An improved genetic linkage map has been constructed for cowpea (Vigna unguiculata L. Walp.) based on the segregation of various molecular markers and biological resistance traits in a population of 94 recombinant inbred lines (RILs) derived from the cross between 'IT84S-2049' and '524B'. A set of 242 molecular markers, mostly amplified fragment length polymorphism (AFLP), linked to 17 biological resistance traits, resistance genes, and resistance gene analogs (RGAs) were scored for segregation within the parental and recombinant inbred lines. These data were used in conjunction with the 181 random amplified polymorphic DNA (RAPD), restriction fragment length polymorphism (RFLP), AFLP, and biochemical markers previously mapped to construct an integrated linkage map for cowpea. The new genetic map of cowpea consists of 11 linkage groups (LGs) spanning a total of 2670 cM, with an average distance of 6.43 cM between markers. Astonishingly, a large, contiguous portion of LG1 that had been undetected in previous mapping work was discovered. This region, spanning about 580 cM, is composed entirely of AFLP markers (54 in total). In addition to the construction of a new map, molecular markers associated with various biological resistance and (or) tolerance traits, resistance genes, and RGAs were also placed on the map, including markers for resistance to Striga gesnerioides races 1 and 3, CPMV, CPSMV, B1CMV, SBMV, Fusarium wilt, and root-knot nematodes. These markers will be useful for the development of tools for marker-assisted selection in cowpea breeding, as well as for subsequent map-based cloning of the various resistance genes.

Genetic diversity and phylogenetic relationships among 141 cowpea (Vigna unguiculata L. Walp.) accessions collected throughout the nine geographical regions of Ghana were evaluated using simple sequence repeat (SSR) molecular markers. Twenty-five primer combinations pre-selected by their ability to polymerase chain reaction amplify SSRs from a set of test cowpea germplasm were evaluated. Of these, 20 primer combinations gave reproducible polymorphisms among 97.2% of the cowpea accessions tested, with the remaining accessions being found to be genetically identical. The informative primer combinations revealed a total of 74 alleles at 20 loci with an average of 3.8 alleles detected per locus. Variation in heterozygosity among cowpea SSRs ranged from 0.01 to 0.84 with an average occurrence of 0.19. The polymorphism information content varied from 0.07 to 0.66 with an average of 0.38. The Ghanaian cowpea accessions clustered into five main branches, each of which was loosely associated with the geographical regions from which samples were obtained. Accession GH2288 was found to be the most divergent cowpea accession compared with all others including the outgroup IT84S-2049, a breeding line from Nigeria. Our results provide a framework for future studies aimed at the conservation and management of cultivated cowpea germplasm in Ghana, and a good starting point for the selection of parental lines for genetic improvement programmes.

ABSTRACT. Genetic diversity and phylogenetic relationships among 22 local cowpea (Vigna unguiculata) varieties and inbred lines collected throughout Senegal were evaluated using simple sequence repeat molecular markers. A set of 49 primer combinations were developed from cowpea genomic/expressed sequence tags and evaluated for their ability to detect polymorphisms among the various cowpea genotypes. Forty-four primer combinations detected polymorphisms, with the remaining five primer sets failing to yield PCR amplification products. From one to 16 alleles were found among the informative primer combinations; their frequencies ranged from 0.60 to 0.95 (mean = 0.79). The genetic diversity of the sample varied from 0.08 to 0.42 (mean = 0.28). The polymorphic information content ranged from 0.08 to 0.33 (mean = 0.23). The local varieties clustered in the same group, except 53-3, 58-53, and 58-57; while Ndoute yellow pods, Ndoute violet pods and Baye Ngagne were in the Genetic relationship of Vigna unguiculata second group. The photosensitive varieties (Ndoute yellow pods and Ndoute violet pods) were closely clustered in the second group and so were inbred line Mouride and local cultivar 58-57, which is also one of the parents for inbred line Mouride. These molecular markers could be used for selection and identification of elite varieties for cowpea improvement and germplasm management in Senegal.

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SummaryStudies of the initial interactions of Striga asiatica with the non-host plant species Tagetes erecta (marigold) established that parasite penetration through the root is arrested most frequently in the cortex. The arrest of parasite ingress is associated with browning and necrosis of root cortical cells¯anking the invading endophyte and with increased intracellular wall appositions on the root cell walls directly adjacent to the plant-parasite interface. Using a polymerase chain reaction-based differential cDNA ampli®cation strategy followed by 5¢-RACE, we have identi®ed several gene products whose expression is induced in marigold roots during attempted parasitism by Striga. Among these was a 917 bp cDNA encoding a 221 amino acid protein with signi®cant homology to proteins encoded by disease resistance genes from other plant species, including N, RPP5, L6 and M. This cDNA was subsequently used to isolate a nuclear gene, designated NRSA-1, for non-host resistance to Striga asiatica. NRSA-1 is a member of a small gene family in marigold consisting of two to four members. RNA gel blot analysis showed that NRSA-1 transcripts accumulate to high levels in roots near the site of Striga invasion within 120 h after parasite attachment, and appear at lower levels throughout the rest of the plant under Striga parasitism. NRSA-1 expression is rapidly induced by treatment with jasmonic acid (JA), but not by mechanical wounding, treatment with salicylic acid, paraquat or ABA. A possible role for NRSA-1 in the nonhost resistance mechanism is discussed.