This is the last in my series where I have attempted to argue the case for hybridisation as a significant force in evolution and also to describe interesting hybridised plants.
In the last three posts of my series I attempt to explore some of the secrets of the three renowned ‘fossil trees’ Ginkgo, Taxodium and Wollemia.
Today I conclude with Wollemia
We all recognise the contribution of hybridisation to beauty and nourishment from the garden and food from the farm. Why do people think that somehow hybridisation is foreign to nature? Our resistance may be as inconsistent as “it does not work in nature” or “hybrid plants don’t survive in the wild” to “the alien monsters will take over” or “its an unnatural abomination”.
Which is it to be, it does not work or it is too successful?
Certainly many of man’s creations by hybridisation, be it gaudy plants or flat faced dogs would not survive in the wild.
When the power of natural selection - rather than mans’s fickle choices - choses between success or failure when plants or animals hybridise the result is organisms that are honed for survival.
Some folk believe that hybridisation just does not happen in nature. In some people’s eyes hybridisation by definition is somehow unnatural! Even if they get over this hurdle they think that there is no way a hybrid can pass on its new gene combinations.
Documented cases of successful hybridisations in nature are legion and on an evolutionary timescale their number almost infinite.
It is beyond the scope of this post today to explain how hybrids in nature survive to pass on their genes. It is a real difficulty of comprehension. Read my inspiration Eugene McCarthy to understand stabilisation of a new life-form. You might take some time!
For almost two billion years cellular life on this planet was limited to single celled organisms. There were and still are two distinct forms, archeae and bacteria. So distinct that some scientists have hypothesised that they might even have had separate evolutionary geneses. Many scientists consider that multi-celled animals might have arisen by some kind of fusion between an archaeon and a bacteria.
Another example of a game-changing fusion of two organisms, now a mainstream theory, is that chloroplasts and mitochondria were created when bacteria were subsumed by another organism.
The above two examples are not hybridisation. Hybridisation requires the sexual sharing which came with multicellular life. However if these hypotheses are correct, that a process of joining together generated the genetic genesis of all multi-cellular life then this is very convincing evidence indeed that sharing of distant genetic information helped power evolution.
Some people are repelled by the idea of genes moving across barriers when distinct species hybridise. Standard theory states that genes only pass down in a straight line from life’s very beginnings.
Yet now there are countless examples of ‘horizontal transfer’ of genes. Even in this week’s New Scientist it is suggested in a matter of fact and casual way, that through evolutionary time that 9% of our genome arrived via virus transfer!
The Wollemi Pine, Wollemia nobilis
Found in an isolated canyon by climber David Noble in Australia in 1994 this was the last of my three featured fossil trees to be discovered. Just like the others it raises fascinating evolutionary questions not least in this case about its remarkable genetic uniformity. It might be that every single Wollemi in the world is genetically identical!
The Australian authorities seem to have done everything to both preserve and distribute the tree. Not least to secrete the exact location of where it resides in the wild. Conventional opinion is that genetically uniform material is particularly vulnerable to disturbance and invasion by introduced pathogens. The wollemis might be endangered by public exposure.
The wollemi pine is easily propagated and huge numbers have been propagated from cuttings and micropropagation. Many have also been raised from seed and several trees have now set seed in foreign places. Seeds have been collected for example in Bangor in Wales.
The wollemi has several peculiar botanical features. It has strange ‘bubbly bark’ and distinct juvenile and adult leaves. The juvenile leaves are low on the tree and are able to photosynthesise in the extremely low light levels deep down in the shade of its Australian canyon. Seed is able to germinate in these low light conditions but in the absence of any forest clearings it was thought that there was little evidence of seed making new trees - but this is now disputed. The intense shade is made worse by the wollemi’s efficient vegetative propagation when several trees in a stand might be a coppice of a single plant.
The adult leaves higher on a tree extend on a single branch looking rather like thick arrowheads. It is described as ‘phased growth’. The life of a branch might be as little as ten years before restricted xylem at trunk connections fail.
