In May, researchers led by Karen Nelson (The Institute for Genomic Research) reported obtaining the complete 1.8-Mb genomic sequence of the heat-loving bacterium Thermotoga maritima, first isolated from geothermally heated marine sediment in Vulcano, Italy. Early analysis reveals some unusual features that could affect our understanding of how earth's simplest life forms evolved.
The three major life groups or kingdoms are eubacteria and archaea, which include the simplest life forms lacking a central nucleus; and the more complex eucaryota, which include animals and plants. T. maritima has been considered one of the deepest and most slowly evolving lineages in the eubacteria kingdom.
Authors of the May 27 article in Nature (399, 323-29) reported that almost a quarter of T. maritima's genes are similar to those found in archaea, with 81 archaeal-style genes clustered in 15 genomic regions. These results pose new questions about defining organisms that have mosaic-like genomes, with features shared across two domains. The authors note that these findings do not necessarily reflect a closely shared common ancestor but could point instead to lateral gene transfers.
Because of growing evidence for a high frequency of gene transfers and a lack of agreement in different phylogenetic (evolutionary) analyses on individual genes, the authors suggest that sequence comparisons of individual genes may be inaccurate indicators of organismal evolution. Relationships among the eubacteria and archaea willbe understood better as other microbial genomes are fully sequenced and analyzed. |
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