Project description:Evolutionary outcomes depend not only on the selective forces acting upon a species, but also on the genetic background. However, large timescales and uncertain historical selection pressures can make it difficult to discern such important background differences between species. Experimental evolution is one tool to compare evolutionary potential of known genotypes in a controlled environment. Here we utilized a highly reproducible evolutionary adaptation in Saccharomyces cerevisiae to investigate whether experimental evolution of other yeast species would select for similar adaptive mutations. We evolved populations of S. cerevisiae, S. paradoxus, S. mikatae, S. uvarum, and interspecific hybrids between S. uvarum and S. cerevisiae for 200-500 generations in sulfate-limited continuous culture. Wild-type S. cerevisiae cultures invariably amplify the high affinity sulfate transporter gene, SUL1. However, while amplification of the SUL1 locus was detected in S. paradoxus and S. mikatae populations, S. uvarum cultures instead selected for amplification of the paralog, SUL2. We measured the relative fitness of strains bearing deletions and amplifications of both SUL genes from different species, confirming that, converse to S. cerevisiae, S. uvarum SUL2 contributes more to fitness in sulfate limitation than S. uvarum SUL1. By measuring the fitness and gene expression of chimeric promoter-ORF constructs, we were able to delineate the cause of this differential fitness effect primarily to the promoter of S. uvarum SUL1. Our data show evidence of differential sub-functionalization among the sulfur transporters across Saccharomyces species through recent changes in noncoding sequence. Furthermore, these results show a clear example of how such background differences due to paralog divergence can drive changes in genome evolution.
Project description:Evolutionary outcomes depend not only on the selective forces acting upon a species, but also on the genetic background. However, large timescales and uncertain historical selection pressures can make it difficult to discern such important background differences between species. Experimental evolution is one tool to compare evolutionary potential of known genotypes in a controlled environment. Here we utilized a highly reproducible evolutionary adaptation in Saccharomyces cerevisiae to investigate whether experimental evolution of other yeast species would select for similar adaptive mutations. We evolved populations of S. cerevisiae, S. paradoxus, S. mikatae, S. uvarum, and interspecific hybrids between S. uvarum and S. cerevisiae for ~200-500 generations in sulfate-limited continuous culture. Wild-type S. cerevisiae cultures invariably amplify the high affinity sulfate transporter gene, SUL1. However, while amplification of the SUL1 locus was detected in S. paradoxus and S. mikatae populations, S. uvarum cultures instead selected for amplification of the paralog, SUL2. We measured the relative fitness of strains bearing deletions and amplifications of both SUL genes from different species, confirming that, converse to S. cerevisiae, S. uvarum SUL2 contributes more to fitness in sulfate limitation than S. uvarum SUL1. By measuring the fitness and gene expression of chimeric promoter-ORF constructs, we were able to delineate the cause of this differential fitness effect primarily to the promoter of S. uvarum SUL1. Our data show evidence of differential sub-functionalization among the sulfate transporters across Saccharomyces species through recent changes in noncoding sequence. Furthermore, these results show a clear example of how such background differences due to paralog divergence can drive changes in genome evolution.
Project description:Throughout evolution, the duplication and functional divergence of transcription factors (TFs) has driven cellular and organismal complexity. Mechanisms by which paralogous TFs functionally diverge are thus of broad interest yet remain poorly understood. One well-established mechanism underlying TF divergence is the occupation and regulation of distinct sets of genes. Here we test for new mechanisms using CORONA (CNA) and PHABULOSA (PHB), two representative members of the CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIPIII) family of plant TFs. CNA and PHB have largely overlapping binding profiles yet each paralog has hundreds of uniquely regulated targets. Regulation of a given gene thus depends on whether its local binding site is considered primed (inactive) or regulated (active) by CNA or PHB. This decision appears to be controlled, at least in part, by their lipid binding START domain, proposing a model in which HD-ZIPIII TFs use information integrated by their START domain to generate paralog-specific transcriptional outcomes at commonly bound genes. Taken together, our study identifies a new mechanism of TF paralog divergence and proposes the ubiquitously distributed START evolutionary module as a driver of functional divergence.
Project description:Throughout evolution, the duplication and functional divergence of transcription factors (TFs) has driven cellular and organismal complexity. Mechanisms by which paralogous TFs functionally diverge are thus of broad interest yet remain poorly understood. One well-established mechanism underlying TF divergence is the occupation and regulation of distinct sets of genes. Here we test for new mechanisms using CORONA (CNA) and PHABULOSA (PHB), two representative members of the CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIPIII) family of plant TFs. CNA and PHB have largely overlapping binding profiles yet each paralog has hundreds of uniquely regulated targets. Regulation of a given gene thus depends on whether its local binding site is considered primed (inactive) or regulated (active) by CNA or PHB. This decision appears to be controlled, at least in part, by their lipid binding START domain, proposing a model in which HD-ZIPIII TFs use information integrated by their START domain to generate paralog-specific transcriptional outcomes at commonly bound genes. Taken together, our study identifies a new mechanism of TF paralog divergence and proposes the ubiquitously distributed START evolutionary module as a driver of functional divergence.
Project description:A fundamental problem in biology is the molecular basis for divergence among related organisms. We have investigated the level of divergence of transcription factor binding sites for two key factors that regulate developmental processes in the budding yeasts. The genomic binding locations for the Ste12 and Tec1 transcription factors in S. cerevisiae, S. mikatae and S. bayanus were mapped by chromatin immunoprecipitation combined with microarrays (chIP chip)1, 2 and compared to one another. While there was a large core network which was conserved in all three species, there were many instances of binding events whose relative levels differ significantly quantitatively in one species relative to another and as well as species-specific binding events. One interesting class of genes were identified that were bound only in S. mikatae and S. bayanus; many of these genes are targets of Ste12 in haploid strains of S. cerevisiae, suggesting that S. cerevisiae has uniquely acquired the ability to differentially regulate these genes in haploid and diploid cells in these species. To extend these studies, the transcriptional network for the Ste12 homologue (Cph1) in Candida albicans was also mapped and compared to the Saccharomyces species. Again, there were several genes bound by Cph1 which are involved in mating in S. cerevisiae, suggesting that the precise delineation between many mating and pseudohyphal targets by Ste12 may be specific to S. cerevisiae. Overall our results demonstrate that transcription binding sites differ faster than gene content indicating that gene regulation at the level of transcription factor binding is likely to be a major mode of evolutionary divergence between related species. We expect that this divergence is essential for the distinct ecological niches inhabited by these organisms. Keywords: chIP-chip