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Kittipat Ukoskit and Paul G. Thompson

Low-density randomly amplified polymorphic DNA (RAPD) markers of sweetpotato [Ipomoea batatus (L.) Lam.; 2n = 6x = 90] were constructed from 76 pseudotestcross progenies obtained from `Vardaman' × `Regal'. Of 460 primers, 84 generating 196 well-resolved repeatable markers were selected for genetic analysis. `Vardaman' and `Regal' testcross progenies were analyzed for segregation and linkages of RAPD markers. Type of polyploidy, autopolyploidy, or allopolyploidy is uncertain in sweetpotato and was examined in this study using the ratio of nonsimplex to simplex RAPD markers and the ratio of simplex RAPD marker pairs linked in repulsion to coupling. Both measures indicated autopolyploidy. Low-density RAPD linkage maps of `Vardaman' and `Regal' were constructed from simplex RAPD marker linkage analysis. Duplex and triplex markers were then mapped manually into the simplex marker map. Homologous linkage groups were identified using nonsimplex RAPD markers and three homologous groups were found in each of the parent maps. Use of nonsimplex markers increased mapping efficiency. The `Vardaman' map had a predicted coverage of 10.5% at a 25-cM interval of the genome size of 5024 cM. In `Regal', genome coverage was estimated to be 5.6% at a 25-cM interval of the genome size of 6560 cM. Therefore, average chromosome length was ≈56 to 73 cM.

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Liang L. Hong, Kittipat Ukoskit, and Paul G. Thompson

Parents and progeny of four biparental crosses were analyzed for RAPD marker segregation. A range of 57 to 122 primers were tested in each cross, with an average of 82. Average polymorphic primers and band numbers were 22 and 53, respectively. Of the 212 polymorphic bands, phenotypic segregation ratios were as follows: 133 fitted 1 dominant: 1 recessive, 58 fitted 3:1, 11 fitted ratios 4:1 to 19:1 and 10 were distorted. The 1:1 and 3:1 ratios were expected for either diploid or hexaploid segregation, and the 4:1 to 19:1 are exclusive to hexploid. A total of 14 pairs of markers were linked at map distances ranging from 2.1 to 36.5 cM. One common pair of linked markers was found in two separate crosses.

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Paul. G. Thompson, Liang L. Hong, Kittipat Ukoskit, and Zhiqiang Zhu

RAPD marker analyses were completed on parents and progeny of two sweetpotato [Ipomoea batatas (L.) Lam.] crosses to determine the feasibility of genetic linkage map construction. A total of 100 primers was tested and 96 produced amplified genomic DNA fragments. The average number of polymorphisms per primer was 0.69. A total of 134 polyphorphic markers was observed and 74 (60%) segregated 1 band present : 1 band absent as needed for use in genetic linkage mapping of polyploids. The 60% of RAPD markers that segregated 1:1 shows that genetic linkage mapping of the hexaploid sweetpotato by RAPD marker analysis is feasible. Linkage was determined for all markers that segregated 1:1 and five pairs of linked markers were found. These were the first linked molecular markers found in sweetpotato and they show that construction of a genetic linkage map is feasible. A genetic linkage map will be a valuable tool to assist in genetic improvements.

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Kittipat Ukoskit, Paul G. Thompson, Gary W. Lawrence, and Clarence E. Watson

The inheritance of root-knot nematode race 3 [Meloidogyne incognita (Kofoid & White) Chitwood] resistance was studied in 71 progenies of the F1 backcross population produced from the resistant parent `Regal' and the susceptible parent `Vardaman'. The distribution frequency of the progenies measured on total nematode number (eggs + juveniles) indicated a bimodal distribution with a ratio of 4 resistant: 1 susceptible. Based on this phenotypic ratio, the proposed genetic model was duplex polysomic inheritance (RRrrrr = resistant). Bulk segregant analysis in conjunction with the RAPD technique was employed to identify RAPD marker linked to the root knot nematode-resistant gene. Nine of 760 random decamer primers screened showed polymorphic bands. Primer OPI51500 produced a band in the resistant bulk, but not in the susceptible bulk. Estimated recombination frequency of 0.24 between the OPI51500 marker and the root-knot nematode-resistant gene indicated linkage.