The frequency of polyploid speciation in vascular plants

Abstract

Since its discovery in 1907, polyploidy has been recognized as an important phenomenon in vascular plants, and several lines of evidence indicate that most, if not all, plant species ultimately have a polyploid ancestry. However, previous estimates of the frequency of polyploid speciation suggest that the formation and establishment of neopolyploid species is rare. By combining information from the botanical community's vast cytogenetic and phylogenetic databases, we establish that 15% of angiosperm and 31% of fern speciation events are accompanied by ploidy increase. These frequency estimates are higher by a factor of four than earlier estimates and lead to a standing incidence of polyploid species within genera of 35% (n = 1,506). Despite this high incidence, we find no direct evidence that polyploid lines, once established, enjoy greater net species diversification. Thus, the widespread occurrence of polyploid taxa appears to result from the substantial contribution of polyploidy to cladogenesis, but not from subsequent increases in diversification rates of polyploid lines.

Fig. 1.

Polyploid incidence and speciation frequencies across major groups of vascular plants. Polyploid speciation frequencies are the fractions of branching events that were accompanied by a ploidy shift across the studied phylogenetic trees for each group. The speciation frequencies reported here are based on an irreversible model of polyploid evolution. A binomial standard error follows each incidence and frequency estimate. See Fig. S1 for a diagrammatic explanation of estimation methods for polyploid speciation frequencies. Phylogenetic hypothesis/timescale modified from (13), and based on clades defined in refs. –; clade species richness from refs. and . The Higher Monocots are represented by Arecales, Commelinales, Poales, Proteales, Zingiberales; the Basal Monocots by Alistmatales, Asparagales, Dioscoreales, Liliales, Pandanales.

Fig. 2.

Dependence of infrageneric polyploid incidence on the minimum number of chromosomes reported for angiosperm genera (generic base count). Shown are mean percentages (± SE) of polyploid species within genera. Means with different letters are significantly different [P < 0.05 when evaluated within a logistic model and with nonparametric (rank sums) tests]. (N (l − r) = 320, 323, 370, 354).