While many people are familiar with X and Y chromosomes, this isn’t the only system of sex determination in animals. In some species – including Hymenoptera, which include bees, ants, and wasps – males develop from unfertilised eggs (giving them one set of chromosomes, known as haploid) while females develop from fertilised eggs (giving them two sets of chromosomes, known as diploid). This is referred to as haplodiploidy.
Scientists have long debated whether this type of sex determination has promoted the evolution of altruism and eusociality: the highest level of social organisation, which includes cooperative care of offspring and division of labour. The reason for this hypothesis is that haplodiploidy makes females more closely related to their sisters than their own offspring. This means haplodiploidy could give an evolutionary advantage, in terms of getting genes into future generations, for females helping their sisters produce more offspring rather than reproducing themselves.
Now, over half a century after it was initially proposed, researchers have finally been able to carry out the first formal test of the haplodiploidy hypothesis, by examining data from 5159 insect species. In a surprising result, the study does not support the theory that haplodiploidy has promoted eusociality. This adds to the growing evidence that the evolution of eusociality has been promoted not by haplodiploidy but by ecological and life history factors – the pattern and timing of events that make up an organism's life – such as monogamy and extended parental care. Dr Rosa Bonifacii says:
“Contrary to popular assumption for the last 50 years, the evolution of eusociality has not occurred at a higher rate in haplodiploid species. Eusociality is more common in the Hymenoptera, but there are a lot of potential reasons for this – monogamy keeps relatedness high, parental care is common, and they have a 'sting' which could favour group defence.”
The study used phylogenetic analyses, which are comparisons of the evolutionary history of species. These were specifically designed to avoid a problem that previous studies had come up against, called ‘pseudoreplication within lineages’, which can lead to false positives. Professor Stuart West says:
“The pattern by which eusociality has evolved within the Hymenoptera has previously been presented as a case study of the pseudoreplication problem. But despite the debate, the validity of the haplodiploidy hypothesis has often been accepted in textbooks or other overviews. We’ve now shown that haplodiploidy doesn't explain eusociality, so the next step is to work out the factors that do.”
To read more about this research, published in PNAS, visit: https://www.pnas.org/doi/10.1073/pnas.2517458123
