Person:

Davis, Charles

Species of Taxus, particularly Taxus baccata, have long been cultivated as ornamentals in Europe and North America. Ten species of Taxus are generally recognized, but phylogenetic relationships among these species remain unclear. We used sequences of the nuclear ribosomal DNA ITS region to infer phylogenetic relationships. Three of the four New World species form a well-supported clade, within which the Pacific coastal species. T. brevifolia is sister to a clade containing T. floridana and T. globosa of northwestern Florida and northern Mexico, respectively. Taxus canadensis, which is more widely distributed in eastern North America, appears to be more closely related to Old World taxa than to other New World species. This relationship, though weakly supported in our analysis, is consistent with leaf anatomical features. Taxus chinensis and T. mairei of southeast Asia form a clade, which is sister to a clade containing T. cuspidata of Japan and northeastern China, and T. baccata of Europe and North Africa. Our ITS phylogeny implies that intercontinental disjunctions in Taxus entailed at least two vicariance events: an initial split between the New World T. floridana-T. brevifolia-T. globosa clade and the rest, and a later split separating T. canadensis from the Old World species.

Torreya, composed of five to seven species, is distributed disjunctly in easten Asia and the eastern and western United States. In this study, interspecific relationships of Torreya were examined on the basis of sequences of the nrDNA ITS region. Torreya taxifolia and T. californica of the New World form a clade, as do the Old World species. This result suggests that the previous division of Torreya into two sections does not reflect phylogenetic relationships and that the diagnostic character for the two sections, namely rumination of albumen, is homoplastic. It is possible that the present distribution of species of Torreya resulted from a single vicariance event separating the Old World and New World. However, the modern species are apparently young in age, and it will be necessary to integrate older fossils to ascertain the deeper biogeographic history of the genus.

Explanations for biogeographic disjunctions involving South America and Africa typically invoke vicariance of western Gondwanan biotas or long distance dispersal. These hypotheses are problematical because many groups originated and diversified well after the last known connection between Africa and South America (≈105 million years ago), and it is unlikely that “sweepstakes” dispersal accounts for many of these disjunctions. Phylogenetic analyses of the angiosperm clade Malpighiaceae, combined with fossil evidence and molecular divergence-time estimates, suggest an alternative hypothesis to account for such distributions. We propose that Malpighiaceae originated in northern South America, and that members of several clades repeatedly migrated into North America and subsequently moved via North Atlantic land connections into the Old World during episodes starting in the Eocene, when climates supported tropical forests. This Laurasian migration route may explain many other extant lineages that exhibit western Gondwanan distributions.

Fossil data have been interpreted as indicating that Late Cretaceous tropical forests were open and dry adapted and that modern closed-canopy rain forest did not originate until after the Cretaceous-Tertiary (K/T) boundary. However, some mid-Cretaceous leaf floras have been interpreted as rain forest. Molecular divergence-time estimates within the clade Malpighiales, which constitute a large percentage of species in the shaded, shrub, and small tree layer in tropical rain forests worldwide, provide new tests of these hypotheses. We estimate that all 28 major lineages (i.e., traditionally recognized families) within this clade originated in tropical rain forest well before the Tertiary, mostly during the Albian and Cenomanian (112 - 94 Ma). Their rapid rise in the mid-Cretaceous may have resulted from the origin of adaptations to survive and reproduce under a closed forest canopy. This pattern may also be paralleled by other similarly diverse lineages and supports fossil indications that closed-canopy tropical rain forests existed well before the K/T boundary. This case illustrates that dated phylogenies can provide an important new source of evidence bearing on the timing of major environmental changes, which may be especially useful when fossil evidence is limited or controversial.

The Malpighiaceae are a family of similar to ~1250 species of predominantly New World tropical flowering plants. Infrafamilial classification has long been based on fruit characters. Phylogenetic analyses of chloroplast DNA nucleotide sequences were analyzed to help resolve the phylogeny of Malpighiaceae. A total of 79 species. representing 58 of the 65 currently recognized genera. were studied. The 3' region of the gene ndhF was sequenced for 77 species and the noncoding intergenic spacer region trnL-F was sequenced for 65 species' both sequences were obtained for the outgroup, Humiria (Humiriaceae), Phylogenetic relationships inferred from these data,ets are largely congruent with one another and with results from combined analyses. The family is divided into two major clades, recognized here as the subfamilies Byrsonimoideae (New World only) and Malpighioideae (New World and Old World). Niedenzu's tribes are all polyphyletic, suggesting extensive convergence on similar fruit types; only de Jussieu's tribe Gaudichaudieae and Anderson's tribes Acmanthereae and Galphimieae are monophyletic. Fleshy fruits evolved three times in the family and bristly fruits at least three times. Among the wing-fruited vines, which constitute more than half the diversity in the family, genera with dorsal-winged samaras are fairly well resolved, while the resolution of taxa with lateral-winged samaras is poor. The trees suggest a shift from radially symmetrical pollen arrangement to globally symmetrical pollen at the base of one of the clades within the Malpighioideae. The Old World taxa fall into at least six and as many as nine clades.