<p>The relative amounts of secreted proteins in the major molecular weight bands as determi... more <p>The relative amounts of secreted proteins in the major molecular weight bands as determined by SDS-PAGE.<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0147024#t002fn001" target="_blank"><sup>a</sup></a></p
Most models of speciation require gradual change and geographic or ecological isolation for new s... more Most models of speciation require gradual change and geographic or ecological isolation for new species to arise. Homoploid hybrid speciation occurred readily between Saccharomyces cerevisiae and Saccharomyces paradoxus . Hybrids had high self-fertility (about 82%), low fertility when backcrossed to either parental species (about 7.5%), and vigorous growth under different thermal environments that favored one or the other of the parental species. Extensive karyotypic changes (tetrasomy) were observed in the hybrids, although genic incompatibilities accounted for 50% of the variation in self-fertility.
Pseudomonas fluorescens is a model for the study of adaptive radiation. When propagated in a spat... more Pseudomonas fluorescens is a model for the study of adaptive radiation. When propagated in a spatially structured environment, the bacterium rapidly diversifies into a range of niche specialist genotypes. Here we present a genetic dissection and phenotypic characterization of the fuzzy spreader (FS) morphotype—a type that arises repeatedly during the course of the P. fluorescens radiation and appears to colonize the bottom of static broth microcosms. The causal mutation is located within gene fuzY (pflu0478)—the fourth gene of the five-gene fuzVWXYZ operon. fuzY encodes a β-glycosyltransferase that is predicted to modify lipopolysaccharide (LPS) O antigens. The effect of the mutation is to cause cell flocculation. Analysis of 92 independent FS genotypes showed each to have arisen as the result of a loss-of-function mutation in fuzY, although different mutations have subtly different phenotypic and fitness effects. Mutations within fuzY were previously shown to suppress the phenotype...
The distribution of the number of pairwise differences calculated from comparisons between n hapl... more The distribution of the number of pairwise differences calculated from comparisons between n haploid genomes has frequently been used as a starting point for testing the hypothesis of linkage equilibrium. For this purpose the variance of the pairwise differences, VD, is used as a test statistic to evaluate the null hypothesis that all loci are in linkage equilibrium. The problem is to determine the critical value of the distribution of VD. This critical value can be estimated either by Monte Carlo simulation or by assuming that VD is distributed normally and calculating a one-tailed 95% critical value for VD, L, L = E(VD) + 1.645 Var(VD), where E(VD) is the expectation of VD, and Var(VD) is the variance of VD. If VD (observed) > L, the null hypothesis of linkage equilibrium is rejected. Using Monte Carlo simulation we show that the formula currently available for Var(VD) is incorrect, especially for genetically highly diverse data. This has implications for hypothesis testing in ...
Successive adaptive radiations have played a pivotal role in the evolution of biological diversit... more Successive adaptive radiations have played a pivotal role in the evolution of biological diversity. The effects of adaptive radiation are often seen, but the underlying causes are difficult to disentangle and remain unclear. Here we examine directly the role of ecological opportunity and competition in driving genetic diversification. We use the common aerobic bacterium Pseudomonas fluorescens, which evolves rapidly under novel environmental conditions to generate a large repertoire of mutants. When provided with ecological opportunity (afforded by spatial structure), identical populations diversify morphologically, but when ecological opportunity is restricted there is no such divergence. In spatially structured environments, the evolution of variant morphs follows a predictable sequence and we show that competition among the newly evolved niche-specialists maintains this variation. These results demonstrate that the elementary processes of mutation and selection alone are sufficient to promote rapid proliferation of new designs and support the theory that trade-offs in competitive ability drive adaptive radiation.
Proceedings of the National Academy of Sciences, 1994
We followed evolutionary change in 12 populations of Escherichia coli propagated for 10,000 gener... more We followed evolutionary change in 12 populations of Escherichia coli propagated for 10,000 generations in identical environments. Both morphology (cell size) and fitness (measured in competition with the ancestor) evolved rapidly for the first 2000 generations or so after the populations were introduced into the experimental environment, but both were nearly static for the last 5000 generations. Although evolving in identical environments, the replicate populations diverged significantly from one another in both morphology and mean fitness. The divergence in mean fitness was sustained and implies that the populations have approached different fitness peaks of unequal height in the adaptive landscape. Although the experimental time scale and environment were microevolutionary in scope, our experiments were designed to address questions concerning the origin as well as the fate of genetic and phenotypic novelties, the repeatability of adaptation, the diversification of lineages, and ...
