ARS | Publication request: Molecular phylogeny of Daucus

Submitted to: Systematic Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 18, 2013
Publication Date: N/A

Interpretive Summary: Cultivated carrot is the most widely grown crop of the carrot family and is cultivated on 1.1 million hectares globally of which 35,000 hectares are US grown and worth $600M annually. Cultivated carrot was selected from the common weed, Queen Anne’s Lace, which is widely distributed across temperate regions of the globe. Like many vegetables, the time frame and geographic region(s) of the first cultivation of carrots are unclear. This study uses a very powerful technique for investigating crop origins, by the use of a large number of single-copy molecular DNA sequences; here with eight sequence regions from the nucleus and a single sequence from the chloroplast. In total, 7212 DNA sequence data points were generated. Analysis of these data provide very firm conclusions indicating that cultivated carrot is related to a group of wild carrot all possessing the same number of chromosomes (18), unlike all other wild carrots that possess 20, 22, or 44 chromosomes. The results also concur with prior studies showing that the genus Daucus, as currently classified, needs to be reevaluated, because it includes some other genera in the carrot family. This study is useful in documenting relationships that will for the basis of future studies using additional DNA markers and additional collections of Daucus and related genera, to guide breeders into possible wild species to be used on their breeding programs to improve cultivated carrot.

Technical Abstract: We studied the phylogeny of 22 accessions of Daucus: D. broteri, D. capillifolius, D. carota, D. carota subsp. carota, D. carota subsp. commutatus, D. carota subsp. commutatus, D. carota subsp. drepanensis, D. carota subsp. gadecaei, D. carota subsp. gummifer, D. carota subsp. halophilius, D. carota subsp. hispidus, D. carota subsp. maritimus, D. carota subsp. maximus, D. carota var. boissieri, D. crinitus, D. glochidiatus, D. guttatus, D. involucratus, D. littoralis, D. muricatus, D. pusillus, and D. sahariensis, and seven accessions of related species based on prior hypotheses or our observations of phenetic similarity: Astrodaucus littoralis, Caucalis platycarpos, Margottia gummifera, Orlaya daucoides, Pseudorlaya pumila, Torilis sp., and Turgenia latifolia. We used DNA sequences from eight nuclear orthologs and one plastid region, the trnH-psbA intergenic spacer. The aligned data matrix was 7212 characters long and provided high bootstrap support for many clades. Orlaya daucoides was supported as the closest outgroup to Daucus. Concordant with prior molecular results Pseudorlaya pumila was imbedded within Daucus, as was Margottia gummifera, a new finding. All accessions of D. carota, D. sahariensis, and D. capillifolius (all 2n = 18) formed a clade with 100% bootstrap support, with all other species within the Daucus clade with chromosome numbers of = 20, 22, and 44 (D. glochidiatus). Sister to the D. carota clade was a clade containing Margottia gummifera and Pseudorlaya pumila, sister to these species was D. crinitus, sister to all the above was D. muricatus, and sister to all of the above was a clade containing the remaining Daucus species. Our research, and parallel morphological studies, highlight the difficult taxonomy of this group, and pointed out misidentifications in germplasm collections caused by similarity of many of these species and the lack of a reliable comprehensive taxonomic monograph of Daucus.