Ty-2, a gene on chromosome 11 conditioning geminivirus resistance in tomato

 

Hanson, P., Green, S.K., and Kuo, G.

AVRDC-The World Vegetable Center, P.O. Box 42, Shanhua, Tainan 74151, Taiwan

 

     Whitefly-transmitted geminiviruses drastically reduce tomato yields and increase production costs throughout the tropics and subtropics as well as southern temperate regions. Losses due to geminivirus infection became a major production problem in Southeast Asia and Taiwan by the mid-1990’s and the disease is currently spreading in China as far north as the Yangtze River (T-C Wang, personal communication). Resistance is the cheapest and most effective means of control. Almost all geminivirus resistance genes were introgressed into tomato from wild species. Ty-1, the first reported geminivirus resistance gene, originated from S. chilense accession LA1969 and was mapped to the top of chromosome 6 (Zamir et al., 1994). H24 is a tomato line bred in India (Kalloo and Banerjee, 1990) with geminivirus resistance derived from a S. habrochaites accession called B6013. H24 has demonstrated high levels of resistance to monopartite geminiviruses which prevail in south India and Taiwan (Hanson et al., 2000), Japan, and north Vietnam, and has been used as a source of resistance by AVRDC and other breeding programs. Banerjee and Kalloo (1987) reported that two genes acting epistatically conditioned resistance in H24 although our mapping study found a single wild introgression of over 14 cM on the bottom of chromosome 11 between TG393 and TG36 associated with resistance (Hanson et al., 2000).

 

RFLP probing of resistant lines developed at AVRDC derived from crosses with H24 revealed that some lines carry the entire introgression on the bottom of chromosome 11 while other lines contained a smaller introgression with the region from TG26 to TG393 replaced with S. lycopersicum DNA. We conducted a study in 2001 to compare the resistance of lines with the entire introgression and lines with the shorter introgression. Entries included thirteen F₆ lines from three AVRDC double crosses (Table 1). Cross codes and parents were as follows: CLN2114= (CL5915-93D4 x H24) x (PT4671A x CRA84-58-1); CLN2116= (CL5915-93D4 x H24) x (UC204A x CRA84-58-1); CLN2131= (CL5915-93D4 x CRA84-58-1) x (PT4664 x H24). H24 was the only source of geminivirus resistance in these crosses. F₆ entries and checks were evaluated for reaction to Tomato leaf curl Taiwan virus (ToLCTWV) and probed with RFLP markers TG393, TG26, and TG36 to determine the presence or absence of S. habrochaites DNA. Each plot included five plants. Entries were replicated five times and arranged in a RCBD. Fifteen-day-old seedlings of entries were exposed to viruliferous whiteflies maintained in a plastic house and scored for disease incidence and severity at two, four, and six weeks after exposure. 

 

Almost 100% of plants of the susceptible check CL5915 and the two entries homozygous for S. lycopersicum alleles at the three regions developed severe stunting, yellowing and curling by two weeks after whitefly exposure. The three entries possessing the full S. habrochaites introgression and the four entries containing the shorter introgression around TG36 were almost completely free of virus symptoms until conclusion of the experiment. Our results indicate that no geminivirus resistance genes are located on that portion of the S. habrochaites introgression from to TG26 to TG393 and resistance was associated with presence of wild alleles at TG36. Fine-mapping of geminivirus resistance in this region is ongoing at AVRDC.

 

The mapping paper of Hanson et al. (2000) did not propose a name for the geminivirus resistance on chromosome 11 because of the possibility that such a large wild DNA fragment might harbor several resistance loci. However, the gene has already been referred to as ‘Ty-2’ informally and in several literature citations (Grube et al., 2000; Giordano et al., 2005). In order to avoid confusion we propose the name ‘Ty-2 ‘for the geminivirus resistance gene on chromosome 11 located around TG36. 

 

Literature cited:

Banerjee, M.K. and Kalloo. 1987. Inheritance of tomato leaf curl virus resistance in Lycopersicon hirsutum f. glabratum. Euphytica 36: 581-584.

Giordano, L.B., Silva-Lobo, V.L., Santana, F.M., Fonseca, M.E.N., and Boiteux, L.S. 2005. Inheritance of resistance to the bipartite tomato chlorotic mottle begomovirus derived from Lycopersicon esculentum cv. ‘Tyking’. Euphytica 143: 27-33.

Grube, R.C., Radwanski, E.R. and Jahn, M. 2000. Comparative genetics of disease resistance within the solanaceae. Genetics 155: 873-887.

Hanson, P.M., Bernacchi, D., Green, S., Tanksley, S.D., Muniyappa, V., Padmaja, A.S., Chen, H.M., Kuo, G., Fang, D., and Chen, J.T. 2000. Mapping of a wild tomato introgression associated with tomato yellow leaf curl virus resistance in a cultivated tomato Line. J. Amer. Soc. Hort. Sci. 125: 15-20.

Kalloo and Banerjee, M.K. 1990. Transfer of tomato leaf curl virus resistance from Lycopersicon hirsutum f. glabratum to L. esculentum. Plant Breeding 105: 156-159.

Zamir, D., Ekstein-Michelson, I., Zakay, Y., Navot, N., Zeidan, M., Sarfatti, M., Eshed, Y., Harel, E., Pleban, T., van-Oss, H., Kedar, N., Rabinowitch, H.D., Czosnek, H. 1994. Mapping and introgression of a tomato yellow leaf curl virus tolerance gene, TY-1. Theor. Appl. Genet. 88: 141-146.

 

Table 1. RFLP genotype and geminivirus reactions of F₆ lines derived from crosses with H24, AVRDC, 2000.

	RFLP1			Incidence (weeks after exposure)
Entry	TG393	TG26	TG36	2	4	6
CLN2131-5M-1	LL	LL	LL	100	100	100
CLN2131-5M-22	LL	LL	LL	100	100	100
CLN2114-2-17	HH	HH	HH	0	0	0
CLN2114-1-9	HH	HH	HH	0	0	0
CLN2116-39-4	HH	HH	HH	0	0	0
CLN2116-25-8	LL	LL	HH	4	4	4
CLN2116-25-22	LL	LL	HH	0	0	0
CLN2116-11-12	LL	LL	HH	0	0	0
CLN2131-4M-15	LL	LL	HH	0	0	0
CLN2114-23-14	LL	LH	HH	0	0	0
CLN2114-23-21	LH	LH	HH	0	0	0
CLN2114-16-5	HH	LH	HH	0	0	0
CLN2131-4M-11	LL	LL	LH	16	16	16
CL5915 (S ck)	LL	LL	LL	96	100	100
H24 (R ck)	HH	HH	HH	0	0	0
LSD (0.05)	-	-	-	7	6	5

 

¹ LL=homozygous for S. lycopersicum alleles at that locus; HH=homozygous for S. habrochaites alleles; LH=heterozygous. TG36, TG26, and TG393 are RFLP markers mapped to the bottom of chromosome 11.

² n=25 plants per entry with 5 plants per plot and 5 replications