Systematics Section / ASPT
Campbell, Christopher .
Subrepeats in Pinaceae nrDNA internal transcribed spacer 1 (ITS1)..
ITS1 in Pinaceae ranges from 944 bp in Pseudolarix to 3271 bp in Picea abies, the longest reported in plants. Much of the great length of Pinaceae ITS1 is attributable to the presence of subrepeats. These blocks of similar sequence make up 10 to 50% of Pinaceae ITS1s, range in number from two (Larix and Pseudolarix) to six (Pinus), and vary from 53 bp (Picea) to 265 bp (Tsuga). Pinaceae subrepeats contain a 13-bp motif, 5’-GGCCACCCTAGTC, that is conserved across the family and synapomorphic for it. In the four known genera containing three subrepeats – Abies, Picea, Pseudotsuga, and Tsuga – two subrepeats are tandem (except in Abies), remote from the third, and more closely related and significantly more similar to one another than either is to the third subrepeat. This correlation between similarity and proximity may be the result of duplication or concerted evolution within ITS1s. ITS1 secondary structure has been modeled for seven Pinaceae genera using the software Mfold. Subrepeats are generally major contributors to extended hairpins in these models, and part of the conserved motif is situated in the terminal loop of at least one long hairpin in each genus. In genera with two subrepeats (Larix and Pseudolarix), both subrepeats form long hairpins with their conserved motifs partially exposed in the terminal loop. In the other five genera, two adjacent subrepeats pair with one another over most of their lengths. In Picea, in which the most detailed study of ITS1 secondary structure has been undertaken, models are supported by some compensating base changes (CBCs). The great stability of Pinaceae ITS1 secondary structure models, which is up to about an order of magnitude greater than those of angiosperms, may reduce the importance of CBCs in conservation of secondary structure.
1 - University of Maine, Department of Biological Sciences, Orono, Maine, 04473-5751, USA
compensating base changes.
Presentation Type: Paper
Location: Cottonwood B (Snowbird Center)
Date: Wednesday, August 4th, 2004
Time: 5:00 PM