The Evolutionary Implication of Gene Expression Variation in Eukaryotes: From Yeast to Human

The expression level of a single gene can vary substantially within and between species, which might facilitate the emergence and fixation of novel expression patterns in the course of evolution. With rapidly accumulating data from genome-wide expression profiling, dense genotyping and individual genome re-sequencing, it is now possible to pinpoint the genetic loci that potentially give rise to gene expression variation. However, what remains elusive is how expression changes could be attributed to the differences in genetic elements, and our understanding of the phenotypic manifestation resulting from gene expression variation is far from comprehensive. In this thesis, I aim to answer these questions in budding yeast and in human. I first studied duplicated genes in budding yeast, which usually shared the identical expression patterns immediately upon duplication events. I searched for the cis-elements, whose divergence might explain the substantial expression variation between the extant paralogs, and established the role of nucleosome occupancy in driving expression differentiation between yeast duplicates. I next investigated the role of trans-factors in establishing species- or population-specific gene expression, and my study was specifically focused on primate microRNAs as a special class of regulators in trans. I first delineated the evolutionary trajectory of an X-linked primate microRNA cluster, and then proposed its function in regulating primate epididymal physiology. I extended this study to human by identifying several microRNAs with highly differentiated regulation among human populations, and such regulatory differentiation was driven by positive selection during recent human evolution. This study for the first time demonstrated high plasticity of the microRNA regulatory interactions in modulating expression variation of their target messengers. Beyond exploring the elements that control gene expression variation, I examined phenotypic manifestation of the observed expression variation in human populations, and my analysis revealed significant implication of expression variation towards differential disease susceptibility among individuals. Lastly, I examined gene expression variation at a micro scale among isogenic cell populations in budding yeast, which is termed “expression noise”. Though expression noise originates from stochasticity, my analysis demonstrated strong topological constraints on expression noise in yeast cellular networks, with which I was able to predict gene expression noise with high accuracy. These observations suggest that the seemingly stochastic gene expression may have been evolutionarily constrained. Taken together, my study presented in this thesis investigates the origin, consequence and evolutionary significance of gene expression variation in eukaryotes.Ph