In social Hymenoptera, the determination of sex is intertwined with the determination of social role. For sociality to be effective, the balance of workers and reproductives (reproductively able individuals) must be modulated. Females develop into one of several castes, each of which perform specific tasks. Different taxa have different numbers of castes, and different degrees of caste specification. In some groups caste determination is both reversible and barely distinguishable on a morphological level, while in other taxa caste differentiation is irreversible and marked by extreme morphological differences. The mechanisms of caste determination may vary widely. The molecular details are becoming increasing clear, although much work remains to be done.
Apis mellifera, the honey bee, has the best studied system of caste differentiation. Differences in caste specific behavior have been understood for many years (Michener, 1974), but recent molecular studies have shed new light on the mechanisms by which it occurs. In honey bees, the primary determination is between worker bees and gyne (future queens). Gynes are given a special diet that activates queen specific development. Workers assume different roles in the nest as they age, a pattern known as temporal polyethism. Young workers stay in the nest, and as they age they replace foragers, and are replaced by younger workers within the nest. Foragers die by about a month of age, either from predation or old age. The timing of the progression through the tasks is not fixed. The progression can be delayed, or even reversed, if young workers die. Over the winter, the progression is also delayed, so that there are workers to staff the hive early in the spring.
An increase in titers of juvenile hormone (JH) throughout a worker's life causes temporal polyethism in Apis mellifera(Robinson, et. al. 1989). JH levels increase with age, but changes in the conditions of the colony can alter the pace at which JH levels increase. Robinson, et. al. (1989) demonstrated the plasticity of worker development by establishing in the laboratory colonies of same age individuals. Workers quickly assumed different tasks, many precociously. As new workers were artificially removed from the colony, workers remained at their initial tasks well beyond when they would normally be expected to assume new tasks. When bees were sacrificed to measure their JH levels, foragers were found to have more JH despite being the same age as other workers. The addition of JH analogue to worker bees caused them to assume tasks reserved for older bees prematurely, showing that JH can cause temporal polyethism.
Some ants also have age-correlated division of labor. In some species there is simply an early developmental decision between gynes and workers (Bourke and Franks, 1995). Gynes will grow wings, and either remain in the colony (to replace or coexist with their mother) or establish a new colony. Workers, all with a similar morphology, will begin working in the nest near their place of birth. As they age, they will move to other work in other parts of the colony, then begin foraging. Eventually they will become guards who patrol the extreme boundaries of a colony's territory. In ants with multiple worker castes, age related occupation is still common (Bourke and Franks, 1995). Different morphological types assume different tasks (usually soldiers versus workers), but within each morphological type, work is divided in a temporal fashion. The mechanisms by which this age related division of labor occurs are less well understood, but are probably similar to honeybee maturation.
In other Hymenopteran species, such as many halictid bees, caste differences between workers and gynes are fairly plastic (Michener, 1974; Yanega, 1990). Females become either workers or reproductives dependent on dominance hierarchies within the nest. The oldest or largest bee becomes reproductively active, while others act as workers. If the reproductive bee should perish, the largest worker will replace her (Yanega, 1990). Caste specific programs are not activated until adulthood, and are not activated permanently. Yet, the queen can influence caste by providing daughters with more or less food (Richards and Packer, 1994). With less food during larval growth, bees are more likely to be smaller and to become workers.
In honeybees and many ants there are also overlaps between the reproductive role of the queen and her workers. Worker honeybees will lay eggs (all male because they have not mated) if the queen dies (Michener, 1974). In some ant taxa, workers will lay a significant number of male offspring (Bourke and Franks, 1995). These interpenetration of caste roles demonstrate that even if species with fairly rigid caste differentiation, there is some plasticity in development and behavior.
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