The phylogenetic position of taxaceae based on 18S rRNA sequences

Abstract

The evolutionary position of the yew family, Taxaceae, has been very controversial. Some plant taxonomists strongly advocate excluding Taxaceae from the conifer order and raising its taxonomic status to a new order or even class because of its absence of seed cones, contrary to the case in the majority of conifers. However, other authors believe that the Taxaceae are not fundamentally different from the rest of the conifers except in that they possess the most reduced solitary-ovule cones. To resolve the controversy, we have sequenced the 18S rRNA genes from representative gymnosperms: Taxus mairei (Taxaceae), Podocarpus nakaii (Podocarpaceae), Pinus luchuensis (Pinaceae), and Ginkgo biloba (Ginkgoales). Our phylogenetic analysis of the new sequence data with the published 18S rRNA sequence of Zamia pumila (a.cycad) as an outgroup strongly indicates that Taxus, Pinus, and Podocarpus form a monophyletic group with the exclusion of Ginkgo and that Taxus is more closely related to Pinus than to Podocarpus. Therefore, Taxaceae should be classified as a family of Coniferales. Our finding that Taxaceae, Pinaceae, and Podocarpaceae form a clade contradicts both the view that the uniovulate seed of Taxaceae is a primitive character and the view that the Taxaceae are descendants of the Podocarpaceae. Rather, the uniovulate seed of Taxaceae and that of some species of Podocarpus appear to have different origins, probably all reduced from multiovulate cones.

Phylogeny of Taxaceae and Cephalotaxaceae Genera Inferred from Chloroplast matK Gene and Nuclear rDNA ITS Region

“…These results also reinforce our earlier inference (Chaw et al, 1993(Chaw et al, , 1995 based on nr18S rRNA sequences that the uniovulate seeds of the taxads and Cephalotaxceae have different origins than those of Podocarpaceae.…”

“…In most other conifers the ovules are borne axillary on cone scales (Hart, 1987;Price, 1990;Chaw et al, 1993, and references therein). Because of this distinction, the yew family was historically excluded from the Coniferales, notably by Sahni (1920) and Florin (1948Florin ( , 1951, and was placed in its own order, Taxales.…”

Phylogenetics of Taxus Using the Internal Transcribed Spacers of Nuclear Ribosomal DNA and Plastid trnL-F Regions

“…Molecular phylogenetic approaches offer the potential to better understand and resolve the relationships within taxonomically difficult groups including Taxus and a number of studies exist that provide insight into the evolution of the genus and its relatives. Chaw et al [21] and Cheng et al [22] used DNA sequences of 18S ribosomal DNA, internal transcribed spacer regions of nuclear ribosomal DNA and matK plastid DNA to clarify the phylogenetic position of Taxaceae in the gymnosperm order Taxales. There is also evidence for the monophyly of Taxus [21,23], but the relationships of its species remain understudied and not fully resolved [10,[21][22][23].…”

Species separation of Taxus baccata, T. canadensis, and T. cuspidata (Taxaceae) and origins of their reputed hybrids inferred from RAPD and cpDNA data

“…Recent biomedical studies, showing the potential for using paclitaxel (an alkaloid constituent of yews) as an anti-cancer treatment, have highlighted these deficiencies (Dempsey and Hook, 2000). Long-standing questions of placing the order Taxales in the gymnosperms and the position of the family Taxaceae, as it relates with other families of conifers, have been clarified by recent molecular studies (Chaw et al, 1993;Cheng et al, 2000). Yet species delimitation remains a problem, and progress in phytochemical and pharmacological studies has suffered as a result (Appendino, 1995).…”