||This article provides insufficient context for those unfamiliar with the subject. (November 2008)|
The analysis of SINEs – Short INterspersed Elements – LINEs – Long INterspersed Elements – or truncated LTRs – Long Terminal Repeats – as molecular cladistic markers represents a particularly interesting complement to DNA sequence and morphological data.
The reason for this is that retrotransposons are assumed to represent powerful noise-poor synapomorphies. The target sites are relatively unspecific so that the chance of an independent integration of exactly the same element into one specific site in different taxa is not large and may even be negligible over evolutionary time scales. Retrotransposon integrations are currently assumed to be irreversible events; this might change since no eminent biological mechanisms have yet been described for the precise re-excision of class I transposons, but see van de Lagemaat et al. (2005). A clear differentiation between ancestral and derived character state at the respective locus thus becomes possible as the absence of the introduced sequence can be with high confidence considered ancestral.
In combination, the low incidence of homoplasy together with a clear character polarity make retrotransposon integration markers ideal tools for determining the common ancestry of taxa by a shared derived transpositional event. The “presence” of a given retrotransposon in related taxa suggests their orthologous integration, a derived condition acquired via a common ancestry, while the “absence” of particular elements indicates the plesiomorphic condition prior to integration in more distant taxa. The use of presence/absence analyses to reconstruct the systematic biology of mammals depends on the availability of retrotransposons that were actively integrating before the divergence of a particular species.
Examples for phylogenetic studies based on retrotransposon presence/absence data are the definition of whales as members of the order Cetartiodactyla with hippos being their closest living relatives, hominoid relationships, the strepsirrhine tree, the marsupial radiation from South America to Australia, and the placental mammalian evolution.
Inter-Retrotransposons Amplified Polymorphisms (IRAPs) are an alternative valuable retrotransposon-based markers. In this method, PCR oligonucleotide primers facing outwards from the LTR or other regions of retrotransposons are made and amplify between two retroelements inserted into the genome. As discussed above, the insertion of elements into the genome mean that the number of sites amplified and sizes of inter-retroelement fragments differ between different lines, and can be used as markers to detect genotypes, measure diversity or reconstruct phylogeny.
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