Genetic structure in the endosymbiont Breviolum 'muscatinei' is correlated with geographical location, environment and host species.

Brendan H. Cornwell,Luis Hernández

doi:10.1098/rspb.2020.2896

Brendan H. Cornwell, Luis Hernández

Open Access

https://doi.org/10.1098/rspb.2020.2896

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Abstract

Corals and cnidarians form symbioses with dinoflagellates across a wide range of habitats from the tropics to temperate zones. Notably, these partnerships create the foundation of coral reef ecosystems and are at risk of breaking down due to climate change. This symbiosis couples the fitness of the partners, where adaptations in one species can benefit the holobiont. However, the scales over which each partner can match their current—and future—environment are largely unknown. We investigated population genetic patterns of temperate anemones (Anthopleura spp.) and their endosymbiont Breviolum ‘muscatinei’, across an extensive geographical range to identify the spatial scales over which local adaptation is possible. Similar to previously published results, two solitary host species exhibited isolation by distance across hundreds of kilometres. However, symbionts exhibited genetic structure across multiple spatial scales, from geographical location to depth in the intertidal zone, and host species, suggesting that symbiont populations are more likely than their hosts to adaptively mitigate the impact of increasing temperatures.

Highlights

There is mounting evidence that associations with microbial symbiont partners can dramatically expand the niche space of a host species [1,2,3]
In the Anthopleura spp.–B. ‘muscatinei’ partnership, a combination of low rates of gene flow and strong selective gradients across metres in the intertidal zone as well as across their geographical range shape the genetic structure of the symbiont
0.4 covariance population genetic signals of local adaptation across any of the spatial scales we examined in this study

Summary

Introduction

There is mounting evidence that associations with microbial symbiont partners can dramatically expand the niche space of a host species [1,2,3]. We examine the degree to which differentiated 2 symbiont populations are associated with varying environmental conditions at the macroscale of the geographical range and the mesoscale of the intertidal zone (using the surrounding benthic community as a proxy for environmental variation at this scale). We evaluated population genetic patterns in the hosts across their geographical ranges, and symbionts across a set of nested spatial scales (see electronic supplementary material for details). We assessed genetic differentiation across smaller spatial scales by developing a proxy for the microhabitat of each polyp using the surrounding biotic community (see electronic supplementary material for details) and used this as an environmental measure in a principle components analysis, a partial Mantel test, as well as using the R package BEDASSLE [28] to identify the relative contributions of geographical and environmental distance in structuring populations. We re-calculated the covariance after removing the host species effect on symbiont allele frequencies to quantify the role of species-specific interactions in shaping covariance between host and symbiont loci

Results

Findings

Discussion