Detailed analysis and comparison of de novo identified Cassandra retrotransposons in Asteraceae species with their 5S rDNA genic equivalent
The 5S rRNA gene is one of the most conserved nucleotide sequences across all species. Nevertheless, in plants, especially within Asteraceae species, two main variants of 5S architecture can be observed depending on the co-localization of 5S rRNA with other ribosomal gene clusters: linked and separated arrangements. Coherent with the 5S preservation we observe the occurrence of 5S related non-autonomous LTR-retrotransposons, so called Cassandras, all across the plant kingdom. Cassandra sequences also show a unique LTR retrotransposon structure as they harbor the highly conserved 5S rRNA promoter genes within their long terminal repeats, advantageously providing them with a very strong promoter. However, the dynamics between Cassandra retrotransposon evolution in context of 5S rRNA genes are still undescribed. Here we show first evidence for co-evolution of Cassandra sequences with their corresponding 5S genic region. Interestingly Cassandra sequences show 5S rDNA promoter box mutations equivalent to their host. But although all analyzed plant Cassandras show a highly conserved region similar to the genomic 5S rDNA, they differ in overall sequence statistics. Plant Cassandra sequences can be condensed into families according to their species-dependent plant family association. We discuss these observations in the light of Cassandra evolution and more important the possibility of their single or multiple origin within the angiosperms. Furthermore, our findings provide new insights into dynamics of non-autonomous LTR retrotransposons, especially in light of the question if we observe co-evolution or the domestication of highly conserved genic regions by Cassandras.