Systematic Analysis Of Intron Size And Abundance Arameters In Diyerse Lineages

Introns are important for the structure of the eukaryotic genome, and strongly coupled with gene expression, modification and transportation of transcripts. The size, number and sequence of introns shows different feature in different eukaryote. It shows different features in processing and routing the spliced transcripts with different size of introns. Introns can be divided into four types, i.e. ultra-short introns (less than50nt), small intron (50～200nt), big introns (200-4000nt) and ultra-long intron (more than4000nt). We collected159eukaryotes, including22protozoa,26fungi,20plants,38invertebrates and53vertebrates, and analized distribution characteristics of introns in their genomics. In general, there are more and longer introns with higher eukaryotes, and there are more and longer introns in higher animals than higher plants. There are usually two introns per containing-intron-gene in protists and fungi on average, five in higher plants, and ten in vertebrate. Ultra short intron exists mainly in the protists, fungi and part of lower animals, and small introns take the largest proportion of all introns in eukaryotes other than vertebrates,and the number of big introns nearly is half of all introns in higher plants, and ultra-long introns are mainly in higher animals. Parameters of the number and size of introns is the most conserved in higher plants and vertebrates among all species, there is same curve shape and peak-value in density distribution of the length of introns.Througth systematic analysis of introns, we found that intron length increased over evolutionary time, and expansibility of the length of introns is much more flexible in animal than plants, and small introns are highly conserved, show special distribution and functions,and are important in evolution. The different characteristics of introns, relates to different splicing mechanism and path of introns processing, and we put forward the hypothesis about the transcription-splicing-export coupling of eukaryotic introns. Firs, the small size of ultra short intron need simple structure of spliceosomes. Second, the size of small introns suits intron-definition splicing patterns, i.e. supper complexes shapping in the intron ends of the super spliceosome complex, forming a "marker" in the transcript, so protein factors related can be accurately and efficiently regulate transcripts, and export trancripts separatly from those intronless or only containing long intron. Small introns with special structure and function are select by natural selection in size and sequence with biological functions, which shows that the processing route of small introns in evolution also tend to make the corresponding gene function more efficiently. Finally, splicing about long introns and ultra-long introns suit exon-definition patterns,which more likely to occur alternative splicing, and these intron often divided by multi-step, by circular or nested splicing. Longer the intron is, longer time transcription spend, and more likely to be interference termination, so processing timeof long introns is unpredictable. In conclusion, the different characteristics of introns tend to be more efficient in evolution, in order to adapt to the genome and individual environment.