Wednesday, 1 April 2015

Hybridity’s contribution to evolution: featuring fossil trees

Part 4

Most of our garden plants have been created or hugely modified by hybridisation. Most farm and domestic animals do not exist in nature. Even those that do occur naturally have been ‘improved’ by mixing diverse forms together. Plant and animal breeders routinely find hybridisation to be a quick and easy tool to create new forms. Crossing of even quite diverse species and ‘varieties’ is relatively easy.
I reported last time how cycads are potently promiscuous. In my research for my post on horsetail, I found the several species of equisetum - that most ancient of plants - mix their genes very readily indeed. Clearly prehistoric plants and animals hybridised too. Why don’t evolutionists recognise the implications?

The Dawn Redwood, Metasequoia glyptostroboides and its later relations

My twelve year old metasequoia will soon get too big
Living metasequoias were identified for the first time in China in 1941 and in the same year its fossils were first found. An expedition to China in 1948 found a remote forest of a thousand more trees. Like those other fossil trees ginkgo and the Wollemi pine it was easy to propagate and specimens were very soon planted all round the world. Ginkgo and metasequoia are now relatively common here in the UK. Known to rapidly grow into a large tree the early specimens have not been here long enough to know how large they will get.

I do not want to use these relics of former forest in the time of the dinosaurs to merely argue my case for hybridity. I want to also consider them as fascinating plants. In terms of hybridity’s place in evolution they probably raise more questions than answers. I have allowed myself to be tempted to also discuss the Dawn Redwood’s very close relations, Sequoiadendron giganteum, Sequoia sempervirens (the coast redwood) and Taxodium distichum (the swamp cypress). They are all very big, exceedingly long lived trees and in terms of plant classification are placed close together.

The dawn redwoods clock-in as fossils ninety million years old. Even on an evolutionary timescale this is not small, albeit compared with ginkgo, relatively recent. Unlike ginkgo it can be classified together with well known younger relations. In common with ginkgo it has provided enough puzzles to lead some botanists to suggest it might  be a hybrid. In common with ginkgo modern specimens are almost identical to the earliest known fossils. It is truly amazing how so many plants and animals appear as fossils ‘out of nowhere’ and then continue to be found in succeeding strata that show sequences over millions of years with very little change until extinction or the present day.

To me, the startling and significant thing is that the coast redwood, Sequoia sempervirens, that giant of all trees, achieving more than 350ft high, is thought by more enlightened botanists to be a hybrid. Metasequoia is thought to be one of the parents, the other probably Sequoiadendron giganteum or perhaps another now extinct sequoia. Taller than either purported parent it is a supreme example of hybrid vigour.

Taxodium distichum
My own swamp cypress Taxodium distichum 

Although the swamp cypress is not thought to be a hybrid - but who knows - its fossil ancestry and an unusual characteristic make it a very interesting plant.
It’s a plant with a mystery. It possesses upright ‘pneumatophores’ that grow from the ground or from the muddy floor of a lake. Its not called ‘swamp cypress’  for nothing and I am privileged to have seen this phenomenon on many fine trees in the US Everglades. The pneumatophores look rather like wooden traffic cones and may elongate to several feet long.
I learnt on my metaphorical mother’s knee that their function is to facilitate oxygen uptake - and pneumatophores truly fill this role on trees such as mangroves. The strange thing is that despite very extensive study in the case of the swamp cypress they have no obvious function at all.

