Ancient Genome Duplication
The topic of genome duplication gets revisited often in genome analysis and Lamprey genome paper is the most recent example. We are going through past literature and decided to keep notes here for anyone else interested.
Biological Implication
In 1967, Ohno wrote in his article titled “Evolution From Fish to Mammals by Gene Duplication” -
While speciation from an immediate common ancestor can be explained by allelic mutations at already existing gene loci, when evolution of the sub- phylum Vertebrata is considered in toto, allelic mutations are no longer sufficent to account for all the changes that have taken place during the past 300 million years. Gene duplication now emerges as the prime factor of evolution. It is becoming increasingly clear that in higher organisms such as mammals, one particular function is more often assigned to a group of several gene loci rather than to a single gene locus. Products of these several genes which arose by duplication from an ancestral gene perform the same function but in slightly different ways. These slight differences are exploited during ontogeny. Each somatic cell type at a given stage of development preferentially activates a few from the group of duplicated genes which fit the particular need of that cell type.
With passage of time, various changes take place in the the genome.
Random mutation of nucleotides
At the lowest timescale, various nucleotides in the genome get randomly mutated. Some As turn into Gs, Cs or Ts, some Cs turn into As and so on. Depending on where the mutation occurs, this change gives offspring slightly different property. At times, random mutations within non-essential genes may turn them into pseudo-genes.
Insertion and deletion of nucleotides
At the next higher time scale, nucleotides get randomly inserted or deleted from here and there in the genome.
Insertion and deletion of genes
From time to time, large chunks of nucleotides get duplicated or deleted too, and some of those chunks may have functional elements such as protein-coding genes. Over time, the newly created proteins may accept new functions.
Rearrangement of the genome
Rarely chunks of genome break and join other parts of the genome. Half of chromosome 7 may merges with 1/3rd of chromosome 2 to create a new chromosome. If the break point has two neighboring essential genes, who function through coexpression from a single promoter, splitting of chromosome can make them non-functional. Selection pressure would demand those two genes to stick together through all trials and tribulations.
Genome duplication
Genome duplication is the rarest of all possible genome rearrangements. In this case, the entire genome of an organism duplicates to create two copies of every single gene. Eventually, one copy of each gene continues to do what it was doing before, while the other one may get recruited to perform completely new tasks, or may get removed from the genome.
Ohno proposed that whole genome duplication was an important evolutionary mechanism to get so much diversity in animal kingdom, but confirmation of its validity had to wait for another ~30 years.
Bioinformatic Challenges in Finding Genome Duplication
Let us say someone analyzes a genome and tells you that it went through whole genome duplication two times in the past. Why should we need to take such analysis with a grain of salt?
Quality of genome assembly
We know that most published genomes are not fully assembled chromosomes, but rather a large group of scaffolds waiting to be turned into chromosomes some day in future. Few genes may be present twice or more due to inaccuracy in assembly. For example, polymorphic regions are very difficult to assemble. What if two copies of a gene are merely polymorphic versions of the same genomic region?
Gene Duplication
Even if the entire genome got duplicated in the past, many genes disappear after such duplication event. Let us say genes 1, 2, 3, 4, 5 and 6 get duplicated into genes (1a, 2a, 3a, 4a, 5a, 6a) and (1b, 2b, 3b, 4b, 5b, 6b). Then 1a, 2b, 3b and 6b disappears and we are left with (1b, 2a, 3a, 4a, 4b, 5a, 5b and 6a). How do we know that 4a and 4b or 5a and 5b came from whole genome duplication, and not merely duplication of 5 and 6?
A good bioinformatic analysis should address those issues.
Literature
It is no surprise that the first genome duplication paper in eukaryote came obviously after first eukaryotic genome was published (abstract, pdf).
**Molecular Evidence for an Ancient Duplication of the Entire Yeast Genome -
Kenneth H. Wolfe & Denis C. Shields**
Gene duplication is an important source of evolutionary novelty. Most duplications are of just a single gene, but Ohno proposed that whole-genome duplication (polyploidy) is an important evolutionary mechanism. Many duplicate genes have been found in Saccharomyces cerevisiae, and these often seem to be phenotypically redundant. Here we show that the arrangement of duplicated genes in the S. cerevisiae genome is consistent with Ohnos hypothesis. We propose a model in which this species is a degenerate tetraploid resulting from a whole-genome duplication that occurred after the divergence of Saccharomyces from Kluyveromyces. Only a small fraction of the genes were subsequently retained in duplicate (most were deleted), and gene order was rearranged by many reciprocal translocations between chromosomes. Protein pairs derived from this duplication event make up 13% of all yeast proteins, and include pairs of transcription
factors, protein kinases, myosins, cyclins and pheromones. Tetraploidy may have facilitated the evolution of anaerobic fermentation in Saccharomyces.
My bioinformatician colleagues and I used to discuss the above paper in 2001, and most were distrustful of the results. Why? It was because of the bioinformatics challenges mentioned above. Wolfe and Shields did not have to deal with incomplete chromosomes, but they had to address the question of gene deletion after whole genome duplication versus isolated gene duplication. This paragraph from methods section was what we used to debate about -
Statistical analysis. Chi-square tests (data not shown) indicate that duplicated genes in yeast are distributed in a highly non-random manner with regard to both the order in which homologous genes occur on pairs of chromosomes and the transcriptional orientations of those genes. A simultaneous origin of duplicate regions, as opposed to 55 independent duplications, is supported by a chi-square test on block orientations and by the lack of triplicated regions. The Poisson expectation if blocks were duplicated sequentially is for approximately 40 duplicated blocks, and 7 blocks that are replicated more than once (mainly triplicated). There is only one possible candidate for a triplicated region…
Another reason we used to discuss the paper was because rarely pure bioinformatics paper got accepted in Nature (here was another). Poor bioinformaticians like us did not have money to do experiments, and we could not get that money without papers in Science and Nature. Heard the problem before?
