Physiological Control Of Reproduction

The initiation and termination of some reproductive events often depend on environmental factors, such as temperature, humidity, photoperiod, or availability of food or a suitable egg-laying site. Additionally, these external influences may be modified by internal factors such as nutritional condition and the state of maturation of the oocytes. Copulation also may trigger oocyte development, oviposition, and inhibition of sexual receptivity in the female via enzymes or peptides transferred to the female reproductive tract in male accessory gland secretions (Box 5.4). Fertilization following mating normally triggers embryogenesis via egg activation (Chapter 6). Regulation of reproduction is complex and involves sensory receptors, neuronal transmission, and integration of messages in the brain, as well as chemical messengers (hormones) transported in the hemolymph or via the nerve axons to target tissues or to other endocrine glands. Certain parts of the nervous system, particularly neuro-secretory cells in the brain, produce neurohormones or neuropeptides (proteinaceous messengers) and also control the synthesis of two groups of insect hormones: the ecdysteroids and the juvenile hormones (JHs). More detailed discussions of the regulation and functions of all of these hormones are provided in Chapters 3 and 6. Neuropeptides, steroid hormones, and JH all play essential roles in the regulation of reproduction, as summarized in Fig. 5.13.

Fig. 5.13 A schematic diagram of the hormonal regulation of reproductive events in insects. The transition from ecdysterone production by the pre-adult prothoracic gland to the adult ovary varies between taxa. (After Raabe 1986.)

JHs and/or ecdysteroids are essential to reproduction, with JH mostly triggering the functioning of organs such as the ovary, accessory glands, and fat body, whereas ecdysteroids influence morphogenesis as well as gonad functions. Neuropeptides play various roles at different stages of reproduction, as they regulate endocrine function (via the corpora allata and prothoracic glands) and also directly influence reproductive events, especially ovulation and oviposition or larviposition.

The role of neuropeptides in control of reproduction (Table 3.1) is an expanding area of research, made possible by new technologies, especially in biochemistry and molecular biology. To date, most studies have concentrated on the Diptera (especially Drosophila, mosquitoes, and houseflies), the Lepidoptera (especially the tobacco hornworm, Manduca sexta), locusts, and cockroaches.

5.11.1 Vitellogenesis and its regulation

In the ovary, both nurse cells (or trophocytes) and ovarian follicle cells are associated with the oocytes (section 3.8.1). These cells pass nutrients to the growing oocytes. The relatively slow period of oocyte growth is followed by a period of rapid yolk deposition, or vitellogenesis, which mostly occurs in the terminal oocyte of each ovariole and leads to the production of fully developed eggs. Vitellogenesis involves the production (mostly by the fat body) of specific female lipoglycoproteins called vitellogenins, followed by their passage into the oocyte. Once inside the oocyte these proteins are called vitellins and their chemical structure may differ slightly from that of vitellogenins. Lipid bodies - mostly triglycerides from the follicle cells, nurse cells, or fat body - also are deposited in the growing oocyte.

Vitellogenesis has been a favored area of insect hormone research because it is amenable to experimental manipulation with artificially supplied hormones, and analysis is facilitated by the large amounts of vitel-logenins produced during egg growth. The regulation of vitellogenesis varies among insect taxa, with JH from the corpora allata, ecdysteroids from the prothoracic glands or the ovary, and brain neurohormones (neuro-peptides such as ovarian ecdysteroidogenic hormone, OEH) considered to induce or stimulate vitellogenin synthesis to varying degrees in different insect species (Fig. 5.13).

Inhibition of egg development in ovarian follicles in the previtellogenic stage is mediated by antigonado-tropins. This inhibition ensures that only some of the oocytes undergo vitellogenesis in each ovarian cycle. The antigonadotropins responsible for this suppression are peptides termed oostatic hormones. In most of the insects studied, oostatic hormones are produced by the ovary or neurosecretory tissue associated with the ovary and, depending on species, may work in one of three ways:

1 inhibit the release or synthesis of OEH (also called egg-development neurohormone, EDNH); or

2 affect ovarian development by inhibiting proteolytic enzyme synthesis and blood digestion in the midgut, as in mosquitoes; or

3 inhibit the action of JH on vitellogenic follicle cells thus preventing the ovary from accumulating vitellogenin from the hemolymph, as in the bloodsucking bug Rhodnius prolixus.

Originally, it was firmly believed that JH controlled vitellogenesis in most insects. Then, in certain insects, the importance of ecdysteroids was discovered. Now we are becoming increasingly aware of the part played by neuropeptides, a group of proteins for which reproductive regulation is but one of an array of functions in the insect body (see Table 3.1).

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.

Get My Free Ebook


Post a comment