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Megan Ulmer, Regina Ali, Conrad Bonsi, Louis Jackai and Bryon Sosinski

The sweetpotato weevil (SPW), Cylas formicarius, is the most devastating insect pest of sweetpotato worldwide. In the U.S., the devastation by this pest costs the sweetpotato industry several million dollars in crop loss and lost income each year. Many growers in highly infested areas have simply abandoned growing sweetpotatoes. The overall project goals are to elucidate the routes used for the spread of the SPW, and to determine the existence of intra-specific variation in the SPW population in the US and selected overseas countries. These results will lead to more effective and targeted management of the SPW. Results will also make quarantine enforcement more efficient. We are examining the highly conserved and phenotypically neutral rDNA sequences of both the 18S and ITS1 regions of the SPW genome as a way to determine the population structures and origins of SPW in the US. Here, the molecular genetic aspects of the project are outlined, and preliminary results are presented.

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Jim C. Cervantes-Flores, G. Craig Yencho, Kenneth V. Pecota, Bryon Sosinski and Robert O.M. Mwanga

Resistance to root-knot nematodes [Meloidogyne incognita (Kofoid & White) Chitwood] in sweetpotato [Ipomoea batatas (L.) Lam.] was studied in a mapping population consisting of 240 progeny derived from a cross between ‘Beauregard’, the predominant cultivar in the United States, and ‘Tanzania’, an African landrace. Quantitative trait loci (QTL) analyses to locate markers associated with resistance to root-knot nematodes (RKN) were performed using genetic maps based on parental segregation in ‘Beauregard’ and ‘Tanzania’ consisting of 726 and 947 single-dose amplified fragment length polymorphism (AFLP) markers, respectively. RKN resistance in the progeny was highly skewed with most of the progeny exhibiting medium to high levels of resistance. Single-point analysis of variance and interval mapping revealed seven consistently significant QTL in ‘Tanzania’ and two significant QTL in ‘Beauregard’. In ‘Tanzania’, three QTL were associated with reduction in resistance as measured by the number of RKN egg masses and explained ≈20% of the variation. Another four QTL had positive effects on resistance and explained ≈21% of the variation. Other minor QTL explained ≈2% or less of the variation but were not always consistent across geographical locations. In ‘Beauregard’, two QTL had positive effects on RKN resistance and explained ≈6% of the observed variation. Based on molecular and phenotypic data, RKN resistance in sweetpotato is hypothesized to be conferred by several genes, but at least nine AFLP markers (seven from ‘Tanzania’ and two from ‘Beauregard’) are associated with genomic regions that have the biggest effect in the number of egg masses of RKN produced in the root system.

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Bryon Sosinski, W.V. Baird, S. Rajapakse, R.E. Ballard and A.G. Abbott

We have developed a highly saturated genetic linkage map in peach (diploid, 2n = 16) using two separate crosses. The first population consists of 48 randomly selected F2 individuals which were generated by selfing an F1 from the cross of `New Jersey Pillar' x KV 77119. This progeny set exhibits segregation for gross morphological traits including: canopy shape, fruit flesh color, and flower petal color, size, and number. The second population contains 48 F2 progeny derived from the cross of `Suncrest' x `Bailey'. These progeny segregate for quality traits such as fruit diameter, weight, flesh color, cling vs. free stone, soluble solids, pH of juice extract, and fruit developmental period. Nine linkage groups were identified in the first cross, which cover 590 cM of the genome. In the second cross, eight linkage groups were found that contain several significant chromosomal intervals contributing to fruit quality characteristics by QTL analysis. Anchor loci present in both maps were used to join the linkage groups to create a single combined map of the peach genome. Physical mapping is currently underway to assign the each linkage group to the appropriate chromosome.