American Society of Naturalists

Dynamics of Mixed-Ploidy Populations under Demographic and Environmental Stochasticities

Michelle L. Gaynor, Nicholas Kortessis, Douglas E. Soltis, Pamela S. Soltis, and José Miguel Ponciano: Read the article

When you think of a mutation, you might think of the classic “spelling mistake” in the form of As, Ts, Cs, and Gs. But sometimes, errors during reproduction can be much bigger, affecting entire genomes – and even multiplying them. Whole genome duplication creates individuals with extra sets of chromosomes, or “polyploids”, which are distinct from their pre-duplication counterparts, diploids. Often, polyploids vanish from populations, brief blips in evolutionary history. Yet, in some cases, they persist alongside their diploid progenitors. Despite past research, what determines whether polyploids and diploids can coexist remains unclear.

In a recent American Naturalist study, Gaynor et al. developed a mathematical model to explore how multiple ploidy levels might persist in a population. Their model expands on previous work by incorporating underexplored factors – most notably, randomness. Traditional matrix models assume fixed rates of reproduction, maturation, and survival, but in reality, these processes all fluctuate due to chance or individual differences. Gaynor et al. account for this in their model by allowing these rates to vary with population density and random environmental shifts. They also model biologically relevant complexities like overlapping generations (common in perennial plants where polyploids frequently occur) and incomplete reproductive isolation, where diploids and polyploids still experience some gene flow.

To test their elaborate model, Gaynor et al. ran millions of simulations, evaluating what happens to populations under different conditions over thousands of years. They found that environmental instability plays a crucial role. Under more unpredictable conditions, polyploids are more likely to outcompete and replace diploids. However, both ploidy types can persist together over time, and coexistence is stabilized by reproductive isolation. An exception arises when competition drives diploids to extinction; since polyploids can arise from diploids but not vice versa, once diploids are lost, they’re gone for good, leading to purely polyploid populations. By integrating random chance and realistic biological features, this study offers a more nuanced view of how polyploids establish and endure.

Though once considered evolutionary oddities, polyploids are increasingly recognized as common in natural populations and potentially influential in speciation. They also play key roles in ecosystems, including those of important and beloved plant species like big bluestem and quaking aspen. Thus, understanding polyploid dynamics doesn’t just advance our knowledge of evolutionary and ecological processes – it may have real implications for conservation in at-risk habitats. Gaynor et al.’s work brings us closer to deciphering these hidden yet powerful evolutionary processes.

Regina A. Fairbanks is a Ph.D. candidate in the Population Biology Graduate Group at the University of California, Davis. Regina’s research aims to integrate evolutionary genomics with archaeological science to better understand crop domestication and plant-people relationships. For their dissertation, Regina uses population genomic data to study the evolutionary history of maize. In addition to their research, Regina participates in a wide range of science communication and mentorship programs, including Letters to a Pre-Scientist and the UC Davis Evolution & Ecology Graduate School Preview Program. When not staring at a computer screen, Regina can be found in museums, botanical gardens, and farmers markets.