Three orange-mesocarp derivatives of the xishuangbannan cucumber (Cucumis sativus L. var. xishuangbannanesis Qi et Yuan), P100, P101, and P104; and NPI (P105), an unrelated cucumber (Cucumis sativus L.) reported to have orange flesh, were selected as parents for a diallel experiment to evaluate inheritance of orange cucumber mesocarp pigment over 3 years. Visual color intensity and carotenoid content were closely related. A preponderance of additive genetic effects for cucumber mesocarp pigmentation was observed in grade size 2 fruit (immature fruit used for pickling). Both additive and nonadditive genetic effects were important in grade size 4 fruit (mature). Years and yea× genotype interactions were highly significant for pigmentation of size 2 fruit, indicating the importance of environment on the expression of pigmentation in this size class. In contrast, color development was stable among years for size 4 fruit. P104 exhibited high general combining ability (GCA) estimates for size 4 fruit pigmentation across years, while P101 had high GCA estimates for size 2 fruit. The diallel analysis illustrated high fruit carotene content of parents per se. However, most hybrid combinations of the diallel reduced carotenoid content relative to parents, indicating both dominance for low carotenoid content for both fruit sizes and lack of genetic complementation among parents to enhance fruit color. Genetic control of pigmentation in size 2 fruit appeared to be independent of that for size 4 fruit.
Diverse carrot (Daucus carota L.) inbreds were evaluated as young plants in the greenhouse and mature plants in the field for resistance to the pathogen Alternaria dauci (Kühn) Groves and Skolko. Persistence of leaves after infection was the criterion used to estimate disease damage by the pathogen. Partial resistance was identified. A five-parent diallel evaluated for resistance indicated a preponderance of additive variation with some dominant gene action for resistance. Reciprocal cross differences were significant for certain crosses. Responses of most inbreds were useful in predicting resistance in their hybrids, but exceptions to this trend did occur. Field resistance ratings generally correlated well with resistance ratings obtained in greenhouse, even in grower fields where fungicides were applied.
Carrot tissue cultures, germinating seed, and dry seed were exposed to gamma radiation. Irradiation accelerated germination of carrot seed in the M1 generation at low doses (0.5 and 1 krad), whereas higher doses delayed germination. A high negative correlation was observed between dose and survival of plants after seed irradiation. Plant size and root weight were 20 % to 35% greater than control plants after seeds, but not tissue cultures, were exposed to low doses of gamma irradiation. Higher doses reduced M1 plant size by >50% in germinating seed and tissue culture treatments but less for the dry seed treatment. Seed production decreased while phenotypic variation of M1 plants increased with increasing gamma ray dosage. Root weight and total dissolved solids were highly variable in M2 families. Less variation was observed in total carotene content and none was seen in sugar type (reducing vs. nonreducing sugars). Induced variation in root color and root shape was also observed. Irradiation of germinating seed and tissue cultures yielded more M2 variation than irradiation of dry seed. Putative point mutations were not observed. Unirradiated carrot tissue cultures did not yield significant M2 somaclonal variation. Average root weight of M2 plants increased with increasing gamma ray dosage, especially for the dry seed treatment.
Current classifications of the genus Daucus are based on morphological and anatomical characteristics. We have used single to low copy nuclear restriction fragment length polymorphisms (nRFLPs) to describe the phylogeny and relationships of eight Daucus species including cultivated carrot (D. carota L.). Parsimony analysis of 247 characters (DNA fragments from 58 probe-enzyme combinations) yielded a tree in which accessions were grouped into three major clades and phenetic analysis using Jaccard's coefficient yielded two major clusters. The phylogenetic relationships from the nuclear RFLP data generally agreed with an earlier morphological classification. Resolution and placement of D. guttatus and D. muricatus were not consistent with the morphological classifications. Molecular variation among carrot inbreds was large.
An anatomical comparison of florets from fertile, brown anther partially fertile, petaloid male-sterile, and 3 brown anther male-sterile carrot (Daucus carota L.) lines indicated that most pollen was well-developed and deeply stained in fertile lines. Partially fertile plants contained fertile and sterile anthers in which meiosis had occurred. The 3 lines of brown anther steriles examined failed to enter meiosis. In early development of the fine foliage brown anther sterile, all cells of the anther, including sporogenous cells, hypertrophied. Later, sporogenous tissue collapsed. The anatomical structure of the stamens in petaloid florets was leaf-like and similar to petals.
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 × HCM and B493 × 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.
