Abstract
The rymv1-2 and rymv1-3 alleles of the RYMV1 resistance to Rice yellow mottle virus (RYMV), coded by an eIF(iso)4G1 gene, occur in a few cultivars of the Asiatic (Oryza sativa) and African (O. glaberrima) rice species, respectively. The most salient feature of the resistance breaking (RB) process is the converse genetic barrier to rymv1-2 and rymv1-3 resistance breakdown. This specificity is modulated by the amino acid (glutamic acid vs. threonine) at codon 49 of the Viral Protein genome-linked (VPg), a position which is adjacent to the virulence codons 48 and 52. Isolates with a glutamic acid (E) do not overcome rymv1-3 whereas those with a threonine (T) rarely overcome rymv1-2. We found that isolates with T49 had a strong selective advantage over isolates with E49 in O. glaberrima susceptible cultivars. This explains the fixation of the mutation T49 during RYMV evolution and accounts for the diversifying selection estimated at codon 49. Better adapted to O. glaberrima, isolates with T49 are also more prone than isolates with E49 to fix rymv1-3 RB mutations at codon 52 in resistant O. glaberrima cultivars. However, subsequent genetic constraints impaired the ability of isolates with T49 to fix rymv1-2 RB mutations at codons 48 and 52 in resistant O. sativa cultivars. The origin and role of the amino acid at codon 49 of the VPg exemplifies the importance of historical contingencies in the ability of RYMV to overcome RYMV1 resistance.
Highlights
Rice yellow mottle virus, of the genus Sobemovirus, causes a major disease in Africa [1]
The mutant CIa*49E was obtained from the infectious clone of the isolate CIa (T49) to establish the causal role of the amino acid at codon 49 in the ability to break RYMV1 resistance alleles
The first objective of this study was to establish that the amino acid at codon 49 of the Viral Protein genome-linked (VPg) was a molecular ‘signature’ but was a key genetic determinant of the ability to overcome rymv1-2 and rymv1-3 resistance alleles
Summary
Of the genus Sobemovirus, causes a major disease in Africa [1]. The virus has a narrow host range restricted to the two cultivated rice species, Oryza sativa and O. glaberrima, and a few wild grasses [2]. The Asiatic rice O. sativa, which was introduced to East Africa and Madagascar in the 11th century and to West Africa in the 15th century, is grown throughout Africa. A few rice cultivars exhibit a high resistance to RYMV characterized by an absence of symptoms and no viral detection by ELISA [7]. The inheritance of this high resistance is recessive. Four rymv resistance alleles have been characterized, one in O. sativa (rymv1-2) and three in O. glaberrima cultivars (rymv, rymv, rymv1-5) [9]. The genetic determinants of the ability to break resistance alleles rymv, rymv and rymv were investigated in this study
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