Species-specific variation of alternative splicing and transcriptional initiation in six eukaryotes

The genome-wide detection of alternative splicing and transcriptional initiation (ASTI) was conducted in six eukaryotes (human, mouse, fruit fly, nematode, cress and rice) whose genome sequencing has been completed or nearly completed. Transcriptional isoforms were collected by mapping a batch of full-length cDNA sequences onto the respective cognate genomic sequences. Isoforms mapped on the same gene locus were compared pair-wise, ASTI patterns were segmented into minimal spans, and then the minimal patterns (ASTI units) were classified into unique types, such as the cassette type or the alternative donor site. All these procedures were performed automatically under the same conditions so that the results obtained from different species could be compared directly. The fraction of loci that underwent ASTI of the total mapped loci was the largest for mammals and fruit fly, and the smallest for plants. Exactly the same trend was observed for the number of unique ASTI types found in each species. The observed fractional representations of the ASTI types were similar between evolutionarily close species, such as human and mouse or cress and rice. On the other hand, the relative orders of abundance in individual ASTI type were considerably different between evolutionarily distant species, such as between mammals and plants. In human and mouse, alternative splicing other than the retained introns tended to occur within the protein coding sequence (CDS) regions rather than within the untranslated regions (UTRs), whereas this tendency was obscure in the other four species. In all the species examined, the difference in alternative exon lengths was most likely in multiples of three, and this tendency was most prominent when the alternative exons were embedded within the CDSs. These observations are generally consistent with the idea that higher organisms utilize the ASTI mechanisms more extensively and in a more complicated manner than lower organisms, and that ASTI actively participates in the enhancement of the functional and structural diversity of products generated from a limited number of genes on a genome.