Chapter 5 Genetic Analysis of Apomixis - cimmyt

Breedllt of Apomktk Species 141As accessions were transferred to CIAT­Colombia and released from quarantine, largeportions of this collection were subsequentlyforwarded to Brazil (approximately 400accessions), Costa Rica (approximately 280accessions), and Peru (approximately 260accessions) for agronomic evaluation (Grof etal. 1989a; CIAT 1992). Because large numbersof accessions were involved, evaluationmethodology needed to be simple andefficient, such as that proposed by Toledo(1982), in order to discard poorly performingmaterials. Since then, more detailed andintensive evaluation has been conducted at theEmbrapa Beef Cattle Research Center inCampo Grande, MS, Brazil. Agronomicevaluation began with accessions planted insmall plots with three replications. A periodiccutting regime was imposed for three years toestimate overall and seasonal production,regrowth vigor, seed production, andresistance to spittlebug and diseases (Valle etal. 1993a). At one harvest each year, sampleswere analyzed for crude protein content andin vitro digestibility. The range of variationobserved within this collection is remarkable(Table 10.1). Nineteen selected accessions werethen evaluated in regional trials in differentecosystems and superior genotypes wereidentified (Valle et a1.1 997). The next evaluationstep involved studying the effects of livestockon the pasture. Eight apomictic accessions werecompared in paddocks to the commercialcultivar. Four of these were selected (Euclideset al. 2001) to undergo animal performancetrials, the last step prior to release as a newcultivar.Morphological characterization, applyingnumerical taxonomy and using 26 descriptors,was carried out for all 340 accessions in theBrazilian Brachiaria collection (Valle et al.1993c). The objectives were to study thediversity of the accessions, analyze thedispersion and genetic distance betweenaccessions and species, and organize thegermplasm into groups of morphologicallysimilar accessions, regardless of taxonomicclassification. This type of study helpsresearchers define closely related accessionswithin and among groups from whichindividual progenitors may be selected forfuture crosses. This analysis revealed thecontinuous polymorphism that exists amongthree species (B. deCllmbens, B. bri:mntha, andB. ruziziensis) and clearly separated typicalaccessions of B. humidicola, B. dictyoneura, andB. jubata (Figure 10.2). The selection ofaccessions for pasture trials was based on anassociation of agronomic traits withmorphological characteristics.Cytology, Reproductive Mode, andInheritance of ApomixisBasic information about mode of reproductionand cytogenetics of sexually reprod ucingaccessions was also ascertained from theTable 10.1 Agronomic evaluation of Brachiariaaccessions in BrazilN LDMY (kg/ha) OfoDSPB. brizantharange 96 2040·9420 9·27 1.9·3.8overage callee. 96 4797 19 2.6overage select. 10 7503 18 3.1B. decumbensrange 35 1348-5543 10-25 1.5·3.2overage (olle