Baby Beetles Evolve in 13 Generations

Sexton beetles, Nicrophorus vespilloides and larva

Credit: tomhouslay.com

Neglected baby beetles evolve greater self-reliance | Cambridge University Research News

How do you make your offspring grow big and strong?

If you are a sexton beetle, you feed them on dead mice, neglect them and let them fend for themselves. Well, not exactly their immediate offspring but the offspring of your 11 time great grandchildren who will evolve to be better at fending for themselves than those who are cared for, as this observed example of evolution in a laboratory shows.

Sexton beetles, Nicrophorus vespilloides, provide a useful service in the British countryside by burying the dead bodies of mice and other small vertebrates to create food for their offspring. They represent an intermediate level of parental care, between providing for all their needs and providing no care at all. Normally, the females remove the hair from the corpse and make cuts into the skin so their larvae can get into the body more easily. They will also bite off pieces of flesh and feed it to their larvae.

Scanning EM of the head of a larva.

Credit: Claudia Grossman

However, the larvae are also capable of fending for themselves, so can cope if the parent is killed or neglectful.

Researchers at Cambridge University's Zoology Department, led by Benjamin Jarrett, realised that this behaviour presented an opportunity to study how parental care drives evolution so they set up two populations of sexton beetles, one in which parents were removed as soon as they had laid their eggs, and a control group in which parents could fend for their young in the normal way. The experiment ran for 13 generations.

The group's results are published open access today in Nature Communications.

Abstract
Although cooperative social interactions within species are considered an important driver of evolutionary change, few studies have experimentally demonstrated that they cause adaptive evolution. Here we address this problem by studying the burying beetle Nicrophorus vespilloides. In this species, parents and larvae work together to obtain nourishment for larvae from the carrion breeding resource: parents feed larvae and larvae also self-feed. We established experimentally evolving populations in which we varied the assistance that parents provided for their offspring and investigated how offspring evolved in response. We show that in populations where parents predictably supplied more care, larval mandibles evolved to be smaller in relation to larval mass, and larvae were correspondingly less self-sufficient. Previous work has shown that antagonistic social interactions can generate escalating evolutionary arms races. Our study shows that cooperative interactions can yield the opposite evolutionary outcome: when one party invests more, the other evolves to invest less.

Benjamin J. M. Jarrett, Emma Evans, Hannah B. Haynes, Miranda R. Leaf, Darren Rebar, Ana Duarte, Matthew Schrader & Rebecca M. Kilner
Variation in the supply of parental care drives rapid adaptive evolution of offspring morphology.
Nature Communications. doi:10.1038/s41467-018-06513-6

Copyright: © 2018 The Authors
Published open access
Reprinted under the terms of Creative Commons Attribution 4.0 International License (CC-BY 4.0)

This elegantly simple experiment showed that, after just 13 generations, the offspring of the 'neglected' population had evolved larger mandibles in proportion to their body mass and were more self-reliant than those which had been cared for. In a kind of reverse arms race, when one partner in a cooperative social partnership (in this case, the parent) contributes more, the other partner (the larva) will evolve to contribute less. Parental care, or it's absence, can act as a driver of evolutionary change.

The populations of sexton beetles are continuing to evolve and are now in their 35^th generation, enabling the team to examine different levels of parental care and how this influences the evolution of the beetles.

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