Ancient polyploidization predating divergence of the cereals

The tribe is morphologically diverse and contains some of our most economically important crop plants [maize ().

Here, we focus on the tribe Andropogoneae (subfamily Panicoideae), a group of about 1,200 species in 90 genera.

ancient polyploidization predating divergence of the cereals-53ancient polyploidization predating divergence of the cereals-71

The parental genomes of most Andropogoneae polyploids diverged in the Late Miocene coincident with the expansion of the major CPolyploidy (whole-genome duplication) is often linked with the acquisition of new traits and subsequent species diversification, particularly in plants (1, 2).

This number is a minimum, and the actual frequency could be considerably higher.

This phylogenetic pattern has led to the hypothesis that polyploidy causes or promotes diversification.

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).

However, because of the large scale of the analyses, these studies necessarily had to rely on inference of polyploidization events from chromosome numbers.

As noted by Stebbins (13), “polyploidy has been important in the diversification of species and genera within families, but not in the origin of the families and orders themselves,” implying that polyploidy is only a minor force in diversification (see also ref. Surveys of angiosperms and ferns have found no evidence for increased speciation after polyploidization (15, 16), supporting Stebbins's hypothesis.

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The role of polyploidy, particularly allopolyploidy, in plant diversification is a subject of debate. The pattern apparently indicates a rapid radiation.

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.

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