marker-assisted selection in wheat

94Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishthe cassava seed, most processing methodsallow a rapid release and elimination of thecyanide. Depending on the end use, highor low cyanide clones are preferred. Otherrelevant traits for the roots are dry matter,protein and carotenoid content (Chávez etal., 2005).Breeding schemesGenetic improvement of clonally propagatednon-inbred crops such as cassavais made possible by the fact that a superiorgenotype can be fixed at any stage inthe breeding scheme, even after a singlecross, the equivalent of an F 1 in commercialhybrids such as maize. Therefore, nonadditivegene actions including dominanceand epitasis become important componentsof the genetic variance to be manipulatedby the breeder (Jaramillo et al., 2005; Calleet al., 2005; Perez et al., 2005a). Large effectivebreeding population sizes are requiredto retain favourable dominant alleles andepistatic loci combination.As in most crop breeding activities, cassavagenetic improvement starts with theproduction of new recombinant genotypesderived from selected elite clones. Scientificcassava breeding began only a few decadesago, and the divergence between landracesand improved germplasm is not as wide asin other crops. Therefore, accessions forgermplasm bank collections from differentresearch institutions play a more relevantrole in cassava than in other crops that havebeen scientifically bred for longer periodsof time. Parental lines are selected basedmainly on their performance per se andlittle progress has been made to use generalcombining ability (Hallauer and MirandaFo, 1988) as a criterion for parental selection.Sexual seeds obtained by the differentcrossing schemes are germinated to initiatea new cycle of selection. The multiplicationrate of cassava planting material is lowas five to ten cuttings can be obtained fromone plant. This implies a lengthy selectionprocess, and in fact it takes about six yearsfrom the time the botanical seed is germinateduntil enough planting material isavailable for multilocation replicated trials.Table 1 illustrates a typical selectioncycle in cassava. It begins with the crossingof elite clones and finishes when the fewclones surviving the selection process reachthe stage of regional trials across severallocations. It should be emphasized thatthere is some variation among the fewcassava-breeding programmes in the worldwith respect to the number of genotypesTable 1Typical selection cycle in cassava beginning with the crossing of elite clones to the point when fewclones surviving the selection process reach the stage of regional trials across several locationsYear Activity Number Plants per genotype1-2 Crosses among elite clones planned, nurseriesplanted and pollinations made3 F 1 : Evaluation of seedlings from botanical seeds.Strong selection for African cassava mosaic virus(ACMV) in Africa.Up to 100 000100 000 a ; 50 0000 b ; 150 000 c4 Clonal evaluation trial (CET) 20 000–30 000 a, b 700 c 6–8 (1 rep, 1 location)5 Preliminary yield trial (PYT) 100 a ; 300 b ; 80 c 20–60 (3 reps, 1 location)6 Advanced yield trial (AYT) 25 a ; 100 b ; 20–25 c 100–500 (3 reps, 2–3location)7-9 Regional trials (RT) 5-30 a, b, c 500-4 000 (3 reps,3–4 locations)Figures for cassava breeding at a IITA (Ibadan, Nigeria); b CIAT (Cali, Colombia) and c CIAT and Rayong Field Research Stationfrom Department of Agriculture (Thailand).Source: adapted from Jennings and Iglesias, 2002.