Evolutionary Relationships within Tribe Maxillarieae

ON: What can you tell us about the modifications inside this tribe due to the DNA sequence data?
MW: At the subtribal level, the DNA data revealed a few surprises. For example, Cryptarrhena (an odd Central American genus of about 2 species) traditionally has been placed in its own subtribe, but DNA data show it is firmly embedded within the Zygopetalinae. Its vegetative habit and many-flowered inflorescence are aberrant within Zygopetalinae, but if one looks at the flowers, they have an anchor-shaped lip, similar to Dichaea, and a hooded column (clinandrium) similar to Huntleya. So, we no longer recognize subtribe Cryptarrheninae. Similarly, the subtribes Pachyphylliinae, Telipogoninae (Andean groups) and Ornithocephalinae (Ornithocephalus, Sphryrastylis) traditionally have been separated from Oncidiinae on the basis of pollinium number (four vs. two). The DNA data clearly show that these clades are embedded within the Oncidiinae, indicating that there has been an evolutionary increase in pollinium number within Oncidiinae from two to four. This provides an important lesson to not rely on a few key morphological characters a priori to construct a classification.

ON: As you said many spectacular and horticulturally important genera such as Oncidium, Odontoglossum, Stanhopea, Zygopetalum, Maxillaria are found in this tribe, which are the others?
MW: Well, if those are not enough, you can include genera such as Lycaste, Ida, Anguloa (Lycastinae) and Bifrenaria (Bifrenariinae). The genus Xylobium seems to be distinct and not clearly sister to any other subtribe, and should probably best be treated as a separate subtribe. Xyobium is not very showy, but is a moderately large and interesting genus that would make a nice thesis topic for a student.

ON: Do you think there will be many changes inside the genus Oncidium? Do you have any idea of how many others genera should be created?
MW:Oh, yes! The Oncidiinae is the subtribe that is suffering the most taxonomic changes. There are clear biological reasons for this (see next question), but the changes are especially contentious because the Oncidiinae (like the Laeliinae) contains so many horticulturally important species. Orchid growers and hybridizers dont like to have to learn new names for their familiar plants, so it is important that:
1) any nomenclatural changes are based upon solid phylogenetic evidence and
2) generic changes are made with an emphasis to minimize nomenclatural transfers (name changes).
Norris Williams, Mark Chase, and Cida Farina are the primary researchers on Oncidiinae phylogenetics, and have sampled over 500 species for three DNA regions. Combined with careful morphological analyses, we believe this will ultimately lead to a more stable and predictive classification, instead of one based largely on taxonomic authority and opinion.
Taxonomists have all agreed for years that many generic distinctions were artificial, such as Oncidium vs. Odontoglossum (based on the angle of the lip and column).
Now, we must collaboratively gather enough data that can be analyzed objectively to delimit evolutionary groups (monophyletic clades), and then we must decide how to name these clades to minimize nomenclatural changes.

ON: You said that the DNA data support traditional classifications within subtribes that produce legitimate floral rewards (e.g., Stanhopeinae); on the other hand, subtribes that contain many species with deceit-based pollination systems (e.g. ), the DNA-based trees conflict strongly with traditional classifications. Could you tell us a little more about this?

Sievekingia reichenbachiana

MW: Yes, Stanhopeinae are taxonomically well-behaved; for the most part, the genera are easy to recognize (e.g., Gongora vs. Stanhopea vs. Coryanthes). As far as we know, all Stanhopeinae are pollinated by fragrance-collecting male euglossine bees that collect perfumes from the flowers.

Malphighia sp

Oncidium ampliatum

The different genera represent different pollination mechanisms that have evolved to attach the pollinaria to different sites on the bees body (head or thorax or leg). The DNA-based trees agree very well with the traditional generic classification based on floral morphology.In contrast, Oncidiinae contain many species that do not produce a legitimate floral reward to the pollinators; they deceive the insects by mimicking the appearance of other species that produce a floral reward such as nectar or oil. The clearest example is provided by the stereotypical yellow Oncidium flower. These are probably mimics of yellow flowers of Malpighia, which produce an oil reward. This oil is collected by female anthophorid bees and used to feed their larvae. Many different species within Oncidium have evolved a yellow flower with clawed sepals and petals that superficially resembles a yellow Malpighia; these orchid flowers produce no legitimate reward, but deceive the bees into an occasional visit that results in pollination.

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