Classification of Orchidaceae in the Age of DNA data英文电子书.pdf

CLASSIFICATION OF ORCHIDACEAE IN THE AGE OF DNA DATA Mark W. Chase Summary. The relationships of orchid tribes and subtribes are discussed in the light of recent DNA studies. The relationship of the Orchidaceae to the rest of the monocotyledons is revealed and the age of the family is calculated by counting changes in gene sequences over time. A summary cladogram of orchid relationships based on published DNA studies is given. The family Orchidaceae is one of the two largest families of flowering plants; the other, Asteraceae (Compositae), is sometimes stated to be the larger, but this point has been hotly debated over the years. No clear resolution is in sight, so my first sentence is one that system- atists on both sides of the issue can agree. Among monocotyledons, the orchids (with perhaps as many as 25,000 species) have about twice the number of species as the next largest family, Poaceae (Graminae). Orchids are particularly diverse in the wet tropics world- wide, and they are absent only from the polar regions and the driest of deserts. Their highly modified flowers are, in the minds of most people, their hallmark, but the best and only truly universal char- acter for the family is their unusual early developmental (post- germination) stage, the protocorm, which is the point at which orchids form their linkage with a symbiotic fungus. The higher-level classification of Orchidaceae has traditionally been based on the construction of the fused gynoecium and androe- cium (gymnostemium or column), which is, in its details, unique to the family. The number of anthers has been the primary trait emphasised, which has resulted in the family being split into three groups, often recognised as subfamilies. Those with two anthers or one anther were