Ancient polyploidization predating divergence of the cereals
Alternatively, allopolyploidy could correlate with diversification of a major clade or even the entire tribe (the diversification hypothesis), in which case most allopolyploids would be the product of a single polyploidy event followed by radiation at the tetraploid or hexaploid level; we would see only a handful of polyploidization events deep in the tree (Fig. Previous attempts to resolve relationships within Andropogoneae, using both chloroplast and nuclear markers, have produced phylogenies that are poorly supported, with intergeneric relationships unresolved (30–36).
We therefore sought to test whether each allopolyploidy event leads only to one or a few species (the Stebbins hypothesis), in which case, the allopolyploids should appear in the phylogeny as the result of numerous independent polyploidy events.
About 80% of its species are polyploid (24), and Hunziker and Stebbins (25) have noted that the grasses are the “only large family in which high frequency of polyploids prevails throughout the family.” The polyploids that Stebbins refers to and that we discuss in this paper all formed after the well-documented whole-genome duplication at the origin of the family (26, 27); the grass WGD () occurred about 80–90 million years ago (Mya) (28, 29).
The grass family is a powerful system in which to study allopolyploidization.
10% of the species of the clade, based on sequences of four concatenated low-copy nuclear loci.
We focus here on the grass tribe Andropogoneae, an economically and ecologically important group of C.
Ancient polyploidy correlates with major land-plant radiations (3) and the origins of orders, large families, and major clades (4–7) although, in many cases, sharp changes in diversification rates are delayed for millions of years after the polyploidization event (1). Phylogenetic trees of nuclear genes can reliably identify allopolyploidization events (17–21) because they produce characteristic double-labeled tree topologies in which the polyploid species appears twice (Fig. In such trees, allopolyploids are readily identified and can be discovered even when no chromosome counts are available (e.g., ref. This tree-based method will recover genetic allopolyploids; following the terminology of Doyle and Egan (23), these polyploids may be descendants of two species (taxonomic allopolyploids) or one (taxonomic autopolyploids). Although unavoidable, inferences from chromosome numbers require assumptions about which numbers represent polyploids and when the polyploids arose. Of the species listed in the preceding paragraph, all but sorghum are allopolyploid.
In short, the Andropogoneae feed, and increasingly fuel, the planet.At least 32% of the species sampled are the result of genetic allopolyploidy and result from 28 distinct tetraploidy events plus an additional six hexaploidy events. We found a remarkable number of allopolyploidization events, none of which correlate with shifts in diversification rate or with the origin of unique morphological characters.