Plant Genome IV Conference Plant Genome IV Conference
Genetic maps of citrus (2n=2x=18) are being developed in laboratories in California, Florida, France, Japan, and Spain. Markers linked to genes for citrus tristeza virus resistance, citrus nematode resistance, fruit acidity, rootstock dwarfing, and salt-tolerance have been identified. The primary map developed at Riverside is based on a cross between two different Citrus x Poncirus hybrids and currently includes 264 loci (10 isozymes, 58 RFLPs, and 196 RAPDs) in 12 linkage groups with more than 3 loci at LOD=3.5. Comparison with the map developed in Florida shows considerable similarity in locus order, but some differences in map distances. Although Poncirus trifoliata is one grandparent of both mapping populations, few RAPD markers occur on both maps. Bulk segregant analysis with RAPDs has been very successful for identifying markers linked to major genes, but conversion of these RAPDs to RFLP or SCAR markers useful in a wide range of genetic backgrounds has had limited success. Current research at Riverside is adding ISSR markers to the map. Most ISSR primers amplify several to more than 20 polymorphic products. We have also started developing a physical map using fluorescent in situ hybridization with rDNA probes. Development of citrus maps is facilitated by collaborations: many of the cDNAs mapped have been sequenced in Japan and the Riverside population is being used in Australia to map microsatellite markers.