The ‘Mystery Tree’: finding a new genus in a Central American rain forest
21 October 2010
By Dr Daniel Kelly
First collected by TCD Botanist Daniel Kelly in 2004, a new kind of tree has just been published in the Annals of the Missouri Botanical Garden.
The tree was found in the process of surveying the rain forest vegetation of Cusuco National Park in the north-west of Honduras. The flora of Central America is very rich and by no means well-studied, so another unknown tree did not attract immediate attention. Returning to the same forest in 2006, another tree of the same species was found, this time with fruits (Pictures 1,2). Each fruit was about 2 cm across, with a cup-like structure projecting at the apex, and a single large seed inside. The flowers, in contrast, were tiny – only 2 mm across. Male and female flowers were produced on separate trees. Microscopic examination showed that the male flower was also peculiar. The stamens have three radiating pollen sacs, reminiscent of a minute clover-leaf - an arrangement apparently unique among flowering plants (Picture 3).
The tree is known by local people as “guayabillo” because of the superficial resemblance of the fruit to a guava (Spanish guayaba). However, the fruit is not succulent. It appears to function as a nut; it is eaten by small mammals, which hoard the seeds and probably act as dispersal agents.
Specimens were studied at the Natural History Museum in London, in collaboration with Ms Caroline Whitefoord, and making use of the extensive collections and taxonomic expertise there and at Kew. However, no-one could figure out what group of plants the ‘Mystery Tree’ belonged to. The breakthrough came in May 2007, when a specimen was sent to Carmen Ulloa Ulloa, at Missouri Botanical Garden. Dr Ulloa Ulloa is a specialist in the Order Santalales, a relatively little-known group that includes the family Santalaceae (Sandalwood family). She confirmed that the Mystery Tree belonged to this group; however, not only was it new to her, it showed no obvious affinity to any of the known species.
On a third visit, in 2008, fresh leaf samples of the Mystery Tree were collected, dried over silica gel and posted to Dr Daniel Nickrent at S. Illinois University, Carbondale, a geneticist who has worked on this group. DNA was extracted and molecular analysis carried out on four different genes. The result is a phylogenetic ‘tree’ which confirms that the Mystery Tree comes closest to a group in the Santalales now placed in the family Aptandraceae. Related species hale from countries as far apart as Peru, Gabon and Indonesia. However, these are ‘cousin’ rather than ‘sister’ species. A separate genus had to be recognized: a new category of tree. To quote Dr Ulloa Ulloa, “Although many botanists describe numerous species as part of our scientific work, to describe a new genus is perhaps a once in a lifetime experience”.
For the genus, the name Hondurodendron was chosen, meaning ‘tree of Honduras’ (dendron being Greek for tree). The Latin specific epithet urceolatum means shaped like a pitcher or urn, and alludes to the diagnostic shape of the fruit.
Hondurodendron urceolatum is known only from a single mountain range. With further searching, it might be found in adjacent mountainous areas, but it is certainly rare. It is, in itself, an exciting discovery. It is also a spur to fresh endeavour – who knows what other mysteries are still hidden in those still extensive but steadily shrinking forests?
The survey of the biodiversity of Cusuco National Park is being carried out under the aegis of the educational charity Operation Wallacea, working with volunteers – mainly students - from a range of countries (Picture 4) . The following staff and students of TCD Department of Botany have participated in this research: Dr Alison Donnelly, Mr David Brady, Mr Sean Feeney, Ms Meadhbh Costigan, Ms Annabelle Bergoënd and Dr Anke Dietzsch.
Carmen Ulloa Ulloa, Daniel L. Nickrent, Caroline Whitefoord and Daniel L. Kelly. Hondurodendron, a new monotypic genus of Aptandraceae from Honduras. Annals of the Missouri Botanical Garden 97: 461-471. 10 October 2010.
Pic. 1. The Forest Botany team, Cusuco National Park, June 2008. From L: Emmanuel (Honduras); Dr Kate Fagan (U.K.); Sr Pastor Cortés (Honduras: local guide); Ms Elin Jones (U.K.); Dr Daniel Kelly (TCD); Ms Tegan Haslan (Canada); Dr Alison Donnelly (TCD).
Pic. 2. Fruit of Hondurodendron urceolatum in section Photo Dr K. Snarr, June 2005
Pic. 3. Drawings of Hondurodendron urceolatum showing male flowers, by Ms A. L. Arbeláez.
Pic. 4. Fruit of Hondurodendron urceolatum on branch. Photo Dr J. Kolby, July 2010