A polymerase chain reaction-based linkage map of broccoli and identification of quantitative trait loci associated with harvest date and head weight

A set of 216 polymerase chain reaction-based molecular markers was screened for polymorphisms using two morphologically dissimilar broccoli (Brassica oleracea L. ssp. italica Plenck) lines, 'VI-158' and "Brocolette Neri E. Cespuglio'. Fifty-nine of these simple sequence repeat (SSR) and sequence-related amplified polymorphic (SRAP) primer pairs generated 69 polymorphisms that were used to construct a linkage map of broccoli from a population of 162 F_2:3 families derived from the cross between these two lines. Ten linkage groups were generated that spanned a distance of 468 cM with an average interval width of 9.4 cM. The map was used to identify quantitative trait loci (QTL) associated with differences in harvest date maturity and head weight in the population grown in the same location over 2 years. Heritability estimates for days to maturity and head weight were 0.84 and 0.64, respectively. Four QTL for harvest maturity were identified that described 55.6% of the phenotypic variation in the first year with two of these QTL also detected in the second year of the experiment that described 29.2% of the phenotypic variation. Five QTL were identified as associated with head weight in 1999 and accounted for 71.8% of the phenotypic variability. Two of these QTL accounted for 24% of the phenotypic variability in head weight in 2000. To our knowledge, this is the first linkage map of broccoli and the first combined SSR and SRAP map of B. oleracea, which should provide a useful tool for the genetic analysis of traits specific to ssp. italica.