Progress 09/15/00 to 09/30/04
Outputs
The tomato gene Mi-1, which confers resistance against three of the most damaging species of root-knot nematodes, as well as to some isolates of potato aphids and white flies, was previously cloned in our laboratory. Molecular and genetic analyses indicate that there are seven Mi-1 homologs in both resistant and susceptible tomato closely linked on the short arm of chromosome 6. In both cases the genes are present in two clusters, one of 3 and the other of 4 copies. Using DNA markers that flank the resistance genes, we ascertained that a genetic inversion is present between the two clusters in susceptible, cultivated tomato compared to the wild tomato species from which resistance was derived. We have cloned and sequenced all seven Mi homologs from both resistant and susceptible tomato. In each case we found two pseudogenes and one truncated gene among the homologs. The intact copies are highly conserved, over 92% identical, in sequence. Comparison of the sequences
indicates that extensive DNA exchange or gene conversion has occurred. Analysis of the flanking sequences identified regions that are homologous to those present in resistance genes on other chromosomes in several solanaceous species. Transcript analysis indicates that all of the Mi-1 homologs, except the pseudogenes, are expressed. DNA blots indicate that Mi homologs are confined to solanaceous plants but are widely distributed within this group. We have cloned and sequenced the central, conserved region of 70 Mi homologs from a range of solanaceous plant species. We have used the sequence information to develop markers that can be used to monitor introgression of nematode resistance and of other important resistance genes on chromosome 6.
Impacts
This research has provided insights into the evolution of host resistance to nematodes and other pathogens. Our findings provide the groundwork for development of strategies to generate novel resistance by molecular manipulation of the cloned genes. Resistance to a number of important tomato and potato diseases has been found to map to the same region of the genome as Mi. This work has produced DNA markers for breeders who are introducing these new resistance traits into commercial cultivars.
Publications
- Mehrach, E., Chouchane, S.G., Mejia, L., Williamson, V.M., Vidavski, F., Hatimi, A., Salus, M.S., Martin, C.T. and D. P. Maxwell. 2004. PCR-based methods for tagging the Mi-1 locus for resistance to root-knot nematode in begomovirus-resistant tomato germplasm. Horticultura Acta, in press.
- Seah, S., Fort, K. P., Yaghoobi, J. and Williamson, V. M. 2005. Comparison of Mi-1 genome context and homolog expression in nematode resistant and susceptible tomato. Manuscript in preparation.
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Progress 01/01/03 to 12/31/03
Outputs
The tomato gene Mi-1, which confers resistance against three of the most damaging species of root-knot nematodes, as well as to some isolates of potato aphids and white flies, was previously cloned in our laboratory. Molecular and genetic analyses indicated that there are seven Mi-1 homologs in both resistant and susceptible tomato on the short arm of chromosome 6. In both cases the genes are present in two clusters, one of 3 and the other of 4 copies. Using DNA markers that flank the resistance genes, we have ascertained that a genetic inversion has occurred between the two clusters in susceptible, cultivated tomato compared to those of wild tomato species from which resistance was derived. We have cloned and sequenced the Mi homologs from both resistant and susceptible tomato. Our analysis indicates that several of the Mi-1 homologs are expressed in both leaves and roots and carry open reading frames that may code for unknown resistance functions. Mi homologs appear
to be confined to solanaceous plants but are widely distributed within this group. We have cloned and sequenced the central, conserved region of 70 Mi homologs from a range of solanaceous plant species. Analysis of evolutionary relationships is underway. We are focusing on genetic characterization of the homologs in the wild potato species Solanum bulbocastanum and in other germplasm where resistance phenotypes are found in the corresponding genetic region.
Impacts
This research addresses the evolution of host resistance to nematodes and other pathogens. Our findings should provide the groundwork for development of strategies to generate novel resistance by molecular manipulation of the cloned genes. The information will also be useful to breeders who are introducing new traits and combinations of traits into tomato and similar crops.
Publications
- Seah, S., Yaghoobi, J., Rossi, M., Gleason, C.A. and Williamson, V.M. 2004. The nematode resistance gene, Mi-1, is associated with an inverted chromosomal segment in susceptible compared to resistant tomato. Theor. Appl. Genet. In press.
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Progress 01/01/02 to 12/31/02
Outputs
The tomato gene Mi-1, which confers resistance to three of the most damaging species of root-knot nematodes as well as to some isolates of potato aphids, was previously cloned in our laboratory. Southern blot analysis indicated that there are seven Mi-1 homologs in susceptible tomato, Lycopersicon esculentum, on chromosome 6. These genes are organized in two clusters about 200 kilobases apart. The corresponding region of resistant tomato that was introgressed from the wild species Lycopersicon peruvianum also carries seven Mi-1 homologs, which are present in two gene clusters on chromosome 6. We have cloned each of these genes and determined their sequence and arrangement in the genome. Using DNA markers that flank the resistance genes, we have ascertained that a genetic inversion has occurred between the two clusters in cultivated tomato compared to wild tomato. This inversion may account for the severe repression in recombination that occurs in the Mi-1-region. We
have identified additional copies of Mi-1 homologs on a different chromosome in tomato. We have partially characterized about 70 homologous sequences from several other solanaceous species. These genes are located near disease resistance genes that function against other pathogens. We are focusing on genetic characterization of the homologs in the wild potato species Solanum bulbocastanum. Our analysis indicates that several of the Mi-1 homologs in tomato and its wild relative are expressed in leaves and roots and carry open reading frames that may code for unknown resistance functions.
Impacts
Root-knot nematodes cause serious damage to many crops worldwide. This research also addresses the evolution of host resistance to nematodes and lays the groundwork for development of strategies to generate novel resistance by molecular manipulation of the cloned genes. Our findings provide information that will be useful to breeders who are introducing new traits from wild species.
Publications
- No publications reported this period
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Progress 01/01/01 to 12/31/01
Outputs
The tomato gene Mi, which confers resistance to three of the most damaging species of root-knot nematodes as well as to some isolates of potato aphids was previously cloned in our laboratory. Southern blot analysis has indicated that there are 7 Mi-homologs in susceptible tomato, Lycopersicon esculentum, on chromosome 6. The corresponding region of resistant tomato was introgressed from the wild species Lycopersicon peruvianum also carries seven Mi homologs. We have cloned each of these genes and determined their arrangement in the genome. We have also sequenced the coding region of all 14 genes. In resistant tomato three members of the gene family carry open reading frames suggesting that they encode functional genes. In susceptible tomato, four of the family members may encode intact genes. Characterization of the organization of the family members in each parent indicates that gene conversion among family members has been frequent. Our studies indicate that there
are major differences in gene organisation between the two tomato species and we are working toward understanding how these differences may have occurred.
Impacts
Root-knot nematodes cause serious damage to many crops world wide. This research will enrich genetic maps of Solanaceous plants and provide markers for plant resistance breeding. The project also addresses the evolution of host resistance to nematodes and lays the groundwork for development of strategies to generate novel resistance by molecular manipulation of the cloned genes.
Publications
- No publications reported this period
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