Two pathways to genome duplication

Photo Credit

USDA photo by Bryce Richardson

Growth rate differences between tetraploid (middle rows) and diploid (right row) big sagebrush. Tetraploids slow grow rate may be adaptive for growing in more arid climates where tetraploids are most dominant.

Understanding the processes involved in polyploid formation is critical for assessing plants' abilities to adapt to unprecedented rates of environmental change. From a scientific perspective, genome duplication was traditionally thought to be a rare event, and neoploids were typically viewed as ephemeral and reproductively isolated from diploid progenitors due to chromosome incompatibilities. Our study, however, supports a different paradigm.

We demonstrate that Artemisia tridentata (and likely other plants) utilize dual pathways, described as one- and two-step. Our findings reinforce the paradigm that polyploidy is an important, recurrent mechanism for generating genomic diversity on generational timescales. By screening seedlings, our research directly measures polyploidy—a method more accurate than previous studies that merely estimated the rate of unreduced gametes. This work is also unique in showing that a single species is capable of using both pathways to polyploidization.

Genome duplication can be achieved via two pathways: a direct one-step pathway, where unreduced gametes produce neoploids, or a two-step pathway, where reduced and unreduced gametes form triploids that subsequently reproduce with either diploids or other polyploids. We document that triploids can produce near-diploid to near-tetraploid offspring, potentially acting as a gene flow bridge between diploids and polyploids. From a management perspective, this suggests polyploids can represent a sizable portion of a species' distribution and can reform quickly after a disturbance. Specifically, we show that the rate of polyploidization in big sagebrush is 0.22 percent. While this may appear small, when you consider that the yearly reproductive output of a single plant can exceed 1 million seeds, this rate accounts for about 2,000 polyploid seeds. This may partly explain the prevalence of tetraploid big sagebrush and its landscape dominance.