NEW YORK (GenomeWeb News) – In a study appearing online last night in PLoS Biology, a Duke University duo reported that they have identified a chromosomal inversion polymorphism in the yellow monkeyflower that appears to contribute to local adaptations in that plant.
The team used a combination of genetic markers and plant crossing experiments to look at the genetic differences between plants from yellow monkeyflower ecotypes that have become reproductively isolated from one another. In the process, they identified a chromosomal inversion — encompassing millions of bases of DNA and hundreds of genes — that seems to influence a range of monkeyflower adaptations, from plant hardiness to flowering time.
"Our results demonstrate for the first time in nature the contribution of an inversion to adaptation, an annual/perennial life-history shift, and multiple reproductive isolation barriers," Duke University genetics and genomics researcher John Willis and graduate student David Lowry wrote.
"Such a mechanism may be partially responsible for the observation that closely related species often differ by multiple chromosomal rearrangements," they added.
The yellow monkeyflower species Mimulus guttatus includes perennial ecotypes found in damp, coastal environments, annual ecotypes that grow in dry, desert-like locales, and additional locally adapted ecotypes in between. But the genetics underlying this variability is still poorly characterized, the researchers explained.
"When you look at one plant species across a broad landscape with lots of different habitat conditions, you find differences in the genes from one place to the next," lead author Lowry said in a statement. "The cause of these differences has been a source of contention among evolutionary biologists for decades as they've tried to figure out what mechanisms drive the origin of species."
In an effort to understand how a single plant species can adapt to such diverse surroundings and adopt distinct reproductive patterns, Lowry and Willis focused in on a monkeyflower linkage group that was thought to contain a strong quantitative trait locus based on previous genetic mapping studies of the plant.
Through a series of crosses within and between annual and perennial monkeyflower ecotypes, combined with genotyping analyses of plants stemming from these crosses, the researchers identified a 3.35 million base inversion containing an estimated 362 genes.
Based on their follow-up experiments — which included field studies of monkeyflower plants from one ecotype containing a form of the inversion typically found in the other ecotype — the researchers argue that genes within the chromosomal inversion influence several adaptive features in monkeyflower, including the plant's appearance, flowering time, and reproductive isolation between ecotypes.
"The inversion investigated here is involved in a classic life-history shift in plants that results from differences in the seasonal availability of water resources," Lowry and Willis wrote.
"The [inland annual] arrangement of the inversion promotes rapid flowering over sustained vegetative growth, leading to an annual life-history strategy that allows for avoidance of hot seasonal drought," they explained. "In contrast, the [coastal perennial] arrangement of the inversion promotes greater vegetative growth early in the season, followed by later flowering and survival into subsequent years, and therefore a perennial life-history."
While the adaptations have not yet caused a split within the monkeyflower species, Lowry explained in a statement, they are "going in that direction."
"We actually showed through experimentation that the inversion contributes to adaptation and reproductive isolation," he added.
Based on their findings so far, the pair postulated that such chromosomal inversions may play a larger role in adaptive processes than previously appreciated.
"Inversions are going to be seen as an important part of local adaptation as more people look for them," senior author Willis said in a statement. "This is an extremely important argument and could explain a lot of the inversions that people are finding."