Nematode resistance in present day cultivars has been derived from one Lycopersicon peruvianum source, PI 128657. This gene (Mi) was introgressed by embryo culture in the early 1940's. In 1986, Ammati et al. reported four L. peruvianum accessions (PI 126443, PI 129152, PI 270435, and LA 2157) were resistant to nematodes under high soil temp. (32 C) conditions under which the Mi resistance was ineffective. In 1988, we crossed PI 126443 and PI 129152 with 10 tomato lines and obtained 536 fruit. Fruit were harvested between 15 and 65 days after anthesis for embryo rescue. There were 140 embryos plated (26% of fruit #) and 36 plants were obtained (7% of fruit #, 26% of the embryos plated). 'Campbell 28', 'Fla. 7217', and 'Fla. 7182' were the most efficient tomato genotypes for producing F1 plants. There was no difference in F1 plant production efficiency between the two L. peruvianum accessions. No age interval at harvest could be singled out as superior for embryo or plant production, but no plants were obtained from fruit greater than 57 days old. Thirteen percent of the fruit had culturable embryos and many of these fruit had more than 1 embryo plated. For most potential seeds, it appeared that either fertilization failed or embryo abortion occurred very early after anthesis, thus there was no benefit to harvesting fruit at an early (15-30 day) stage.
The first backcross (BC1) was done in the fall under higher temperatures which possibly were less favorable to embryo formation. Of 248 fruit, 94 embryos were plated (40% of fruit #), and 15 plants were obtained (6% of fruit #, 17% of embryos plated). It cannot be concluded that embryo rescue is more efficient in the backcross generation. The F1 pollen source may not have been critical but 'Campbell 28' and 'Suncoast' F1's had the greatest percentage of plants per fruit crossed, 10 and 8% respectively. Pollen from 'Walter' and Fla. 7182 F1's had plants result from only 3 and 2% of the fruit, respectively. The female parents with the best percentage of BC1 plants per number of fruit were Fla. 7217, Fla. 7215, and 'Campbell 28' with 15, 8, and 7%, respectively. No plants were obtained from 45 fruit of Fla. 7182. As with the F1, many fruit had more than 1 embryo per fruit. If one could identify something unique about these fruit (position on truss, time of pollination, etc.) then rescue efficiencies might be improved.
In a field test under high temperatures, the galling on the two L. peruvianum accessions and their F11's were not different from each other and were significantly less than the galling of both susceptible and Mi resistant genotypes. This indicated resistance was due to dominant gene action. In a seedling test under high (>28 deg.F) and low (<28 deg.F) soil temperatures, galling was significantly less on the two peruvianum accessions and the 3 homozygous Mi lines than the susceptible 'Horizon'. Only the Mi/+ hybrid had significantly greater galling under high temperatures, but it still had significantly less galling than 'Horizon'. In each of two seasons, data from segregating F1BC1S3 and F1BC1S4 lines derived from each L. peruvianum source were pooled to obtain enough plants for Chi square tests. In both seasons, the data from lines derived from PI 126443 fit the 3:1 ratio of resistant to susceptible plants expected for control of resistance by a single dominant gene. For PI 129152, the 3 resistant to 1 susceptible ratio fit in the spring but not in the fall. In the fall, there was an excess of susceptible plants even though the fall lines were derived from the spring lines. Still, the conclusion from these preliminary data is that resistance is controlled by single dominant genes for each PI source. More definitive work is underway to verify the inheritance and determine allelic relationships between the genes from each L. peruvianum and between these genes and Mi. Due to low fertility, seed has been hard to obtain for many of the introgressed lines but it will be made available to those with interest.
Literature cited:
Ammati, M. et al. 1986. J. of Nematology 18(4):491-495.