Biogeography of thermophilic cyanobacteria and the importance of isolation to the evolution of microorganisms.

摘要

Evolutionary theory predicts the divergence of populations when they become geographically isolated. However, Baas Becking's theory that "everything is everywhere and the environment selects" excludes geographic isolation for microorganisms. In previous diversity and distribution studies, the sequencing of 16S rRNA genes acquired from natural Synechococcus populations residing in hot spring mats from Yellowstone National Park revealed that a single morphology concealed a rich 16S rRNA genotypic diversity. Predominating within that diversity is a group of closely related 16S rRNA genotypes (the A/B cluster) that are uniquely distributed along thermal and light gradients. Curiously, the upper temperature limit for cyanobacterial mat formation is different in globally disparate sites suggesting barriers to dispersal for some populations. I hypothesized that either members of the A/B cluster are distributed globally, but the highest temperature adapted forms (A types) are limited in their dispersal capabilities, or alternatively, globally disparate hot springs are dominated by unrelated Synechococcus genotypes. To test these hypotheses, I performed phylogenetic analysis on PCR-amplified, cloned, 16S rDNA genes recovered from Synechococcus populations residing in hot spring mats in Italy, New Zealand, Japan and the northwest U.S.A. The abundance of detected lineages was determined using lineage-specific oligonuleotide probes; low-abundance genotypes were sought using the same probes as PCR primers. I also assessed 20 different hot spring physical/chemical properties to determine whether adaptation was important to the local and global distributions of Synechococcus populations. Results revealed that: (1) AB cluster 16S rDNA sequences were not detected outside of the U.S., (2) each country had unique dominating Synechococcus genotypes, (3) within the U.S. and Japan there exist local geographic clades for A/B and C1 lineages, respectively, at the 16S rRNA and internal transcribed spacer region loci, (4) Oscillatoria amphigranulata, a filamentous thermophilic cyanobacterial species also demonstrated unique geographical distributions, and (5) genetic variation did not correlate with tested hot spring physical/chemical parameters. The results revealed that all cyanobacterial lineages had a different dispersal capability, but even the most widely dispersed exhibited substantial evidence of geographic isolation. Additional evidence for isolated prokaryotic populations is reviewed and the general importance of isolation in microbial evolution is emphasized.