A rather tall tale
Taxodium distichum might very well be a hybrid. A property of organisms that arise from ‘more distant’ hybrid crosses is that they have not been directly shaped by the pressures of conventional natural selection. They arise as a new organism and are a result of thousands of genes in new combinations. They ‘sink or swim’ in the place that they find themselves or if animals any suitable niche they can find.
Taxodium has formidable characteristics that have enabled its survival over millions of year. Should useless pneumatophores have no selective disadvantage they might very well stay.
This is analogous to conventional ‘genetic drift’ where many characteristics such as hair colour or the shape of a nose or the kind of twirl on an ammonite has no selective pressure to remain or go.
It is well known that where animal and plant organs and tissues are no longer useful that natural selection will select against them. Evolutionary theory recognises that many organs are vestigial. Some organs such as the human appendix and tonsils were thought to be so. It is now known that this is incorrect and it would seem that they both bear a role in our immune systems.
One can perhaps better explain ‘useless’ pneumatophores in terms of having evolved in conditions where in the past they did incur advantage. In which case they would be truly vestigial in Darwinian terms.

As is usual  on tricky botanical issues I consulted scientist Peter. He reminded me that any  biological attribute comes at a ‘cost’ in terms of resources. Natural selection would tend  to remove anything that failed to contribute to survival.

With regard to taxodium’s funny stumps we find it very hard to  believe they have no function at all!

Like metasequoia, taxodium is a deciduous tree

Taxodium fossils
Petrified forests have been found in several locations. A famous one is in 65million year Cretacious deposits in Prince William Forest in Virginia. Such fossils are of course made of stone.

Scuba divers made a fascinating find of submerged taxodium forest sixty foot deep off the coast of Mobile two years ago. The trees were seemingly so fresh that the you could smell their characteristic resin. It would seem that they only have a very short time left to observe and record these trees  before they succumb to predation and decay. Research findings I believe have not yet been published. I presume the sea floor must have elevated to expose these ancient pristine trees?

The Soviet connection
Writing these posts have led me down some fascinating avenues. This article written by A.S. Yablokov on wide hybridisation and translated in 1960 gave me both interesting material and a chill shudder. It covers work in the USSR in the period dominated by that misguided man Lysenko. Lysenko promoted the Lamarckian view of the inheritance of acquired characteristics. Such ideas complemented Stalin’s ambitions and Lysenko rose to high status and dominated Soviet genetics for several decades. Not only did many distinguished geneticists die in the gulags, Lysenko’s crackpot ideas led to several failures in Soviet agriculture and the death of millions.

On further investigation I learnt that up to about 1925 Soviet geneticists were the best in the world and many things taken for granted in breeding and genetics today were discovered in Russia.
You need to read between the lines when reading the Yablokov link. Mindful of the fate of many colleagues they had to be careful.
It does seem to me that Yablokov’s associates were fine breeders of forest trees and fruit and many of them were proud to be called horticulturists. The link describes their work with distant hybridisation which they define as either crosses between different species or crosses within a species but from diverse geographic sources. The article contains references to many successful intergeneric crosses between different conifers including sequoia, sequioadendron and taxodium. Had metasequoia been discovered they would have hybridised it too!
Consistent with Lysenko doctrine the article also refers to grafting as a special case of hybridisation. This is of course nonsense! Never-the-less the Russians were skilled grafters and the results they report do not in most cases conflict with the known benefits of grafting. We recognise today that rootstocks do exert powerful effects on the scions but in no way interfere with their genes.

Could it be that because hybridisation was a tenet in Lysenko’s poisonous doctrine, that as a force in evolution this concept has been tarnished?

Metasequoia and Taxodium as garden plants



I group them together as both these close deciduous relations can easily be grown and share similar features. Both can grow in poorly drained conditions but also in almost any normal garden soil.
Potentially too big for many sites, dwarf and more slender forms are available. For example Metasequoia ‘Sheridan’s Spire’ is more slender than the usual form. Taxodium ‘Celeste’ is slow growing and spreads to make a low shrub. Even that giant of all trees, sequoia comes as ‘Little Ted’ in an exaggerated dwarf form.



I find it amazing that plants are so ‘plastic’ that trees with the genetic constitution to be so huge, with tiny genetic tweaks can be so small. In the case of bonsai trees their diminution requires no genetic change at all.
My own two metasequoias and lone taxodium are the normal vigorous quick growing kind. RHS quote 25 metres as the eventual height of metasequoia in the UK. It could be very much more!
Initially quite slender my own dawn redwood will do me for perhaps twenty years. Should I still be around I will chop it down and plant another. I wonder if the stump has the capacity to regenerate?



