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How does inbreeding avoidance evolve in plants?
Case study of Leavenworthia suggests that loss of complex traits may be reversed.
Published: 10 June 2013
Many flowering plants are able to recognize and reject their own pollen, thereby preventing inbreeding despite the plants鈥� hermaphroditic nature. This mechanism is a complex trait that involves the interaction of a gene that tags the pollen with an identifier molecule, and a gene that produces a molecule capable of detecting pollen produced by the same plant.
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Evolutionary biologists have often argued that once complex traits are lost, they are seldom regained. But a new study, led by biologists at McGill University and published in the journal PLOS Biology, suggests that this may not be the case for self-pollen recognition.
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In the evolutionary lineage leading to the genus听Leavenworthia听(a plant group related to canola and cole crops such as broccoli and cabbage), the ancestral genes that code for self-pollen recognition were lost. But the self-pollen recognition function in听Leavenworthia听appears to have been taken up by two other genes that originally may have had a different role -- for example, in pathogen recognition.
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鈥淪elf-incompatibility,鈥� the pollen-recognition system that enables plants to avoid the inbreeding caused by self-pollination, involves a pair of tightly linked genes known as theS听locus. In this study, the researchers analyzed the gene sequence, genome organization, and gene evolutionary history of听S听loci in members of the Brassicaceae family, which includes plants of the genus听Leavenworthia.
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鈥淲e conclude that both genes that comprise the ancestral S locus in the Brassicaceae were lost in听Leavenworthia,鈥� says McGill researcher Sier-Ching Chantha, lead author of the study. Our analyses show, however, that plants of this genus have two other linked genes that exhibit patterns characteristic of an听S听locus. These genes occupy the same genomic position in听Leavenworthia听as do two non-S-locus genes in a related species. We suggest that these genes have evolved to assume the function of the pollen recognition system of self-incompatibility in听Leavenworthia.鈥�
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How plants avoid inbreeding, and the related topic of听S听locus evolution have been important research subjects for plant biologists.听 There can be hundreds of variants of a single S-locus in individual plant populations鈥攁 very unusual situation. In the animal world, a similar pheomenon is the many variations in immune-system genes. Immune system genes in animals, like the听S听locus in plants, are also involved in recognition, though in the case of immune genes it is foreign antigens rather than pollen types that are recognized. It seems that the recognition function can act in both systems to allow the evolution of large amounts of genetic diversity.
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鈥淔ran莽ois Jacob, the famous French biologist, once compared the action of natural selection to that of a tinkerer who uses the materials around him to produce a working object,鈥� notes McGill biology professor Daniel Schoen, the corresponding author of the study. 鈥淭he evolution of the genes involved in self-pollen recognition in听Leavenworthiaprovides a compelling example of this idea, and lends credence to the notion that the loss of complex traits may not always be irreversible.鈥�
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The study鈥檚 other co-authors are Adam C. Herman and Adrian E. Platts of McGill鈥檚 Department of Biology and Xavier Vekemans of Universit茅 Lille 1 in France.
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The research was supported by the Natural Sciences and Engineering Research Council, Genome Canada, Genome Quebec, and France鈥檚 Agence Nationale de la Recherche.
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