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Arthur Q. Villordon and Don R. LaBonte

Our research examined whether plants originating from adventitious sprouts from fleshy sweetpotato roots are genetically more variable than plants that arise from pre-existing meristematic regions, i.e., nodes. Our study compared one plant each of `Jewel', `Sumor', and L87-95 clonally propagated for seven generations both nodally and through adventitious sprouts. PCR-based analysis of 60 samples (10 nodal and 10 adventitiously derived plants/genotype) showed 20% polymorphism among adventitious materials vs. 6% among nodally derived plants. An “analysis of molecular variance” showed that differences between propagation methods accounted for 30% of the total marker variability. Our results support previous findings that, relative to non-meristematic materials, meristematic regions strictly control cell division and DNA synthesis that exclude DNA duplication and other irregularities.

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Durel J. Romaine and Don R. LaBonte

Narrow-sense heritability (h2) estimates for sugars were determined to assess the feasibility of breeding for a sweeter baked sweetpotato. Roots of parents and half-sib progeny were baked (190°C for 75 minutes) 16 weeks after harvest. Sugars from 10 gram root samples were extracted in ethanol for HPLC sugar quantification. Alcohol insoluble solid (AIS) residues (starch) were also measured from the samples. Dry matter was determined on a separate 10-g sample. Narrow-sense heritability estimates based on variance components analysis for AIS and percent dry matter were 0.20 and 0.32, respectively. Estimates for sugar data were 0.05 for sucrose, 0.52 for maltose, and 0.52 for total sugars (fructose, glucose, sucrose and maltose). These heritability estimates for maltose and total sugars imply a breeder could expect a moderate gain in sweetness over several cycles of selection.

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Arthur Q. Villordon and Don R. LaBonte

Polymorphism analysis and yield tests were conducted among `Jewel' sweetpotato clones [Ipomoea batatas (L.) Lam] obtained from eight state foundation seed programs. Initially, 38 arbitrary primers generated a total of 110 scorable DNA fragments in a sample of virus-indexed plants from each clone source. The number of marker loci scored for each primer varied from one to eight with an average of 2.89. Twenty-one bands (19.1%) were scored as putative polymorphic markers based on the presence or absence of amplified products. Further estimation of variability within each clone source was accomplished by an assay of 10 sample plants per clone group by 14 marker loci generated by four selected primers. Polymorphic bands ranged from 7.1% to 35.7 % in five of eight clone groups. Field studies show variation in nearly all yield grades measured. In three tests during the 1991 and 1992 seasons, yield differences ranged from 27% to 46% within the economically important U.S. no. 1 root grade. The results suggest the usefulness of arbitrarily-primed markers in detecting intra-clonal sweetpotato DNA polymorphisms and indicate an underlying genetic cause for phenotypic variability in the crop.

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Cecilia E. McGregor and Don R. LaBonte

`White Jewel' is a yellow-and-orange fleshed spontaneous mutant of the orange-flesh sweetpotato [Ipomoea batatas (L.) Lam.] cultivar Jewel. Mutations in storage root flesh color, and other traits are common in sweetpotato. The orange flesh color of sweetpotato is due to β-carotene stored in chromoplasts of root cells. β-carotene is important because of its role in human health. In an effort to elucidate biosynthesis and storage of β-carotene in sweetpotato roots, microarray analysis was used to investigate genes differentially expressed between `White Jewel' and `Jewel' storage roots. β-carotene content calculated from a* color values of `Jewel' and `White Jewel' were 20.66 mg/100 g fresh weight (FW) and 1.68 mg/100 g FW, respectively. Isopentenyl diphosphate isomerase (IPI) was down-regulated in `White Jewel', but farnesyl-diphosphate synthase (FPPS), geranylgeranyl diphosphate synthase (GGPS), and lycopene β-cyclase (LCY-b) were not differentially expressed. Several genes associated with chloroplasts were differentially expressed, indicating probable differences in chromoplast development of `White Jewel' and `Jewel'. Sucrose Synthase was down-regulated in `White Jewel' and fructose and glucose levels in `White Jewel' were lower than in `Jewel' while sucrose levels were higher in `White Jewel'. No differences were observed between dry weight or alcohol insoluble solids of the two cultivars. This study represents the first effort to elucidate β-carotene synthesis and storage in sweetpotato through large-scale gene expression analysis.

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Arthur Q. Villordon and Don R. LaBonte

Genetic uniformity was assessed among sweetpotato (Ipomoea batatas) clones propagated through adventitious and nodal procedures. A single sprout each of `Jewel,' `Sumor,' and L87-95 was used as source of clonal plants that were simultaneously propagated through conventional adventitious procedures and a tissue culture-based nodal culture technique. A sample of 15 decamer primers generated 64 scorable amplified fragments in a PCR-based assay, 29 of which were putatively polymorphic across n = 60 samples (10 each of nodal and adventitiously derived plants/genotype). Within adventitiously derived materials, putative polymorphisms ranged from 4.7% to 31.3% depending on the genotypic class. In contrast, putative polymorphisms ranged from 0.0% to 3.1% among nodally derived samples. Marker loci differentiated genotypes as well as putative marker phenotype variants through a multidimensional scaling analysis of the genetic similarity matrix. An `analysis of molecular variance' shows that genotypic effects accounted for 88.7% of the total molecular marker variability, while propagation effects (within genotypic groups) accounted for 11.3%. Results confirm that clonal plants derived from preexisting meristematic regions are more genetically uniform than plants propagated from adventitious origins.

