Concerted evolution

Concerted evolution is the phenomenon where paralogous genes within one species are more closely related to one another than to members of the same gene family in closely related species. It is possible that this might occur even if the gene duplication event preceded the speciation event . High sequence similarity between paralogs may be maintained by homologous recombination events that lead to gene conversion, effectively copying some sequence from one and overwriting the homologous region in the other. Another possible hypothesis that has yet to be disproved is that rapid waves of gene duplication are responsible for the apparently "concerted" homogeneity of tandem and unlinked repeats seen in concerted evolution.

Example

An example can be seen in bacteria: Escherichia coli has seven operons encoding various Ribosomal RNA. For each of these genes, rDNA sequences are essentially identical among all of the seven operons (sequence divergence of only 0.195%). In a closely related species, Haemophilus influenzae its six ribosomal RNA operons are entirely identical. When the 2 species are compared together however, the sequence divergence of the 16S rRNA gene between them is 5.90%.[1]

Hypotheses to explain concerted evolution
  1. Rapid amplification of a gene, usually assisted by recombination events in IS elements, in bacteria, or in other repetitive genetic elements (ERV, LINE, SINE, etc.), for example, in eukaryotes. Unchecked transposition events of these transposable elements are thought to be associated with increases in the copy number of the gene.
  2. In sexually reproducing organisms unequal crossing over during meiosis may be responsible for amplification due to misalignment of repeated sequences.
  3. Redistribution of genes requires transposition, probably assisted by the same repetitive genetic elements as in 1).
  4. Homogenization of alleles by gene conversion may also play a role in sexually reproducing organisms. Some genes may be more prone to gene conversion than others, thus reinforcing the unity of the genes within a gene family of a species.

Evolution and speciation

Findings of concerted evolution, particularly in ribosomal DNA genes, led the Cambridge molecular geneticist Gabriel Dover to his controversial proposal of molecular drive, which in his view was an evolutionary principle distinct both from natural selection and from genetic drift. Closely related species or even populations may differ in their nucleolus organizing regions (NORs), which are genomic regions that contain many copies of ribosomal RNA genes in eukaryotes, typically found within or adjacent to highly repetitive parts of the genome such as centromeres or telomeres in mammals such as the house mouse Mus musculus [2] or insects such as the grasshopper Podisma pedestris.[3]

So this begs the question: are NORs and ribosomal genes located on different genomic locations in different species as a result of natural selection for the good of the genomes that bear them? Or are these changes merely a by-product of selfish DNA, such as transposable element amplification? If repeats are relocated to different parts of the genome in related species, what happens to the ancestral copies? How are tandem arrays of new ribosomal genes born, and how do existing tandem arrays become lost? We don't yet know the answers, but it will be the task of future evolutionary genomics and molecular genetics studies to understand the mechanisms of concerted evolution.

Does concerted evolution or molecular drive play a role in speciation? We don't yet know, but it seems possible that for example some hybrids or backcrosses between species with different nucleolar organizing regions/ribosomal DNA repeat regions may have reduced fitness as a result of over- or under-expression of ribosomal RNA.

References
  1. Liao, D (1999). "Concerted evolution: molecular mechanism and biological implications". Am J Hum Genet. 64 (1): 24–30. doi:10.1086/302221. PMC 1377698. PMID 9915939.
  2. Britton-Davidian, J (2012). "Chromosomal dynamics of nucleolar organizer regions (NORs) in the house mouse: micro-evolutionary insights". Heredity. 108: 68–74. doi:10.1038/hdy.2011.105.
  3. Bella, JL (1991). "Sex chromosome and autosome divergence in Podisma (Orthoptera) in western Europe". Genetics Selection Evolution. 23: 5–13. doi:10.1186/1297-9686-23-1-5.
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