A study out of Harvard and Purdue universities is starting to unravel the genetic mechanisms that allow some plants to duplicate their entire genomes and continue to reproduce.
Most plants, including crops, at some point in history have duplicated their genomes, giving them two or more copies of each of the instructions to build the plant. These plants have few problems reproducing normally.
When scientists manipulate individuals and induce duplicate genomes, however, it is difficult – sometimes impossible – for the organism to sexually reproduce. Being able to create polyploids, organisms with three or more genomes, such as the crops peanut, strawberry, banana, canola, cotton, wheat and others, may allow scientists to improve those crops more quickly through breeding and create new polyploidy crop species. Breeding programs that use new polyploids are cumbersome because many do not reproduce easily.
"The machinery that divides the genome during sex is designed to pull paired chromosomes apart and create two cells from one. When you have four chromosomes to work with, they aren't always divided correctly," says Brian Dilkes, a Purdue assistant professor of horticulture and co-author of the findings published in the journal PLoS Genetics. "What this paper demonstrates is that we can use evolution as a tool to find the genes that allow plants to tolerate being tetraploids, which have four copies of their genomes."
The scientists found a species that does reproduce as both a diploid and tetraploid - Arabidopsis arenosa, a cousin of the standard research plant Arabidopsis thaliana. By comparing the DNA sequences of the whole genomes of plants, they detected the genetic differences between the tetraploid and diploid versions of the species.
Many genes known to play a role in meiosis, or cell division, were different in the tetraploids as compared to the diploids. In particular, the gene Asynaptic1, which controls the organization of chromosome pairs during reproduction, was mutated in tetraploids.
Of the plants tested, 95 percent of the tetraploids shared the same mutation in Asynaptic1, while 95 percent of the diploids did not contain this variant. This suggests that the mutation in Asynaptic1 is involved in the adaptation of the meiotic machinery needed to work with four copies of the genome.
"We're actually learning the mechanisms that were used in evolution to solve challenges faced by plants dealing with tetraploidy," Dilkes says. "With this understanding we will be able to manipulate crops and crop relatives to accelerate plant breeding and the inclusion of a wider genetic base in the improvement of many existing crops."
Dilkes' laboratory at Purdue was involved in analyzing the DNA of each plant tested to determine whether it was a diploid or tetraploid. He says the work would continue to determine which genes and mutations allow for sexual reproduction in tetraploid plants.
Kirsten Bomblies, an assistant professor of organismic and evolutionary biology at Harvard and principal investigator for the project, says the results are also important for human health.
"Several of the genes have been shown to be critical for survival of tetraploid, but not diploid yeast, and they are also implicated in human polyploid cancers and genome instability syndromes," Bomblies says.
The research was funded by the Purdue Univ. College of Agriculture. Bomblies was supported by a Harvard Univ. William F. Milton Fund award.