Evolution Of Eusociality 2 Transition

It is reasonable to assume that a termite ancestor packaged its eggs in oothecae, since the basal termite Mas-totermes does so (Nalepa and Lenz, 2000). If the timing of oviposition in this ancestor was similar to that of Crypto-cercus—a reproductive burst, with several oothecae laid within a relatively short time frame—nymphs in the family also exhibited age differentials. It is likely that repro duction was suspended as adults fed and otherwise cared for their dependent neonates, as reproductive stasis occurs in extant young termite families when adults are nurturing their first set of offspring (reviewed by Nalepa, 1994). This suggests that, as in Cryptocercus, parental care during colony initiation in the termite ancestor was costly.

The crucial step, and one that occurs during the ontogeny of extant termite colonies, is that older nymphs assume responsibility for feeding and maintaining younger siblings, relieving their parents of the cost of brood care and allowing them to invest in additional offspring (Fig. 9.8B). All defining components of eusociality (Michener, 1969; Wilson, 1971) follow. First, relieved of her provisioning duties, the female can redirect her reserves into oogenesis, and the result is a second cohort that overlaps with offspring produced during the first reproductive burst. Second, the assumption of responsibility for younger siblings by the oldest offspring in the family constitutes brood care. Third, by trophallactically feeding younger siblings, fourth instars are depleting reserves that could have been channeled into their own development, thus delaying their own maturation (Nalepa, 1988b, 1994). A single behavioral change, the switch from parental to alloparental care, thus represents the pathway for making a seamless transition between adaptive points, accounting with great parsimony for the defining components of the early stages of termite eusociality (Nalepa 1988b, 1994). A key life history characteristic in a Crypto-cercus-like termite ancestor would be the extraordinarily extended developmental period the first workers face, even prior to assuming brood care duties. Tacking an addition developmental delay onto the half dozen or so years these nymphs already require to reach reproductive maturity may be a pittance when balanced against the additional eggs their already reproductively competent mother may be able to produce as a result of their alloparental behavior. A preliminary mathematical model indicates that when a key resource like nitrogen is scarce, the costs of delayed reproduction in these first workers are outweighed by the benefits accrued by their labor in the colony (Higashi et al., 2000).4 A cockroach-like developmental plasticity supplied the physiological underpinnings for the social shift, as high-demand metabolic processes such as reproduction and development are tightly modulated in response to nutritional status in Blattaria. It is of particular interest, then, that in extant termites (Reticulitermes) two hexamerin genes may signal nutritional status and participate in the regulation of caste polyphenism (Zhou et al., 2006).

4. Masahiko Higashi was tragically killed in a boating accident in March 2000 (Bignell, 2000b) and never completed the study.

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