A genetic linkage map of cassava has been constructed with 132 RFLPs, 30 RAPDs, 3 microsatellites, and 3 isoenzyme markers segregating from the heterozygous female parent of an intraspecific cross. The F cross was made between 'TMS 30572' and 'CM 2177-2', elite cassava cultivars from Nigeria and Colombia, respectively. The map consists of 20 linkage groups spanning 931.6 cM or an estimated 60% of the cassava genome. Average marker density is 1 per 7.9 cM. Since the mapping population is an F cross between heterozygous parents, with unique alleles segregating from either parent, a second map was constructed from the segregation of 107 RFLPs, 50 RAPDs, 1 microsatellite, and 1 isoenzyme marker from the male parent. Comparison of intervals in the maleand female-derived maps, bounded by markers heterozygous in both parents, revealed significantly less meiotic recombination in the gametes of the female than in the male parent. Six pairs of duplicated loci were detected by low-copy genomic and cDNA sequences used as probes. Efforts are underway to saturate the cassava map with additional markers, to join the male-and female-derived maps, and to elucidate genome organization in cassava.

The cassava core collection (601 genotypes)

For many crops with preferential vegetative propagation, or which are sterile, or have seed unresponsive to standard seed storage techniques, in vitro methods can provide a valuable adjunct to other germ plasm conservation strategies. Two types of in vitro gene banks are proposed: (i) an in vitro active gene bank where cultures are maintained under slow growth; of the few of these that exist, the cassava (Manihot) in vitro active gene bank of the Centro Internacional de Agricultura Tropical (CIAT) consists of over 4000 clones; (ii) an in vitro base gene bank, where cultures are cryopreserved; at present, none of this type exist. In vitro conservation offers a means of maintaining valuable gene combinations in a small space, protected against pest and disease attack, soil problems, and climatic changes, and with high multiplication potential. The main limitations are inability to regenerate plants of many important crops and the risk of genetic instability of cultures. A joint project of the International Board for Plant Genetic Resources and CIAT is underway to develop an in vitro based set of procedures for handling germ plasm, using cassava as a model, with culture maintenance, stability monitoring, and management data bases being considered. Results of recent work on in vitro conservation and related activities, i.e., germ-plasm collection, exchange, and characterization, will be presented and discussed, with special reference to Manihot.Key words: in vitro gene banks, cryopreservation, isozyme electrophoresis, genotype stability, Manihot esculenta Crantz.

No abstract

No abstract