Merging carrot linkage groups based on conserved dominant AFLP markers in F2 populations

Markers were placed on linkage groups, ordered, and merged for two unrelated F2 populations of carrot (Daucus carota L.). Included were 277 and 242 dominant Amplified fragment-length polymorphism (AFLP) markers and 10 and eight codominant markers assigned to the nine linkage groups of Brasilia x HCM and B493 x QAL F2 populations, respectively. The merged linkage groups were based on two codominant markers and 28 conserved dominant AFLP markers (based upon sequence and size) shared by both populations. The average marker spacing was 4.8 to 5.5 cM in the four parental coupling phase maps. The average marker spacing in the six merged linkage groups was 3.75 cM with maximum gaps among linkage groups ranging from 8.0 to 19.8 cM. Gaps of a similar size were observed with the linkage coupling phase maps of the parents, indicating that linkage group integration did not double the bias which comes with repulsion phase mapping. Three out of nine linkage groups of carrot were not merged due to the absence of common markers. The six merged linkage groups incorporated similar numbers of AFLP fragments from the four parents, further indicating no significant increase in bias expected with repulsion phase linkage. While other studies have merged linkage maps with shared AFLPs of similar size, this is the first report to use shared AFLPs with highly conserved sequence to merge linkage maps in carrot. The genome coverage in this study is suitable to apply quantitative trait locus analysis and to construct a cross-validated consensus map of carrot, which is an important step toward an integrated map of carrot.