The Tree of Flowering Plants
15 April 2008
Professor Simon Hiscock, Director of the Botanic Garden, explains how over the past 20 years there has been a revolution in the plant world.
Molecular phylogenetics uses the sequences of an organism’s genes to gain a better understanding about its evolutionary relationships with other related organisms. Previously plants were classified largely according to their morphological characters, i.e. the outward appearance of their vegetative and reproductive structures, such as leaves and flowers, respectively. Today, the taxonomic classification of plants is largely based on DNA sequences, in combination with morphological characters. This new classification has had a major impact on all groups of organisms, but particularly the angiosperms – flowering plants. DNA sequencing has adjusted many plant relationships and in the process the more ancient lineages have become evident.
Some time before 140 million years ago, the angiosperms diverged from the gymnosperms – non-flowering seed plants such as pine trees and other evergreens – but since fossil gymnosperms go back at least 350 million years, the birth of flowering plants could have occurred anytime between 350 and 140 million years ago. Botanists and evolutionary biologists are keen to narrow that gap.
It appears that water lilies are one of the closest living relatives to the first flowering plants
Among today’s flowering plants, the earliest, or ‘basal’ angiosperm lineages are now quite well understood and it appears that water lilies are one of the closest living relatives to the first flowering plants. Their extinct ancestors were once terrestrial, but at some point these became adapted to an aquatic lifestyle that has given water lilies an advantage to survive all these millions of years. The most basal angiosperm, however, is now recognised as a plant called Amborella trichopoda, which is only found on New Caledonia, a small tropical island in the Pacific. Amborella trichopoda is the only remaining species of a lineage that first appeared on Earth more than 140 million years ago, while dinosaurs still ruled the planet.
There’s only one species in the genus Amborella; there’s only one genus in the family Amborellaceae; and there’s only one family in the order Amborellales. Amborella is the closest living relative of the ancestor that gave rise to all modern day flowering plants, which now number over 350,000 different species. Amborella and water lilies are the first two branches on the family tree of flowering plants, and the University’s Botanic Garden is one of the first in Britain to acquire an Amborella. It was collected in New Caledonia by Hiscock and French colleagues last March.
Previously, all flowering plants were split into two main groups; monocotyledons (monocots) and dicotyledons (dicots), but pollen morphological research, combined with the DNA data, has completely removed that divide. Now, two main lineages of flowering plants emerge from the basal angiosperms (or basal monocolpates), which together contain about 90 per cent of all flowering plant diversity. These two lineages are the true dicots, which we now call the eudicots (roses, daisies, and many other familiar flowers) and the monocots (plants like grasses, lilies and orchids).
Bristol University's Botanic Garden is the first in the UK to lay out a garden so people can walk along the 'family tree' of angiosperms
When Hiscock arrived at the University in 2001, plans to relocate the University’s botanical collections to a new site were being discussed and it appeared that the Botanic Garden would be moved for the fourth time in its 120-year history. This was a golden opportunity to create a modern Botanic Garden and present the new classification of flowering plants as a unique display – one of the things Hiscock was most keen to do. Previously, botanic gardens had their family beds, but now you never quite know how the plants they contain relate to one another. Here was a chance to show a simplified version of the phylogentic tree in the form of paths, so people could actually walk along the evolutionary ‘family tree’ of angiosperms.
Five years down the road, Bristol’s Botanic Garden at the Holmes was the first in the UK to lay out a garden in this way. The recent visit of a Chinese delegation, which included Professor He Shanan, Director of the network of botanic gardens in China and Professor Xia Bing, Director of the Nanjing Botanical Garden, provided an opportunity to show off the results of years of hard work. The Chinese had heard about Bristol’s display and were seeking inspiration and advice for the angiosperm phylogeny display they are planning in Nanjing. During their visit Professor Xia Bing planted a sacred bamboo at the moongate, a traditional Chinese front entrance to the Chinese medicinal herb collection in the garden. Previously He Shanan had kindly donated a magnificent collection of Sacred Lotus (Nelumbo nucifera) varieties and cultivars to the Bristol garden – probably the best collection of Sacred Lotus in the UK. Thanks to this visit a partnership has been established between the University of Bristol Botanic Garden and Nanjing Botanic Garden which will ensure future collaborative projects and plant exchange.
As well as enhancing the teaching of plant sciences within the University, the Bristol Botanic Garden provides a unique teaching resource for local schools and a new cultural attraction for the City of Bristol. The gardens are open to the public most days.