Claude E. Thomas

Genetic diversity among watermelon (Citrullus lanatus and Citrullus colocynthis) accessions

Genetic diversity was estimated among 42 U.S. PlantIntroduction (PI) accessions of the genusCitrullus (of these, 34 PIs are reported tohave disease resistance), and 5 watermelon cultivars, using 30RAPD primers. These primers produced 662 RAPD markers that could berated with high confidence. Based on these markers, geneticsimilarity coefficients were calculated and a dendrogram wasconstructed using the unweighted pair-group method witharithmetic average (UPGMA). The analysis delineated threemajor clusters. The first cluster consisted of a group of fivewatermelon cultivars, a group of C.lanatus var. lanatusaccessions, and a group of C.lanatus var. lanatusaccessions that contained some C.lanatus var. citroidesgenes. The second cluster consisted of the C.lanatus var. citroidesaccessions, while the third cluster consisted of theC. colocynthis accessions.The two C. lanatus clustersdifferentiated from each other and from the C.colocynthis cluster at the level of 58.8%and 38.9% genetic similarity, respectively. Assessment ofgenetic diversity among accessions that have been reported to havedisease resistance indicated that resistance to either anthracnose,downy mildew, powdery mildew, or watermelon mosaic virus is foundamong all major groups of Citrullus PIs.Additionally, resistance to gummy stem blight or Fusarium wilt mayexist among C. lanatus var.citroides PIs. This study demonstrates thatmolecular markers can be useful in assessing genetic diversity, andin sorting Citrullus PIs into phylogeneticgroups prior to their evaluation for disease or pestresistance.

A genetic map of melon (Cucumis melo L.) based on amplified fragment length polymorphism (AFLP) markers

Abstractu2002Genetic maps facilitate the study of genome structure and evolution, and the identification of monogenic traits or Mendelian components of quantitative traits. We evaluated 228 RAPD, microsatellite and AFLP markers for linkage analysis in melon (Cucumis melo L.) varieties MR-1 (resistant to Fusarium wilt, powdery and downy mildews) and Ananas Yokneum (AY; susceptible to these diseases) and constructed a detailed genetic map. The mapping population consisted of 66 backcross progenies derived from AY×(MR-1×AY). Despite a relatively low level of polymorphism in the species, AFLP markers were found to be more efficient in mapping the melon genome than RAPD or microsatellite markers. The map contains 197 AFLPs, six RAPDs and one microsatellite marker assigned to 14 major and six minor linkage groups, and covers 1942u2005cM with the average distance between adjacent markers of approximately 10u2005cM. The maximum distance allowed between markers is 27.5u2005cM. About 11% of the intervals (20 out of 173) are over 20u2005cM (but less than 27.5u2005cM). The map has immediate utility for identifying markers linked to disease resistance genes that are suitable for marker-assisted breeding. The use of microsatellite markers for integration with other maps is also discussed.

Analysis based on RAPD and ISSR markers reveals closer similarities among Citrullus and Cucumis species than with Praecitrullus fistulosus (Stocks) Pangalo

A cucurbit species named Praecitrullus fistulosus (Stocks) Pangalo, which thrives in India, is considered to be a distant relative of watermelon. Recent experiments indicated that it has mild resistance to whiteflies (Bemisia tabaci). However, our attempts to cross various US plant introductions (PIs) of P. fistulosus with watermelon or other Citrullus PIs have not been successful. Thus, to determine genetic relatedness among those species, phylogenetic analysis [based on simple sequence repeat (SSR)–anchored (also termed ISSR), and randomly amplified polymorphic DNA (RAPD) markers] was conducted among PIs of P. fistulosus, Citrullus lanatus var. lanatus (watermelon), C. lanatus var. citroides and the wild Citrullus colocynthis. Phylogenetic relationships were also examined with Cucumis melo (melon), Cucumis sativus (cucumber), and wild Cucumis species including C. africanus, C. metuliferus, C. anguria, C. meeusei, and C. zeyheri. Wide genetic distance exists between Citrullus and Cucumis groups (8% genetic similarity). Phylogenetic relationships among Citrullus species and subspecies are closer (25–55% genetic similarity) as compared with those among most Cucumis species (14–68% genetic similarity). P. fistulosus appeared to be distant from both Cucumis and Citrullus species (genetic similarity between P. fistulosus and Cucumis or Citrullus groups is less than 3%). Although wide genetic differences and reproductive barriers exist among cucurbit species examined in this study, they are still considered as potential germplasm source for enhancing watermelon and melon crops using traditional breeding and biotechnology procedures.

Genetic mapping of a fusarium wilt resistance gene (Fom-2) in melon (Cucumis melo L.)

Fusarium wilt caused by Fusarium oxysporum f.sp. melonis is one of the most devastating diseases in melon production worldwide. The most effective control measure available is the use of resistant varieties. Identifying molecular markers linked to resistance genes can serve as a valuable tool for the selection of resistant genotypes. Bulked segregant analysis was used to identify markers linked to the Fom-2 genes, which confers resistance to races 0 and 1 of the fungal pathogen. Pooled DNA from homozygous resistant or homozygous susceptible progeny of F2 cross between MR-1 and AY was screened using 240xa0PstI/MseI and 200xa0EcoRI/MseI primer combinations to identify AFLP markers linked to Fom-2. Fifteen markers potentially linked to Fom-2 were identified, all with EcoRI/MseI primer pairs. These were mapped relative to Fom-2 in a backcross (BC) population of 60 progeny derived from MR-1 × AY with AY as recurrent parent. Two AFLP markers (ACT/CAT1 and AAC/CAT1) flanked the gene at 1.7 and 3.3xa0cM, respectively. Moreover, AFLP marker AGG/CCC and the previously identified RAPD marker 596-1 cosegregated with Fom-2. These two dominant markers were converted to co-dominant markers by designing specific PCR primers that produced product length polymorphisms between the parents. A survey of 45 melon genotypes from diverse geographic origins with the co-dominant markers demonstrated a high correlation between fragment size and the resistance phenotype. These markers may therefore be useful in marker-assisted breeding programs.

