Identification of simple sequence repeat markers for sweetpotato weevil resistance

The development of sweetpotato [Ipomoea batatas (L.) Lam] germplasm with resistance to sweetpotato weevil (SPW) requires an understanding of the biochemical and genetic mechanisms of resistance to optimize crop resistance. The African sweetpotato landrace, ‘New Kawogo’, was reported to be moderately resistant to two species of SPW, Cylas puncticollis and Cylas brunneus. Resistance has been associated with the presence of hydroxycinnamic acids esters (HCAs), but the underlying genetic basis remains unknown. To determine the genetic basis of this resistance, a bi-parental sweetpotato population from a cross between the moderately resistant, white-fleshed ‘New Kawogo’ and the highly susceptible, orange-fleshed North American variety ‘Beauregard’ was evaluated for SPW resistance and genotyped with simple sequence repeat (SSR) markers to identify weevil resistance loci. SPW resistance was measured on the basis of field storage root SPW damage severity and total HCA ester concentrations. Moderate broad sense heritability (H2 = 0.49) was observed for weevil resistance in the population. Mean genotype SPW severity scores ranged from 1.0 to 9.0 and 25 progeny exhibited transgressive segregation for SPW resistance. Mean genotype total HCA ester concentrations were significantly different (P < 0.0001). A weak but significant correlation (r = 0.103, P = 0.015) was observed between total HCA ester concentration and SPW severity. A total of five and seven SSR markers were associated with field SPW severity and total HCA ester concentration, respectively. Markers IBS11, IbE5 and IbJ544b showed significant association with both field and HCA-based resistance, representing potential markers for the development of SPW resistant sweetpotato cultivars

Effects of hydroxycinnamic acid esters on sweetpotato weevil feeding and oviposition and interactions with Bacillus thuringiensis proteins

Sweetpotato weevil (SPW) pest management is challenging because the pest target is sub-terranean, so the application of pesticides is impractical and usually ineffective. Host plant resistance and the genetic transformation of sweetpotatoes to produce entomotoxic Bt proteins offer potential for environmentally benign pest control. Resistance can be conferred by naturally occurring hydroxycinnamic acids which protect against oviposition by adults, but these compounds are restricted to the root surface so do not protect against the cortex bound larvae where the greatest damage occurs. Resistance could be enhanced if combined with expression of Bt proteins in transformed plants but interactions between hydroxycinnamic acids and Bt proteins remain unknown. Here the bioactivity of Cry7Aa1 protein and hydroxycinnamic acid esters was evaluated individually and in combination against SPW larvae and mortality determined. Low and high concentrations of hydroxycinnamic acid esters alone caused significantly higher mortality of both weevil species in all experiments compared to the control. SPW larval mortality was greater when tested as a combination of hydroxycinnamic acid esters and Bt protein but this effect was additive not synergistic. Although we report no evidence of antagonistic interactions the antifeedant effects of the plant compounds conferring host plant resistance could have reduced consumption of the Bt protein in our assays leading to a lower efficacy when combined. Further work is required to determine if the toxic effects of Bt proteins function alongside host plant resistance in sweetpotato under field conditions