Apr. 15, 2002 HOUSTON—APRIL 11, 2002 — Two Rice University biologists believe social insects like ants and bees could provide clues to why some animals -- including humans -- have developed a curious quality in which the genes of their parents vie in direct competition, waging a kind of biochemical war.
Social insects are best known as exceptions to the general rule that relations between individuals are competitive. They have evolved social organizations that are so harmonious that the colony itself is often considered a single individual, or "superorganism", for purposes of biological study. In an article in the April 12 issue of Science, David C. Queller and Joan E. Strassmann propose that social insects may also provide extraordinary exceptions to another general rule: that relations within individuals are completely harmonious.
The suggestion grows out of theories that aim to explain possible genetic conflicts between the genes that individuals receive from their mothers and fathers. For mom's and dad's genes to battle it out requires both means and motive. Until recently, the means were thought to be absent because genes could not tell which parent they came from. However, this "veil of ignorance" is sometimes lifted through a process called "imprinting" in which mothers and fathers label some of their genes before passing them on to their young.
The "motive" comes, as always in evolution, from how genes best reproduce themselves. In the classic mammalian case of imprinting, paternally derived genes strive to create a large placenta that favors the developing child -- who carries the father’s genes -- over the health of the mother -- who doesn’t share the paternal genes. Maternally derived genes favor a smaller placenta that would protect the mother’s health and her ability to produce future offspring who will carry on her genetic line.
In most other respects, however, maternal and paternal genes should be in agreement. Queller and Strassmann, both professors of ecology and evolutionary biology, believe that certain social insects -- ants, bees, and wasps -- are an exception because they have a peculiar genetic system called haploidiploidy, in which all the males result from unfertilized eggs. This results in unusual genetic patterns where different groups of siblings and cousins share more genetic links on their father’s side than their mother’s, or vice versa. For example, haploidiploid sisters always share paternal genes, but they have only a 50 percent chance of sharing maternal genes.
This pattern of asymmetrical relatedness provides the motive for conflict between maternal and paternal genes. The many kinds of social interactions in social insects provide the opportunity. Evolutionary theory predicts individuals with a deeply divided self: maternal and paternal genes should battle over sex ratios, kin recognition, who should be the queen, and who should lay eggs.
"We don’t yet know if bees, ants or wasps have imprinted genes, let alone what the imprinted genes do, but this presents biologists with an unusual opportunity" said Queller. "Evolutionary theories are sometimes accused of simply explaining what we already know rather than predicting the unknown. Here is a rare chance where evolutionary theory makes a complex set of predictions, completely unpolluted by what we already know."
Finding imprinting in social insects might also provide clues to a fuller understanding of how imprinting affects higher-order species such as mammals. In humans, imprinting is known to underlie several genetic diseases, and is also thought to be a possible barrier to cloning.
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