Also known as retrotransposons, these employ a strategy of self-copying via RNA transcriptase and subsequently inserting themselves into a new site within the host genome. The presence or absence of transcriptase (the enzyme which allows for self-copying) within the coding of the transposon defines class I elements as autonomous or non-autonomous. Class I transposons can take the form of:
* LTRs, long terminal repeats, which contain immensely repetitive code (hundreds or thousands of the same few nucleotides)
* Non-LTRs, which lack lengthy repetitive coding, and can be LINEs, long interspersed nuclear elements, which code for their own transpositional machinery, and SINEs, short interspersed nuclear elements, which piggy-back off of LINE machinery
Curiously, retrotransposons have been discovered to be the predominant form of transpositional element in plants with large genomes, such as maize and wheat, potentially indicating rapid success of this class of transposon in the creation of hybrids, such as wheat, peppermint and, in the distant past, maize. Plant hybridization often creates polyploids, with double, triple, quadruple or more the number of chromosomes present in the parent generation. Polyploid hybrids seem to be particularly susceptible to genetic intrusion by retrotransposons, as supported by a study in sunflower hybridization, which showed that the hybridized flowers possessed genomes that were about 50% larger than that of their parents, with the majority of this increase linked to the amplification of a single retrotransposon class