DNA evidence for global dispersal and probable endemicity of protozoa

David Bass,Thomas Cavalier-Smith,Victoria Marsh,Lena Matthai,Thomas A Richards

doi:10.1186/1471-2148-7-162

David Bass, Thomas Cavalier-Smith + Show 3 more

Open Access

https://doi.org/10.1186/1471-2148-7-162

Copy DOI

Abstract

BackgroundIt is much debated whether microbes are easily dispersed globally or whether they, like many macro-organisms, have historical biogeographies. The ubiquitous dispersal hypothesis states that microbes are so numerous and so easily dispersed worldwide that all should be globally distributed and found wherever growing conditions suit them. This has been broadly upheld for protists (microbial eukaryotes) by most morphological and some molecular analyses. However, morphology and most previously used evolutionary markers evolve too slowly to test this important hypothesis adequately.ResultsHere we use a fast-evolving marker (ITS1 rDNA) to map global diversity and distribution of three different clades of cercomonad Protozoa (Eocercomonas and Paracercomonas: phylum Cercozoa) by sequencing multiple environmental gene libraries constructed from 47–80 globally-dispersed samples per group. Even with this enhanced resolution, identical ITS sequences (ITS-types) were retrieved from widely separated sites and on all continents for several genotypes, implying relatively rapid global dispersal. Some identical ITS-types were even recovered from both marine and non-marine samples, habitats that generally harbour significantly different protist communities. Conversely, other ITS-types had either patchy or restricted distributions.ConclusionOur results strongly suggest that geographic dispersal in macro-organisms and microbes is not fundamentally different: some taxa show restricted and/or patchy distributions while others are clearly cosmopolitan. These results are concordant with the 'moderate endemicity model' of microbial biogeography. Rare or continentally endemic microbes may be ecologically significant and potentially of conservational concern. We also demonstrate that strains with identical 18S but different ITS1 rDNA sequences can differ significantly in terms of morphological and important physiological characteristics, providing strong additional support for global protist biodiversity being significantly higher than previously thought.

Highlights

It is much debated whether microbes are dispersed globally or whether they, like many macro-organisms, have historical biogeographies
In 1934, Baas-Becking stated with respect to microbes that 'everything is everywhere' with the important qualification 'the environment selects' [1]. This ubiquitous dispersal hypothesis (UDH) of global dispersal with transient local occurrence is the focus of increasingly active debate [2,3,4,5]
Our culture-independent, molecular screening approach provides a robust framework for testing the moderate endemicity model at high phylogenetic resolution, as it does not distinguish between protists which differ in ease of propagation in the laboratory, or ease of detection and study by microscopy

Summary

Introduction

It is much debated whether microbes are dispersed globally or whether they, like many macro-organisms, have historical biogeographies. The ubiquitous dispersal hypothesis states that microbes are so numerous and so dispersed worldwide that all should be globally distributed and found wherever growing conditions suit them This has been broadly upheld for protists (microbial eukaryotes) by most morphological and some molecular analyses. In 1934, Baas-Becking stated with respect to microbes that 'everything is everywhere' with the important qualification 'the environment selects' [1] This ubiquitous dispersal hypothesis (UDH) of global dispersal with transient local occurrence is the focus of increasingly active debate [2,3,4,5]. Our culture-independent, molecular screening approach provides a robust framework for testing the moderate endemicity model at high phylogenetic resolution, as it does not distinguish between protists which differ in ease of propagation in the laboratory, or ease of detection and study by microscopy

Methods

Results

Conclusion