Molecular Divergence Between Disjunct Taxa in Eastern Asia and Eastern North America

Abstract

Eastern Asian-eastern North American disjuncts in four genera were examined for allozyme divergence and sequence divergence of the internal transcribed spacers (ITS) of nuclear ribosomal DNA. The disjunct pairs of taxa include Caulophyllum robustum-C. thalictroides, Menispermum dauricum-M. canadense, Penthorum chinense-P. sedoides, and Phryma leptostachya var. asiatica-P. leptostachya var. leptostachya. Allozyme divergence was comparable in Caulophyllum and Penthorum (genetic identities of 0.534 and 0.546) and was considerably higher than between pairs of taxa in Menispermum (0.273) and Phryma (0.291). Caulophyllum and Penthorum, which have the highest genetic identities at allozyme loci, also have low ITS sequence divergences (1.30 and 1.65%, respectively). Phryma, which has low isozyme identity, also has the highest ITS sequence divergence (4.46%). The two taxa of Menispermum have low ITS sequence divergence (0.93%) despite having a low identity (0.273) at allozyme loci. The results suggest that divergence between the taxa in the four genera are not the result of a single historical event. Estimated divergence times are reasonably consistent with a late Miocene disjunction for Caulophyllum and Pen tho rum, whereas the age of the Phryma disjunction is calculated at over 20 million years. The nonconcordant divergences between allozymes and ITS sequences in Menispermum may be caused by concerted evolution in the latter or possibly longer generation time in the woody plants. Additional molecular data are needed to clarify the situation.

Integrating fossils in a molecular-based phylogeny and testing them as calibration points for divergence time estimates in Menispermaceae

“…4). This date is older than the one estimated with internal transcribed spacer sequences (2.35 Myr; Lee et al, 1996), and than the date reported by Xiang et al (2000) at less than 0.28 Myr. The latter authors only used rbcL sequences and did not find any substitution between either species of Menispermum.…”

Geographical distribution and ecological conservatism of disjunct genera of vascular plants in eastern Asia and eastern North America

“…Area parallelism in herbaceous disjunct genera together with the fact that these genera occur farther to the north than woody disjunct genera suggests that the persistent correlations in geographical range of the disjunct congeners might be due to later separation of herbaceous disjunct genera compared with woody genera. This hypothesis is marginally supported by divergence times estimated from genetic distance: 8.5 ± 6.1 × 10 6 years for herbaceous genera (n = 7) and 17.8 ± 5.6 × 10 6 years for woody genera (n = 6) (t = 2.8, P < 0.05) (Parks & Wendel 1990;Wen & Jansen 1995;Lee et al 1996;Schnabel & Wendel 1998;Donoghue et al 2001). Further resolution of the timing of disjunction and its relationship to the degree of correlation between traits in descendant lineages awaits further analyses of genetic distance.…”

PHYLOGENY, BIOGEOGRAPHY, AND MOLECULAR DATING OF CORNELIAN CHERRIES (CORNUS,CORNACEAE): TRACKING TERTIARY PLANT MIGRATION

“…These views of differential importance of the two bridges in different geological times for plant exchanges receive some support from phylogenetic and molecular evidence. For ex-ample, studies of temperate and cold-tolerant taxa suggested a dominant connection via the BLB between eastern Asia and North America during the Miocene (Qiu et al 1995;Lee et al 1996;Wen et al 1998;Xiang et al 1998a;Wen 2000;Xiang et al 2000;Manos and Stanford 2001;Xiang and Soltis 2001;Donoghue et al 2001;Wen et al 2002; also see summary in Wen 1999;Milne and Abbott 2002). However, fossil evidence indicates that the early Tertiary floras from Greenland and Spitzbergen, part of the NALB, did not contain many thermophilic taxa (Manchester 1994).…”