Linkage of random amplified polymorphic DNA markers to downy mildew resistance in cucumber (Cucumis sativus L.)

Marker assisted selection (MAS) may improve the efficiency of breeding downy mildew resistant cucumber cultivars. A study was conducted to identify random amplified polymorphic DNA (RAPD) markers linked to the downy mildew resistance gene (dm) which would be suitable for MAS. A total of 145 F3 families from two populations (55 from the WI 1983G × Straight 8 population and 90 from the Zudm1 × Straight 8 population) were evaluated over five locations in North America and Europe. Resistant and susceptible F3 families were identified and mean family resistance ratings were used to type individual F2 plants. No evidence for race differences in the pathogen (Psuedoperonospora cubensis (Berk. & Curt.) Rostow) between North America and Europe was found. Phenotypic correlations between locations ranged from 0.3 to 0.7. Of the 135 polymorphic RAPD markers identified from 960 primers, five were linked to dm - G14800, X151100, AS5800, BC5191100, and BC5261000. In the WI 1983G × Straight 8 population, G14800 was linked to dm at 16.5 cm, AS5800 at 32.8 cm, BC5191100 at 9.9 cm, and BC5261000 at 19.2 cm. In the Zudm1 ×Straight 8 population, G14800 was linked at 20.9 cm, X151100at 14.8 cm, AS5800 at 24.8 cm, and BC526_1000 at 32.9 cm. MarkersG14800 and BC5191100 were linked in repulsion to the dm allele, and X151100, AS5800, and BC5261000 were linked in coupling phase. These genetic markers may be exploited to develop an efficient MAS strategy for breeding resistant cucumber cultivars.

A genetic linkage map for watermelon derived from a testcross population: (Citrullus lanatus var. citroides × C. lanatus var. lanatus) × Citrullus colocynthis

Abstract.A genetic linkage map was constructed for watermelon using a testcross population [Plant Accession Griffin 14113 (Citrullus lanatus var. citroides) × New Hampshire Midget (NHM; C. lanatus var. lanatus)] × U.S. Plant Introduction (PI) 386015 (Citrullus colocynthis). The map contains 141 randomly amplified polymorphic DNA (RAPD) markers produced by 78 primers, 27 inter-simple sequence repeat (ISSR) markers produced by 17 primers, and a sequence-characterized amplified region (SCAR) marker that was previously reported as linked (1.6xa0cM) to race-1 Fusarium wilt [incited by Fusarium oxysporum Schlechtend.:Fr. f. sp. niveum (E.F.Sm.) W.C. Synder & H.N. Hans] resistance in watermelon. The map consists of 25 linkage groups. Among them are a large linkage group that contains 22 markers covering a mapping distance of 225.6xa0cM and six large groups each with 10–20 markers covering a mapping distance of 68.8 to 110.8xa0cM. There are five additional linkage groups consisting of 3–7 markers per group, each covering a mapping distance of 36.5 to 57.2xa0cM. The 13 remaining linkage groups are small, each consisting of 2–11 markers covering a mapping distance of 3.5–29.9xa0cM. The entire map covers a total distance of 1,166.2xa0cM with an average distance of 8.1xa0cM between two markers. This map is useful for the further development of markers linked to disease resistance and watermelon fruit qualities.

Polymorphisms Among Chloroplast and Mitochondrial Genomes of Citrullus Species and Subspecies

Twelve and six DNA clones representing various parts of chloroplast and mitochondrial genomes, respectively, were used to detect polymorphism among five watermelon cultivars and 21 U.S. Plant Introductions (PIs) collected from diverse geographical locations and representing major groups of Citrullus species. Cluster analysis based on 20 chloroplast DNA (cpDNA) and 10 mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP) markers differentiated the accessions into three major phenetic groups: PIs and watermelon cultivars of Citrullus lanatus subsp. vulgaris (Schrad. ex Eckl. et Zeyh.) Fursa (also designated as C. lanatus var. lanatus) (group I), PIs of C. lanatus var. citroides (of C. lanatus subsp. lanatus Schrad. ex Eckl. et Zeyh.)(group II), and C. colocynthis (L.) Schrad. PIs (group III). The chloroplast and mitochondrial genomes of watermelon cultivars are distinct, but closely related to those of the C. lanatus var. lanatus PIs. On the other hand, the chloroplast and mitochondrial genomes of the wild species C. colocynthis are more similar to those of C. lanatus var. citroides. Polymorphic cpDNA and mtDNA markers identified in this study can complement isozyme and nuclear DNA data used in earlier phylogenetic and phenetic classifications of Citrullus PIs. These cpDNA and mtDNA markers are being used in experiments designed to enhance watermelon cultivars by replacing the chloroplast and mitochondrial genome of cultivated watermelon with those of the wild species C. colocynthis.