Genetic improvement of the desiccation tolerance of the entomopathogenic nematode Heterorhabditis bacteriophora through selective breeding

The commercial exploitation of the entomopathogenic nematode Heterorhabditis bacteriophora in biological control is limited due to its relatively short shelf life, that is related to its low tolerance of environmental extremes such as desiccation. Desiccation is a result of evaporation at low humidity or hypertonic osmotic conditions. For storage and transportation infective dauer juveniles (DJs) are mixed with clay minerals. To maintain DJ quality their metabolism is reduced by transfer of the DJs into a quiescent state, which is induced by moderate desiccation. If the nematodes were more resistant to desiccation stress, they could be further desiccated to prolong DJ survival and shelf life of nematode-based products. The reduction of the nematode metabolism and the survival in the quiescent stage could be enhanced by an increased resistance to desiccation stress. The objective of this investigation was to determine the genetic variability of the desiccation tolerance of H. bacteriophora and to exploit this variability for an enhancement of desiccation tolerance by breeding. A hybrid strain resulting from crosses of eight H. bacteriophora isolates from different geographic origins was used in our investigation. The desiccation stress was adjusted by hygroscopic polyethyleneglycol (PEG 600) solutions. By lowering the water activity (aw-value) of this solution, the removal of water from the DJs is enhanced. The influence of an adaptation phase on the desiccation tolerance was investigated. The lowest mean tolerated aw-value (0.85) was achieved with an adaptation phase of 72 h at an aw-value of 0.96. The variance of the desiccation tolerance increased with the reduction of the aw-value during adaptation. The heritability of the trait, determined by using homozygous inbred lines, was 0.46 for non-adapted and 0.48 for adapted populations. A negative heterosis effect could be observed for the desiccation tolerance because nearly all of the inbred lines had a higher tolerance to the desiccation stress than the hybrid strain. Improvement of the desiccation tolerance by breeding was only obtained when the adaptation process was included in the selection process, which was related to a higher phenotypic variance in the populations after adaptation. A total of eight selection and breeding steps were carried out. Without previous adaptation, the mean of the tolerated aw-value remained almost constant between 0.94 and 0.93. In contrast, when we adapted the DJs prior to the exposure to desiccation stress, the tolerable aw-values continuously dropped from 0.89 to 0.81.