Membranous wing order haploid diploid sex determination

Haplodiploid sex determination is the reproductive system described for ants, bees, and wasps: unfertilized eggs develop as males with one chromosome set, while fertilized eggs develop as females with two. Its evolutionary significance in The Selfish Gene lies in the kinship pattern it creates. Because full sisters share, on average, three-quarters of their genes, a female may propagate more of her genes by helping her mother produce sisters than by reproducing herself. The system supplies a striking explanation for the evolution of worker sterility, while also setting up conflict over the proportion of female and male reproductive offspring.

The reproductive mechanism

A typical Hymenopteran nest has one mature queen. After a mating flight when young, she stores sperm and uses it to fertilize eggs over a long reproductive life, which may last ten years or more. Eggs that are not fertilized develop into males; fertilized eggs develop into females. Thus a male has no father and carries a single chromosome set, all inherited from his mother, rather than the two sets found in a female and in the usual account of human inheritance.

Female development branches into different castes. A female has a father and the usual double chromosome set. Whether she becomes a worker or a queen depends not on possession of a different set of genes, but on how she is reared, especially on the food she receives. The account describes females as carrying genetic potential for queen-making and for the specialized worker castes, with rearing conditions determining which developmental program is expressed. Termites are explicitly excluded: they are not Hymenoptera and do not share this sex-determination system.

How the system changes relatedness

The unusual chromosome pattern complicates simple measures of relatedness. A male’s sperm all carry the same complete set of genes, because he has only one set to transmit. This differs from the account of ordinary sexual reproduction, in which a male’s sperm are not all genetically identical. The genetic relation between a queen and her offspring can therefore look paradoxical if expressed without specifying whose genes are being followed.

A male inherits all his genes from his mother, so every gene present in him came from her. Yet a queen passes only half of her genes to any one son, just as she passes half to a daughter. From the queen’s perspective, therefore, offspring of either sex have the same relatedness to her as human children have to their mother. The apparent contradiction disappears once the comparison is made from the perspective of a particular gene rather than by treating the male’s single chromosome set as equivalent to a full double set.

The pattern is different between full sisters. They share the same father, and because that father’s sperm are genetically identical, each sister receives the same paternal genes. For a gene in one sister, if it came from her mother, there is a one-half chance that her sister inherited it too; if it came from their father, the chance is complete. Averaged across those possibilities, full sisters are related by three-quarters, rather than the one-half given for full siblings in the usual comparison among sexually reproducing animals.

Worker sterility and reproductive interests

This elevated sister-to-sister relatedness gives the system its evolutionary force in the book’s argument. A Hymenopteran female is more closely related to her full sisters than to her own offspring of either sex. If a worker’s genes are better represented in sisters produced by her mother than in children she could produce herself, selection can favor helping the queen produce reproductive sisters. In this sense, the workers can be understood as directing their mother’s reproductive activity toward producing copies of the workers’ genes. The proposed consequence is worker sterility: a female may increase the propagation of her genes not by breeding directly but by contributing to the production of close relatives.

The account connects this explanation to the repeated evolution of social organization. True sociality involving worker sterility is said to have evolved at least eleven times independently within the Hymenoptera, compared with a single occurrence elsewhere in the animal kingdom, in termites. This comparison makes the sex-determination system more than a curiosity: it offers a framework for understanding why sterile workers could evolve repeatedly in one major group, while the text treats the origin of the underlying reproductive peculiarity itself as unknown.

Conflict over sex ratio

The workers’ potential advantage has a limit. A queen producing equal numbers of male and female reproductive offspring would not necessarily maximize the workers’ representation in future generations. From a worker’s perspective, a brother has only a one-quarter chance of carrying any particular one of her genes, whereas a sister has the higher relatedness created by the shared, identical paternal contribution. Workers therefore have an evolutionary interest in favoring reproductive females over reproductive males.

Trivers and Hare applied calculations associated with Fisher’s sex-ratio analysis to this special case. The resulting contrast is between a stable investment ratio of one to one from the mother’s perspective and a ratio of three to one in favor of sisters from a sister’s perspective. A female worker’s most efficient route, in this model, is to refrain from reproducing and induce her mother to produce reproductive siblings in that female-biased proportion. If the female instead has offspring of her own, the model gives her a one-to-one investment ratio between sons and daughters.

The system thus links a concrete mechanism of reproduction to a broader argument about competing genetic interests. The queen’s relation to sons and daughters is symmetrical when considered gene by gene, but a worker’s relation to sisters and brothers is not. That asymmetry helps explain both cooperation among workers and conflict over the sex ratio: the same arrangement that can make sterile assistance advantageous also gives workers reason to resist a queen’s equal allocation to male and female reproductives.