WRIGHT, WESLEY A., MARGARET COX, and CHRISTOPHER S. CAMPBELL.* Department of Biological Sciences, University of Maine, Orono, ME 04469-5722, USA. - Internal transcribed spacer 1 (ITS1) in Picea (Pinaceae): structure and sequence divergence.
Sequences of ITS1 of Picea mariana (black spruce) and P.
rubens (red spruce) are at least 2750 base pairs long, longer than
the longest previously known conifer ITS1 (Pinus pinea, 2631
bp). All studied Pinaceae ITS1 contain subrepeats, regions up to 265
bp in length, containing an embedded conserved motif (GGCCACCCTAGTC),
and thought to be involved in secondary structure. In Pinaceae, there
is a strong association between ITS1 length and number of subrepeats.
Pinus has six subrepeats, but the two Picea species
reported here are exceptional in having only three subrepeats with the
conserved motif. Two of these subrepeats are 189 bp in length, and the
third is roughly 50 bp long. Picea ITS1 also contains two
regions about 480 bp long, with high sequence similarity, and lacking
the conserved motif. Variation in Picea ITS1 appears to be
complex in our sample, which includes multiple clones from four
individuals, representing two populations from each species. We find
variation between populations and within individuals, suggesting
ineffectiveness of concerted evolution as in some other conifers.
Black spruce and red spruce are considered sister species, hybridize
frequently, and are sometimes difficult to distinguish
morphologically. ITS1 sequence divergence between these two species is
2-4%, more than that between sister species in Tsuga (three
pairs of sister species), Larix (two pairs), and
Pseudotsuga (one pair), but similar to that between sister
species in one other pair in Pseudotsuga.
Key words: ITS, Picea, Pinaceae, sequence divergence, sister species, subrepeats