Published online: 11 January 2007; | doi:10.1038/news070108-10
Giant stinker finds place in plant family tree
Pinning down the rotting-flesh plant could reveal the roots of gigantism.
Lucy Odling-Smee
This flower may have evolved to become so large in order to spread its rotten scent far and wide.
Jeremy Holden
With blooms that stink of rotting flesh and span up to a metre across, a flowering Rafflesia arnoldii
is hard to miss in the tropical forests where it grows. But it has
taken taxonomists nearly 200 years — since when the odd plant was first
described — to find its place in the family tree.
Knowing
the plant's closest relatives not only solves a centuries-old mystery,
but could also help scientists to work out how floral gigantism evolved.
The
plant's taxonomic position, along with that of other species in the
Rafflesiaceae group, has been hard to pin down because of a lack of
clues in the morphological and genetic data, says Charles Davis of
Harvard University Herbaria in Cambridge, Massachusetts.
The
plants are parasitic, using fungal-like threads to extract nutrients
and water from host tissues. As a result, they have no leaves or stems
— the morphological features normally used to identify plants.
Even
DNA has proved unhelpful in classifying this group. DNA from
food-producing chloroplasts is commonly used to identify plant
relationships because it is easy to extract and evolves at a rate that
allows scientists to track evolutionary changes (fast enough so that
variation can be easily detected, but not so fast that later changes
conceal earlier ones). But in parasitic plants, the genes for food
production are redundant, and so are either missing or are severely
truncated.
Family roots
Davis and his collaborators tackled the taxonomic challenge by instead looking at mitochondrial DNA.
Broad-scale studies using mitochondrial DNA had already shown that Rafflesiaceae are members of the Malpighiales order1.
But by analysing about 11,500 base pairs of DNA from more than 100
species representing all families of the Malpighiales, Davis and his
team show that Rafflesiaceae are nestled within the Euphorbiaceae, or
spurge family.
The
spurge family includes more than 6,000 species — among them the
tropical cassava - which mostly have small flowers measuring just
millimetres across.
"To
find Rafflesiaceae being bedfellows with a small-flowered family blew
us away," says Daniel Nickrent of Southern Illinois University in
Carbondale, a co-author of the study, which was published in this
week's Science2.
"The
scale of change is unheard of in the plant kingdom and is even pushing
anything in the biological world," says Nickrent. "It's like comparing
man to the Great Pyramids of Giza."
They must be giants
All the Rafflesiaceae plants have giant flowers that smell of rotting meat, with R. arnoldii
being the queen stinker. The plants probably experienced an
evolutionary pressure to generate larger and larger flowers to
broadcast their powerful stench to distant pollinating flies in the
damp, still conditions of a tropical forest, explains Nikrent.
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By looking at the fossil record of Euphorbiaceae, the
researchers showed that the enormous flower size evolved mainly during
a 46-million-year period — after Rafflesiacea diverged from the rest of
the Euphorbiaceae, but before it diversified into multiple lines.
Unfortunately, however, there are no Rafflesiaceae fossils. So it is
impossible to determine whether the dramatic shift in flower size
occurred during a tiny period of time within this window, or in some
slower or more stepwise fashion, points out Doug Soltis, a botanist as
the University of Florida in Gainesville who was not involved in the
study.
Knowing
the taxonomic status of Rafflesiaceae could still, however, prove an
important first step in understanding what developmental genes are
involved in floral gigantism. "If you know Rafflesia's closest
relatives, you can ask more specific questions from a
developmental-genetics viewpoint, as you know what to compare it with,"
says Soltis. "This research is a springboard for the next studies."
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