Zhang, Li-Bing; Renner, Susanne S.
The deepest splits in Chloranthaceae as resolved by chloroplast sequences.
In: International journal of plant sciences, Vol. 164, No. 5: S383-S392
Evidence from the fossil record, comparative morphology, and molecular phylogenetic analyses indicates that Chloranthaceae are among the oldest lineages of flowering plants alive today. Their four genera (ca. 65 species) today are disjunctly distributed in the Neotropics, China, tropical Asia, and Australasia, with a single species in Madagascar but none in mainland Africa. In the Cretaceous, Chloranthaceae occurred in much of Laurasia as well as Africa, Australia, and southern South America. We used DNA sequence data from the plastid rbcL gene, the rpl20-rps12 spacer, the trnL intron, and the trnL-F spacer to evaluate intra-Chloranthaceae relationships and geographic disjunctions. In agreement with earlier analyses, Hedyosmum was found to be sister to the remaining genera, followed by Ascarina and Chloranthus + Sarcandra. Bayesian and parsimony analyses of the combined data yielded resolved and well-supported trees except for polytomies among Andean Hedyosmum and Madagascan-Australasian-Polynesian Ascarina. The sole Asiatic species of Hedyosmum, Hedyosmum orientale from Hainan, China, was sister to Caribbean and Neotropical species. Likelihood ratio tests on the rbcL data set did not reject the assumption of a clock as long as the long-branched outgroup Canella was excluded. Two alternative fossil calibrations were used to convert genetic distances into absolute ages. Calibrations with Hedyosmum-like flowers from the Barremian-Aptian or Chloranthus-like androecia from the Turonian yielded substitution rates that differed by a factor of two, illustrating a perhaps unsolvable problem in molecular clock–based studies that use several calibration fossils. The alternative rates place the onset of divergence among crown group (extant) species of Hedyosmum at 60 or 29 Ma, between the Paleocene and the Oligocene; that among extant Chloranthus at 22 or 11 Ma; and that among extant Ascarina at 18 or 9 Ma, implying long-distance dispersal between Madagascar and Australasia-Polynesia.