No abstract

Simple sequence repeats (SSRs), also referred to as microsatellites, represent a PCR-based marker system that has been described in mammalian and plant genomes in recent years. In self-pollinating crop plants they have been shown to be superior to other DNA markers with respect to their level of polymorphism. The technical advantages compared with RFLP markers should also facilitate marker analysis in outcrossing crops like rye. In order to determine the usefulness of SSR markers in rye genetics and breeding, several genomic libraries were screened for (CT/GA)n and (GT/CA)n dinucleotide repeats. It was estimated that these motifs occur at a frequency of one per 268-519 kb. Seventy four out of 182 positive clones were sequenced, and the majority (56.8%) revealed perfect repeats, predominantly of the type (GT/CA)n (61.9%). Fifty seven primer pairs were designed and 27 (47.4%) resulted in specific SSR markers, of which 20 were genetically mapped or assigned to chromosomes or chromosome arms, respectively. The level of polymorphism of four SSR and three RFLP markers was assessed in two open-pollinated rye cultivars. On average, the SSR markers showed larger values of expected heterozygosity (0.62 vs. 0.43) and allele number (5.9 vs. 3.4) than RFLP markers in both cultivars.

The potential of microsatellite markers for use in genetic studies in eggplant, Solanum melongena, has been evaluated. A genomic library of eggplant was screened for GA and GT repeat motifs to isolate microsatellite clones. The frequency of each repeat motif in the eggplant genome was found to be every 3200 kb for GA repeats and every 820 kb for GT repeats. Sixty-one per cent of GT repeats were found to directly flank AT repeats. A total of 37 polymerase chain reaction (PCR) primer pairs were designed, 23 of which amplified a single product or several products. The level of microsatellite polymorphism was evaluated by using S. melongena lines and related Solanum species. Two to six alleles per primer pair were displayed in the S. melongena lines and two to 13 alleles were displayed in the Solanum relatives. Seven microsatellites showed polymorphism between parental lines of the mapping population and segregated in a codominant Mendelian manner. These microsatellite loci were distributed throughout the linkage map.

The witchesÕ broom disease caused by the fungus Crinipellis perniciosa is the main limiting factor for cocoa production in South America and the Caribbean. In Brazil, this disease affects almost all cocoa-growing regions, causing serious economic, social and ecological damage. The aim of this study was to map genomic regions associated with resistance to C. perniciosa using an F 2 population derived from a cross between ÔScavina-6Õ (resistant) and ÔICS-1Õ (susceptible). The phenotypic index was determined as the average number of vegetative witchesÕ brooms per canopy area of each plant, the witchesÕ brooms were counted and eliminated during six field evaluations between May 1998 and August 1999. A total of 124 random amplified polymorphic DNA (RAPD) and 69 amplified fragment length polymorphism (AFLP) markers were mapped along 25 linkage groups covering 1713 cM of cocoa genome. After employing single factor and composite interval mapping analyses, a major quantitative trait loci (QTL) flanked by the marker AV14.940 was identified in the linkage group 11, explaining almost 35% of the resistance to witchesÕ broom. The present result suggests that this QTL acts as a major dominant component of resistance to this pathogen, with great potential for use in marker-assisted selection procedures in cocoa breeding programmes.

The linkage relationship between eighteen isozyme loci and the morphological markers hypocotyl colour (R-r), monogerm character (M-m), pollen fertility (X) and stem fasciation (Verb.) are tested. Tliree linkage groups could be set up, involving all morphological marker loci and eight of the isozyme loci. Est-2, R-r, Fdp-2, Got2 and led-] belong to linkage group I, linkage group II includes the loci Fas-fas Mm, Est-3 and Aeo-1, linkage group III contains the loci X, Mdh-1 and Est-5.When analysing the inheritance of isozymes and RFLPs, deviations are usually found in some lines from the expected frequencies of a 3 : 1 or 1 : 2 : 1 segregation at single marker loci. In many cases these data can still be used for the estimation of recombination values with linked loci under the control of selection. Procedures to estimate linkage in such cases are given and applied to experimental data in Beta vulgaris.