Here we present a progress report. Details of primer sequences, PCR protocols, DNA sequences of genome segments, and complete maps will be published elsewhere. As in the earlier studies, we found that Osmunda and Tri-chomanes both have the Angiopteris gene order. Marsilea, Dicksonia, and Pteridium have the Adiantum gene order. Lygodium appears to have only one of the two inferred inversions, thus it has an "intermediate" gene order (Figure 6.1). The gene order of Gleichenia appears to be distinct, but has yet to be fully characterized. It may have only one of the two major inversions but with additional rearrangements, perhaps unique to the gleichenioid lineage. These results are similar to those of Raubeson and Stein (1995), except that the latter inferred the Adiantum structure for Lygodium, whereas we find the intermediate structure. We found that both Lygodium and Gleichenia share the gene order chlL, trnN, trnR, rrn5 and rpl32, trnN, trnR, rrn5, as one moves from each end of the SSC into the IR, i.e., the Angiopteris structure. This indicates that the second inversion has not occurred. The problem then is determining the gene order at the LSC/IR boundary. This region is clearly a hotspot of structural evolution since the IR is subject to "ebb and flow" at its boundary (Goulding et al., 1996). We are currently experimenting with additional primer combinations to test for distinct gene orders.
Figure 6.5 illustrates the evolutionary order of the taxa being studied as proposed by Pryer et al. (2004). Marking the changes in gene order described above on the phylogeny, we find three possible points for rearrangements. Angiopteris, Osmunda, and Trichomanes all share the ancestral gene order, similar to that of other vascular plants. Inversion 1 occurred at one of two possible positions: either on the branch leading to the common ancestor of the schizaeoid clade and its sister group, or on the branch leading to the common ancestor of the rp!2 ndhB rrn5 rrn16 rps7 psbA rp!32
chIL psbA rps7_rrn16 rrn5 ndhB chIB
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