Friday 14 September 2018

Evolution of Social Insects

Ooceraea biroi clonal raider ants with larvae.
Photo: Romain Libbrecht
A study of ants provides information on the evolution of social insects.

Far from the forlorn claims of creationist that the Theory of Evolution is a theory in crisis, as this recent paper shows, other than citing magic (which by making anything possible, makes any claim meaningless and useless), evolution is the only way to make sense of the natural world.

This example comes from the insect world and in particular the world of eusocial species of the insect sub-order, hymenoptera. It explains how some species of the bees, wasps and ants for colonies in which one female, or sometimes a small number of females, are the reproducing members of the colony while the vast bulk of the colony are sterile worker females.

Why this system arose is easy to explain in evolutionary terms - it gives more descendants. What has been harder to explain is just how this system arose. As the press release from Johannes Gutenberg University, Mainz, Germany explained:

One of the great puzzles of evolutional biology is what induced certain living creatures to abandon solitary existence in favor of living in collaborative societies, as seen in the case of ants and other social, colony-forming insects. A major characteristic of so-called eusocial species is the division of labor between queens that lay eggs and workers that take care of the brood and perform other tasks. But what is it that determines that a queen should lay eggs and that workers shouldn't reproduce? And how did this distinction come about during the course of evolution?

Now evolutionary biologist Dr. Romain Libbrecht of Johannes Gutenberg University together with researchers at Rockefeller University in New York City, NY, USA have discovered that it is all down to one single gene called insulin-like peptide 2 (ILP2). This gene, which is probably activated by better nutrition, stimulates the ovaries and triggers reproduction.

They reached this conclusion by examining 5,581 genes in seven ant species in four different subfamilies. They found they all had one thing in common - all the reproducing females had a higher level of ILP2 in their brains than non-reproducing females. Further study showed that ILP2 is found only in the brain and is produced by a small cluster of 12-15 cells.

The hypothesis is that an ancestral wasp female would lay a batch of eggs and then stop laying while she cared for the eggs and grubs until they were adults, then she would revert to her egg-laying phase, with changes in ILP2 levels controlling the phases. In a social group of wasps there could be females in different phases and over time this division of labour could have progressed to the point where only a few had ILP2 and could lay eggs; the others would be a permanent worker caste.

Queens and workers of eusocial Hymenoptera are considered homologous to the reproductive and brood care phases of an ancestral subsocial life cycle. However, the molecular mechanisms underlying the evolution of reproductive division of labor remain obscure. Using a brain transcriptomics screen, we identified a single gene, insulin-like peptide 2 (ilp2), which is always up-regulated in ant reproductives, likely because they are better nourished than their nonreproductive nestmates. In clonal raider ants (Ooceraea biroi), larval signals inhibit adult reproduction by suppressing ilp2, thus producing a colony reproductive cycle reminiscent of ancestral subsociality. However, increasing ILP2 peptide levels overrides larval suppression, thereby breaking the colony cycle and inducing a stable division of labor. These findings suggest a simple model for the origin of ant eusociality via nutritionally determined reproductive asymmetries potentially amplified by larval signals.

Chandra, V., Fetter-Pruneda, I., Oxley, P. R., Ritger, A. L., McKenzie, S. K., Libbrecht, R., & Kronauer, D. J. C. (2018).
Social regulation of insulin signaling and the evolution of eusociality in ants.
, 361(6400), 398–402.

Copyright: © 2018 The Authors
Reprinted with kind permission under licence #4427611383230

To test this further the team studied a species of small ant, Ooceraea biroi, which is unusual in that it represents what would be an intermediate stage in this evolution of full eusocial status. Instead of queens, the females are all fertile but alternate between breeding and work phases. Each female breed parthenogenetically and in a strict cycle consisting of 18 days laying eggs, followed by 16 days gathering food and tending the larvae. The presence of larvae suppresses ovarian activity and as the larvae pupate this suppression is removed.

The team showed that this was moderated by the presence of ILP2. By giving worker-phase ants a synthetic form of ILP2, egg-laying could be restored in the presence of larvae. They also showed that ants in their egg-laying phase could be stopped from laying eggs and converted to brood-care phase by simply introducing larvae to the colony. Conversely they could be reverted to egg-laying by removing larvae. In each case the behavioural and physiological change was associated with a change in ILP2 activity in the brain.

Additionally, colonies have a small number of 'intercaste' individuals that are larger and have eyes. They are more reproductive. They can be regarded as an intermediate between the usual queens and the normal workers. The only thing that differs in these intercaste individuals is that they receive higher quality food. They also have higher levels of ILP2 in their brains.

The suggestion is then that the asymmetry between workers and queens in eusocial hymenopterans could have arisen from a small, chance asymmetry in nutrition.

Here is a perfectly clear explanation of the evolution of eusocial hymenopterans from a non-social ancestral wasp and all due to the activity of a single gene. And what's more, there does not need to be any change in the genetic information, merely a change in how and when a gene is expressed, moderated by chemical and/or visual stimuli.

Yep! Evolution is a theory in crisis! Except it isn't. It's creationism - a notion that hasn't even made it to a hypothesis yet - that is in crisis. The facts keep falsifying it.

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