Genetic variation for big-vein symptom expression and resistance to Mirafiori lettuce big-vein virus in Lactuca virosa L., a wild relative of cultivated lettuce
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Lactuca virosa L. is a wild relative of lettuce that is potentially an important source of resistance to big-vein disease, an economically damaging disease of lettuce. Identification of L. virosa accessions with resistance to Mirafiori lettuce big-vein virus (MLBVV), the disease causing agent, may be useful for lettuce breeding. The objectives of this research were to determine the genetic variation for big-vein symptom expression and MLBVV accumulation in diverse L. virosa accessions. Greenhouse testing was conducted to characterize variation for symptom expression 90–100 days after planting (DAP) with 70 L. virosa accessions in unreplicated experiments in 2001 and 2003, and with 10 accessions in an experiment with 3 replications conducted in 2004. In 2005, six replications of seven accessions were evaluated for the percentage of symptomatic plants 120 DAP and 180 DAP in a growth chamber experiment. Reverse transcription-polymerase chain reaction or nucleic acid spot hybridization was used to determine MLBVV presence or absence at each reading date. Genetic variation for symptom expression was confirmed among the L. virosa accessions, although the majority of tested accessions did not express big-vein symptoms. Symptomless infections were discovered, although accumulation of MLBVV to detectable levels appeared to be a slow process in L. virosa. Genetic variation for the incidence of MLBVV positive plants was identified within symptomless accessions, and suggests that symptom expression and MLBVV resistance may be independent factors contributing to big-vein resistance. Regardless, symptomless accessions with low MLBVV incidence were identified, and should be useful for breeding new big-vein resistant cultivars.
KeywordsLactuca sativa L. Breeding Compositae Disease resistance Virus resistance Ophiovirus MLBVV
This research was supported in part by the California Lettuce Research Board and the Arizona Iceberg Lettuce Research Council.
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