The Evolution of Colonial Insects

       Figure 1 - Haplodiploid sex determination.            .

While esociality has evolved in different orders of animals, from mammals like the naked mole rat (Jarvis, 1981) to marine sponge-dwelling shrimp (Duffy, 1996), it has been most commonly observed in the order Hymenoptera (the sawflies, wasps, bees, and ants), where it has evolved at least eleven separate times (Wilson, 1975). This has been hypothsised to be due a result of the peculiar genetic arrangement present in the order, haplodiploidity.

Haplodiploidy is a sex-determination system where males develop from unfertilized eggs and are haploid (only a single set of chromosomes, similar to gametes like sperm or eggs), and females develop from fertilized eggs and are diploid (two sets of chromosomes). In this system males have no father, and females that have not mated will produce only male offspring (Figure 1). The evolutionary history of this peculiar adaptation is murky, but haplodiploidy seems to have evolved most commonly in insect species that nest in bark and are exposed to Wolbachia, a bacteria that infects the ovaries of insects (Kawasaki et al, 2016). 

Of particular evolutionary interest is how haplodiploidity effects the relatedness between family members. Due to this genetic configuration, a male’s sperm are all identical, and the offspring of his matings all bear the identical genetic contribution from their father. The relatedness among sisters is particularly noteworthy in lifetime monogamous haplodiploids, with sisters having an average relatedness of r = 0.75 (Hamilton, 1964). Dubbed 'supersister relatedness' in these haplodiploid species, sisters are more related to each other than to their mother, their daughters or to any other family member.

The haplodiploidity hypothesis (Hamilton, 1964) argues that haplodiploid inheritance generates relatedness asymmetries that promote the evolution of altruism by females, due to the high relatedness between sisters (via Hamilton's rule of kin selection). While the haplodiploid hypothesis has fallen from favour over the years, and although haplodiploidity is clearly not essential for the evolution of eusocialality, there is undoubtedly correlation with the likelihood of it occurring. Recent studies have shown that haplodiploid females should be more prone to altruism than diplodiploid females (Kennedy and Radford, 2020), strengthening the idea that the an altruistic social structure is a more likely evolutionary outcome when you have high relatedness like these supersisters.


References: 
Duffy, J., 1996, Eusociality in a coral-reef shrimp. Nature 381, 512–514 (1996). https://doi.org/10.1038/381512a0

Hamilton W.D., 1964, The genetical evolution of social behaviour. II. J Theor Biol. Jul;7(1) 17-52. doi:10.1016/0022-5193(64)90039-6. PMID: 5875340.

Jarvis, J., 1981, Eusociality in a Mammal: Cooperative Breeding in Naked Mole-Rat Colonies. Science, 212(4494), 571-573. Retrieved May 16, 2020, from www.jstor.org/stable/1686202

Kawasaki, Y., Schuler, H., Stauffer, C., Lakatos, F., Kajimura, H., 2016, Wolbachia endosymbionts in haplodiploid and diploid scolytine beetles (Coleoptera: Curculionidae: Scolytinae): Wolbachia infection in scolytine beetles. Environmental Microbiology Reports. 8. 680-688. 10.1111/1758-2229.12425. 

Kennedy, P, Radford, A.N., 2020, Sibling quality and the haplodiploidy hypothesis, Biol. Lett.1620190764, http://doi.org/10.1098/rsbl.2019.0764

Wilson, E. O., 1975, Sociobiology: The New Synthesis. Cambridge, Mass: Har­vard Univ. Press. 697 pp. £13-60. Millennium, 5(2), 208–212. https://doi.org/10.1177/03058298760050020204

Figure:
Haplodiploid sex determination, http://www.sas.rochester.edu/bio/labs/WerrenLab/WerrenLab-HaplodiploidSexDetermination.html, 14/05/2020