Systematics Section / ASPT
Jarrell, David C. , Pittman, Kimberly B. , Bryant, Curtis M. .
matK pseudogene origins in the chloroplast DNA of subtribe Aeridinae, Orchidaceae: real or artifact?, phylogenetic analysis and screening of MATK-dependent group II introns.
The chloroplast locus encoding the putative RNA maturase, MATK, is arguably the fastest evolving protein coding sequence of the chloroplast genome and is often used alone or in conjunction with various introns and spacers sequences in the analysis of inter- and intra-generic relationships. The high rate of nucleotide substitution and variation in the position of start codons that might enable complete translation of the protein has caused some to suggest that most, if not all, plants carry matK as a pseudogene. However, this disregards research that demonstrates the presence of both transcripts and proteins from this locus. In addition, while many other pseudogenes exist as truncated or deleted forms in comparison to functional copies, there is little evidence of this for matK. However, matK sequences from several sister taxa in the orchid subtribe Aeridinae have frameshift deletions of 7, 8 or 10 bp mostly in the 5' end of the gene. In addition, no inframe start codons are available for the translation of a protein of sufficient size. Taxon sampling has been greatly expanded to examine the evolutionary history of pseudogene formation. We have also begun testing several hypotheses to explain their origin. These hypotheses include: 1) MatK-dependent intron loss, 2) duplication of matK to another chloroplast or nuclear location, 3) functional replacement by another maturase and 4) insertional RNA editing. To screen the presence of MatK-dependent introns, primers were designed to intron flanking regions such that in cases of loss a shorter length product would be amplified. To date, all screened introns appear to be present in our sample taxa. Our progress on the second hypothesis and future directions will be discussed.
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1 - University of Mary Washington, Biological Sciences, 1301 College Av., Jepson Science Center, Fredericksburg, Virginia, 22401, USA
group II introns
Presentation Type: Oral Paper
Date: Tuesday, August 16th, 2005
Time: 4:30 PM