An observed trend was revealed between exon length and its ordinal position in all species analyzed (Fig. 2B ) and a decay curve was observed in mammal, plant and fish genomes ( P < 0.001 mostly, except for Fly P = 0.017).
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Patterns of exon-intron architecture variation of genes in eukaryotic genomes.
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If the amount of exons or introns follows an increasing trend (during the process of genome-size expansion), the first exon and intron are older than the next ones; the older introns have more time to be inserted and become longer; the inserted sequences in introns generally have a lower GC content; and the later occurring introns cut the coding sequences into shorter ones except for the first and the last exon, which are required by splicing-related factors; the subsequent recruited exons (or parts of them), compared with the first exons, have a higher possibility of coming from intron sequences and therefore have a lower GC content.
When sorted by exon length from short to long like introns, a negative trend of average GC content of every 1000 introns was present in mammals, although the tendency was not close as that observed for introns (Additional file 1 , Fig.