Barley yellow mosaic virus (BaYMV) causes the most serious soil-borne disease to barley, and differentiated into several strains (BaYMV-I, II and III in Japan, and BaYMV-1 and 2 in Europe). The strain of BaYMV-II is prevalent in Ibaraki Prefecture of eastern Japan grown a six-rowed covered barley cv. Kashimamugi for five or more years. Iida et al. (1993) found the resistance to BaYMV-II is related to esterase isozyme genotypes in Japanese two-rowed malting barley. Furthermore, Iida and Konishi (1994) demonstrated that the resistance gene is linked to the esterase isozyme complex locus (Est1, Est2 and Est4) with 6.97 to 10.15% of recombination values on chromosome 3H. We have focused our attention to map the resistance gene on the chromosome.

A linkage stock susceptible to BaYMV-II, Choshiro-hen, which carries three markers (uz, cu2 and Est1.Af) on chromosome 3H, was crossed with a resistant cv. Amagi Nijo, and F₂ plants were grown in the BaYMV-free field and harvested individually. A total of 227 F₃ lines were sown in each one hill-plot of the BaYMV-II infested field, and investigated their reaction to the virus at the seedling stage. As is difficult to distinguish between susceptible homozygous and heterozygous lines, they were scored to be the susceptible lines against the resistant ones. Genotypes for esterase isozymes were judged after starch gel electrophoresis using a tip of the first leaf of the seedlings. Segregation of uz and cu2 for uzu type and curly growth was observed in ten seedlings per F₃ line, and converted to indicate as their genotypes in the F₂ generation.

Table 1 shows every segregation of BaYMV reaction and the three markers well fit to a ratio of 3:1 or 1:2:1. As F₁ plants between the parents were susceptible and resistance to BaYMV-II is governed by one gene, the resistance gene is newly designated as rym6 after rym5 (formerly ym5) derived from Mokusekko 3 (Konishi et al. 1997). Linkage analysis was performed between four loci for BaYMV reaction and linkage markers on chromosome 3H. As shown in Table 2, rym6 is linked to Est1 and cu2 with recombination values of 11.4 and 30.4% respectively, independent of uz. This indicates that the position of rym6 is distal from Est1 on the long arm of chromosome 3H, and that rym6 is not allelic to rym5 that is linked to Est1 with 1.9% (Konishi et al. 1997). The arrangement of the four loci on chromosome 3H is illustrated in Fig. 1.

Further studies on the allelism of BaYMV resistance genes were conducted in the field infested with BaYMV-II. F₁ plants crossed between barley cultivars carrying different resistance genes were grown and observed their reactions to BaYMV-II. However, all the F₁ plants including rym4/rym6 and rym5/rym6 did not show any disease symptoms, whereas the susceptible parent, Choshiro-hen, was severely damaged. Suppression of the disease symptoms might be caused by the non-allelic interaction of different resistance genes and/or genetic backgrounds. This suggests that it is difficult to test the allelism of the resistance genes against BaYMV-II based on the symptoms only.

References:

Iida, Y., K. Watanabe, I. Toshima, and K. Ogawa. 1993. Relationships between reaction to infection with barley yellow mosaic virus strains and genotypes for esterase isozymes on two-rowed barley (Hordeum distichum L.). Japan. J. Breed. 113-122.

Iida, Y. and T. Konishi. 1994. Linkage analysis of a resistance gene to barley yellow mosaic virus strain II in two- rowed barley. Breeding Science 44:191-194.

Konishi, T., T. Ban, Y. Iida, and R. Yoshimi. 1997. Genetic analysis of disease resistance to all strains of BaYMV in a Chinese barley landrace, Mokusekko 3. Theor. Appl. Genet. 94:871-877.

Locus A    B	Segregation1	Total	X2	Recombination value (%)
rym6-Est1	47:107:17:1:7:48	227	127.82**	11.4 ± 2.2
rym6-cu2	42:94:35:1:28:27	227	21.64**	30.4 ± 3.6
rym6-uz	40:83:48:10:27:19	227	1.06	45.9 ± 4.0
Est1-cu2	26:20:2:16:70:28:1:32:32	227	59.35**	25.8 ± 2.5
Est1-uz	14:21:13:27:54:33:9:35:21	227	4.37	44.6 ± 3.2
cu2-uz	12:19:12:28:64:30:10:27:25	227	7.12	42.7 ± 3.4

¹Segregation=A-BB:A-Bb:A-bb:aaBB:aaBb:aabb or AABB:AABb:Aabb:AaBB:AaBb:Aabb:aaBB:aaBb:aabb.
** Significant at the 1% level.

Fig 1. A linkage map of chromosome 3H including rym6 and three loci

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