Evolutionary Relationships within Tribe Maxillarieae


Mark Whitten
is a botanist at the Florida Museum of Natural History in Gainesville Florida.
His research interests include molecular systematics of orchids and chemical ecology of orchids Euglossine bees.
He is presently working on the molecular systematics of Maxillaria in collaboration with colleagues in Mexico, Costa Rica, Ecuador and Brazil.
   




ON: Your lecture concerns Evolutionary Relationships within Tribe Maxillarieae: Lessons from Molecules, Morphology, and Pollination Biology. How long have you been studying the tribe to get those results?
MW: In fact, I developed this work with Norris H. Williams.
Our research was originally focused on studying floral fragrances of orchids pollinated by male euglossine bees, such
as Gongora and Stanhopea, using gas chromatography/mass spectrometry. This work built upon the pioneering work of Dodson, Dressler, Norris Williams, and others. Species in the Stanhopeinae usually have species-specific floral fragrances, and the pollinating bees seemed to be better orchid taxonomists than the botanists. Analyzing the floral fragrances helped to define species, but it was not very useful for determining how orchid species and genera were related. When DNA sequencing became a taxonomic tool in the mid 1990s, Mark Chase invited me to visit his lab to learn

Sievekingia butcheri
the techniques and we began a molecular phylogenetic study ofStanhopeinae. We have gradually expanded our studies to other subtribes, including Oncidiinae (with Norris Williams), Bifrenariinae (with Samantha Koehler), Zygopetalinae (with Robert Dressler and Guenter Gerlach)and now Maxillariinae (with German Carnevali, Samantha Koehler, and Rodrigo Singer).
In the most recent orchid classification proposed by Chase et al. (2003), these subtribes are now included in a broader tribe Cymbidieae, instead of Maxillarieae.Chase, M.W., K.M. Cameron, R.L. Barrett, and J.V. Freudenstein. 2003. DNA data and Orchidaceae systematics: A new phylogenetic classification. Pp. 69-89 in K.W. Dixon, S.P. Kell, Rl. Barrett and P.J. Cribb (eds.) Orchid Conservation. Natural History Publications (Borneo), Kota Kinabalu, Sabah.

Tolumnia urophylla / Onc ampliatum

Max. gentryi
Max. witsenioides
Max. platypetala
Max. hillsii
Max. notylioglossa
Chrysocycnis schlimii.

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.

Max. horichii
Max. witsenioides
Max scalariformis

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.
To make matters more confusing, some Oncidiinae (e.g., Cyrtochilum and Sigmatostalix) have oil glands that provide a legitimate reward for the oil collecting bees. In the Andes of Ecuador, yellow flowered Calceolaria are common along roadsides and are pollinated by bees collecting oil secreted by trichomes inside the lip. We suspect that at least some Oncidiums [e.g., Oncidium harlingii (now Otoglossum)] are mimics of Calceolaria. Other Oncidiinae have evolved long spurs that produce nectar to attract butterflies or hummingbirds (Comparettia), and others have long spurs but do not produce any nectar (Trichocentrum s.s.). So, it seems that most of our taxonomic confusion is caused by our heavy focus on floral colors and shapes, and by not recognizing that these floral syndromes may have evolved several times within a lineage of orchids.
Another good example is the genus Cryptocentrum, whose flowers are green, fragrant at night, and have a long nectariferous spur. The DNA data show that Cryptocentrum is embedded within Maxillaria (as currently defined).
Cryptocentrum
is simply an odd Maxillaria that has evolved moth pollinated flowers, the only group with this pollination syndrome within Maxillariinae.
Similarly, Trigonidium, Chrysocycnis, and Mormolyca represent independent evolutionary adaptations to pseudocopulation from within a polyphyletic Maxillaria.

Cryptocentrum latifolium

ON: You also affirm that convergent evolution may produce very similar floral morphologies in unrelated orchids; classifications based primarily upon gross floral characters may be misleading. Which are the consequences for modern classification of orchids?
MW: I think everyone would agree that classifying orchids based solely upon flower color is simplistic and unnatural; for example, we all agree that an orange Epidendrum is not closely related to an orange Sophronitis.
I think that the evidence from DNA-based trees is teaching us that floral morphology in orchids is very evolutionarily plastic and labile, and that we have relied too much on simplistic floral traits in constructing our previous classifications. Interestingly, in the Oncidiinae, we are discovering that vegetative features often provide better taxonomic characters at the generic level than do floral features.

ON: We know that you have an important project that will really change the classification of Orchidaceae. What could you tell us about it?
MW: Well, right now its just a proposal for a research project. The US National Science Foundation has a special grant program called Assembling the Tree of Life. Its aim is to provide funds for phylogenetic studies for major branches of the tree of life, such as the orchid family. This year, we submitted a proposal to construct a generic level phylogeny of the entire orchid family, based on a sampling of about 1500 carefully chosen species sequenced for six DNA regions. This would be a large collaborative project over five years with participants from all over the world, including Cassio van den Berg, Samantha Koehler, Rodrigo Singer, and many others. Even if this project is funded (the awards will be announced this August), it will not provide a final, definitive classification. Hopefully, it would provide a solid framework that would help guide more focused, intensive studies of orchid genera or subtribes.
With 25,000 species of orchids, there will be plenty of work for future students.




ON: Thank you, Mark Whitten


Photos by Mark Whitten


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