There are about a hundred wollemi pines in the National Park forest They achieve forty metres high. Conditions are described as ‘rainforest’ and the soil is an extremely acid pH 4. Remarkably and in common with ginkgo and metasequoia it now grows very well in temperate conditions around the world.
It has been reported as frost hardy in the UK to -7C, in Japan to -10C. We might not be warm enough in the UK for long term survival other than in very warm places! It’s too soon to know.
|This wollemi at Kew is outgrowing it’s prison. I wonder if the cage gave early frost protection to the young growth?|
The genetic mystery surrounding Wollemi noblis
The wollemi is know as a ‘fossil tree’ on the basis that fossils were already known before the ‘extinct tree’ was discovered.
The earliest fossils are about 90 million years old. Even that long ago they seem to have had juvenile and adult foliage identical to that of the modern tree. Until seen together on the newly discovered wollemis these variations were thought to be separate trees. Yet again a fossil tree shows remarkable lack of change from its fossil.
Wollemi nobilis is not the oldest tree in its lineage. It belongs to the ancient gymnosperm family Araucariaceae which includes the even older genera Agathis and Araucaria. Araucaria dates back 150 million years.
The mystery is that all Wollemi pines are genetically identical. They can be raised from seed yet every known plant is effectively a single clone. (Inbreeding which comes with self pollination within isolated populations leads to genetic similarity but nothing like this. We are familiar with seed of a cultivar such as an inbred tomato ‘Ailsa Craig’ giving for practical purposes seemingly identical young plants but such plants are not completely the same).
Inbreeding and consequent uniformity is associated with ill health and lack of vigour. This is not always so as deleterious genes can be ‘selected away’ as in the case of the all conquering harlequin ladybird. The Wollemi pine too is remarkable for its health and vigour.
This fascinating piece in this New Scientist link is not entirely consistent with the above about the harlequin lady bird. Indeed the picture might show the first step to producing a hybrid
No one has an adequate explanation for wollemis seeming to be genetically a single entity. It is quite understandable that as a result of vegetative propagation wollemis growing together might be a single identical plant but not those several miles apart in the roughest terrains. Could it be that their seed is apomictic and propagates vegetatively? This phenomenon is almost unknown in conifers. Is this consistent with needing pollen for fertilisation? It is possible as variations of apomixis are known where fertilisation is needed to produce seed endosperm but leaves the clonal embryo intact.
Perhaps the Wollemi pine has always been a vegetatively propagated clone? Could it be a clone of an infertile hybrid?
Possible hybrid origin.
I don’t think my case for hybridity being the origin of new genera is much advanced by the Wollemi pine.
Possible indications are that the viability of wollemi seed is a very low 11%. Sometimes seed from a hybrid is not very fertile. The original botanist who examined specimens brought to him by David Noble, said they looked a bit like a cycad or a fern. Straws in the wind.
If there is any evidence about hybridity coming out of my story it is provided by wollemi’s close relation, Araucaria. Members of the family Araucariaceae are known to have a genetic inheritance of low genetic diversity. (Araucaria araucana our own monkey puzzle tree which was imported from Chile lacks genetic variability because of geographical isolation).
|A fine monkey puzzle tree at Dial Farm down our road|
Araucaria over it’s more than 100 million year history has diverged into several species. Ecologists are very concerned that with climate change plants lacking genetic diversity are particularly vulnerable. Genetic investigation of several araucaria species in South America showed introgression and probable hybrids. Recall that introgression occurs when different species with overlapping distribution create a hybrid which as a result of repeated backcrossing merges with one of the parents.
If biodiversity is promoted by the extra genetic mixing brought about by hybridity we need to know. There is too much about preserving native species unsullied by hybridisation. Hybridisation apart from it’s past huge significance might be the way of the future.
The more I read about the work of ecologists and geneticists I realise that they do embrace hybridity. It is a recognised significant component of evolution. Why don’t they rock the boat a bit more and get it into the evolutionary text books?
This is an interesting but redacted article about the story of wollemia that you might not easily find in a routine google search. It is surprising what you can find on a high numbered search page. (You will need to scroll back to page 1 of the site)
To find my previous four articles on hybridity use the link ‘hybridity’ in ‘Themes’ in the right hand column