Can interaction specificity in the fungus-farming termite symbiosis be explained by nutritional requirements of the fungal crop?

Rafael R Da Costa,Michael Poulsen,Jonathan Z Shik,Duur K Aanen,Sabine M.E Vreeburg

doi:10.1016/j.funeco.2018.08.009

Rafael R Da Costa, Michael Poulsen + Show 3 more

Open Access

https://doi.org/10.1016/j.funeco.2018.08.009

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Abstract

Fungus-growing termites are associated with genus-specific fungal symbionts, which they acquire via horizontal transmission. Selection of specific symbionts may be explained by the provisioning of specific, optimal cultivar growth substrates by termite farmers. We tested whether differences in in vitro performance of Termitomyces cultivars from nests of three termite species on various substrates are correlated with the interaction specificity of their hosts. We performed single-factor growth assays (varying carbon sources), and a two-factor geometric framework experiment (simultaneously varying carbohydrate and protein availability). Although we did not find qualitative differences between Termitomyces strains in carbon-source use, there were quantitative differences, which we analysed using principal component analysis. This showed that growth of Termitomyces on different carbon sources was correlated with termite host genus, rather than host species, while growth on different ratios and concentrations of protein and carbohydrate was correlated with termite host species. Our findings corroborate the interaction specificity between fungus-growing termites and Termitomyces cultivars and indicate that specificity between termite hosts and fungi is reflected both nutritionally and physiologically. However, it remains to be demonstrated whether those differences contribute to selection of specific fungal cultivars by termites at the onset of colony foundation.

Highlights

Mutualisms are widespread in nature, with cooperation between species often providing entry into ecological niches that could not support either species alone (Moya et al, 2008)
Vertical transmission of symbionts generally leads to a high degree of interaction specificity and co-evolution, whereas horizontal transmission typically leads
We studied Termitomyces fungi from mature colonies for which selection of the resident fungal cultivar has already taken place

Summary

Introduction

Mutualisms are widespread in nature, with cooperation between species often providing entry into ecological niches that could not support either species alone (Moya et al, 2008). There are varying degrees of interaction specificities; i.e., possible combinations of hosts and symbionts (Aanen et al, 2007). It is often observed that (metabolic) traits of a symbiont are lost because their functions become redundant if the other partner reliably provides the resources (Visser et al, 2010; Ellers et al, 2012). Such reciprocal specialization can favour obligate symbiotic partnerships and foster cocladogenesis of symbionts, even in the absence of vertical transmission (Aanen et al, 2007)

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