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Parallel evolution of hull colour in domesticated and weedy rice

Parallel evolution of hull colour in domesticated and weedy rice

In an article entitled, ‘The role of Bh4 in parallel evolution of hull color in domesticated and weedy rice’ published in July 17, 2013, issue of Journal of Evolutionary Biology (DOI: 10.1111/jeb.12171), the lead author C C Vigueira with two other associates affiliated to the Department of Biology, Washington University, have examined DNA sequence variation by using Black hull 4 (Bh4) as a hull color candidate gene to study the parallel evolution of hull color variation in the domesticated and weedy rice system. Worldwide, most of the cultivated rice is Asian rice, Oryza sativa, which was bred from its wild progenitor Oryza rufipogon in southern Asia within the past 10,000 years. Whether the familiar indica and japonica subspecies of Asian rice also represent independent domestication is controversial. Most of the rice grown in the U.S. is japonica rice which is genetically pretty different from indica rice. In any event there was a second unambiguous domestication event about 3,500 years ago when African cultivated rice (O. glaberrima) was bred from the African wild species O. barthii in the Niger River delta. However, when a plant is domesticated, it acquires a suite of traits called the domestication syndrome that made it easier to grow as a crop. In rice, the syndrome includes loss of shattering (the seeds don’t break off the central grain stalk before harvest), increase in seed size, and loss of dormancy (the seeds all germinate at once and can be harvested at once). In current study, they  revealed that independent Bh4 coding mutations have arisen in African and Asian rice that are correlated with the straw hull phenotype, suggesting that the same gene is responsible for parallel trait evolution. For the U.S. weeds, Bh4 haplotype sequences support current hypotheses on the phylogenetic relationship between the two biotypes and domesticated Asian rice; straw hull weeds are most similar to indica crops, and black hull weeds are most similar to aus crops. Tests for selection indicate that Asian crops and straw hull weeds deviate from neutrality at this gene, suggesting possible selection on Bh4 during both rice domestication and de-domestication. However, the most important part of this story is that the genetic histories of the crops and the weeds are closely intertwined. This means the weedy forms can draw on both ancestral genes and crop genes as they respond to the selection pressures exerted by factors of modern agriculture. [summarized by Research Associate: Samsad Razzaque at pbtlabdu.net.]

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