Michael Lynch, Ph.D.
University of Indiana
The Origins of Genome Complexity
Wednesday - May 20, 2009
Pacific Forum – 3:00 p.m.
Most of the phenotypic diversity that we perceive in the natural world is directly attributable to the peculiar structure of the eukaryotic gene, which harbors numerous embellishments relative to the situation in prokaryotes. These include introns that must be spliced out of precursor mRNAs, transcribed but untranslated leader and trailer sequences (UTRs), modular regulatory elements that drive patterns of gene expression, and expansive intergenic regions that harbor control mechanisms. Explaining the origins of these features is difficult because they each impose an intrinsic fitness disadvantage by increasing the genic mutation rate to defective alleles. To address these issues, a general hypothesis for the emergence of eukaryotic gene structure will be discussed. Extensive observations on population sizes, recombination rates, and mutation rates strongly support the view that eukaryotes have reduced genetic effective population sizes relative to prokaryotes, with especially extreme reductions occurring in multicellular lineages. The resultant increase in the power of random genetic drift is sufficient to overwhelm the weak mutational disadvantages associated with most novel aspects of the eukaryotic gene, supporting the idea that most such changes arose as nonadaptive by-products rather than direct products of natural selection. However, by establishing a population-genetic environment permissive to the genome-wide repatterning of gene structure, the eukaryotic condition also promoted a reliable resource from which natural selection could secondarily build novel forms of organismal complexity.
Time permiting, I will also discuss: 1) the implications of recent theoretical and empirical results that suggest that multicellular organisms are particularly vulnerable to evolutionary increases in the mutation rate; and 2) evidence that mammalian genomes have experienced parallel evolutionary changes in genome architecture, most notably genome-size contraction, following the Cretaceous-Tertiary boundary.
Next: June 3 - Deborah Cramer