Intraspecific trait variation provides ecological opportunity for polyploids

One of the features of some plants is they can be polyploid. Humans have a pair of chromosomes carrying our genetic information. Plants can vary between one or two chromosomes, known as haploid or diploid. However, they can add extra copies of chromosomes, making them polyploid. For example, a plant with four copies of a chromosome is tetraploid.

Niche divergence between polyploids and their lower ploidy ancestors is one of the primary ways that polyploids establish themselves. Occupying different niches also drives adaptive divergence. However, within-species chromosomal and reproductive variability have usually been neglected in community ecology and biodiversity analyses, even though they have been recognised to play a role in the adaptive diversification of lineages. It’s no surprise then that whole genome duplication, creating polyploidy, is often found before the rapid diversification of species.

Map displaying all collection localities of P. intermedium and ploidy levels determined in the present study
Map displaying all collection localities of Paspalum intermedium and ploidy levels determined in this study. The North–South cytotype cline is apparent, together with an East–West transition zone where cytotypes occur intermingled in pure and mixed populations. See Karunarathne et al. 2018 for more details.

Karunarathne and colleagues used Paspalum intermedium and a combination of ploidy and reproductive assessments and environmental niche modelling to explore the ecological consequences of within-species trait variation.

What they found was that the older, diploid, populations were adapted to a narrower set of habitats than the tetraploid grasses. It meant that sexual diploid specialists displace polyploids in core areas, while apomictic (asexual) tetraploid generalists are successful in peripheral areas promoting polyploid range expansions.

The study also found some triploid populations of the grass. The authors note: “Previous and present findings of rare triploid cytotypes of P. intermedium do not provide much information, but rather an opportunity to study the role of triploids in polyploid formation and population dynamics that need to be addressed in a set of new experiments.” And this has started.

In a further paper in Frontiers in Plant Science one of the authors, Diego Hojsgaard, examines how a temporary apomixis (switching sexual for asexual reproduction) could help triploids establish themselves. For Karunarathne and colleagues in their, this ability to switch to apomixis explains quite a bit about how polyploids can establish and the difference in ranges that they observed. Asexual reproduction prevents crossing with the diploid ancestors, and the extra genes allow the most adapted plants to expand into new environments.

Reference List

Soltis, D. E., Albert, V. A., Leebens-Mack, J., Bell, C. D., Paterson, A. H., Zheng, C., … Soltis, P. S. (2009). Polyploidy and angiosperm diversification. American Journal of Botany, 96(1), 336–348. https://doi.org/10.3732/ajb.0800079

Karunarathne, P., Schedler, M., Martínez, E. J., Honfi, A. I., Novichkova, A., & Hojsgaard, D. (2018). Intraspecific ecological niche divergence and reproductive shifts foster cytotype displacement and provide ecological opportunity to polyploids. Annals of Botany, 121(6), 1183–1196. https://doi.org/10.1093/aob/mcy004

Hojsgaard, D. (2018). Transient Activation of Apomixis in Sexual Neotriploids May Retain Genomically Altered States and Enhance Polyploid Establishment. Frontiers in Plant Science, 9. https://doi.org/10.3389/fpls.2018.00230