The Paraphyletic Origins of Genetic Resistance to Cabbage Stem Flea Beetle in Brassica oleracea

The cabbage stem flea beetle (CSFB) poses a growing threat to winter Brassica crops in Europe, yet the genetic basis of resistance remains poorly understood. To clarify the genetic architecture and evolutionary origins of resistance to CSFB adult feeding, we conducted a genome-wide association study (GWAS) by combining high-throughput pool-sequencing and a large-scale non-choice feeding assay on 113 Brassica oleracea accessions from wild (or feral) populations and major domesticated morphotypes. We demonstrate that resistance displays moderate heritability with a predominantly polygenic basis, revealing strong phenotypic divergence among morphotypes: B. oleracea var. capitata was generally susceptible, whereas var. botrytis and wild populations showed markedly higher resistance. Despite this polygenic background, we identified a major-effect candidate QTL on chromosome C01 with strong enrichment of resistance alleles in wild populations and susceptible alleles in var. capitata. Genome-wide F-statistics and heterozygosity scans revealed a recent selective sweep at this locus in wild lineages. Considering current evidence for the feral origin of contemporary "wild" populations, our results suggest that resistance evolved after domestication and subsequent feralization, independently of resistance in var. botrytis. This paraphyletic distribution underlines the critical importance of integrating demographic history into quantitative genetic analyses of domesticated plant systems.