Wednesday, 6 May 2015

Hybridity’s contribution to evolution - featuring the Wollemi Pine

Part 5

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, Metasequoia 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.

Conclusion

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

15 comments:

  1. My understanding of ecosystems and evolution is that it needs to happen over a long period of time to allow the arms race between species to take place. I’m guessing that hybridisation can be part of that, but it needs to happen slowly, and the problem occurs when it’s too fast and species don’t have time to evolve. The argument for native species is perhaps intended to keep the pace of change slow, rather than prevent it altogether? My favorite example is the Purple Emperor butterfly, as it prefers the biggest sallow leaves for it’s eggs - the caterpillars are large and to survive predation they need these big leaves. If over a thousand years or more, the leaves were to slowly become smaller, then by natural selection the butterfly would likely do the same. But do it suddenly, and it you lose the butterfly.

    I think it also depends what you are arguing for. In the long term, millions of years, then new species would hopefully evolve. They would need a stable environment in which to do so, i.e. not one that man is constantly changing. Perhaps that will happen, who knows. If I wait long enough, perhaps something will evolve with an appetite for horsetail!

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    1. Thank you for your as ever thoughtful and questioning comment, Sarah.
      My comments about hybridity refer to the long game - life from the very beginning!
      I think hybridity contributes to the long slow grind of evolution when a hybrid between near species backcrosses multiple times, generation on generation to be absorbed into the never-the-less changed parent species.
      But most significantly it can bring dramatic rapid change too.

      Perhaps we do need rapid change as mankind trashes the natural environment. Examples of rapid change are when alien species cross hitherto natural barriers. The public image is that this is a very bad thing - and sometimes it really is. In other cases the feared effects do not arise or the initial harmful effects become ameliorated as native flora and fauna adapt. Sometimes the alien contributes substantially to improvement of habitats disturbed by man
      Take one of the hated invaders, the Zebra mussel. I am not qualified to say whether they are the deadly enemies as many jurisdictions fear but it is very interesting that in polluted Lake Erie, water that was opaque at six inch depth is now clear to 3 foot and NATIVE wildlife is returning.

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  2. Very interesting post, though some of it is a bit deep for me. It does seem that there are naturally occurring hybrids in many genera - for example Aquilegia. Some genera seem to hybridize more readily than others.

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    1. If ever difference in species as a barrier to easy breeding was ever a sensible concept it does not apply to aquilegia Jason!

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  3. Very, very interesting post Roger. I too do believe that hybridity can occur naturally. I am not at all knowledgable on this subject, so appreciate all your research and postulation. The wollemi tree article is rather thought provoking too.

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  4. Thanks Donna
    Brenda says I am very good at provocation and postulation and for good measure gesticulation!

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  5. I took a photo of the Wollemi pine in that cage in 2005, it was a bit smaller back then. I wonder if all the people planting this one in their garden down here have thought about what they will look like when they get 20-30-40m tall?? Small pines usually grow up…..
    Fascinating post as usual Roger, I have followed your series with great interest.

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    1. I wonder if that one at Kew will accelerate it'd growth now it is established. Its not really grown that much in ten years.
      Your comment about getting too big applies to my metasequoia and a number of other trees in my garden. I actually hold a rather ruthless philosophy and am prepared to chop down and replant.
      Perhaps you noticed in my recent post about Bolton Percy cemetery garden where I chop down ten year old birches and let them sprout new trunks!

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  6. Fascinating post, Roger. Thank you so much for sharing.

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  7. Fisrt thing I want to say that it was hard to read easily your mature post due to pink flowering them.
    You deserve high appreciation for you grave work.I don't have much knowledge of the subject but one thing is coming in mind that life began in shape of bacteria and developed in uncountable species in various forms among which we human are best and this is just beginning of the development of human brain .just one hundred year back people were treated inhumanly due to be exist in certain gene pool but thank God that now we know that no race is created different, inferior or high on the bases of colour

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    1. Thank you for your comments Baili.
      Sorry you are having trouble presumably on your phone with the dicentra flowers! Apparently you can do things on most apparatus to get rid of it- certainly on i phones where you just press the three bar logo

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    2. Dicentra were bugging me too, so just followed your instructions, and found 'request desktop site' option on the drop down. That sorted it.

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    3. Thanks for your prompt confirmation. Sarah. What were you doing here? No wonder my figures are good!
      By the way Yorkshire arboretum has a wollemi and John Grimshaw the curator says it survived the 2010 Winter without turning a hair. ((hope I have remembered right from his walk round he hosted recently)

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    4. Don't worry, I'm not a blog stalker! I must have ticked 'notify me' in the past. Once you've ticked that, any new comment gets sent by email. I just happened to notice your reply and as those flipping flowers have driven me nuts in the past I was interested in killing them :-)

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