The origins & evolution of genome complexityPlan of attackPowerPoint PresentationSlide 4Slide 5Slide 6Slide 7Slide 8Increasing genomic complexity in eukaryotes over evolutionary time.Slide 10Slide 11Slide 12What caused this increase in genomic size and complexity?Slide 14But there are problems with this hypothesis...Alternative hypothesisSlide 17Genetic DriftWhat is the [evolutionarily] meaningful size of a population?Effective population size (Ne)Slide 21How does smaller Ne lead to genome complexity?Slide 23DuplicationIncreasing genomic complexity over evolutionary timeSlide 26Slide 27Sources and image creditsThe origins & evolution of genome complexitySeth DonougheLynch & Conery (2003)Plan of attack•Review simplified definitions for: genes, genome, mRNA, codons, introns/exons, transposons–Two-fold purpose•Work through the data, discussing the theory along the wayGene: An inheritable sequence of DNA, which encodes one or more products.Genome: All of the hereditary information encoded in an organism’s DNA (contains all of its genes)DNA is transcribed into single-stranded mRNA.(with A, U, G, and C as the nucleotides)Each set of three nucleotides forms a codon.RNA polymeraseThe “canonical” genetic code. What are “silent sites”?mRNA is translated into a chain of amino acids = protein.Different kinds of diversity. How to infer about evolutionary past.Increasing genome sizeIncreasing genomic complexity in eukaryotes over evolutionary time.1) Introns (and exons)Increasing genomic complexity in eukaryotes over evolutionary time.2) Transposons2) TransposonsWhat caused this increase in genomic size and complexity?•The evolution of single-celled eukaryotes and multicellularity brought:–Increased intracellular structural variety–Cell differentiation and specializationPerhaps genomic complexity evolved as a means to achieve this adaptive diversification.But there are problems with this hypothesis...•Genomic complexity is not the only way to create different functions from the same genes•Some (rare) prokaryotes are capable of cell differentiation with smaller genomes•Increasingly long introns in some multicellular organisms and many transposons do not bring a clear functional advantage.Alternative hypothesis“The transition from prokaryote to unicellular eukaryote to multicellular eukaryotes was associated with orders-of-magnitude reductions in population size”Alternative hypothesisAs population size decreased, genetic drift became an increasingly powerful factor in changing the features of the genome. Why?Genetic Drift20 alleles Initial freq. = 0.5In general, alleles drift to fixation (frequency of 0 or 1) significantly faster in smaller populations.N = 10N = 100What is the [evolutionarily] meaningful size of a population?•Abundance is a coarse measurement•There is a broad trend:–Inverse relationship between population density and the body mass of an individual•We can do better with: –genetic effective population size.Effective population size (Ne)•How “faithfully” gene frequencies are transmitted across generations.–Can be estimated from the rates of mutation at silent sites (read: neutral mutations).–# of neutral mutations = 4Neu–Where u is the mutation rate per nucleotide •We can roughly measure u independently for taxa, allowing us to estimate NeHow does smaller Ne lead to genome complexity?•Gene duplication occurs at roughly the same rate (probably due to the same mechanism across all taxa) but …•Duplicated genes are lost much more slowly in smaller populations•Pairs of partially degenerated genes can fulfill a single functionDuplication•Duplicated genes can acquire new beneficial functions but the findings of this study indicate that this is unlikely to have been the driving cause behind increased genomic complexity.Increasing genomic complexity over evolutionary time•Introns and exons–Origin unknown, probably in the single ancestor of eukaryotes–Average of 4-7 introns per multicellular organism gene–Average of 2 for unicellular eukaryote gene•Virtually none has been found in prokaryotesSources and image
View Full Document
Unlocking...