The Evolution of Human Chromosome 2

#Bio The evolution of human chromosome 2

Clare Sansom We humans like to think of ourselves as members of The reason for this difference can be seen from Downloaded from http://portlandpress.com/biochemist/article-pdf/37/5/40/3805/bio037050040.pdf by guest on 01 October 2021 (Birkbeck College, UK) a species like no other, but, as many of the articles in a straightforward examination of the human and this issue of The Biochemist explain, we are genetically chimpanzee chromosome sets using a light microscope. pretty ordinary. Our genomes contain no more DNA We have not lost any DNA relative to the chimpanzee and no more genes than an average vertebrate, and (or other great apes); rather, the distinct banding our chromosome number – as every high-school pattern of the chromosomes reveals that two of the ape biology student knows, 22 pairs of autosomes plus chromosomes must have fused to form one large one. XX or XY, making 46 in total – is just about in the This ‘new’ human chromosome is, in fact, the second middle of the range for mammals. Interestingly, the largest as viewed under a microscope, and it is therefore domestic dog (Canis lupus familiaris) has one of the named chromosome 2. The chimpanzee chromosomes largest mammalian genomes in terms of chromosome involved in the fusion have been named in derivative number, with 39 pairs. fashion as 2A and 2B. It is actually quite surprising that we have only 23 There are a number of free web-based resources pairs of chromosomes. In taxonomy, Homo sapiens is one available for browsing and comparing genomes from of seven species that make up the family Hominidae, or different species. One of the most widely used is the the great apes. Every other living species in this lineage, Ensembl database (http://www.ensembl.org), which is including our closest relatives, the chimpanzees Pan a joint project of the Sanger Institute and the European paniscus and Pan troglodytes, has 24 chromosome pairs. Bioinformatics Institute, both based on the Hinxton Less than 3% of our DNA sequence differs from that Genome Campus near Cambridge. This focuses on of the chimpanzee, so it might have been expected that vertebrate genomics, and the main part of the site we would share something as basic as our chromosome currently holds data on about 70 species ranging number as well. alphabetically from the aardvark to the zebrafish. Sequence data are released well before the genome of each species can be considered ‘finished’. If you go to Ensembl and click on the icon for the human genome, which can be found in the main species list and also, not surprisingly, as one of the ‘popular genomes’ on the site’s top page, you will be presented with a wide list of resources. To go to chromosome 2, you first select ‘view karyotype’, click on the second chromosome, and then select ‘Chromosome Summary’. Selecting ‘Synteny’ from the left-hand menu will initially produce a complex diagram showing that parts of this human chromosome have significant similarities with no fewer than ten chromosomes from the mouse. Changing species from mouse to chimpanzee while keeping the chromosome number the same reveals a much simpler picture: all of the short arm (2p) and a short segment of the long arm (2q) of human chromosome 2 is significantly similar to the chimpanzee chromosome named 2A, and the rest of the long arm is similar to www.ensembl.org/Homo_sapiens/Location/Synteny?db=core&r=2%3A206444782- the chromosome named 2B. Very short segments of 206544782&otherspecies=Pan_troglodytes the chromosome are marked as similar to three other

40 October 2015 © Biochemical Society #Bio Downloaded from http://portlandpress.com/biochemist/article-pdf/37/5/40/3805/bio037050040.pdf by guest on 01 October 2021

3D rendered illustration of X-chromosomes chimpanzee chromosomes, but, by comparison, these at about the point in evolutionary history when the are not particularly significant. lineages that led to humans and chimpanzees diverged, A detailed examination of the sequences of these i.e. about 6 million years ago. At that time, it seems, one chromosomes provides further evidence for an end-to- of the many species of ape roaming the forests of Africa end chromosome fusion. All eukaryotic chromosomes became split into two isolated populations. This led to have characteristic sequences at their centres (the genetic divergence between the two lineages, including centromeres) and ends (the telomeres). The DNA in the fusion that formed human chromosome 2, and from both of these regions is repetitive in sequence and codes that point onwards, the lineages could no longer breed; for few, if any, genes. An alignment of the human and one, or possibly two, new species had been created. chimpanzee chromosomes shows that the centromere of It would take genetic analysis to distinguish our human chromosome 2 lines up almost exactly with that direct ancestors, the first true hominins, from their of chimpanzee chromosome 2A. Furthermore, there proto-chimpanzee cousins. Over the subsequent is a segment of the long arm of human chromosome 2, 6 million years, the genomes of the two lineages close to its centromere, that has significant similarity to continued to diverge, with the human lineage chimpanzee telomeric DNA. The detailed sequence in gradually acquiring the features that distinguish it this region is exactly what you would expect to see if the from the chimpanzee, including its bipedal posture, two chimpanzee chromosomes had fused end-to-end. It small canine teeth and the large brains that allowed is interesting to note that one of the normal functions of us to develop the technology necessary to colonize the telomeres is to prevent just such an end-to-end fusion. and thrive in all of the major ecological niches that The exact mechanism through which these two this planet allows. So far, at least, our species can be chimpanzee chromosomes evaded their protective considered an evolutionary success. ■ telomeres to fuse is not yet completely understood, although we do know that no genes from the ends of This article draws on Matt Ridley’s excellent Genome: the chimpanzee chromosomes 2A and 2B appear to have Autobiography of a Species in 23 Chapters (Fourth Estate, been lost. We also know that it must have happened London, 2000; ISBN 978-1-857--02835-5).