I find it quite fascinating that a simple mutation can change the leaf colour
Nikolai Vavilov 
Radio Four recently made a programme about how old wheat varieties such as emmer and durum had been preserved and were now maintained as a genetic resource for the future. I learnt for the first time about one of the world’s greatest geneticists, Nikolai Vavilov who travelled the world, building up the greatest ever collection of seeds, roots and tubers. He could talk to local peasants and farmers in fifteen different languages. A giant amongst geneticists and evolutionists he was a world authority on the evolution of plants and ourselves. He dreamed of breeding  – no doubt exploiting hybridisation - high yielding nutritious plants that would feed the world. He rose from peasant stock to become for many years the director of the Lenin Academy of Agricultural Science.

He sponsored a young peasant named Lysenko….
In 1940 Valilov was sentenced to death by Stalin for opposing Lysenko’s views. This was commuted to 20 years hard labour and he died in a prison camp in 1943.
His former staff at the Institute maintained his collection and although several of them died of hunger in the Russian famine they refused to eat their valuable store. Seized in the Nazi advance the collection went to Germany.
Posthumously pardoned, the Vavilov Institute was named in his honour. He lives on today together with his equally brilliant physicist brother in the names of a minor planet, a crater on the far side of the moon and Vavilov II, the name of a ‘Siberian wheatgrass’ widely planted in North America.
In 2010 the Putin regime was in the process of selling for housing development hundreds of acres where The Vasilov Institute’s vast collection of fruits and trees were preserved…. 

A footnote on Sequoia giganteum
Rather like the the three fossil trees the giant sequoias came late to Europe and in the UK became widely known in 1853. When seed raised plants became available every gentleman’s estate had to have one. 
The new introduction was named Wellingtonia! How Victorian to name a fine American tree after a British general. In the UK wellingtonia is still widely used as a  ‘common name’.
Some have grown very quickly and several specimens are now fifty metres high. For a tree that can live for thousands of years their potential UK height is unknown.

We visited an old country estate in Oxford this weekend and I was thrilled to find two fine sequoias.

The wellingtonia towers over other trees at the entrance

The lovely soft red bark is fibrous and fire resistant. In ‘less refined gardens’ the bark is worn where visitors have punched it!

Look carefully and spot the male and female cones
To read my previous posts in this series click 'hybridity' in the side bar

4 comments:

  1. For an amateur gardener like me, the science and history behind everything I see, touch and smell in my garden is fascinating, interesting but rather heavy stuff to take in as I don’t have any enough base knowledge to slot every piece of information into. That said, I read every article you write with interest, you must have put a lot of research into them and I commend you for that!

    Apart from the interesting topic in itself, I have always been fascinated by trees and loved your photos. As I am about to move house, one of the things I will miss from my current garden is my 9-10m tall tree at the bottom of my garden –way too big and tall for such a tiny garden I have. I am not 100% sure what it is as I inherited the tree with the house, but it is probably a Thuja plicata. The tree is now so mature that the bark is peeling and if you touch it during spring and summer, the smell on your hands from the aromatic oils are lovely. I can’t pass it without having a sniff :-)

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    1. You are not the first person to use the word 'heavy'. I am glad you are reading the post! It's fun doing the research and I learn a lot. It's most easier the ones that just come out of my head.
      I am sure you will love your new garden Helene. It sounds if your conifer is quite big. Thuja does have a very characteristic smell and I am sure you have looked it up for pictures.
      I look forward to hearing about your progress in your new garden on your blog!

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  2. I had often read about Wellingtonia but never bothered to look it up. How funny to realize it is Sequoia giganteum!

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    Replies
    1. It's surprising how many people do call them wellingtonias over here and many fail to make the connection with the wonderful American redwoods, Alain.

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