Poster: Genome evolution

Abs # 852: A possible role for large-scale RNA-mediated gene duplication in the evolution of a huge plant superfamily

Presenter:	Vernon, Daniel M., vernondm@whitman.edu
Authors	Vernon, Daniel M. (A) (B)  Hutchison, Delbert  (B)   Anderson, Tovi M. (A)
Affiliations:	(A): Biology Department, Whitman College (B): Program in Biochemistry, Biophysics, and Molecular Biology, Whitman College

Pentatricopeptide Repeat proteins (PPRs) function in organellar gene expression and other cellular & developmental processes. PPRs are found in all eukaryotes but the superfamily is greatly expanded in plants, with more than 400 members in the Arabidopsis genome. To investigate the dramatic expansion of plant PPRs, we have characterized the structure & chromosomal distribution of 439 Arabidopsis PPR genes, comparing them to two control groups: the comparably large LRR-RLK family and a set of 50 unrelated loci. PPR s had a number of unusual features. They had significantly fewer introns (median=0) than genes in either control group, with the majority of PPRs (>67%) having no introns. Also, PPR genes were statistically significantly closer to adjacent loci than control group genes were to their nearest neighbors (mean distance = 454bp). The chromosomal geography of PPR s differed from that of other large Arabidopsis gene families: PPR s were distributed throughout the genome and members of closely-related gene pairs were not found in corresponding duplicated chromosomal blocks. Nor were PPR s found in tandem arrays. Thus, much PPR gene expansion occurred independent of chromosomal duplication events. We propose that the PPR family grew via reverse-transcription of spliced ancestral RNAs, followed by insertion into active chromosomal regions. This model accounts for the characteristics of the superfamily: large numbers of scattered, intronless genes situated very close to neighboring transcription units. While RNA mediated duplication has been a major force in the expansion of non-coding elements in eukaryotic genomes, and has led to some individual gene duplications, PPR s may provide the first example of large-scale RNA-mediated gene family expansion in plants.