L1 transcripts and L1 retrotransposition in sperm and egg cells and early embryo development.
(A) Abundant L1 RNA is present in both developing egg and sperm cells and somatic cells. Some L1 RNA segregates from its encoding DNA. However, L1 retrotransposition events are rare in these cells.
(B) L1 transgene-positive gametes can be fertilized, carrying L1 RNA into the fertilized egg. L1 RNA transcribed either during egg and sperm cell development or during embryogenesis can retrotranspose into the genome.
(C) L1 transgene-negative gametes (both sperm and egg) can carry over L1 RNA that has segregated from its encoding DNA into the fertilized egg. This L1 RNA can be the source of infrequent L1 retrotransposition events during embryogenesis.
Retrotransposition events in scenario B occur more frequently than those in scenario C.
(Tg) L1 transgene; (Rtn) retrotransposition event. Shaded cells represent retrotransposition positive cells.
Credit: Haig Kazazian, PhD, University of Pennsylvania School of Medicine; Genes & Development