Genetic mapping of root development and Resistance to Radopholus similis in a segregating diploid Banana population
Radopholus similis (and other nematodes) cause significant damage on banana worldwide. Genetic resistance to nematodes provides a more sustainable management option in resource poor farming systems. New evidence indicates that there is an effect of root growth characteristics on resistance observations. The main goal of the study was to identify root traits of banana that can be targeted in breeding for resistance to burrowing nematode, Radopholus similis. To do this, an early screening of a segregating population derived from a cross conducted between TMB2x 6142-1 (2x), a diploid genotype susceptible to nematodes, and TMB2x 8075-7 (2x), a diploid genotype resistant to nematodes was done. Root growth and development of 51 clonal genotypes was evaluated in minirhizotron boxes where observations were recorded starting two weeks after planting and after every 2 weeks for six weeks (3 times). Diversity in root architecture for these genotypes was revealed, with tertiary roots contributing the highest proportion of the total roots length (81 %) and surface area (52%) while primary roots contributed the highest proportion of roots volume (61%). Single roots of the 51 macro-propagated genotypes were inoculated with R. similis and observed after eight weeks in a screen-house. One resistant genotype (Yangambi km-5) and one susceptible (Valery) were included as controls in this experiment. Significant differences (P<0.05) in final nematode population and reproduction were observed among the genotypes tested. Twenty-one (21) clones were identified as resistant, 17 susceptible and 15 partially resistant. Important root parameters were identified as root length, surface area and root volume and these were subjected to marker-trait association analyses. One hundred (100) SSR markers were analyzed for association with variables of interest using simple regression analysis. Twenty-three (23) heterozygous markers were identified in the female parent, twenty-five (25) in the male parent, while filthy-two (52) heterozygote markers appeared in both parents. Close associations were observed between marker BaPt780639 and root growth rate (R2=50.3%, P=0.0001); between root surface area and marker BaPt-784290 (R2 =40.5%, P=0.0001), and between root volume and marker BaPt-784290 (R2 =41.1%, P=0.0001). Two markers were closely linked with the final population of nematodes. However, only marker BaPt786236 showed close linkage to both root size and resistance to R.similis (R2=20.4% for length, R2=14.9% for nematodes population. This marker can, therefore, be a good candidate for selecting for root length trait and resistance to R. similis. This study identified two hybrid genotypes (25539S_26 and 25540S_149) to have important root traits that confer tolerance to R. similis and should be targeted in breeding programs.
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