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Spider Mite Resistance Mechanisms in Lycopersicon hirsutum Accession LA2329
Department of Horticulture
Insect damage is responsible for a significant loss of crop yield. Relatives of some crop plants possess genes that deter or resist insects. These genes can be used to improve crop productivity The purpose of this study is to provide a better understanding of how a wild relative of tomato resists a broad array of insects. The knowledge generated will lead to creation of crop plants that are insect resistant.
2009 Project Description
Activities during the past year have centered on characterizing inheritance and mode of action of spider mite resistance in a BC1F2 population of tomato of interspecific origin. Also considerable effort has been expended to advance generations through backcrossing, with the third backcross generation obtained for the other high priority populations. The progenitors of these populations are Lycopersicon esculentum and LA2329, an accession of L. hirsutum.
Unlike populations arising from other accessions of L. hirsutum, those arising from LA2329 have been relatively easy to cross. Prior work has indicated the presence of spider mite resistance that was under dominant control; frequency of resistant individuals was high in F1, F2 and BC1F1 populations.
A small BC1F2 population was evaluated for mite resistance. Leaves from each BC1F2 plant were evaluated in a whole leaf bioassay. One leaflet of each leaf was infested with mites, and then presence of mites and webbing on other leaflets on the leaf were determined 72 h after initial infestation.
Similar to previous results with this bioassay, extent of webbing was the most reliable data for differentiating resistance among these individuals. For the entire population average webbing was 3.5. Webbing ratings for individual plants ranged from 0.5 to 7, with a theoretical minimum of 0 and maximum of 7. When these same BC1F2 individuals were evaluated for repellency of spider mites, as determined by the thumbtack bioassay, nearly 80% of the individuals tested were no more repellent than the non-repellent check. However, more than 20% of the tested individuals were at least as repellent as the resistant parent, LA2329. Even after 72 h of bioassay, for one BC1F2 individual, an average of 7 out of 10 mites remained on the thumbtack, an indication of very high levels of repellency recovered in this hybrid. Previously, we have never recovered this level of repellency in an interspecific hybrid.
Causes of repellency in this BC1F2 population are under investigation. Initial evaluation of the chemical composition of trichome secretions provided little evidence that there was any significant differentiation of trichome secretion compositions among BC1F2 individuals. Causes of repellency remain under investigation. We are also in the process of advancing this particular generation, so that inheritance of resistance and repellency can be further evaluated. However, self seed set has been difficult to obtain.
The reported change in knowledge allows agricultural scientists to reevaluate the genetic potential of insect resistance in Lycopersicon hirsutum. Most of the insect resistance reported in this species has been associated with leaf trichomes. Trichome-based resistance, while effective, has been difficult to utilize by plant breeders. Also typically, trichome based resistance is not a dominant genetic character. Dominant characters are more easily utilized by plant breeders for crop improvement.