| Abstract |
The entomopathogenic nematode脙聝脗垄脙聜脗聙脙聜脗聯bacterium complex Heterorhabditis bacteriophora
Photorhabdus luminescens is one of the most widely used biological control agents against
several insect pests. The dauer juveniles (DJs) are third stage non-feeding and infective
juveniles, which carry the symbiotic bacteria, survive in the soil to search for and penetrate
insect hosts. For their industrial production, DJs are produced in large quantities (up to 40 m 3 ) in
e-nema GmbH (Schwentinental, Germany). Subsequently, DJs are stored, formulated,
transported and applied in the field. However, from production to application, DJs encounter
environmental stresses (desiccation, hypoxia, UV radiation, heat, and oxidative stress) in each
stage that influences longevity before they can actively kill the insect pests. Understanding
therefore the genetic component of the stress responses in H. bacteriophora and improving DJ
longevity are important research tasks. In this study, we combined approaches like classical
breeding and next-generation genomics to acquire knowledge about and improve DJ oxidative
stress tolerance and longevity. We first screen the oxidative stress tolerance in a wide collection
of H. bacteriophora strains and inbred lines from different geographical locations under with and
without oxidative stress inductions. We determined a high variability among strains of this
species and a high heritability for oxidative stress (h 2 > 0.9). Oxidative stress was also found to
have significant positive correlation with DJ longevity and persistence. Thus, oxidative stress can
be used as a predictor for DJ longevity and persistence, permitting a selection process within a
shorter testing period. In order to enhance tolerance to oxidative stress, longevity and
persistence, several genetic crosses, EMS-mutants and homozygous inbred lines were generated.
These genetic crosses, EMS-mutants and homozygous inbred lines were observed to have a
higher tolerance to oxidative stress, extended longevity in water and higher infectivity against
Tenebrio molitor compared to their respective parental (AU1 and HU2), donor (IL3) and
commercial (EN01) lines. Furthermore, the oxidative stress-responsive transcriptome of two
contrasting lines was analysed using Massive Amplification of cDNA Ends (MACE) and
candidate genes were screened for polymorphisms. More than 500 SNPs have been detected
between a high- and a low-surviving inbred line. PCR-based KASP markers were derived from
relevant transcripts and were tested in natural H. bacteriophora materials. Significant correlation
between genotype and phenotype was determined for a subset of KASP markers. This basic
research sets the basis for marker assisted selection and further breeding activities to prolong
shelf-life of the nematodes. |
Mindanao State
University - Iligan Institute of Technology