DNA evidence for global dispersal and probable endemicity of protozoa.
BACKGROUND
It 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.


RESULTS
Here 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.


CONCLUSIONS
Our 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.
Bass
David
D
Department of Zoology, The University of Oxford, South Parks Road, Oxford, OX1 3PS, UK. david.bass@zoo.ox.ac.uk
Richards
Thomas A
TA
Matthai
Lena
L
Marsh
Victoria
V
Cavalier-Smith
Thomas
T
D000818:Animals; D001483:Base Sequence; D044822:Biodiversity; D017931:DNA Primers; D016054:DNA, Protozoan / Q000235:genetics*; D004274:DNA, Recombinant; D021903:DNA, Ribosomal Spacer / Q000235:genetics*; D056890:Eukaryota / Q000145:classification / Q000235:genetics*; D019143:Evolution, Molecular; D015723:Gene Library; D005819:Genetic Markers; D014644:Genetic Variation; D005838:Genotype; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D016053:RNA, Protozoan / Q000235:genetics; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA
D017931:DNA Primers; D016054:DNA, Protozoan; D004274:DNA, Recombinant; D021903:DNA, Ribosomal Spacer; D005819:Genetic Markers; D016053:RNA, Protozoan; D012337:RNA, Ribosomal, 18S