Perkinsus SL RNAs mark earlier emergence and more complex evolution of trans-splicing in alveolates PmaSLRNA sequences are similar to those of dinoflagellate SL RNAs, including the location of an apparent Sm-binding domain in the exon rather than in the intron, as is the case typically in other SL trans-splicing eukaryotes (see [8] , [9] for review). The SL has left its footprints in other parts of the dinoflagellate genome in the form of single and tandem exon repeats located adjacent to the 59 UTRs of protein coding genes [28] . This apparently unproductive phenomenon is postulated to occur when SLcontaining mRNA are reverse-transcribed and integrated to the genome [28] but could also be a result of chromosome cross-over recombination [16] . Likewise, SL exons in single or multiple units were found in some P. marinus genes. The S-type SL with L-type intron was also suggested to exist based on PCR-amplified cDNA sequences of P. marinus SL RNA [29] , although it requires verification by further genomic analysis.