Genetic improvement of oxidative stress tolerance and longevity of the entomopathogenic nematode Heterorhabditis bacteriophora
Poor persistence in the soil, product shelf-life and loss of virulence during storage are major constraints for the use as a biocontrol agent of the entomopathogenic nematode (EPN) Heterorhabditis bacteriophora. A more targeted approach exploits classical and molecular genetics, making use of the diversity among natural populations for improvement of EPN beneficial traits. Using natural and genetic variability, a total of 40 H. bacteriophora wild type strains, inbred and hybrid lines of different geographic origin was assessed for their tolerance to oxidative stress and longevity at 25 and 7°C. A high variability among strains of this species was determined and a high heritability for oxidative stress tolerance (h2 > 0.9) was obtained. Oxidative stress tolerance was strongly positive correlated with DJ longevity. Therefore, the oxidative stress tolerance can be used as a predictor for DJ longevity, permitting a selection process within a shorter testing period. In order to search for improved strains, hybridization and mutagenesis was performed and a significant increase of oxidative stress tolerance and longevity was obtained. To elucidate the underlying mechanisms in response to oxidative stress, a comparative transcriptomic analysis on the early stage of oxidative stress induction (4h) in two inbred lines with contrasting stress-tolerance, HU2-IL1 (stress-tolerant) and PT1-IL1 (stress-sensitive) was performed. Down-regulation may play a crucial factor for survival as it can be seen for the tolerant line. The results of this study will facilitate the selection longevity predictor genes in this EPN. Further analysis will allow the identification of molecular markers and facilitate breeding for improved lines. Nematode strains with prolonged longevity can provide a longer persistence in the soil after application and thus open a door in larger scale markets such as for the control of Diabrotica virgifera virgifera in maize.