Biometrical studies and quantitative trait loci associated with major products of the carotenoid pathway of carrot (Daucus carota L.)

Orange carrots are a top ranked vegetable in terms of pro-vitamin A content. Carotenoid pathway products were investigated in F2 populations from two different carrot crosses: orange Brasilia x dark orange HCM and orange B493 x white wild QAL. Broad sense heritabilities values for all carotenoids were greater than 90% in the B493 x QAL cross and from 35% to 70% among different carotenoids in the Brasilia x HCM cross. The estimated number of factors was 4 for α-carotene, 3 for β-carotene and total carotenes and one for ζ-carotene, lycopene and phytoene in the orange x dark orange cross, and 4 for α-carotene, 1-2 for lycopene and total carotenes and 1 for the other carotenes in the orange x white cross. In comparison to the known biochemical pathways the correct order of substrates and products, phytoene→ζ-carotene→lycopene, was identified in the path analysis of β-carotene in the cross Brasilia x HCM but not in the correlation analysis. Linkage grouping analysis assigned 287 and 250 scored molecular markers to the nine chromosomes of carrots, at LOD scores ranging from 3.0 to 7.0 and the average marker spacing was 4.78, 4.80, 5.54 and 5.13 cM in ii the Brasilia-, HCM-, B493 and QAL-coupling phase maps, respectively. Interval mapping performed with the orange x dark orange cross detected four, eight, three, one, five and three putative QTL associated with accumulation of ζ-carotene, α-carotene, β-carotene, lycopene, phytoene and total carotenoids, respectively, with major QTL explaining from 10.2 to 13.0% of total phenotypic variation. In the B493 x QAL population single marker analysis identified loci explaining 13.8%, 6.8%, 19.3%, 5.7%, 17.5% and 20.2% of total phenotypic variation for ζ-carotene, α-carotene, β-carotene, lycopene, phytoene and total carotenoids content, respectively. Overall analysis showed clustered loci affecting the phenotypic variation of carotenoid pathway suggesting clusters of related-pathway loci as an evolutionary mechanism and supporting an adaptive evolutionary model suggested by H. A. Orr. Path analysis and QTL studies suggested that phytoene biosynthesis, perhaps associated with a root specific signal, are the two key factors limiting the carotenoid pathway in roots of white carrots.