HR: 08:20h
AN: B31D-02    [Abstracts]
TI: Can geological systems be used to calibrate rates of microbial genome evolution?
AU: * Banfield, J
EM: jill@seismo.berkeley.edu
AF: University of California, McCone Hall, Berkeley, CA 94720 United States
AB: An important challenge is to develop absolute time calibrations for molecular-level evolutionary processes. This is especially difficult for free-living microorganisms, which lack an interpretable fossil record. Absolute rates of genome change would enable connection of (for example) environmental perturbations documented in the geologic record to organismal responses. Thus, the stimuli for speciation and mechanisms of speciation could be directly linked. It may be possible to apply comprehensive environmental genomic studies in carefully chosen geological settings in order to tackle this problem. For example, Tyson et al. (2004) assembled sequence data from a low diversity biofilm community from an acidic ecosystem to reconstruct near complete and partial genomes for the dominant strain populations. As sequence data from many individuals were assembled into each composite genome, the dataset provides information about strain-level diversity. Ongoing growth in sequencing capability makes application of this approach to increasingly complex ecosystems feasible. Comparisons among groups of genomes from organisms separated by different evolutionary distances can reveal information about the relative rates at which different forms of genomic change occur. For example, the number of changes in gene content and gene order in the genomes of members of a strain population may be small, but the placement and types of mobile constituents (e.g., transposases and prophage) may differ significantly. Each type of genome change can be quantified (e.g., number of nucleotide polymorphisms, amino acid changes, gene duplications, gene loss/gain, gene order shuffling). These data also can be collected for organisms from the same lineage that diverged earlier (e.g., for species or genera). Statistics for each type of genome change yield semi-independent measures of evolutionary distance or relative rates of genomic change. Absolute time calibration is all that is needed to convert these data to real rates. Genomic studies could be carried out in geologic systems for which a date of onset of an environmental change conducive to colonization (e.g., the appearance of a hot spring or onset of acidification) can be determined. If it is assumed that at many of the current microorganisms descended from the colonists, then evolutionary rates can be estimated. Such analyses could be conducted at multiple sites of the same type to verify findings. Comparisons involving geographically separated sites would also provide information about the rates and pathways of microbial dispersal. The coupling of genomics with geochronology appears to be a logical extension of palaeontological studies, and should advance understanding of biological evolution over Earth history.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting