Lecture 5.docx

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Ecology & Evolutionary Biology
Stephen Wright

Lecture 5: Evolution of Genome Complexity  What would a simple genome look like: 1. Just the protein coding genes ( and a minimal amount of additional regulatory DNA) 2. Protein coding genes not separated into pieces (exons) by other sequence (intron)  Gene number varies much less than gene density: Gene number is the amount of functional genes, and that does not vary as much. Humans have 24000 and yeast has around 6000. However humans gene density is a lot less than yeast since humans has a larger genome size  Variation in genome size and complexity is adaptive?  Maybe smaller organisms need to be able to replicate their genomes more quickly 1. Adaptive hypothesis: Larger organisms need larger genomes to adapt as a direct response to selection 2. Non-adaptive hypothesis: The organisms get bigger physically due to adaptation, and as a result their genomes also get larger (the latter is a non-adaptive consequence).  Mutation Pressure:  Gains of sequence are more common than loss of sequences (transposable elements)  Lynch argues that there is an insertional bias although data are somewhat mixed  Dangers of a larger genome: 1. Metabolic costs of DNA: energy is needed to replicate DNA 2. Increased mutational target size:  Non-functional neutral DNA can be target for deleterious “gain of function “ mutations  Transcription factor (TF) binding motifs are under-represented in non-coding regions, presumably as a result of selection  Insertion of TEs in non-coding regions often alters expression of neighboring genes  Alpha-thalassemia  Human blood disease resulting from two few globins  Affected individuals have Single Nucleotide Polymorphism (SNP) in non-coding region  SNP creates a new promoter-like element that interferes with normal activation of all downstream alpha-like globin genes  Why are there still harmful mutations around? That could be because selection may often be weak and effective neutrality happens if l4NeSl << 1  Lynch’s Hypothesis:  There is directional mutation pressure to increase genome size  Small increases in genome size are deleterious but very weakly so  Increases in genome size are more likely to be effectively neutral with smaller Ne since selection is weak  After genome sizes got larger, more complex patterns of gene regulation were able to evolve  Mutation and drift both play a key role  Mutation pressure is the source of excess DNA  Drift reduces the efficiency of selection  Predicts that
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