Higher intron loss rate in Arabidopsis thaliana than A. lyrata is consistent with stronger selection for a smaller genome

The number of introns varies considerably among different organisms. This canbe explained by the differences in the rates of intron gain and loss. Two factorsthat are likely to influence these rates are selection for or against introns, and themutation rate that generates the novel intron or the intronless copy. Although ithas been speculated that stronger selection for a compact genome might resultin a higher rate of intron loss and lower rate of intron gain, clear evidence islacking, and the role of selection in determining these rates has not beenestablished. Here, we studied the gain and loss of introns in the two closelyrelated species Arabidopsis thaliana and A. lyrata as it was recently shown thatA. thaliana has been undergoing a faster genome reduction driven by selection.We found that A. thaliana has lost six times more introns than A. lyrata since thedivergence of the two species, but gained very few introns. We suggest thatstronger selection for genome reduction probably resulted in the much higherintron loss rate in A. thaliana, although further analysis is required as we couldnot find evidence that the loss rate increased in A. thaliana as opposed to havingdecreased in A. lyrata compared to the rate in the common ancestor. We alsoexamined the pattern of the intron gains and losses to better understand themechanisms by which they occur. Microsimilarity was detected between thesplice sites of several gained and lost introns, suggesting that non-homologousend joining (NHEJ) repair of double-strand breaks (DSBs) might be a commonpathway not only for intron gain but also for intron loss.

Fawcett, J., Rouzé, P., Van de Peer, Y. (2012) Higher intron loss rate in Arabidopsis thaliana than A. lyrata is consistent with stronger selection for a smaller genome. Mol. Biol. Evol. 29(2):849-59.

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