during larval development, to be released during the pupal stage when vitellogenesis occurs.

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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