Polyploidy and Paralogous Chromosome Regions

Lars‐Gustav Lundin, Uppsala University, Uppsala, Sweden
Dan Larhammar, Uppsala University, Uppsala, Sweden
Finn Hallböök, Uppsala University, Uppsala, Sweden

Published online: March 2013

DOI: 10.1002/9780470015902.a0005072.pub3

Polyploidisations, that is, genome doublings, have taken place on numerous occasions during the evolution of animals and plants. The resulting duplicated chromosome regions can be recognised by their similar repertoires of gene families. Duplicated genes or regions within a genome are referred to as paralogous, and each set of related chromosome regions comprise a paralogon. In the ancestor of vertebrates, before the origin of the gnathostomes (jawed vertebrates), two tetraploidisations took place, abbreviated 2R for two rounds of tetraploidisation. A third tetraploidisation, 3R, happened before the radiation of the true bony fishes, the teleosts. Paralogues resulting from tetraploidisations are called ohnologues in honour of Susumu Ohno, who proposed the vertebrate tetraploidisations. Paralogous genes can undergo either subfunctionalisation (become more specialised) or neofunctionalisation (evolve novel functions). The vertebrate tetraploidisations seem to have paved the way for many gnathostome innovations such as jaws, limbs, an advanced nervous system and a complex adaptive immune system.

Key Concepts:

  • Polyploidisations have happened on numerous occasions in plant evolution and several times in vertebrate evolution.
  • A paralogon is a set of related chromosome regions resulting from duplications, usually tetraploidizations.
  • Paralogous genes can undergo either subfunctionalisation (become more specialised), neofunctionalisation (evolve novel functions) or loss.
  • The tetraploidisations in vertebrate evolution are thought to have facilitated evolution of new functions and structures.
  • After tetraploidisations, rearrangements scramble the chromosome regions, thereby obscuring the relationships.
  • The tetraploidisations do not only generate copies of single genes. Since all genes are duplicated, whole networks of genes or pathways are duplicated, such as the phototransduction pathway.
  • The total number of genes are doubled after a tetraploidisation event. However, the number of genes is later reduced due to different mutations such as deletions.

Keywords: paralogue; orthologue; Ohnologue; gene duplication; genome duplication; polyploidy; polyploidisation; tetraploidy; tetraploidisation

Figure 1. Proposed evolutionary history for the chromosomal regions containing the five gene families neurotrophin Trk receptors (NTRK), Src homology 2 domain containing (SHC), tropomodulin (TMOD), hyaluronan and proteoglycan link protein (HAPLN) and chondroitin sulfate proteoglycans aggrecan, versican, neurocan and brevican (CAN). A single ancestral chromosomal region was quadrupled in the basal vertebrate tetraploidisations. Subsequently, one local duplication took place whereupon a few paralogues were lost, leading to the present situation in the human genome. The number below each box shows the chromosomal position in Mb. Note that the gene order has been reshuffled to highlight similarities between the duplicated chromosomes. The number to the side of each line shows the chromosome number. The double dash indicates the breakpoint for a translocation involving chromosomes 9 and 5. Based on data from Hallböök et al. (2006).
Figure 2. Proposed evolutionary history of the chromosomal regions containing voltage-gated sodium channel (SCN) alpha genes and some of the adjacent gene families. A single ancestral chromosomal region was quadrupled in the basal vertebrate tetraploidisations. A third tetraploidisation took place in the teleost fish lineage. Local gene duplications have taken place both before and after the tetraploidisations. Some paralogues have been lost. The present gene repertoire is shown for human and zebrafish. Each gene's name or subtype is shown in its box. TGF, transforming growth factor; IGFBP, insulin-like growth factor binding protein; NPY, neuropeptide Y; and PYY, peptide YY. Each Hox cluster contains a large number of genes. The number below each box shows the chromosomal position in Mb. Note that the gene order has been reshuffled to highlight similarities between the duplicated chromosomes. The number to the side of each line shows the chromosome number. The double dash indicates the breakpoint for a translocation from human chromosome 7 to chromosome 3. In the zebrafish, four breakages and translocations have taken place. Drawn from data in Widmark et al. (2011) and Sundström et al. (2008).
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Related Sub-Topics:

Evolutionary Genomics | Molecular Evolution | Evolutionary Processes | Evolution of Coding and Functional Modules
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Article Title: Polyploidy and Paralogous Chromosome Regions
 
How to Cite close
Lundin, Lars‐Gustav, Larhammar, Dan, and Hallböök, Finn(Mar 2013) Polyploidy and Paralogous Chromosome Regions. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005072.pub3]