Studying the Pattern of Recurrent Intron Loss in an Important Height-Reducing Gene, ABCB1

A typical eukaryotic gene contains protein-coding exons and non-coding introns.   However, the total number of introns, the average number of introns per gene and the average intron length vary vastly between species.   Shared intron positions between distantly related eukaryotes suggest that introns arose very early during eukaryotic evolution.  In closely related species, such as the grasses which diverged from a common ancestor 50-70 MYA, the gene structure is generally highly conserved.  However, this is not the case in ABCB1, a gene that encodes a P-glycoprotein and has been shown to underlie the dw3, br2 and d2 dwarf phenotypes in sorghum, maize and pearl millet, respectively.   ABCB1 in dicots and non-grass monocot species such as banana and datepalm has 9 introns while grasses contain between 2 and 4 introns.  To track the intron loss events in grasses, we used conserved primers to PCR amplify across the nine ABCB1 introns in representative members from eight grass sub-families.  Interestingly, five introns were absent in all grasses and three introns were lost several times independently leading to six unique gene structures within the grass members.  Only the largest intron, intron 7, is conserved suggesting an important gene-regulatory role.  Gene transformation experiments in Arabidopsis are underway to study the functional role of intron 7.