The timetable for allopolyploidy in flowering plants

Our understanding of the processes and dynamics of allopolyploid speciation, the long-term consequences of ploidal change, and the genetic and chromosomal changes in new emerged allopolyploids has substantially increased during the past few decades. Yet we remain uncertain about the time since lineage divergence when two taxa are capable of spawning such entities. Indeed, the matter has seemed intractable. Knowledge of the window of opportunity for allopolyploid production is very important because it provides temporal insight into a key evolutionary process, and a temporal reference against which other modes of speciation may be measured. This Viewpoint paper reviews and integrates published information on the crossability of herbaceous species and the fertility of their hybrids in relation to species divergence times. Despite limitations in methodology and sampling, the estimated times to hybrid sterility are somewhat congruent across disparate lineages. Whereas the waiting time for hybrid sterility is roughly 45 million years, the waiting time for cross-incompatibility is roughly 810 million years, sometimes considerably more. Strict allopolyploids may be formed in the intervening time window. The progenitors of several allopolyploids diverged between 4 and 6 million years before allopolyploid synthesis, as expected. This is the first study to propose a general temporal framework for strict allopolyploidy. This Viewpoint paper hopefully will stimulate interest in studying the tempo of speciation and the tempo of reproductive isolation in general.