Saturday, 29 May 2010

Trees of Bristol 1: The Whitebeams and others

Outside the aquarium at Bristol there is a small garden containing various rare or endemic plants of the Avon Gorge, among them are several trees, including two endemic forms of Sorbus, the genus that contains the Whitebeams and Rowan trees. The South West of England is a major global centre of diversity in Sorbus, with at least 16 native species plus another two naturalised European forms in the Avon Gorge that runs by the zoo, and more being identified all the time. In addition, more endemic species can be found nearby in north Devon, the Cheddar Gorge, and Symonds’ Yat.

This is very surprising, as only 10,000 years ago the Bristol area would have been open tundra, possibly with a few birches or conifers in sheltered areas. The main advance of the ice never reached as far south as Bristol itself, but most of Wales was glaciated, and north of Gloucester the ice sheet would have been many hundreds of metres thick. How did so many endemic forms arise in so short a time?

The secret lays in two characteristics of the genus Sorbus, their willingness to hybridize and their ability to reproduce asexually by a process called apomixis.

Whereas in animals new species usually arise by evolution of a single ancestral form, in flowering plants especially new species often arise by hybridization. There are three significant ancestral species of Sorbus in the UK, the Whitebeam Sorbus aria, the Rowan or Mountain Ash S.aucuparia, and the Service Tree S. torminalis. These species have hybridized with each other, and then the hybrids themselves have hybridized, to generate a swarm of distinct varieties, each with unique characteristics and adapted to particular environments. The Sorbus trees in the Avon Gorge currently stand at the following:

S.aria Common Whitebeam

S.aucuparia Rowan or Mountain Ash

S.torminalis Wild Service Tree

S.x thuringiaca B***rd Service Tree (sorry about the name!)

S.bristoliensis Bristol Whitebeam

S.wilmottiana Wilmott’s Whitebeam

S.anglica English Whitebeam

S.porrigentiformis Grey-leaved Whitebeam

S.eminens Round-Leaved Whitebeam

S. leighensis Leigh Woods Whitebeam

S. rupicola Rock Whitebeam

S.croceocarpa Orange-Fruited Whitebeam

S.intermedia Swedish Whitebeam (naturalised Swedish species)

S.decipiens Sharp-Toothed Whitebeam (naturalised German species)

S. x avonensis Avon Gorge Whitebeam (A hybrid between Common Whitebeam and Grey-leaved Whitebeam)

S. x houstoniae Houstons Whitebeam (A hybrid between the Common Whitebeam and the Bristol Whitebeam)

S. x proctoris Proctor’s Rowan (A new hybrid between Rowan and Sichuan Rowan, which is a garden tree)

S. x robertsonii Robertson’s Whitebeam (A new hybrid between the Common whitebeam and the Round-leaved Whitebeam)

An additional complication is that many of the new forms are polyploid – that is to say they have additional sets of chromosomes. Polyploidy is very common in plants, especially crop plants, as the additional sets of chromosomes allow for more sets of genes to be available to a plant to use in a given environment. One problem is that if there is an odd number of sets of chromosomes the process of meiosis does not work properly, rendering the plant incapable of sexual reproduction. In an animal this would be fatal for the species, but plants have several ways round the problem.

One of these is the apomixis I mentioned earlier. In this process, although the seed is not fertilized, it still develops into a viable plant because cells from the parent plant contribute to the seedling. This results in the seedling being a clone of the parent. This is how the various Sorbus forms spread around the Avon Gorge – Sorbus are distributed by birds that spread the clones of the original hybrid form around the area. Apomixis occurs in many other plants – for example Blackberries Rubus fruticosus and in many forms of Citrus.

If you look at Blackberries, you will see individual plants in an area tend to come in a small range of growth habits, leaf sizes and so on. The different forms are referred to as microspecies, and they arise from apomictic seeds in blackberry fruits. Because of their shear number, they are not split into different forms as with Sorbus, and tend to be lumped together as “aggregate” species. In botanical guides, the Blackberry for example is described as Rubus fruticosus agg.

With Citrus seeds, often both a sexually produced and an apomictic seedling may be produced from the same seed. If you collect and sow some seeds of Lemon or Orange fruits, you can see this occur. Generally the stronger seedling will be the sexually produced one; the weaker seedling will be the clone of the parent.

This phenomenon makes the taxonomy of plants rather different than for animals. An evolutionary tree for animals will be basically tree-like, but for plants, especially flowering plants, the pattern looks more like a network, with a given species having multiple ancestors. In addition, insects and viruses can spread genes horizontally through many species, especially if they are closely related. This is part of the reason the genetically modified crops designed to be resistant to herbicides that kill weeds confer only a temporary advantage – new forms can arise very quickly in the right circumstances, and then spread even more quickly.

(Image from Bristol Zoo website)

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