Garlic (Allium sativum L.) is an asexually propagated crop that displays much morphological diversity. Studies which have assessed garlic diversity with isozymes and randomly amplified polymorphic DNA (RAPD) markers generally agreed with the morphological observations but sometimes failed to discriminate clones. To discriminate among closely related garlic clones in more detail, we introduced amplified fragment-length polymorphism (AFLPs) to evaluate the genetic diversity and phenetic relatedness of 45 garlic clones and three A. longicuspis clones and we compared AFLP results with RAPD markers and isozymes. Three AFLP primer combinations generated a total of 183 polymorphic fragments. Although similarities between the clusters were low (≥0.30), some clones within the clusters were very similar (>0.95) with AFLP analysis. Sixteen clones represented only six different banding patterns, within which they shared 100% polymorphic AFLPs and RAPD markers, and likely are duplicates. In agreement with the results of other investigators, A. longicuspis and A. sativum clones were clustered together with no clear separation, suggesting these species are not genetically or specifically distinct. The topology of AFLP, RAPD, and isozyme dendrograms were similar, but RAPD and isozyme dendrograms reflected less and much less polymorphism, respectively. Comparison of unweighted pair group method with arithmetic averaging (UPGMA) dendrograms of AFLP, RAPD, and isozyme cluster analyses using the Mantel test indicated a correlation of 0.96, 0.55, and 0.57 between AFLP and RAPD, AFLP and isozyme, and RAPD and isozyme, respectively. Polymorphic AFLPs are abundant in garlic and demonstrated genetic diversity among closely related clones which could not be differentiated with RAPD markers and isozymes. Therefore, AFLP is an additional tool for fingerprinting and detailed assessment of genetic relationships in garlic.
Five cycles of phenotypic recurrent selection for total dissolved solids and sugar type (reducing vs. nonreducing) were performed on four carrot (Daucus carota L.) populations of common background. The populations contained high or low percentage of total dissolved solids (HTDS and LTDS, respectively) with high or low levels of reducing sugar (HRS and LRS, respectively). Effective selection for total dissolved solids (TDS) and sugar type was indicated by significant gains over five cycles of selection. TDS decreased in LTDS/HRS and LTDS/LRS populations by 21.9% and 15.9%, respectively. Corresponding increases of 22.4% and 28.2% were observed in HTDS/HRS and HTDS/LRS populations. Mean reducing sugar levels in HRS roots after five cycles of selection were limited to 2.0% of root fresh weight; sucrose was the primary storage carbohydrate. Reducing sugars were not detected in LRS roots. Mean total sugar levels in the HTDS and LTDS populations were 7.1% and 3.1% of root fresh weight, respectively. Realized heritability estimates ranged from 0.40 to 0.45 for the four populations. The onset of flowering was markedly delayed in plants of the two HTDS populations after five cycles of selection.
Carotenoids are isoprenoid compounds synthesized in plants that serve as photoprotectants essential for photosynthesis and provide plant tissues with red, orange, and yellow pigmentation. These compounds are important in human health, because they serve as both vitamin A precursors as well as having antioxidant properties. Carrot (Daucus carota ssp. sativus) provides an important source of carotenoids in the human diet, providing up to 30% of provitamin A in the United States. Although essential to human health, very little is currently understood about the accumulation of carotenoids in carrot. To better understand the molecular mechanism for carotenoid accumulation in carrot, we used reverse-transcription quantitative polymerase chain reaction (PCR) to evaluate the expression of nine genes in the carotenoid biosynthetic pathway in storage root tissue. No significant difference was found among white, yellow, orange, and dark orange carrot roots in seven of the nine genes evaluated. However, increased phytoene synthase 1 (PSY1) and phytoene synthase 2 (PSY2) expression was observed in orange and dark orange carrot roots compared with yellow and white carrots. Increased PSY1 and PSY2 expression was not observed in the leaf tissue of these genotypes, indicating a different mechanism for carotenoid accumulation in the leaf tissue of carrot. This study is the first to demonstrate that naturally occurring mutations that dramatically increase carotenoid accumulation in orange carrot are associated with increased PSY1 and PSY2 expression and it provides insights into the mechanism underlying the biosynthesis of these important photoprotectants and nutrients.
A sample of 124 Daucus carota L. accessions, including cultivated carrot [D. carota ssp. sativus (Hoffm.) Arcangeli] and related wild subspecies, using a variety of molecular markers was examined. Represented within the samples were wild accessions from 18 countries, 14 of 16 major root types of European origin, and examples of major North American and Asian cultivated carrot types. Amplified fragment length polymorphism (AFLP) and inter-simple sequence repeat (ISSR) markers revealed extensive variation within D. carota. Although cultivated carrot and wild D. carota subspecies can cross freely, cultivated and wild carrots clustered separately, supporting the possibility that human selection for desirable horticultural traits has artificially reduced gene flow between cultivated and wild forms. Our analyses support the likelihood that North American D. carota populations arose due to introduction of weedy materials rather than escape of cultivated forms. With the exception of wild vs. cultivated types, no genetic alliances were evident in dendrogram topology. Furthermore, between and even within nonmapped marker classes, dendrogram topology predictions were not consistent. Generally poor correlations among root types, geographic origin, mitochondrial, plastid, and specific nuclear diversity and AFLP/ISSR data were also observed. We concluded that genetic diversity in carrot is extensive and relatively nonstructured in nature.