Polyploidy and Paralogous Chromosome Regions

Abstract

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.