The cause of reproductive isolation between biological species is a major issue in the field of b... more The cause of reproductive isolation between biological species is a major issue in the field of biology. Most explanations of hybrid sterility require either genetic incompatibilities between nascent species or gross physical imbalances between their chromosomes, such as rearrangements or ploidy changes. An alternative possibility is that genomes become incompatible at a molecular level, dependent on interactions between primary DNA sequences. The mismatch repair system has previously been shown to contribute to sterility in a hybrid between established yeast species by preventing successful meiotic crossing-over leading to aneuploidy. This system could also promote or reinforce the formation of new species in a similar manner, by making diverging genomes incompatible in meiosis. To test this possibility we crossed yeast strains of the same species but from diverse historical or geographic sources. We show that these crosses are partially sterile and present evidence that the mismatch repair system is largely responsible for this sterility.
A designed protein complex processively copies one strand of a plasmid with errors to speed up mo... more A designed protein complex processively copies one strand of a plasmid with errors to speed up molecular evolution in vivo.
Archaea are a unique system for investigating the diversity of life. There are the most diverse g... more Archaea are a unique system for investigating the diversity of life. There are the most diverse group of organisms with the longest evolutionary history of life on Earth. Phylogenomic investigations reveal the complex evolutionary history of Archaea, overturning longstanding views of the history of life. They exist in the harshest environments and benign conditions, providing a system to investigate the basis for living in extreme environments. They are frequently members of microbial communities, albeit generally rare. Archaea were central in the evolution of Eukaryotes and can be used as a proxy for studying life on other planets. Future advances will depend not only upon phylogenomic studies but also on a better understanding of isolation and cultivation techniques.
<p>The relative amounts of secreted proteins in the major molecular weight bands as determi... more <p>The relative amounts of secreted proteins in the major molecular weight bands as determined by SDS-PAGE.<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0147024#t002fn001" target="_blank"><sup>a</sup></a></p
Most models of speciation require gradual change and geographic or ecological isolation for new s... more Most models of speciation require gradual change and geographic or ecological isolation for new species to arise. Homoploid hybrid speciation occurred readily between Saccharomyces cerevisiae and Saccharomyces paradoxus . Hybrids had high self-fertility (about 82%), low fertility when backcrossed to either parental species (about 7.5%), and vigorous growth under different thermal environments that favored one or the other of the parental species. Extensive karyotypic changes (tetrasomy) were observed in the hybrids, although genic incompatibilities accounted for 50% of the variation in self-fertility.
Pseudomonas fluorescens is a model for the study of adaptive radiation. When propagated in a spat... more Pseudomonas fluorescens is a model for the study of adaptive radiation. When propagated in a spatially structured environment, the bacterium rapidly diversifies into a range of niche specialist genotypes. Here we present a genetic dissection and phenotypic characterization of the fuzzy spreader (FS) morphotype—a type that arises repeatedly during the course of the P. fluorescens radiation and appears to colonize the bottom of static broth microcosms. The causal mutation is located within gene fuzY (pflu0478)—the fourth gene of the five-gene fuzVWXYZ operon. fuzY encodes a β-glycosyltransferase that is predicted to modify lipopolysaccharide (LPS) O antigens. The effect of the mutation is to cause cell flocculation. Analysis of 92 independent FS genotypes showed each to have arisen as the result of a loss-of-function mutation in fuzY, although different mutations have subtly different phenotypic and fitness effects. Mutations within fuzY were previously shown to suppress the phenotype...
