Burmanniaceae: a remarkable mode of life
Unlike almost all flowering plants, most Burmanniaceae do not use the sun as their source of energy. They lack chlorofyll and rely on mycorrhizal fungi in the soil of the forest floor for their main source of carbon. This way they can challenge the low-light conditions in damp primary forests, on places where photosynthetic plants are unable to grow. This mode of life is known as 'myco-heterotrophic', also often but incorrectly indicated as 'saprophytic'. Some Burmannia species have retained chlorophyll, they are still 'green', and can be found in wet grasslands and swamps, often on a sandy soil.
A remarkable feature of most Burmanniaceae is their extreme rarity. Many Burmanniaceae species and even genera are known from only one or two collections, some species were seen only once, often more than a hundred years ago, and now presumed to be extinct. Apart from being rare, Burmanniaceae are seldom observed and collected even if they are present. This is because of their myco-heterotrophic mode of life: the plants remain underground and emerge only when they are flowering and fruiting usually just a few weeks every year, sometimes even less often. Due to the lack of observations and plant material for scientific study, very little is known about these inhabitants of the tropical forests. Even very obvious questions about their mode of pollination and seed dispersal remains unanswered today.
Remainders of ancient ages?
Another intriguing fact is their biogeography. Burmanniaceae are present in the rain-forests of South America, Africa, Asia and Australia, but are not at all known for their ease of dispersal: the area of some species is only a few square meters. Some genera are characterized by huge disjunctions. A less known, but not less impressive example is the genus Oxygyne. Only four species of this remarkable genus are known, two occurring in Africa and two in Japan! A possible explanation for this is that Burmanniaceae are very old and dispersed a very long time ago. According to this theory the plants observed today are paleo-endemics, the last remainders of a family that was once successful but is now at the edge of extinction...
One extreme, and fairly famous case unites the extreme rarity and anomalous disjunctions that characterizes the Burmanniaceae: the species Thismia americana. Almost all species of the fascinating but very rare genus Thismia grow on decaying wood or leaves in tropical rain-forests. This species, however, was found by Pfeiffer in 1912 on a prairie near Chicago! Norma Pfeiffer observed this surprising Thismia population for several years and concluded that it was strongly related with Thismia rodwayi, a species that can be found in Australia, Tasmania and New Zealand, literately on the other side of the world! Despite recent searches, Thismia americana was last seen in 1916 and is probably extinct. What is left are Pfeiffer descriptions, drawings, photographs and some spirit preserved material of the plant.
Our research
Despite recent monographs (Maas et al. 1986; Cowley 1988; Maas-van de Kamer 1998) and morphological research (Rübsamen 1986; Zhang 2000) the evolutionary relationships within the family remain totally obscure. Our goal is to infer these relationships with the use of both molecular and morphological data. Because chloroplast DNA sequences are not reliable for phylogenetic inference with achlorophyllous taxa, we are constructing a nuclear 18S rDNA and a mitochondrial nad1 b-c intron dataset. Standard molecular techniques (DNA extraction - PCR - Capillary sequencing) are used to obtain these data. At the time of writing we have successfully amplified DNA extractions of 48 Burmanniaceae taxa (40 diferent species across 6 genera). Apart from the delineation of the inter- and intrageneric relationships, the resulting topology will be used to investigate the evolution and origin of myco-heterotrophy and the related loss of chlorophyl in several taxa of the Burmanniaceae. Finally, the phylogenetic scenario will also give an insight into the puzzling biogeography of the family.
An additional objective of our project involves the pollen morphology of the study group. The pollen morphology of about 50 Burmanniaceae species will be described based on observations with light microscopy and scanning electron microscopy. The pollen characters will be evaluated in the context of the systematic relationships resulting from the phylogenetic part of the study.