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Don R. LaBonte and David H. Picha

Six sweetpotato cultivars were evaluated for changes in individual sugar concentration, dry weight, and alcohol insoluble solids (AIS) during growth and development. Measurements were taken at weekly intervals from 7 to 21 weeks after transplanting. Sucrose, the major sugar during all stages of development, generally increased in concentration throughout development for `Heart-o-gold', `Travis', and `Jewel', but peaked at 17 weeks for `Beauregard' and `Whitestar'. The high-dry matter white flesh cultivars of `Rojo Blanco' and `Whitestar' contained the lowest sucrose concentration. The monosaccharides glucose and fructose generally decreased in concentration up to 17 weeks in 4 of 6 cultivars, followed by an increase from 17 to 21 weeks in all cultivars. Glucose concentration was marginally greater than fructose at all stages of development in each cultivar. Little or no increase in total sugar concentration occurred during development in `Whitestar' and `Rojo Blanco'. A substantial increase in total sugars occurred during development with `Jewel', `Beauregard', `Heart-o-gold' and `Travis'. Cultivars differed widely in their individual sugar concentrations during development. Percent dry matter increased in all cultivars from 7 to 14 weeks. Dry matter and AIS decreased during the later stages of development.

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Arthur Q. Villordon and Don R. LaBonte

Clonal propagation assures the maintenance of genetic purity of a sweetpotato variety. The existence of foundation seed programs further contributes to the conservation of favorable genetic constitution in a commercial cultivar. However, the improvement of current maintenance procedures is necessary as shown by the occurrence of mutations and the decline of certain commercial varieties. Information on the nature and extent of changes in sweetpotato would therefore be useful in this regard.

`Jewel' clones obtained from eight state foundation seed programs were subjected to yield tests and a RAPD-based assay. Differences in nearly all yield grades were detected during the 1991, 1992, and 1993 seasons. The yield of U.S. No. 1 grade roots varied from 27% to 46%. The quality factors measured also varied: % alcohol insoluble solids varied by 13%, while sucrose ranged from 9.6% to 19%. Total DNA was extracted from each clone and assayed against 40 primers. All primers produced amplified fragments. A total of 110 reproducible bands was generated by 38 primers. Putative polymorphic markers were scored in 21 (18.58%) of these bands based on the presence or absence of amplified products. The results suggest an underlying cause for the variability observed in phenotypic traits within sweetpotato clones.

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Durel J. Romaine and Don R. LaBonte

Seven compositionally diverse sweetpotato lines were examined for changes in individual sugar concentrations at harvest (green), after curing (7 days at 90% RH and 29.5C), and after 4 and 8 weeks of cold storage (16C) to determine the relationship between raw and cooked root sugar composition. Raw root sucrose concentrations at harvest in two dessert types, `L91-80' and `Heart-O-Gold', were at least 22% higher than other dessert types, such as `Beauregard' and `Jewel', and 26% higher than white starchy types (`Rojo Blanca' and `White Star'). The sucrose concentration remained correspondingly higher for these two lines when baked or microwaved. Total sugar concentration was not significantly correlated between raw vs. baked or microwaved roots. The major sugar in most baked and microwaved roots was maltose, accounting for 18% to 93% of the total sugars. `L91-80' behaved differently from other lines during microwaving, where sucrose was the major sugar. The total sugar concentration of `L91-80' and `Heart-O-Gold' were not statistically greater after baking and microwaving for all dates, including the white, starchy types. These results suggest the need to further evaluate the relative importance of individual sugar concentrations on consumer preference.

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Mario I. Buteler, Don R. LaBonte and Robert L. Jarret

Microsatellites or simple sequence repeats (SSRs) were used to characterize 20 sweetpotato genotypes and to assign paternity for offspring from crosses among them. The PCR amplifications were performed with each of the sweetpotato genotypes and primers flanking a SSR loci previously characterized with the varieties Beauregard and Excel and 20 offspring from a cross among them. The PCR reaction products were separated in nondenaturing 12% acrylamide gels run at 25 V·cm–1 for 5 hours, and DNA fragments were visualized with silver staining. Gels were scanned on a flat bed scanner and analyzed using the Pro-RFLP software package. Three primer pairs were sufficient to produce an allelic profile capable of differentiating the 20 genotypes from each other. More than seven alleles/loci were found using each of the three primer pairs assayed. Occasionally primers produced allelic products clearly localized in two or three regions of the gel. These multiple loci segregated independently in a diploid fashion. This evidence suggests that there is not total homology among the three sweetpotato genomes.

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Mario I. Buteler, Don R. LaBonte and James H. Oard

RAPD and the single dose polymorphic band (SDPB) are powerful tools for genome map construction of higher polyploids, such as hexaploid sweetpotato. Duplication in the genome of higher polyploids results in fewer polymorphisms per primer screened than one would expect in diploids. The Stoffel fragment (Sf) is suggested as an alternative to the most commonly used Taq DNA polymerase to maximize the number of polymorphisms. Genomic DNA from two sweetpotato varieties, `Excel' and `Beauregard', and F1 progeny was isolated using a modified CTAB procedure. The DNA was assayed with twelve primers from Operon Technologies groups A and F. Each enzyme was tested with and without a ramp temperature treatment between the annealing and the extension temperatures. Results are based on three separate amplifications and electrophoretic runs. Band reproducibility was better using Sf than Taq; unfortunately, resolution was lower making bands difficult to score. 8.4% more scorable bands and 20.3% more storable polymorphisms were obtained with Taq. The ramp treatment did not alter results using Sf, but did improve the reproducibility of Taq and ease scoring. The number of bands and their location were the same.