Helpful relatives key to the evolution of social insects (5/31/2008)

Researchers have shed light on a paradox of the evolutionary process that has existed since Darwin's time, why individuals will rear their siblings rather then reproduce themselves?

Researchers have shed light on a paradox of the evolutionary process that has existed since Darwin's time, why individuals will rear their siblings rather then reproduce themselves?

Eusocial behaviour - representing one of the pinnacles of sociality - is characterised by individuals who will altruistically rear siblings and forsake giving birth to their own offspring. The existence of such altruistic behaviour was described by Charles Darwin as such a fundamental paradox that it potentially threatened his whole theory of natural selection. How, Darwin pondered, can a trait that causes sterility spread in a population?

Now, researchers have provided the first evidence that monogamy in ancestral eusocial insects - which produced highly related individuals - was key to the evolution of eusociality in insects.

Dr Bill Hughes from the School of Biological Sciences University Sydney and now at the University of Leeds, together with colleagues from the University of Sydney and the University of Sussex published their findings in the 30 May issue of Science.

In 1964 biologist Bill Hamilton, suggested that an individual can be more successful at passing on its genes by helping relatives to rear their young rather than reproducing personally - a process commonly called kin selection.

However, this theory has recently been challenged by E.O. Wilson - founder of socio-biology - who argued that relatedness is unimportant and that highly social behaviour evolves simply because individuals do better when they cooperate than when they live a solitary life.

Until now there has not been data available to allow us to distinguish which of the Hamilton or Wilson hypotheses is correct.

The study provides an explicit test of the two theories by examining female mating frequency across eusocial Hymenoptera (bees, wasps and ants).

Team member Professor Ben Oldroyd from the University of Sydney said "Sociality has evolved repeatedly in the Hymenoptera and the Thysanoptera (thrips). These orders of insects have an unusual mechanism of sex determination in which males are haploid (one set of chromosomes) and females are diploid (two sets of chromosomes like humans). This haplo-diploidy generates very high relatedness among sisters."

Monogamy (mating with one male) means that female offspring of a haplodiploidy insect are highly related, so if the ancestral social insects were monogamous then that would suggest Hamilton's kin selection theory was correct, whereas if the ancestral social insects mated with multiple males, as do most other animals, then that would indicate Wilson is right.

The researchers combined the mating frequencies of 267 species of living social hymenoptera with an advanced statistical technique that reconstructs ancestral states, to infer just how many males the ancestral social insect females mated with.

In every group that they looked at, the researchers found that the ancestral females were always monogamous. This is a surprising result, given the extreme polyandry (females mating with many males) of many living social insects.

"Multiple mating by females is common, in fact usual, across the animal world. Thus it is quite startling that our analysis shows that monogamy was the ancestral condition for all of the modern social insects," said Prof Oldroyd.

Ancestral monogamy demonstrates conclusively that high relatedness was key to the evolution of social insects - just as Darwin himself suggested at the dawn of evolutionary theory.

Note: This story has been adapted from a news release issued by the University of Sydney

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