This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.
Imagine that one day, you make a pact with your brother or sister, vowing to never have children of your own and instead spend your life raising theirs. You’ll agree to do the grocery shopping, cook for them, clean their rooms and bathe them, until you die.
That seems like a crazy plan, but it’s one that some of the most successful animals in the world – the social insects – have adopted. It’s called ‘eusociality‘ and it’s a puzzle for evolutionary biologists. Why should an animal forgo the chance to reproduce in order to help rear its siblings and their young?
The strategy makes sense if you share enough genes with your close relatives. In helping them, you indirectly ensure the transmission of your own genetic material. But even if this explains the existence of eusociality, it doesn’t explain how such an extreme form of co-operation evolved.
Now, Amy Toth and colleages at the University of Illinois have found a clue in the genes of the paper wasp, Polistes metricus, which suggests that their altruistic actions evolved from motherly behaviour.
Scientists have suggested this theory before as a possible origin for eusociality. It doesn’t take a great leap of imagination to picture how a group of wasp sisters living together and communally looking after their young could become a society in which only a few individuals reproduce and the others share the care. But until now, that theory had never been tested at a genetic level.
Truly eusocial insects like honeybees have physically distinct castes with strongly segregated jobs. The queen’s sole purpose is to lay eggs and she never takes on the menial foraging and brood care of the smaller workers.
Paper wasps are only halfway down the road to eusociality, which makes them an ideal choice for studying its evolution. They have different castes, but they all look much the same and their castes are far less strictly segregated. The roles that individuals perform depends on the age of the colony and fall into four different groups.
Foundresses, females that establish new colonies and care for young as well as laying eggs. After creating the first generation, these females become queens and focus solely on laying more eggs. Their daughters, the workers, take up the task of caring for their new siblings to the exclusion of their own reproduction. Later on in the colony’s life, the queen gives birth to gynes, that neither care for young or lay eggs – their job is to mate with males and become foundresses themselves in the following spring.
Toth decided to look at the patterns of gene activity in these four groups. She reasoned that if the workers’ altruistic actions had originated in maternal care, they would share similar genetic profile to the foundresses, the only other group that also cares for young.
Complex behaviours like caring for young and foraging were hardly going to be the province of a single gene. Toth needed a way to analyse a myriad of genes across the entire wasp genome – a genome that has not yet been fully sequenced.
To overcome this problem, the team took a streamlined approach. They specifically looked at genes that were strongly activated in the brains of 87 wasps from all four groups. Using a powerful sequencing technique from the 454 Life Sciences company, they identified almost 400,000 stretches of relevant DNA across their genomes.
Toth matched these hits to the genome of the closely related honeybee (Apis mellifera), which was fully sequenced last year. They focused on 32 genes, whose honeybee counterparts are involved in worker behaviour. Even though bees and paper wasps started down different evolutionary roads some 100 million years ago, the proteins encoded by these genes have remained very similar.
As predicted, Toth found that the activation pattern of these 32 genes was closest in workers and foundresses, and were distinct from those of queens and gynes, which don’t practice maternal care. Regardless of whether the wasps focused on their siblings or their young, their caring behaviour was governed by similar sets of genes, supporting the idea that eusociality evolved from maternal care.
Today, the vast majority of solitary wasps provide food for their helpless young, often in grisly or murderous ways. During the course of evolution, the twin behaviours of egg-laying and maternal care started to separate.
In the intermediary paper wasps, the behaviours are separated in time – the foundresses practice both at first and then focus on just one when they turn into queens. As this happens, their brain undergo dramatic changes and different sets of genes are switched on.
The final stage down this evolutionary path is the one seen in true eusocial wasps, where egg-laying and maternal care are separated in space, in the bodies of queens and workers.
The study also shows that many evolutionary problems can be addressed without the complete sequence of an animal’s genome. For every full genome we have, we can use next-generation sequencing technology to compare it to the partially sequenced genes of closely-related species, just as the bee and wasp proved here.
Reference: toth, Varala, Newman, Miguez, Hutchison, Willoughby, Simons, Egholm, Hunt, Hudson & Robinson. 2007. Wasp gene expression supports and evolutionary link between maternal behaviour and eusociality. Sceince