Convergent evolution of perenniality in rice and sorghum

Abstract

Annual and perennial habit are two major strategies by which grasses adapt to seasonal environmental change, and these distinguish cultivated cereals from their wild relatives. Rhizomatousness, a key trait contributing to perenniality, was investigated by using an F(2) population from a cross between cultivated rice (Oryza sativa) and its wild relative, Oryza longistaminata. Molecular mapping based on a complete simple sequence-repeat map revealed two dominant-complementary genes controlling rhizomatousness. Rhz3 was mapped to the interval between markers OSR16 [1.3 centimorgans (cM)] and OSR13 (8.1 cM) on rice chromosome 4 and Rhz2 located between RM119 (2.2 cM) and RM273 (7.4 cM) on chromosome 3. Comparative mapping indicated that each gene closely corresponds to major quantitative trait loci (QTLs) controlling rhizomatousness in Sorghum propinquum, a wild relative of cultivated sorghum. Correspondence of these genes in rice and sorghum, which diverged from a common ancestor approximately 50 million years ago, suggests that the two genes may be key regulators of rhizome development in many Poaceae. Many additional QTLs affecting abundance of rhizomes in O. longistaminata were identified, most of which also corresponded to the locations of S. propinquum QTLs. Convergent evolution of independent mutations at, in some cases, corresponding genes may have been responsible for the evolution of annual cereals from perennial wild grasses. DNA markers closely linked to Rhz2 and Rhz3 will facilitate cloning of the genes, which may contribute significantly to our understanding of grass evolution, advance opportunities to develop perennial cereals, and offer insights into environmentally benign weed-control strategies.

Figure 1

Comparative mapping of genes/QTLs affecting rhizome expression and abundance in O. longistaminata and S. halepense. DNA markers in normal Italian were used in gene/QTL mapping; those indicated by arrows are locations inferred relative to closely linked restriction fragment length polymorphism markers from rice (9) and sorghum (2) by using the comparative mapping search tool from the Gramene database (16), where M1, M2, and M5 represent maize chromosomes 1, 2, and 5, respectively. The yellow portions of the chromosomes show the genomic regions of distorted segregation favoring the RD23 (O. sativa) allele, and the pink portions show the genomic regions of distorted segregation favoring the O. longistaminata allele. The two underlined rice QTLs were not matched with corresponding sorghum QTLs.