Identification of mating genes provides clues to evolution

June 29, 2001

Newly identified "mating genes" in the mustard plant (Arabidopsis thaliana) may provide a powerful tool for understanding of the interactions that foster self-recognition and the evolution of new species. These mating genes code for all the major protein components of the Arabidopsis pollen coat.

These genes, whose protein products mediate mating through species-specific recognition and initiation of pollination, were found to reside primarily in two chromosomal clusters by a team of researchers from the University of Chicago.

"We now have an ingredients list to work from. The identification of this complete set of pollen coat proteins allows us to start looking at one protein at a time and find each one's role in self-identification," said Jacob A. Mayfield, PhD, lead author of the paper. "Not much is known about the positive interactions that make mating possible. Most of what we know now is negative, what prevents pollination."

To understand the evolution of these mating clusters the researchers looked at comparable gene clusters in differing ecotypes of Arabidopsis. Ecotypes are populations of plants within a species that have adapted to different growing conditions. Like races, ecotypes are all the same species, but have distinctive subsets of characteristics.

"Genes that are involved in self-recognition systems, like plant or animal immunity, are often found in clusters," said Daphne Preuss , PhD, professor of genetics and cell biology at the University of Chicago and Howard Hughes Medical Investigator. " This arrangement is seen in the most rapidly changing proteins known. The clustering of these genes allows the plant or animal to generate diversity through gene duplication and rearrangement."

"The ecotype clusters showed great variation but all the genes in the cluster were functioning genes," said Aretha Fiebig, graduate student in biochemistry and molecular biology at the University of Chicago and co-author of the paper. "Genes with deleterious mutations that destroy function are common in rapidly evolving clusters. The fact that all the pollen coat genes were functional implies that all of these proteins work together and are essential to successful mating."

When the team looked at the comparable cluster in the closely related species, Brassica oleracea, they found a remarkably fast rate of divergence. The clusters were easily identifiable, as the corresponding genes shared the same structure and characteristics and the flanking genes shared 75 percent identity. But the similarity ended there.

"The mating genes were almost impossible to align," said Preuss. "These species are separated by only about 12 million years. They are as closely related as humans and chimps. The rapid divergence of these mating genes across strain and species boundaries is of significant evolutionary interest."

"With the power of Arabidopsis' genetics, including our ability to generate mutants, a set of well-studied strains and closely related species and now, the complete set of pollen coat proteins, we hope to significantly advance our understanding of the control of mate recognition," said Preuss.

Sarah E. Johnstone, an undergraduate student funded by the Howard Hughes Medical Institute, completed a portion of this study as part of her honors thesis in biology.

This work was supported by a National Science Foundation pre-doctoral fellowship, a Howard Hughes Medical Institute grant for undergraduate education and a National Science Foundation grant.

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