Next major paper on the same topic came about seven years later. It was not a completely bioinformatics paper, but it was also memorable for being the first genome paper with less than 400 authors :). Personally I remember the paper well, because I worked on their sequence data right after the paper came out. That was when I learned for the first time that genomes, even if published, were rarely nice sets of complete chromosomes.
Whole-genome duplication followed by massive gene loss and specialization has long been postulated as a powerful mechanism of evolutionary innovation. Recently, it has become possible to test this notion by searching complete genome sequence for signs of ancient duplication. Here, we show that the yeast Saccharomyces cerevisiae arose from ancient whole-genome duplication, by sequencing and analysing Kluyveromyces waltii, a related yeast species that diverged before the duplication. The two genomes are related by a 1:2 mapping, with each region of K. waltii corresponding to two regions of S. cerevisiae, as expected for whole-genome duplication. This resolves the long-standing controversy on the ancestry of the yeast genome, and makes it possible to study the fate of duplicated genes directly. Strikingly, 95% of cases of accelerated evolution involve only one member of a gene pair, providing strong support for a speci?c model of evolution, and allowing us to distinguish ancestral and derived functions.
Interestingly, Manolis Kellis’ recent ENCODE paper (Ward and Kellis) was the only one mentioned by Dan Graur to be one consistent with evolutionary theory.
Our results suggest continued turnover in regulatory regions, with at least an additional 4% of the human genome subject to lineage-specific constraint.
Literature Review in Fishes
Next, we will go through a set of papers on ancient genome duplication in fishes ending with the Lamprey paper. The topic of genome duplication in fishes has been explored extensively by Axel Meyer, Yves Van de Peer, John S. Taylor and Ingo Braasch. Their first paper on this topic appeared in 2001 (pdf). The main claim was that vertebrates went through ancient genome duplication twice in the past, but teleost fishes had an extra round of genome duplication.
Comparative genomics provides evidence for an ancient genome duplication event in fish
There are approximately 25000 species in the division Teleostei and most are believed to have arisen during a relatively short period of time ca. 200 Myr ago. The discovery of ‘extra’ Hox gene clusters in zebrafish (Danio rerio), medaka (Oryzias latipes), and pufferfish (Fugu rubripes), has led to the hypothesis that genome duplication provided the genetic raw material necessary for the teleost radiation. We identified 27 groups of orthologous genes which included one gene from man, mouse and chicken, one or two genes from tetraploid Xenopus and two genes from zebrafish. A genome duplication in the ancestor of teleost fishes is the most parsimonious explanation for the observations that for 15 of these genes, the two zebrafish orthologues are sister sequences in phylogenies that otherwise match the expected organismal tree, the zebrafish gene pairs appear to have been formed at approximately the same time, and are unlinked. Phylogenies of nine genes differ a little from the tree predicted by the fish-specific genome duplication hypothesis: one tree shows a sister sequence relationship for the zebrafish genes but differs slightly from the expected organismal tree and in eight trees, one zebrafish gene is the sister sequence to a clade which includes the second zebrafish gene and orthologues from Xenopus, chicken, mouse and man. For these nine gene trees, deviations from the predictions of the fish-specific genome duplication hypothesis are poorly supported. The two zebrafish orthologues for each of the three remaining genes are tightly linked and are, therefore, unlikely to have been formed during a genome duplication event. We estimated that the unlinked duplicated zebrafish genes are between 300 and 450 Myr. Thus, genome duplication could have provided the genetic raw material for teleost radiation. Alternatively, the loss of different duplicates in different populations (i.e. ‘divergent resolution’) may have promoted speciation in ancient teleost populations.
Other related papers -
Genome Duplication, a Trait Shared by 22,000 Species of Ray-Finned Fish (2003)
Major events in the genome evolution of vertebrates: Paranome age and size differ considerably between ray-finned fishes and land vertebrates (2004) (h/t: Vandepoele lab ?@plaza_genomics with note - “How a phylogenomics approach can partially deal with bad assemblies”)
Gene Loss and Evolutionary Rates Following Whole-Genome Duplication in Teleost Fishes (2006)
Duplicated Gene Evolution Following Whole-Genome Duplication in Teleost Fish
Reader Josesh Normal kindly suggested few other very relevant papers in the comment section. We are including recent ones here, but one classic text he mentioned is also valuable.
The medaka draft genome and insights into vertebrate genome evolution
Importance of Lamprey Genome
For yeast, the ancient genome duplication was suggested to be sometime after divergence of Saccharomyces from Kluyveromyces, as suggested by Wolfe and Shields through statistical analysis. Many researchers did not want to believe that analysis, but Kellis’ paper confirmed it empirically by sequencing Kluyveromyces yeast. Vertebrates went through two such rounds of genome duplication (2R) and teleost fish went through another extra round (3R). When exactly those duplications happened is an important question that Lamprey paper set out to answer. Their answer -
Analyses of the assembly indicate that two whole-genome duplications likely occurred before the divergence of ancestral lamprey and gnathostome lineages.
but that short sentence twitterifies very hard work by a team of 59 scientists. We will cover those details in a separate commentary.
