When mom’s and dad’s genes compete, whose alleles prevail to make their progeny tall or give her big bones -- and more important, why -- is a matter of intense interest among evolutionary biologists. A recent SFI meeting on genomic imprinting reexamined current theory.
Imprinting, the conditional (on which parent donated it) expression of a gene in an individual, accounts for much of the evolutionary change not adequately described in classical genetic theory. But scientists don’t yet have a solid framework for studying the evolutionary role, or long-term outcomes, of genomic imprinting.
The dominant explanation, at least since David Haig published it in 1991, has been “parental conflict hypothesis,” often referred to as kinship theory. Haig’s theory argues that imprinting is a consequence of opposing selection pressures on maternal and paternal genomes within an individual.
In other words, mom’s genes express in developing offspring for the good of her entire brood -- perhaps by conserving resources for the mother so she can reproduce again -- while dad’s express to make his individual offspring as fit as possible -- by hogging maternal resources, for example.
Although most scientists accept kinship theory as a working principle, with just two decades of empirical study behind it, it hardly takes its place among the stalwarts of evolutionary theory.
“Just how prevalent imprinting is among mammals is still controversial,” says SFI Omidyar Fellow Jeremy Van Cleve. “Even murkier is the question of how much new theory is required to account for known cases of imprinting.”
In February Van Cleve and SFI Professor Jon Wilkins hosted a three-day working group at SFI to take stock of recent experimental and theoretic advances in kinship theory, and to examine situations that seem to fall outside its struggle-for-maternal-resources manifestation. These special cases, Wilkins says, provide an opportunity to test the potentially broader explanatory power of the kinship theory.
In one such case, detailed in a recent study published in the American Naturalist, Van Cleve, SFI Science Board member Marcus Feldman (Stanford), and Laurent Lehmann (University of Neuchatel) modeled over many generations the evolution of traits in a notional population of males and females. They found that minor demographic variables -- such as small differences in male-to-female ratios and frequency of movement among individuals -- seem to play important roles in which genomes dominate, at least on evolutionary time scales.
“We suggest that the evolution of imprinting, within the context of kinship theory, can arise from demographic conditions in a population,” says Van Cleve.
At the meeting a small group of collaborators began to develop a framework for what outcomes kinship theory should predict, which they say will be useful for designing future tests of the theory. The meeting is expected to result in a published review paper.
More about the working group
Read the American Naturalist paper