Divergent thermal specialisation of two South African entomopathogenic nematodes.

Matthew P Hill,John S Terblanche,Antoinette P Malan

doi:10.7717/peerj.1023

Abstract

Thermal physiology of entomopathogenic nematodes (EPN) is a critical aspect of field performance and fitness. Thermal limits for survival and activity, and the ability of these limits to adjust (i.e., show phenotypic flexibility) depending on recent thermal history, are generally poorly established, especially for non-model nematode species. Here we report the acute thermal limits for survival, and the thermal acclimation-related plasticity thereof for two key endemic South African EPN species, Steinernema yirgalemense and Heterorhabditis zealandica. Results including LT50 indicate S. yirgalemense (LT50 = 40.8 ± 0.3 °C) has greater high temperature tolerance than H. zealandica (LT50 = 36.7 ± 0.2 °C), but S. yirgalemense (LT50 = −2.4 ± 0 °C) has poorer low temperature tolerance in comparison to H. zealandica (LT50 = −9.7 ± 0.3 °C), suggesting these two EPN species occupy divergent thermal niches to one another.Acclimation had both negative and positive effects on temperature stress survival of both species, although the overall variation meant that many of these effects were non-significant. There was no indication of a consistent loss of plasticity with improved basal thermal tolerance for either species at upper lethal temperatures. At lower temperatures measured for H. zealandica, the 5 °C acclimation lowered survival until below −12.5 °C, where after it increased survival. Such results indicate that the thermal niche breadth of EPN species can differ significantly depending on recent thermal conditions, and should be characterized across a broad range of species to understand the evolution of thermal limits to performance and survival in this group.

Highlights

Temperature plays a key role in both the survival and activity of terrestrial invertebrates
Information on both basal thermal tolerance and the plasticity thereof can be compared to give insight into hierarchical levels of, and the magnitude and direction of, variation that may exist between species and across different groups (e.g., Hoffmann, Chown & Clusella-Trullas, 2013; Faulkner et al, 2014), and constraints or trade-offs that may be significant for understanding adaptive evolution
We extend on estimating the thermal niche breadth and examine whether short term induced thermal acclimation is able to alter basal thermal resistance in these entomopathogenic nematodes (EPN) species and discuss potential evolutionary trade-offs

Summary

Introduction

Temperature plays a key role in both the survival and activity of terrestrial invertebrates. Thermal tolerance may be characterized through traits such as thermal maxima and minima (absolute limits), processes or rates (e.g., development), as well as the optima thereof (e.g., temperatures for which growth rate and reproduction are maximised). Such basal thermal tolerance of terrestrial invertebrate species may be adjusted through plastic responses induced through acclimation, hardening or acclimatization, How to cite this article Hill et al (2015), Divergent thermal specialisation of two South African entomopathogenic nematodes. Information on both basal thermal tolerance and the plasticity thereof can be compared to give insight into hierarchical levels of, and the magnitude and direction of, variation that may exist between species and across different groups (e.g., Hoffmann, Chown & Clusella-Trullas, 2013; Faulkner et al, 2014), and constraints or trade-offs that may be significant for understanding adaptive evolution

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