Taxonomy & Evolution

Polyploid plants take a leaf out from the invasive species book

The extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little.

The importance of propagule pressure (e.g. number of founder plants or seeds introduced), timing and frequency of introductions are key indicators whether an invasive species can establish and colonise a new environment. Polyploids, plants that have more than two copies of every chromosome, are often better colonisers of different habitats compared to diploid plants.

Dr Donald Levin from the University of Texas, Austin proposed that the success of polyploid population establishment depends on propagule pressure, much like for invasive plants. If a polyploid plant produces more seeds than diploids, polyploids could “invade” diploid populations over time. Over the last couple of years, Levin suggested that polyploid speciation is going to be the dominant evolutionary pathway in plants for the next 500 years and polyploid lineages experience extinction at similar or higher rates than diploids. His new proposition of the importance of polyploid seed rain patterns sets out a new direction to study polyploidisation and hybridisations. 

“In contrast to properties or interactions of polyploids, I propose that insufficient production of polyploid seeds within local diploid populations constitutes an important, perhaps prime, factor limiting the establishment of a persistent polyploid population in suitable sites”, Levin wrote. 

“To be sufficient, the seed rain would have to be substantial within one year or the life time of the seeds. If diploid populations produced many polyploid seeds, but if they were well dispersed in time, the number of potential mates at any given time would be few, and the polyploid population would be below the minimum size for persistence.”

The success of polyploid plant establishment in a diploid population or in a new environment could primarily depend on propagule pressure as well as genetic and phenotypic diversity, and (partial) self-fertilisation.

As most invasive plants fail to establish due to low immigaration rates, propagule pressure can be a more important determinant whether a population can establish than environmental factors, as shown in invasive cacti. Whilst self-fertilisation can help in the beginning of the colonisation for polyploid populations to increase, it can have negative impact in terms of fitness and phenotypic and genetic diversity could also be key for establishment. 

Levin discussed the lack of information on polyploid seed production by diploids (e.g. allopolyploidy) and future research directions and questions.

“How many tetraploid seeds are produced by diploid plants, and to what extent does it vary among plants and populations? How many tetraploid plants occur within diploid populations? How many seeds do tetraploid plants produce, and to what extent does it vary among plants and populations?”, Levin lists.  

Polyploidisation could be a “wash–rinse–repeat” evolutionary cycle but understanding how plant populations evolve can help scientists predict future population dynamics and extinction risks.