Division Of Labor For Reproduction

Females dominate the functioning of insect societies, even in termite societies, in which males play more diverse roles than in hymenopteran societies. There are two types of females in an insect society, queens and workers. Queens specialize in reproduction and may lay up to several thousand worker eggs per day. Workers are either completely or partially sterile, engage in little if any personal reproduction, and perform all tasks related to colony growth and maintenance. Worker sterility occurs because the ovaries do not develop or because critical steps in oogenesis do not occur. Worker sterility occurs either during preadult stages or during adulthood.

In many species of social insects, queens and workers are distinguished by striking morphological differences. A queen can have huge ovaries and a sperm storage organ that maintains viable sperm for years. The most striking morphological differences between queens and workers occur as a result of caste determination, which occurs during preadult stages. Caste determination has an endocrine basis. Research on the honey bee, Apis mellifera, and the bumble bee, Bombus terrestris, has shown that a high hemolymph titer of juvenile hormone (JH) during a critical period of larval development induces queen development. JH and presumably other hormones trigger a variety of processes that ultimately result in the production of either a worker or a queen. For example, caste-specific apoptosis (cell death) occurs in the ovaries of worker-destined honey bees and is associated with low titers of JH and ecdysteroid. Molecular analyses of endocrine-mediated caste determination have just begun. Some of the first findings involve caste-specific differences in the expression of genes that are associated with metabolism and protein synthesis, reflecting the fact that developing queens are metabolically more active than developing workers.

Little is known about how extrinsic factors act on endocrine-mediated developmental processes to influence caste determination. There is a strong circumstantial link between diet and JH in honey bee larvae, but how nutritional information acts to elevate JH levels is still largely unknown. In other species, extrinsic factors that influence caste determination include temperature and social factors such as behavioral interactions and pheromones released by adult colony members. These might affect the larvae directly or might influence the treatment accorded them by a colony's workers.

In societies in which queens and workers have strong morphological differences, the major mechanisms for queen domination of reproduction appear to be primer pheromones produced by queens. However, only one queen primer pheromone has been well characterized, that being the mandibular pheromone of the queen honey bee. Workers exposed to queen pheromones show little or no ovary development or egg-laying behavior. In other species of social insects, the physical differences between queens and workers can be very slight. Division of labor for reproduction in these

"primitively eusocial" species is achieved by a dominance hierarchy that is established and maintained by direct behavioral mechanisms, including pushing, biting, and physical prevention of egg laying. Behavioral domination is an ongoing process because some workers are physiologically capable of producing offspring and do, under some circumstances.

Queen behavior and pheromones affect adult worker neuroendocrine systems to reduce reproductive potential. JH has been implicated in the regulation of division of labor for reproduction in some, but not all, species studied to date, especially B. terrestris; the paper wasp, Polistesgallicus; and the fire ant, Solenopsis invicta. This is consistent with the function of JH as a hormone promoting reproductive development. JH does not appear to play this traditional role in adult A. mellifera. Ecdysteroids and biogenic amines also are suspected of being involved in the regulation of division of labor for reproduction among adult queens and workers, but a clear picture has not yet emerged.

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