Why do highly
expressed genes have small introns? This is an important issue, not least because it provides a testing ground to compare selectionist and neutralist models of genome evolution. Some argue that small
introns are selectively favoured to reduce the costs of transcription. Alternatively, large introns might permit complex regulation, not needed for highly expressed genes. This genome design hypothesis evokes a regionalized model of control of expression and hence can explain why
intron size covaries with
intergene distance, a feature also consistent with the hypothesis that highly expressed genes cluster in
genomic regions with high deletion rates. As some genes are expressed in the haploid stage and hence subject to especially strong purifying selection, the evolution of genes in Arabidopsis provides a novel testing ground to discriminate between these possibilities. Importantly, controlling for expression level, genes that are expressed in pollen have shorter introns than genes that are expressed in the sporophyte. That genes flanking pollen-expressed genes have average-sized introns and intergene distances argues against regional mutational biases and genomic design. These observations thus support the view that selection for efficiency contributes to the reduction in intron
length and provide the first report of a molecular signature of strong gametophytic selection.
More abstracts about the Gametophytic Selection in Arabidopsis thaliana Supports the Selective Model of Intron Length Reducti