The dynamic interplay between genomic DNA and the outside world
Leitch, AR 2007, Heredity, vol. 98, pp. 61 – 62.
Exposure to the external environment has resulted in dynamic changes in the eukaryote genome, at a scale which has only recently been understood. In 2006, Tarlinton and colleagues postulated that the integration of a retrovirus into the koala genome occurred only within the last 100 years, which has led to retroviral endogenisation of some koala populations across Australia. This model provides further evidence that dynamism within the genome can be affected by the extra-genomic environment.
Genomic dynamism as a school of thought is a relatively recent development. The initial discovery of chromosomes suggested that they were stable entities of inheritance, with a simplistic view that gross chromosomal changes shuffle the genes for the next generation. This view was supported by later studies in the field of synteny, the study of how gene order is conserved within the genomes of species with distant common ancestors. Dynamism gained a foothold with the discovery of ‘jumping genes’ or transposons by McClintock in Zea mays (maize), with later analysis suggesting that the eukaryote genome is composed primarily of retrotransposons, such as Helitrons, which are transposable elements that amplify through rolling cycle replication. Further genome comparative analyses revealed that the amount and distribution of retrotransposons are highly variable, with rapid turnover of the elements occurring within the genome, as well as contributing to the dynamism within the genome.
The genome can be considered an ecological community of elements, as put forward by Brookfield in 2005, as new sequences can disrupt the nature of the intragenomic environment. Viral DNA has been detected in several eukaryote genomes, such as geminivirus related DNA (GRD) in Nicotiana species. GRD shares sequence similarity with Helitrons, and geminiviruses also replicate through rolling cycle replication. Rather than ‘invading’ the eukaryote genome, it was suggested that a geminivirus DNA sequence was captured by a Helitron and integrated into the genome.
While there are extensive data to support the dynamism view of change in eukaryote genomes, this phenomenon may be better understood by analysing ‘behind-the-scenes’ mechanisms. Thus, the recent integration but incomplete endogenisation of a retrovirus into the koala genome represents a highly valuable opportunity to study the mechanics of early retroviral integration and gain insights into the dynamics between the genome and the external environment.
Summarised by Daniel Chew
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