The distribution of the number of pairwise differences calculated from comparisons between n hapl... more The distribution of the number of pairwise differences calculated from comparisons between n haploid genomes has frequently been used as a starting point for testing the hypothesis of linkage equilibrium. For this purpose the variance of the pairwise differences, VD, is used as a test statistic to evaluate the null hypothesis that all loci are in linkage equilibrium. The problem is to determine the critical value of the distribution of VD. This critical value can be estimated either by Monte Carlo simulation or by assuming that VD is distributed normally and calculating a one-tailed 95% critical value for VD, L, L = E(VD) + 1.645 Var(VD), where E(VD) is the expectation of VD, and Var(VD) is the variance of VD. If VD (observed) > L, the null hypothesis of linkage equilibrium is rejected. Using Monte Carlo simulation we show that the formula currently available for Var(VD) is incorrect, especially for genetically highly diverse data. This has implications for hypothesis testing in ...
Successive adaptive radiations have played a pivotal role in the evolution of biological diversit... more Successive adaptive radiations have played a pivotal role in the evolution of biological diversity. The effects of adaptive radiation are often seen, but the underlying causes are difficult to disentangle and remain unclear. Here we examine directly the role of ecological opportunity and competition in driving genetic diversification. We use the common aerobic bacterium Pseudomonas fluorescens, which evolves rapidly under novel environmental conditions to generate a large repertoire of mutants. When provided with ecological opportunity (afforded by spatial structure), identical populations diversify morphologically, but when ecological opportunity is restricted there is no such divergence. In spatially structured environments, the evolution of variant morphs follows a predictable sequence and we show that competition among the newly evolved niche-specialists maintains this variation. These results demonstrate that the elementary processes of mutation and selection alone are sufficient to promote rapid proliferation of new designs and support the theory that trade-offs in competitive ability drive adaptive radiation.
Proceedings of the National Academy of Sciences, 1994
We followed evolutionary change in 12 populations of Escherichia coli propagated for 10,000 gener... more We followed evolutionary change in 12 populations of Escherichia coli propagated for 10,000 generations in identical environments. Both morphology (cell size) and fitness (measured in competition with the ancestor) evolved rapidly for the first 2000 generations or so after the populations were introduced into the experimental environment, but both were nearly static for the last 5000 generations. Although evolving in identical environments, the replicate populations diverged significantly from one another in both morphology and mean fitness. The divergence in mean fitness was sustained and implies that the populations have approached different fitness peaks of unequal height in the adaptive landscape. Although the experimental time scale and environment were microevolutionary in scope, our experiments were designed to address questions concerning the origin as well as the fate of genetic and phenotypic novelties, the repeatability of adaptation, the diversification of lineages, and ...
The cause of reproductive isolation between biological species is a major issue in the field of b... more The cause of reproductive isolation between biological species is a major issue in the field of biology. Most explanations of hybrid sterility require either genetic incompatibilities between nascent species or gross physical imbalances between their chromosomes, such as rearrangements or ploidy changes. An alternative possibility is that genomes become incompatible at a molecular level, dependent on interactions between primary DNA sequences. The mismatch repair system has previously been shown to contribute to sterility in a hybrid between established yeast species by preventing successful meiotic crossing-over leading to aneuploidy. This system could also promote or reinforce the formation of new species in a similar manner, by making diverging genomes incompatible in meiosis. To test this possibility we crossed yeast strains of the same species but from diverse historical or geographic sources. We show that these crosses are partially sterile and present evidence that the mismatch repair system is largely responsible for this sterility.
A designed protein complex processively copies one strand of a plasmid with errors to speed up mo... more A designed protein complex processively copies one strand of a plasmid with errors to speed up molecular evolution in vivo.
Archaea are a unique system for investigating the diversity of life. There are the most diverse g... more Archaea are a unique system for investigating the diversity of life. There are the most diverse group of organisms with the longest evolutionary history of life on Earth. Phylogenomic investigations reveal the complex evolutionary history of Archaea, overturning longstanding views of the history of life. They exist in the harshest environments and benign conditions, providing a system to investigate the basis for living in extreme environments. They are frequently members of microbial communities, albeit generally rare. Archaea were central in the evolution of Eukaryotes and can be used as a proxy for studying life on other planets. Future advances will depend not only upon phylogenomic studies but also on a better understanding of isolation and cultivation techniques.
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Papers by Michael Travisano