Frederik Mortier, Ghent Universit, discusses his article: Polyploid—diploid coexistence in the greater duckweed Spirodela polyrhiza
Polyploid establishment is not as easy as it looks
Polyploidy, when organisms have extra sets of chromosomes due to whole-genome duplication, is surprisingly common, especially in plants. Polyploidy can be a dramatic mutation with a huge effect on plant traits, such as stress tolerance in many species and larger fruits and flowers in domesticated plants. However, polyploidy also comes at a huge cost: the doubled genome size inhibits proper cell growth, and sexual polyploids experience a minority disadvantage. As a result, many polyploid species, especially immediately after polyploidization, grow slower compared to their diploid cousins. Then, why do polyploid species nonetheless often establish in natural plant communities? One possibility is that polyploids are better competitors. After all, a polyploid needs to avoid being outcompeted by its progenitor.
Mutual invasion experiments to test polyploid establishment in competition with its progenitor
To test how polyploids compete with their diploid relatives, we used ideas from ecological coexistence theory. This theory asks: can species coexist, or will one exclude the other? One way to find out is through “mutual invasion experiments”, in which species are expected to coexist if both can increase when starting as the rare invading species. The coexistence framework further quantifies the stability of species’ coexistence based on frequency-dependent and -independent effects on growth rate.
We performed mutual invasion experiments using the aquatic plant, greater duckweed (Spirodela polyrhiza). Our lab previously created stable tetraploids from four different diploid strains. Because these tetraploids were recently formed, comparing them to their diploid parents lets us isolate the effect of genome duplication itself from evolution after the polyploidization. Mixed populations of either predominantly diploids or tetraploids were used to test tetraploid invasion and diploid invasion, respectively, by tracking tetraploid frequency across 12 weeks using flow cytometry. We tested this in both benign and salt-stressed conditions to also test if stress tolerance favored the tetraploid.
Measuring the frequency of tetraploids was challenging. Because duckweed is multicellular and flow cytometry sometimes produces noisy data, we calibrated our measurements using advanced data models. Even so, it was hard to detect a decrease in tetraploid frequency when tetraploids were already rare, such as in the tetraploid invasion tests. However, tetraploid frequency clearly declined in all tests. This indicated that all tetraploid duckweed strains would be competitively excluded by their diploid progenitor, independent of stress and starting frequencies.
To better understand these dynamics, we also tried to estimate the fitness differences and niche overlap between diploids and tetraploids using coexistence theory. We explain in the manuscript how this necessitated absolute population sizes and how we obtained those using dry weight measurements of the total population. The data were noisy, but in two cases, this analysis surprisingly suggested stable coexistence.
Take-away
While the mutual invasion experiments clearly showed that all tetraploid duckweed strains struggled to establish, quantitative models using coexistence theory surprisingly hinted at stable coexistence in two out of eight strain-environment combinations. This discrepancy leads us to advocate strongly for measuring absolute population sizes as accurately as possible. Only then can coexistence theory help us understand how polyploids can succeed.
