Eusociality in the Insect Order Hymenoptera
Eusociality is commonly found in the insect order Hymenoptera, which includes such social insects as ants, bees, and wasps. The social system of eusocial insects are described as having three main traits: cooperative care of young by individuals that may or may not be directly related, a division of labor involving sterile castes, and an overlapping of generations. It seems rather odd, in light of natural selection that so many individual organisms within a group would forfeit their ability to pass on their own genes for the betterment of the colony. Why should such a caste system develop in which only a select few breed, while the overwhelming majority undergoes a dramatic reduction of reproductive fitness? It has been proposed that there are certain special qualities of these insects (as well as snapping shrimp and mole-rats) that predisposes them to Eusociality.
In 1972 William Hamilton proposed that the genetic system of Hymenoptera is one such quality favoring the adoption of Eusociality. In this genetic system, males are haploid (developed from unfertilized eggs) and females are diploid (developed from fertilized eggs). As a result of this system of sex-determination, called haplodiploidy, female insects within a colony are more closely related to their sisters than their own offspring (r = 3/4 vs. r = ½). For this reason, most females tend to act as workers, thereby improving the fitness of their reproductive sisters who are most closely related to them. Females are thus able to increase their alleles in the population most effectively by promoting and facilitating the production of more sisters, rather than less related offspring.
The haplodiploidy hypothesis creates a conflict of interest in the determination of offspring sex ratio within a colony. Workers prefer to invest time and energy in facilitating the production of sisters rather than their less related brothers (r = ¾ vs. r = ¼). This favors a 3:1 ratio of female to male sex ratio among reproductive caste offspring. In contrast, queens are equally related to their daughters and sons, thus tending to favor a more even, 1:1 sex ratio. In 1996, Liselotte Sundstroem conducted a study involving wood ants (Formica exsecta) in order to discern which ratio actually occurs. It was found that there exists a sort of compromise in which the queen lays an equal 1:1 ratio of females and males, yet the male eggs are somehow identified by the workers and are largely eliminated prior to hatching. This systematic elimination of males produces a female to male ratio much closer to 3:1. These findings strongly suggest that worker ants tend to have a greater influence on the sex ratio, yet there is clearly a struggle between the two castes.
It is unlikely that the haplodiploidy hypothesis fully explains the altruistic nature of these eusocial insects. The 3:1 sex ratio in reproductive offspring suggests that haplodiploidy does have a significant impact on the structure and behavior of different castes, at least in some colonies. The hypothesis tends to fall apart, however, in those colonies of insects that have workers that are not more closely related to their sisters than their offspring. In fact, in many colonies, the coefficient of relatedness between workers is zero due to two or more queens having founded a single nest.
Through the study of the phylogenetic relationships between hymenopterans, it has been determined that Eusociality evolved in three families (wasps, bees, ants) independently rather than all at once. It was noticed that each of these families exhibited two important traits: they build elaborate nests, and they care for their larval young for relatively long periods of time. It has been proposed that these two ecological traits, rather than the genetic hypothesis of haplodiploidy and relatedness, are the underlying reasons why these families of hymenoptera are eusocial. The construction of massive, complex nests, as well as the gathering of food for the continuously produced larva, requires a great working force. Alone, the breeders would simply not be able to provide for the sheer number of larva they produce, for the amount of time required to care for them. This fact seems to best explain the altruistic nature of these insects, yet is a complex problem, undoubtedly involving a combination of genetic and ecological factors.
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