Relationships of cereal crops and other grasses

Abstract

The grass family includes some 10,000 species, and it encompasses tremendous morphological, physiological, ecological, and genetic diversity. The phylogeny of the family is becoming increasingly well understood. There were two major radiations of grasses, an early diversification leading to the subfamilies Pooideae, Bambusoideae, and Oryzoideae, and a later one leading to Panicoideae, Chloridoideae, Centothecoideae, and Arundinoideae. The phylogeny can be used to determine the direction of changes in genome arrangement and genome size.

Figure 1

Summary (semistrict consensus) of phylogenetic data on the grass family. An appropriately shaded rectangle marks any clade supported by a particular set of data and not strongly contradicted by any other set of data. Joinvilleaceae is the sister group of the grasses; all other taxa are grasses. Triangles indicate a large clade. Numbers refer to number of genera. Sources of data and references are listed in text. Some evidence indicates that Bambusoideae, Oryzoideae, and Pooideae actually form a single clade (20, 25), but this grouping is not yet well supported, so the relationship is shown as ambiguous.

Figure 2

Possible evolutionary patterns of genome rearrangements. (a) Tree congruent with that in Fig. 1, pruned to include only those branches with genome maps. Note that no inference about the ancestral arrangement is possible. (b) Same tree, but with Oryzoideae and Pooideae as sister taxa. Note that now it is possible to infer that linkage groups 10 and 3 were unlinked in the common ancestor of rice and the pooids, but it is still impossible to determine the ancestral state for all grasses. (c) More detailed tree. Broken lines indicate ambiguity in the time of origin of linkage groups 3c-10-3b-3a and 7a-9-7b. Note that some of this ambiguity will be resolved with a map for finger millet.

Figure 3

Summary (semistrict consensus) of phylogenetic data for the subfamily Pooideae. An appropriately shaded rectangle marks any clade supported by a particular set of data and not strongly contradicted by any other set of data. Triangles indicate a large clade. Numbers refer to number of genera. Sources of data and references are listed in the text. Chromosome base numbers are indicated on the right.

Figure 4

One possible model for evolution of genome size in the grasses. Data on genome size were extracted from papers by Bennett and colleagues (–39). The cladogram was based on references cited above; phylogeny in the Triticeae is from Kellogg et al. (40), that in the Poeae/Aveneae is from J. I. Davis (personal communication), that in the Chloridoideae is from Clayton and Renvoize (1), and that in the Panicoideae is from Kellogg and Watson (33) and Doebley (41). Ancestral genome sizes were reconstructed according to squared-change parsimony (42), implemented in MacClade 3.0 (43).