The relationships of the slipper orchids (subfamily Cypripedioideae: Orchidaceae).

Atwood, J.T.
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Slipper orchids, phylogenetic, taxonomy, Phragmipedium,Paphiopedilum, Cypripedium, Criosanthes, Selenipedium-Cypripedium, Brachypetalum, chromosome, fission, fusion, aneuploids, karyological, Miocene, evolution
The phylogenetic relationships of the Cypripedioideae are adumbrated by the Wagner Groundplan-divergence method. The mutual resemblance between paleotropical Paphiopedilum and neotropical Phragmipedium is doubtfully superficial since both share several derived vegetative character states and similar floral bud development. For these genera a simpligied classification is proposed, and one new taxonomic combination is provided. Selenipedium grades into Cypripedium via C. irapeanum and C. californicum. The correlation of reduced vegetative features with increasing latitude suggests that temperate climates have altered the northern taxa most. By eliminating these features from cladistic analysis, C. arietinum (for which the genus Criosanthes is recommended) is parsimoniously distinct from the Selenipedium-Cypripedium clade. Similar flowers of Phragmipedium schlimii and Paphiopedilum subgenus Brachypetalum related to neoteny, since the floral parts resemble those of young flower buds of related species. Their superficial floral similarity is interpreted as a convergence. Chromosome numbers are reported for five species of Phragmipedium and nine species of Paphiopedilum. Since the basal chromosome number is interpreted as 2n = 20 for Cypripedium, Criosanthes, and Phragmipedium, this is probably the basal number for the subfamily. Although centric fission accounts for the upward aneuploid series in Paphiopedilum (2n = 26-44), higher metacentric arm ratios in the aneuploids than in the non-aneuploids suggest that centric fusion has occurred as well. It is argued that both fission and fusion have karyologically repatterned Paphiopedilum section Barbata with concurrent adaptation to florest floors. Under a vicariance model, the cladograms reveal geographic patterns in Paphiopedilum, Phragmipedium, and the Selenipedium-Cypripedium clade with the primitive taxa southernmost. To explain the seemingly incongruous patterns with a necessary Laurasian origin, it is suggested that the southernmost populations have followed habitats similar to the ancestral ones in response to post-Miocene cooling. this mechanism may offer a partial explanation for accumulation of primitive angiosperms on the Asiatic islands which have largely arisen since the Miocene. Contrary to popular belief, some slipper orchids are very advanced, and there is little evidence that the Cypripedioideae is a relic group at an evolutionary dead end.