New genus found in Honduran rainforest

The botany team in Honduras

A new kind of tree, first collected on an Operation Wallacea expedition in 2004, has just been published in the Annals of the Missouri Botanical Garden.

The tree was found in the course of a systematic survey of the rainforest vegetation of Cusuco National Park, carried out by the Forest Botany team under the leadership of Dr Daniel Kelly of Trinity College Dublin.

The flora of Central America is very rich and by no means well-studied, so another unidentified tree did not attract immediate attention. Back in 2006, the Operation Wallacea group found another tree of the same species, that time with fruits. Each fruit was about 2cm across, with a cup-like structure projecting at the apex, and a single large seed inside. The flowers, in contrast, were tiny – only 2mm across. Male and female flowers were produced on separate trees. Microscopic examination showed that the male flower was also peculiar. The stamens have three pollen sacs, reminiscent of a minute clover-leaf – an arrangement apparently unique among flowering plants.

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. According to local information, it is eaten by small mammals. These hoard the seeds and probably act as dispersal agents.

Mystery tree breakthrough

Having a genus of tree that is known from nowhere else in the world is something that should impress authorities, visitors and local people with the international importance of this site

Specimens were studied at the Natural History Museum in London in collaboration with Ms Caroline Whitefoord who made use of the extensive collections and taxonomic expertise there and at Kew. However, for a long time nobody could ascertain to what group of plants the ‘Mystery Tree’ belonged.

The breakthrough came in May 2007, when a specimen, thought to belong to the family Santalaceae (Sandalwood family), was sent to Dr Carmen Ulloa Ulloa, at Missouri Botanical Garden. Dr Ulloa realised that this specimen had no obvious affinity with any known members of this family. She contacted Dr Daniel Nickrent, Southern Illinois University, Carbondale, who had both morphological and molecular experience with the Sandalwood order (Santalales).

Dr Nickrent extracted DNA from a dry herbarium sample and in a preliminary analysis the ‘Mystery Tree’ appeared to be related to Aptandraceae, not Santalaceae. Dr Ulloa examined extensive herbarium material of the family and relatives in this group, but none matched the specimens from Honduras. Fresh leaf material was needed to complete the DNA sequences and confirm the preliminary taxonomic position.

“To describe a new genus is perhaps a once in a lifetime experience”

DNA identification

On a third Operation Wallacea expedition, in 2008, a fresh leaf sample of the ‘Mystery Tree’ was collected and dried on silica gel. This sample was mailed to Dr Nickrent, DNA was extracted and sequences from four different genes were determined. These sequences, in combination with others from related plants, allowed the generation of a phylogenetic ‘tree’ which confirmed its position in Aptandraceae. These related species came from countries as distant as Peru, Gabon and Indonesia.

Hondurodendron fruit, by Dr Jonathan Kolby

By comparing the genetic distance between each of the included genera, Dr Nickrent realised that the ‘Mystery Tree’ should be recognised as a new genus. To quote Dr Ulloa Ulloa, “Although many botanists describe numerous species as part of their scientific work, to describe a new genus is perhaps a once in a lifetime experience”.

Naming the tree

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.

At present Hondurodendron is only known from the Cusuco National Park, having not been found yet in adjacent mountainous areas. This plant is thus considered a rare endemic, an exciting discovery.

Hondurodendron thus represents a ‘flagship species’ for Cusuco National Park. Having a genus of tree that is known from nowhere else in the world is surely something that should impress authorities, visitors and local people the international importance of the site. It should also stimulate further exploration – for who knows what other mysteries are hidden in these still extensive but, sadly, still shrinking forests?


Acknowledgements

Particular thanks go to Caroline Whitefoord for her support and assistance.

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