Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2evaluated at CIAT and CORPOICA for whiteflyresistance. At present, the major source of hostresistance in cassava is the genotype MEcu-72 (Bellottiand Arias, 2001) (Fig. 1D). When feeding on MEcu-72A. socialis had less oviposition, longer developmentperiods, reduced size, and higher mortality than whenfeeding on the susceptible genotype (Fig. 2). Due to theimportance of whiteflies as a pest and virus vector, it isimportant to understand the nature of genes that conferresistance in the resistant genotype, MEcu-72. To studythe genetics of this resistance, a cross was madebetween MEcu-72 (resistance genotype) x MCol-2246(a very susceptible genotype), to evaluate F1segregation, using molecular markers. This willaccelerate the selection of whitefly resistant germplasmand isolate resistant genes.MATERIALS and METHODSPlant Material: For the present work we have used thecross MEcu-72 (as the resistant parent) x MCol-2246(as the susceptible parent). A total F1 offspring of 286genotypes (family CM8996) was produced from thiscross. These materials were sowed and evaluated in thefield during May 2001, March and August 2002 at twodifferent locations: Espinal-Tolima, Colombia(CORPOICA-NATAIMA) at 350 m.a.s.l. andSantander de Quilichao, Cauca, Colombia, at 990m.a.s.l. With this evaluation we will identify genesegregation in the offspring and we will be able toselect the resistant and susceptible materials. Theevaluation was performed in the field using populationand damage scales ranging between 1-6, where 1 is anabsence of damage and population and 6 is highpopulation and damage(curling, chlorosis, sooty moldfungus, etc.) (Table 1). Threeevaluations were performed in2002 using the highest damageand population data forinformation processing.genome equivalents. The whitefly resistance will betarget for map-based cloning using the BAC librariesas tools. We are using silver staining to visualize theallelic segregation of the markers.RESULTSField Evaluation: The whitefly resistant variety CG489-31 (Fig. 2), a progeny from the MEcu 72 x MBra12 cross, has been released to cassava farmers byCORPOICA, Colombia under the name Nataima-31(Fig. 3).Initial field evaluations showed that thesematerials (family CM 8996) had low levels of the pest,because test plants (materials very susceptible to A.socialis) did not present high levels of damage andpopulations (scale of 4 to 6 Table 1). The harvestevaluation showed that the root yield was between 4.5and 86.5 ton/ha, and many materials presenteddesirable characteristics (high percentage of dry matter,palatability, etc.). Currently, the family is under asecond sowing cycle at the same locality from Tolima,and high pressure exerted by the pest has been detectedsince test materials have high degrees of damage (from4 to 5).Preliminary evaluations have demonstrated thatsome materials from the family present low levels ofdamage and population (up to 2) (Fig. 4 and 5).Molecular Analysis: Both parents MEcu-72 and MCol-2246 were evaluated with 343 cassava SSR markers(Mba et al, 2001) including 156 cDNA SSRsdeveloped (Mba et al, by submitted). Approximately155 of the SSRs were polymorphic in the parentals andMolecular Analysis: We areusing Simple Sequences Repeat(SSR) to find markersassociated with resistance formapping the resistant gene(s).As part of a collaborativeproject with ClemsonUniversity funded by USAID aBAC library for cassava usingthe clone MEcu 72 wasconstructed. The librarycontains 73,728 clones with anaverage insert size of 93 kb.Based on a genome size of 760Mb, library coverage isapproximately 10 haploid39