marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 83Common beans: importance andgeneticsCommon beans (Phaseolus vulgaris L.)are the most important grain legume fordirect human consumption, especially inLatin America and eastern and southernAfrica. They are seed-propagated, true diploids(2n = 22) and have a relatively smallgenome (650 Mb) (Broughton et al., 2003).Originating in the Neotropics, commonbeans were domesticated in at least twomajor centres in Mesoamerica and theAndes (Gepts, 1988) and possibly in athird minor centre in the northern Andes(Islam et al., 2002). Wide DNA polymorphismis expressed between the twomajor gene pools. Mesoamerican beanstypically have small to medium size seedsand can be classed into four races that aredistinguished by randomly amplified polymorphicDNA (RAPD) polymorphisms(Beebe et al., 2000). Andean beans usuallyhave medium to large seeds, and landraceshave been classed into three races basedon plant morphology and agro-ecologicaladaptation (Singh, Gepts and Debouck,1991). These can be differentiated by microsatellites(M. Blair, unpublished data) butthe genetic distance among Andean races isnarrower than that among Mesoamericanraces (Beebe et al., 2001). A large numberof gene tagging studies have been conductedin common beans, predominantlywith RAPD markers, some of which havebeen converted subsequently to sequencecharacterized amplified regions (SCARs;reviewed most recently by Miklas et al.,2006).Beans display a wide range of growthhabits (Van Schoonhoven and Pastor-Corrales, 1987), from determinate bushtypes, to indeterminate upright or vinybush types, to vigorous climbers. Bushtypes are the most widely grown, and are arelatively short season crop, maturing in aslittle as 60 days from seeding in a tropicalclimate and yielding from 700 to 2 000 kg/ha on average. On the other hand, in smallholderagriculture where land is scarce,labour-intensive, high-yielding climbingbeans enjoy continuing or even expandingpopularity. Climbing beans can mature in100 to 120 days at mid-elevations, but candelay as long as ten months at higher elevationsand can produce the highest yields forthe crop, up to 5 000 kg/ha. These featureshave significant implications for breedingprogrammes. In bush types it is possibleto obtain up to three cycles per year inthe field, or even four cycles in greenhouseconditions. Breeding bush beans isthus quite agile with regard to advance ofgenerations, although seed harvest of individualplants is sometimes limited. Withclimbing beans, on the other hand, at bestit is possible to obtain two cycles per yearwith field grown plants, while managingclimbing beans in the greenhouse is logisticallydifficult. However, while bush beansproduce on average 20 to 50 seeds/plant,individual plants of climbing beans oftenproduce enough seeds to plant several rows(100 to 150 seeds).Beans are self-pollinating and thusbreeding methods for autogamous cropsare employed. Pedigree selection or someadaptation thereof is most common, andboth recurrent (Muñoz et al., 2004) andadvanced (or inbred) backcrossing (Sullivanand Bliss, 1983; Buendia et al., 2003; Blair,Iriarte and Beebe, 2003) have been used.Recurrent selection has also been employed(Kelly and Adams, 1987; Beaver et al.,2003) but seldom in a formal sense with adefined population structure. Singh et al.(1998) suggested a system that they calledgamete selection in which individual F 1plants of multiple parent crosses give rise