Improvement of heat and desiccation tolerance in Heterorhabditis bacteriophora through cross-breeding of tolerant strains and successive genetic selection

Abstract

Genetic selection can be a powerful tool to increase beneficial traits in biological control agents. In this study the heat and desiccation tolerance of the entomopathogenic nematode Heterorhabditis bacteriophora Poinar (Rhabditidomorpha: Strongyloidea) were significantly increased by cross breeding tolerant parental strains and successive genetic selection. These strains originated from a prior screening among 60 strains for increased stress tolerance. During genetic selection, the selection pressure was constantly increased and only the most tolerant 10% of the nematode populations were propagated for further selection steps. Assessment of tolerance and selection for both traits was performed with and without prior adaptation to the stress conditions. Eleven selection steps were performed to increase heat tolerance. A final overall increase in mean heat tolerance of 5.5°C was achieved when nematodes had been adapted to heat stress. For non-adapted tolerance an increase of 3.0°C from 40.1°C to 43.1°C was recorded. For comparison, a commercial strain had a mean tolerated temperature after adaptation of 38.2°C and of 36.5°C without adaptation. For assessment of the desiccation tolerance the mean tolerated water activity (a w -value) of a population was measured. Cross-breeding most tolerant strains reduced the a w -value from 0.67 to 0.65 after adaptation and from 0.9 to 0.7 without prior adaptation. The following six selection steps could not increase the tolerance whether nematodes had been adapted to stress or not. In comparison, the commercial strain tolerated a mean a w -value of 0.985 after adaptation and 0.951 without adaptation. Further investigation will have to assess trait stability and possible trade-off effects. This study is a first important step on the road towards domestication of the entomopathogenic nematode H. bacteriophora.

Fitness of heat and desiccation tolerant hybrid strains of Heterorhabditis bacteriophora (Rhabditidomorpha: Heterorhabditidae)

“…The H. bacteriophora hybrid strains (Table 1) had been produced within the study reported by (Mukuka et al 2010c). The virulence and reproduction potential was compared with the hybrid strain EN 01, which is the commercially used strain of e-nema GmbH (Schwentinental, Germany).…”

“…Penetration into G. mellonella was always less than 20%, however, significant differences were recorded (F = 4.08; Table 1 Description of hybrid strains, their mean tolerated temperature (°C) and/or water activity (a w ) after adaptation or without prior adaptation before exposure to stress conditions according to Mukuka et al (2010c) and mean ranking of fitness according to results presented in Table 2 Strain The highest penetration was recorded for HA1 (17.7%) followed by H3 (17.1%) and then DD2 (16.1%). The lowest number of penetrated DJs was recorded for the commercial strain EN01 (4.6%), although not significantly different from HH1 (6.2%), DA1 (7.1%), D3 (7.7%) and HD4 (9.6%) (Fig.…”

“…The three most tolerant strains were then crossed by using only those individuals of a population for crosses, which had tolerated the highest stress conditions. These resulting hybrids provided additional progress in stress tolerance (Mukuka et al 2010c). The maximum mean tolerated temperature (survival of 50% of the DJ population after heat treatment) reached 42°C, and the desiccation tolerance decreased to a minimum mean tolerated water activity (a w ) of 0.65 (Mukuka et al 2010c).…”

Virulence comparisons of high-temperature-adapted Heterorhabditis bacteriophora, Steinernema feltiae and S. carpocapsae

“…There are also several formulation types that contain different media and have different storage durations (Grewal, 2000). Recent studies have generally focused on the effectiveness of EPNs on different insect pests and the improvement of several characteristics with genetic studies, such as selective breeding or hybridization (Mukuka et al, 2010a(Mukuka et al, , 2010cNimkingrat et al, 2013;Susurluk et al, 2013;Ulu & Susurluk, 2014). At present, one of the major obstacles in EPN commercialization is the high application cost due to short shelf life and high production expenses.…”

“…Generally, the optimal temperature for EPNs is approximately 25 °C; however, some strains can handle temperatures up to 40 °C without any adaptation, or genetic selection (Koppenhöfer, 2000;Ulu & Susurluk, 2014). Several studies have achieved improvement in these characteristics with hybridization (Mukuka et al, 2010a). For a commercial EPN strain, higher heat tolerance alone may not be enough.…”