Recombination creates novel L1 (LINE-1) elements in Rattus norvegicus

Area of Science:

• Genomics
• Molecular Evolution
• Retrotransposon Biology

Background:

• Mammalian long interspersed repeat elements (LINE-1) are mobile genetic sequences crucial for genome evolution.
• L1 elements possess regulatory 5' UTR, protein-coding ORFs (ORF I, ORF II), and a 3' UTR.
• The continuous evolution of L1 elements generates novel subfamilies in extant species.

Purpose of the Study:

• To characterize the youngest known L1 subfamily in Rattus norvegicus, designated L1mlvi2.
• To investigate the evolutionary origins and potential mechanisms shaping the L1mlvi2 subfamily's unique structure.

Main Methods:

• Phylogenetic analysis of L1 element sequences.
• Comparative genomics to trace the ancestry of L1mlvi2's 3' UTR and ORF I.
• In silico modeling of potential recombination mechanisms.

Main Results:

• The L1mlvi2 subfamily exhibits dual ancestry, with its 3' UTR tracing a recent lineage and its ORF I originating from an older precursor.
• The ORF I sequence of L1mlvi2 is derived from an ancestral ORF I that predates its nearest chronological relatives, L13 and L14.
• Recombination, potentially via reverse transcriptase template strand switching, is proposed as the mechanism for ORF I recruitment.

Conclusions:

• The L1mlvi2 subfamily's dual ancestry highlights the dynamic nature of L1 element evolution in mammals.
• Recombination mechanisms, such as template strand switching, can explain the mosaic structure and acquisition of novel sequences in L1 elements.
• This study provides insights into the processes driving L1 5' UTR and ORF I evolution, including the acquisition of novel 5' UTRs.