Molecular mapping and candidate gene analysis for resistance to powdery mildew in Cucumis sativus stem.

Powdery mildew (PM) of cucumber (Cucumis sativus), caused by Podosphaera xanthii, is a major foliar disease worldwide and resistance is one of the main objectives in cucumber breeding programs. The resistance to PM in cucumber stem is important to the resistance for the whole plant. In this study, genetic analysis and gene mapping were implemented with cucumber inbred lines NCG-122 (with resistance to PM in the stem) and NCG-121 (with susceptibility in the stem). Genetic analysis showed that resistance to PM in the stem of NCG-122 was qualitative and controlled by a single-recessive nuclear gene (pm-s). Susceptibility was dominant to resistance. In the initial genetic mapping of the pm-s gene, 10 SSR markers were discovered to be linked to pm-s, which was mapped to chromosome 5 (Chr.5) of cucumber. The pm-s gene's closest flanking markers were SSR20486 and SSR06184/SSR13237 with genetic distances of 0.9 and 1.8 cM, respectively. One hundred and fifty-seven pairs of new SSR primers were exploited by the sequence information in the initial mapping region of pm-s. The analysis on the F2 mapping population using the new molecular markers showed that 17 SSR markers were confirmed to be linked to the pm-s gene. The two closest flanking markers, pmSSR27and pmSSR17, were 0.1 and 0.7 cM from pm-s, respectively, confirming the location of this gene on Chr.5. The physical length of the genomic region containing pm-s was 135.7 kb harboring 21 predicted genes. Among these genes, the gene Csa5G623470 annotated as encoding Mlo-related protein was defined as the most probable candidate gene for the pm-s. The results of this study will provide a basis for marker-assisted selection, and make the benefit for the cloning of the resistance gene.

Chromosomal Mapping and QTL Analysis of Resistance to Downy Mildew in Cucumis sativus.

Downy mildew of cucumber (Cucumis sativus), caused by Pseudoperonospora cubensis, is a major foliar disease worldwide. The cucumber inbred lines K8 (resistant to downy mildew) and K18 (susceptible) were used to study the inheritance of resistance to downy mildew. Chromosomal mapping of the resistance genes was completed to provide a theoretical basis for the resistance mechanisms and for marker assisted selection (MAS). Inoculation was used to test the level of resistance to P. cubensis in the F2 and F2:3 families derived from the cross K8 × K18. Simple sequence repeat (SSR) analysis, combined with bulked segregation analysis (BSA), was done with the DNA of F2 plants using 2,360 pairs of SSR primers. JoinMap Version 3.0 and MapInspect were used to construct SSR linkages and to verify the relationships between these SSR linkages and cucumber chromosomes. Quantitative trait locus (QTL) analysis of downy mildew resistance was done using MapQTL Version 4.0. Inheritance of resistance to downy mildew in K8 was quantitative. Five QTLs for resistance to downy mildew were detected: dm1.1, dm5.1, dm5.2, dm5.3, and dm6.1. The loci of dm1.1 and dm6.1 were on chromosomes 1 and 6, respectively. The loci of dm5.1, dm5.2, and dm5.3 were on chromosome 5, and were linked. Six linked SSR markers for these five QTLs were identified: SSR31116, SSR20705, SSR00772, SSR11012, SSR16882, and SSR16110. Six and four nucleotide binding site (NBS)-type resistance gene analogs (RGAs) were predicted in the region of dm5.2 and dm5.3, respectively. These results will be of benefit for fine-mapping the major QTLs for downy mildew resistance, and for MAS in cucumber.

Construction of a watermelon BAC library and identification of SSRs anchored to melon or Arabidopsis genomes.

A bacterial artificial chromosome (BAC) library was constructed for watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus) with an average insert-size of 106 kb, providing 21 haploid genome equivalents. The library was used to identify BAC clones that are anchored to probes evenly distributed on the genomes of melon or Arabidopsis. Twenty eight probes (representing 66% of the tested probes) from melon and 30 probes (65%) from Arabidopsis identified positive BAC clones. Two methods were implemented to identify SSRs from the positively hybridizing BAC clones. First, analysis of BAC end sequences revealed 37 SSRs. For the second method, pooled DNA of BACs identified by the melon probes was used to develop a shotgun library. The library was then screened with synthetic SSR oligonucleotides by hybridization. Sequence analysis of positively hybridizing shotgun clones revealed 142 different SSRs. Thirty eight SSRs were characterized using three watermelon cultivars, five plant introduction (PI) accessions of C. lanatus var lanatus and four PIs of C. lanatus var citroides. Of these, 36 (95%) were found to be polymorphic with up to six alleles per marker. Polymorphism information content values for polymorphic markers varied between 0.22 and 0.79 with an average of 0.53. The methods described herein will be valuable for the construction of a watermelon linkage map with SSRs evenly distributed on its genome that is anchored to the genomes of melon and Arabidopsis.

Inheritance of egusi seed type in watermelon.

An unusual seed mutant in watermelon (Citrullus lanatus var. lanatus) has seeds with a fleshy pericarp, commonly called egusi seeds. The origin of the phenotype is unknown, but it is widely cultivated in Nigeria for the high protein and carbohydrate content of the edible seeds. Egusi seeds have a thick, fleshy pericarp that appears during the second to third week of fruit development. We studied the inheritance of this phenotype in crosses of normal seeded Charleston Gray and Calhoun Gray with two plant introduction accessions, PI 490383w and PI 560006, having the egusi seed type. We found that the egusi seed type is controlled by a single recessive gene, and the symbol eg was assigned.

Effects of Root Decay on the Relationship between Meloidogyne spp. Gall Index and Egg Mass Number in Cucumber and Horned Cucumber.

A greenhouse study was conducted to determine if root necrosis had an effect on the relationship between root-knot nematode gall index and egg mass number. Thirty-four cultigens of Cucumis (14 accessions, 12 cultivars, and six breeding lines of C. sativus, and two accessions of C. metuliferus) were evaluated against four root-knot species (Meloidogyne arenaria race 2, M. incognita race 1, M. incognita race 3, and M. javanica) measuring gall index, root necrosis, and egg mass number. Root necrosis affected the gall index-egg mass relationship. At lower root necrosis values, a stronger relationship existed between gall index and egg mass number than at higher root necrosis values. Root tissue was destroyed by root necrosis, and normal root-knot nematode reproduction would not occur, even though root galling was still observed. The races of M. incognita tested had a greater effect in predisposing C. sativus and C. metuliferus to root necrosis than did M. arenaria race 2 or M. javanica. This study showed that root necrosis had an adverse affect on the relationship between gall index and egg mass number in cucumber.

Resistance to Root-knot Nematodes in Cucumber and Horned Cucumber.

Two experiments were conducted in the greenhouse. In one experiment, cucumber (Cucumis sativus) and horned cucumber (C. metuliferus) cultigens were evaluated for resistance to four root-knot nematode species (Meloidogyne arenaria, M. hapla, M. incognita, and M. javanica), and, in a second experiment, a standard (12-week) test was compared with a rapid (6-week) test. In the first experiment, horned cucumber cultigens varied in response to the Meloidogyne species. 'Sumter' cucumber was more susceptible than the horned cucumber to Meloidogyne incognita, M. javanica, and M. arenaria. All cultigens were more resistant to M. hapla than to the other root-knot nematode species. In the second experiment, best results were obtained when the test was run for 12 weeks rather than 6 weeks after planting (or 10 and 4 weeks after inoculation, respectively). All cultigens were more resistant to M. arenaria than to either M. incognita or M. javanica.