Introduction

The legume genus Arachis is of exclusively South American origin, containing about 69 described species. Six or more species have been cultivated, and one of them, Arachis hypogaea (commonly called peanut or groundnut in English, amendoim in Portuguese and mani in Spanish) has become an important food legume throughout the World.

Although there is high morphological diversity among varieties of A.hypogaea, genetic diversity is low and it lacks sources of resistance to many important diseases. This is possibly because A. hypogaea has its origin in the doubling of chromosomes of a sterile hybrid between two wild diploid species. The resultant tetraploid plant, containing two distinct genomes which are designated A and B, could not cross with its dipoid wild relatives. Therefore, all modern peanuts are probably derived from a single plant. In contrast to cultivated peanuts, wild Arachis species have high genetic diversity, and be a rich source of pest and disease resistances, and genes controlling other characteristics of interest such as drought tolerance and early seed set.

Objectives of the research

Disease resistance in wild Arachis

The work aims to identify, in wild Arachis germplasm, sources of resistance against pests and diseases of peanut in South America. The pests and diseases targeted will be the fungal leaf spots Cercospora arachidicola (mancha castanha, brown spot), Cercosporidium personatum (mancha preta, black spot), Puccinia arachidis (ferrugem, rust) and root-knot nematodes (Meloidogyne ssp.).

Genetic Mapping

To enable the efficient exploitation of resistance genes (R-genes) from wild germplasm, the generation of genetic maps is fundamental. This is because they enable marker-assisted selection which speeds traditional breeding. Genetic markers are also essential for map-based cloning of R-genes. However, A.hypogaea presents a number of difficulties, including being tetraploid and having low genetic diversity making polymorphic molecular markers difficult to find.

To address these difficulties, we propose to map and identify molecular markers linked to R-genes in segregating populations derived from crosses of wild diploid Arachis. At least two mapping populations will be used, one derived from a cross between two A genome species, and one between two B genome species. Various types of molecular markers will be used as appropriate, but in the first instance mapping will be done using microsatellite markers. Microsatellites have a series of advantages as molecular markers, they are co-dominant, abundant and uniformly distributed in plant genomes, and of particular importance for this project, they typically are transportable among closely related species. Disease resistance genes and markers derived from genes will also be placed onto this map.

Development of introgression lines

Introgression lines will be developed by crossing A and B plants which harbour desirable R-genes, colchicine treating the resultant hybrid, and making crosses with A.hypogaea. Markers linked to R-genes will then be used for marker-based selection to speed the introgression of wild genes from both A and B genomes into A.hypogaea, and provide the starting point for map-based cloning.

Investigation of Synteny and in-situ hybridisation

The project also aims to explore the collinear gene relationships (synteny) between Arachis and Lotus by comparative genomics. Synteny is useful because within plant families genome size varies enormously. This seems to be largely due to the accumulation of more repetitive DNA elements (such as retrotransposons) in some evolutionary lines compared to others. If gene organisation and order are preserved, then a small genome can be used as a reference for marker development and study of larger genomes. The genome size of wild diploid Arachis species is about five times larger than that of the model legume Lotus japonicus. The generation of new markers for breeding and the cloning of genes conferring traits of interest in Arachis should be greatly facilitated by means of comparative genomics.

Synteny will be investigated by the development and mapping of "genome anchor markers" which should, ideally, represent unique single copy genes. In parallel with the placement of genome anchor markers onto the genetic maps of Arachis and Lotus, in-situ hybridisation using Arachis and Lotus chromosomes will allow the correlation of linkage groups and physical chromosomes, and will aid in the identification of orthologous chromosome regions. We anticipate that this analysis between the taxonomically highly derived Lotus and the more basal tropical Arachis will be particularly informative and will benefit the understanding of legumes.