supplementary figures s1s11 and tables s1s4 are available at molecular biology and evolution online ( http://www.mbe.oxfordjournals.org/ ) .
l1 elements are mammalian non  long terminal repeat retrotransposons , or long interspersed elements ( lines ) , that significantly influence the dynamics and fluidity of the genome . 
 a series of observations suggest that plant l1-clade lines , just as mammalian l1s , mobilize both short interspersed elements ( sines ) and certain messenger rna by recognizing the 3-poly(a ) tail of rna . 
 however , one l1 lineage in monocots was shown to possess a conserved 3-end sequence with a solid rna structure also observed in maize and sorghum sines . 
 this strongly suggests that plant lines require a particular 3-end sequence during initiation of reverse transcription . 
 as one l1-clade line was also found to share the 3-end sequence with a sine in a green algal genome , i propose that the ancestral l1-clade line in the common ancestor of green plants may have recognized the specific rna template , with stringent recognition then becoming relaxed during the course of plant evolution .