The origin and success of polyploids in the boreal circumpolar Flora: A new analysis

Summary

Data are summarised that support the secondary contact-balanced hybridity of polyploid evolution in plants. This states that polyploids are most likely to arise and achieve initial success in regions where alternate isolation plus differentiation followed by reunion and hybridisation, in the broadest sense of the word, are most likely to occur. The initial polyploids vary greatly in their evolutionary success. With the passage of time, a small proportion of the original neopolyploids evolve into mesopolyploids, that are widespread, aggressive colonisers, and become abundant over a broad geographical and ecological range. Even later, some of these become diploidised. The resulting paleopolyploids are more like diploids than their original polyploid ancestors with respect to gene pools, geographical distribution and different ecological situations occupied in any region. These conclusions are well supported by a preliminary analysis of polyploidy in the flora of Alaskan Beringia, but more extensive and thorough analyses are needed. Additional data support the hypothesis that chromosome doubling by itself does not adapt plants to withstand severe ecological conditions, such as cold and drought. Finally, high chromosome numbers in some species of angiosperms, plus even higher numbers in a few groups of spore bearing vascular plants, indicate that there is no generally valid ceiling to chromosome numbers. Polyploidisation occurs in cycles. If favourable adjustments of gene dosage occur between cycles, later cycles can include multiplication of base numbers that are already polyploid with reference to the original number of the genus or family.