Evolución Bacteriana! Cumbre Mundial de Evolución

Las bacterias son un grupo fascinante para el desarrollo de estudios evolutivos debido a su relativa facilidad de estudio bajo condiciones controladas de laboratorio. Los avances en las tecnologías de análisis genético han abierto un mundo de posibilidades que nos permten entender mejor como se han desarrollado sus procesos evolutivos. Edward Feil, Roberto Kolter y John R. Roth presentaron excelentes charlas sobre la evolución bacteriana.

El Dr. Edward Feil del Departamento de Biología y Biochímica de la University de Bath, Reino Unido, habló sobre nuevas técnicas que permiten una mejor aproximación a la genómica poblacional en bacterias: "A pesar de que multiples secuencias genomicas completas bacterianas estan disponibles, la muestra de cadenas secuenciadas tiende a ser muy pequeña y poco representativa para permitir analisis poblacionales robustos. Técnicas como el MLST (multilocus sequence typing) han sido ampliamente usadas para analisis poblaciones. Nuevas plataformas de secuenciamiento son prometedoras para mejorar los análisis poblacionales al identificar el repertorio total SNP dentro de un genoma. Mis estudios ilustran el poder de esta técnica usando datos generados para 65 aislamientos de distribución global de una sola variante de Staphylococcus aureus resistente a la metacilina (MRSA). Estos datos revelan una significativa estructura geográfica, confirmando una transmisión intercontinental" dijo Edward Feil.

El Dr. Roberto Kolter de la Escuela de Medicina de Harvard, EE.UU., habló sobre la ecología y evolución de interacciones entre especies bacterias:

"Las interacciones entre especies de bacterias pueden ser mediadas por pequeñas moléculas de productos naturales. Para estudiar los roles ecológicos de estas moléculas se han desarrollano numerosos sistemás manipulativos de laboratorio. Estudios con bacterias del suelo como Streptomyces coelicolor y Bacillus subtilis han revelado que muchas de estas moléculas pueden tener efectos letales en otras especies, pero no en todos los casos. Por lo que se especula, que las ventajas evolutivas que se ganan a través de la evolución de la habilidad de hacer estos compuestos químicos puede incluir no solamente su habilidad para medir en interacciones antagonístas pero tambien en interacciones mutualistas".

El Dr. John Roth de la Universidad de California en Davis, EE.UU., hablo sobre un particular tema "Porqué es la selección natural dificil de vencer y cuando necesitas vencerla?".

A continuación, los resumenes de estas charlas:

Why is natural selection hard to beat and when do you need to beat it? by John R. Roth and Dan I. Andersson
Bacterial genetics defeats natural selection -- it uses positive selection to detect largephenotype mutants without influencing their frequency. Metazoans maintain organism integrity by defeating natural selection on somatic cell growth. Bacterial genetics relies on selection strong enough to prevent growth of both the parent and common slightlyimproved mutants. When selective stringency is reduced, frequent small-effect mutations allow growth and initiate a cascade of successive improvements. This rapid response rests on the unexpectedly high formation rate of small-effect mutations (particularly duplications and amplifications). Duplications form at a rate 104 times that of null mutations. The high frequency of small-effect mutations reflects features of replication, repair and coding that minimize the costs of mutation. The striking effect of small-effect mutations is seen in a system designed by John Cairns to test the effect of growth limitation on mutation rate. A leaky E. coli mutant (lac) is plated on lactose medium. Revertant (Lac+) colonies appear over 6 days above a lawn of (108) non-growing parent cells. These colonies have been attributed to stress induced mutagenesis of the non-growing parent. This conclusion ignores natural selection, assuming that only large-effect mutants appear– as is true for lab genetic selections. However, selection is not stringent in the Cairns system -- small increases in lac enzymes allow growth. Common cells with a lac duplication (and 2x the mutant enzyme level) initiate slow-growing colonies, in which selection drives a multi-step adaptation process – higher amplification, reversion to lac+ and loss of mutant lac alleles. The high yield of revertant colonies under selection does not reflect mutagenesis, but rather the high spontaneous rate of gene duplication (10-5), amplification (10-2/step) and the selective addition of mutation targets (more cells with more mutant lac copies/cell). Metazoan somatic cells may escape natural selection by the same mechanism. Metazoans reduce the basal level of unexpressed genes 1000-fold (compared to bacteria) by their epi-genetic modification of DNA and histones – making it impossible for small-effect mutations to provide growth.
Genome-wide SNP discovery in bacteria – population genomics comes of age by Edward J Feil, Simon Harris, Matthew Holden, Sharon Peacock, Herminia de Lencastre, and Stephen Bentley
There are currently nearly 3,000 completed or ongoing genome sequencing projects in bacteria, and these data have shed light on the relative roles of mutation, horizontal gene transfer and genomic rearrangements in bacterial diversification and adaptation. Whilst multiple complete genome sequences are available for a number of species, the sample of sequenced strains sequenced tends to be too small and unrepresentative for robust population level analysis. Sequenced-based typing techniques, such as multilocus sequence typing (MLST), have been widely used for population level analyses, and MLST datasets for several important pathogenic species number thousands of strains. However, MLST assays variation in only a tiny proportion
of the genome. New sequencing platforms promise to bridge the gap between genomewide data on small samples, and genome-limited data on large samples by identifying the total SNP repertoire within a query genome as mapped to a reference sequence. I will illustrate the power of this technique using data generated from 65 globally distributed isolates of a single widespread strain of methicillin resistant Staphylococcus aureus (MRSA). These data reveal substantial geographic structure, confirm intercontinental transmission.

Ecology and evolution of bacterial interspecies interactions by Roberto Kolter
Interspecies interactions amongst bacteria can be mediated by small molecule natural products. To study the ecological roles of these molecules we have devised numerous systems that can be easily manipulated in the laboratory. By studying interactions between the soil bacteria Streptomyces coelicolor and Bacillus subtilis we defined interactions mediated by secondary metabolites. Several molecules made by B. subtilis have profound effects on S. coelicolor development. Surfactin, a lipopeptide made by B. subtilis, specifically inhibits S. coelicolor’s ability to produce the peptide SapB, which is required for aerial mycelium formation. We discovered B. subtilis’ bacillaene, a linear hybrid polyketide/non-ribosomal peptide, because of its ability to repress pigment production (prodigiosin and actinorhodin) in S. coelicolor. Finally, we demonstrated that the dipeptide bacilysin from B. subtilis induces the production of these same pigments. We also searched for small molecule natural products made by soil bacteria that affected the development of B. subtilis biofilms. We discovered that several such molecules, among them nystatin, valinomycin, and gramicidin, induced the transcription of B. subtilis’ genes involved in producing the biofilm’s matrix. Many of the molecules thus discovered can have lethal effects on other species. But, as described above, we discovered that they can have significant non-lethal effects on other bacteria, even at high concentrations. This leads to the speculation that the selective advantages that are gained through evolving the ability to make these compounds may include not only their ability to mediate antagonistic interactions but also the ability to mediate mutualistic interactions.


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