Lima Bean Research to Support Breeding for Lygus Tolerance.

In Lima bean (Phaseolus lunatus), lygus bug (Lygus hsesperus) can cause flower and young pod abortion as well as seed damage resulting in significant losses to California growers. The purpose of this research is to increase understanding of mechanisms involved in lygus tolerance in lima beans while simultaneously developing improved lygus tolerant cultivars and genetic tools for a crop which historically has had relatively little research investment. To do this UC-Davis is developing a genetic population and maps using agronomically valuable cultivars of Lima bean with different genetic and phenotypic profiles.  A population of 230 Recombinant Inbred Lines (RILs) has been made from reciprocal crosses between a lygus tolerant (UC Haskell) and susceptible (UC 92) parent. The lygus tolerant parent is a vine type, baby Lima bean from the Mesoamerican genepool and the susceptible parent is a bush large Lima from the Andean genepool. The RILs in this population segregate for traits including development, growth habit, seed size and pest resistance. Using next-generation DNA sequencing we developed a physical Single Nucleotide Polymorphism (SNP) map for Lima bean based on the two parents which currently has ~50,000 putative SNPs spread across all 11 chromosomes with at least 5x coverage. UC-Davis researchers will take advantage of the large amount of genetic variation from the two parents combined with the segregating phenotpyes in the RIL population to do Quantitative Trait Loci (QTL) mapping for lygus resistance in lima beans. RILs are being evaluated in the field under lygus pressure, with and without insecticide treatment for two seasons (currently in second season). An increase in lygus damage on seeds has been correlated with reduced yield within genotype. We anticipate lygus tolerance to be highly complex and involve multiple plant traits such as flowering time, plant architecture and secondary metabolite defense compounds.