Tuesday, 26 July 2016

Hybridisation as the engine of evolution

Horizontal transfer of genes

In Mendelian terms these thrum flowers are dominant. Why do they appear in equal number to recessive pins?
The scale dropped from my eyes when I realised that hybridity which has shaped virtually all the animals in the farmyard and most of the plants in the garden must also be a force of nature. It is nonsense that hybridisation needs mankind to direct it and since I have read Gene McCarthy I have learned that hybridity between closely related species in nature is an everyday occurrence. More distant crosses are rare but on an evolutionary time scale happen, can be fertile and are hugely significant. The more we learn of our own human evolution, the evidence for natural hybridisation comes closer to home!
Too long has hybridisation in the wild  been thought of as some kind of aberration. The truth is that It is a fundamental part of evolution.

My issue today is the idea that nature has separately evolved almost identical features and processes in very diverse animal and plant groups. I have no quarrel with the fact or the concept of convergent evolution but increasingly wonder if in very many cases when nature has had a ‘good idea’ it has been passed on! 
In an earlier post about mistletoe my headline was ‘so good nature invented it five times’. That was before my Paulian conversion!

I was starting to think that ‘common features’ might not all have independently evolved ‘in a straight line’ as declared in the Darwinian mandatory mantra. I was consequently delighted when I heard  a professional botanist declare in a horticultural lecture that for the pin-eyed and the thrum eyed flower condition that not only had it had evolved independently many times over in numerous distinct families but that the condition was controlled by the same linked multiple genes. He described it as a genetic mystery that the same genes controlled the condition in genetically distinct families.
There is no doubt that the thrum and pin condition is a shared feature. I am less sure about the genes being the same ones and in my naive net searches cannot find confirmation that the genes are really the same. Nor do my searches deny it. There is an immense amount of highly technical literature which frankly is beyond my comprehension on this much researched phenomenon. After all the thrum and pin eyed condition gave Darwin some of his great insights.

How hybridity theory might explain shared clusters of genes across species, genera  and family boundaries.

The process is called introgression. It is when genes flow between closely related species. It is a widely recognised mechanism and the fact that it occurs is not controversial. We ourselves share denisovan and neanderthal genes our ancestors received by introgressive gene flow.

I want to suggest how by introgression genes might travel a long way from species to species to species!  Natures’s good ideas can be passed on!  Many life characteristics are controlled by multiple genes frequently ‘linked’ in their places along the same chromosome. Such gene clusters frequently remain intact after the genetic exchange of sexual liaison.  

The classic circumstance of introgression is where the distribution of species and genera overlap in hybrid zones. Hybridisation across hybrid zones can occur for all types of life that reproduces sexually - plants, animals, fungi et al. (And we know to our cost how simple bacteria even more directly pass on genes for such as antibiotic resistance. Gene-flow across species lines from the earliest times has been a fundamental part of evolution)

Hybrid zones might be meters, kilometres up to thousands of kilometres wide. They might exist for hundreds or thousands of years. The precise line of such boundaries sometimes frequently change, even from season to season. They are places where similar species overlap and sometimes cross fertilize. Such sexual exchange is rare but where large communities overlap intermediate hybrids are frequently found. The closer the two species are in their genetic make up, particularly where they share the same number of chromosomes, the more often such hybrids occur.
Such hybrids do not usually become separate stable species. Low fertility and the mathematics of future coupling almost invariably mean that they successfully mate with the geographically nearest parent species. The process is one of repeated back crossing. After not many generations evident evidence of the initial hybridisation can disappear. 

Crucially what does remain is the fact that packages of genetic information has been passed on in either direction across the zone and in due course, especially where introgressed information is advantagous it persists in the parent population.

Where the process gets really interesting is when introgressed genes are  passed on by hybridisation to a third party. Let’s say that they moved from species A to species B and then moved on to species C. It may well be that species A could never cross with species C directly but advantageous characteristics never-the-less cross the line. Such a progression might take place in as little as a few hundred or a thousand years. That is as nothing in evolutionary terms. Good ideas can potentially be passed on ad infinitum.

I do not suggest that nature cannot reinvent processes such as mistletoe’s ability to tap into a host’s nutrient supply or the subtle advantages of cross pollination achieved by the pin eyed and thrum eyed condition. These things might arise anew in organisms genetically distant. Nature does not have a rulebook and I suspect some things are new innovations and some have been passed on.

An example where a new species has arisen in a hybrid zone
The Appalachian tiger swallowtail butterfly has been shown to be a mosaic of combined traits of the Eastern tiger swallowtail and the Canadian tiger swallowtail. Although in limited areas of shared habitat it will hybridise with either parent it is now sufficiently isolated to be recognised as a new true breeding species sharing distinct traits of both parents.
It would appear that in a period of past climate change hybrids produced within a changing hybrid zone not only displayed superiority in a new habitat but as a result of subsequent isolation intrabred to stabilise as a true breeding species.
Seemingly the Canadian and the Eastern tiger diverged 500,000 years ago and the Appalachian hybrid emerged 100,000 years ago. It has remained stable ever since and inhabits a very different and perhaps harsher habitat than either parent,
The authors of this fairly recent research provide copious genetic evidence that the Appalachian tiger swallowtail is hybrid. Interestingly they assert that the Canadian tiger butterfly and Eastern tiger much earlier had separately evolved mimicry to the same butterfly and had also evolved a phenomenon known as ‘obligatory pupal diapause’…… 
I wonder…..

(Mimicry)
Just to remind you that mimicry is exemplified when a none toxic butterfly in effect ‘pretends’ it is poisonous by displaying similar colours to one that is distasteful or even deadly. The tiger butterflies in the above case mimic their relative the pipe-vine tiger butterfly.
As I have some Maderia holiday pictures of the Monarch butterfly I will use this as a further example. In this case its mimic is the viceroy butterfly.

The monarch is toxic to predators and advertises its presence
It accumulates it’s toxins from poisonous milky sap in this case from poinsettias
Caterpillars accumulate alkaloids in their body tissues and also shout out their presence
Toxic leaves have been stripped by their larvae from this host plant
Intergrading
In my zeal to pass on the concept of information passing across a hybrid zone without obvious change to organisms living in that zone I have neglected to mention a different  situation where populations in effect merge. Across a hybrid zone apart from an initial huge variation in mutual fertility between organisms - no two systems are the same - there are large distances across zones and often changing environmental conditions. Sometimes breeding between hybrids within the zone is much more significant than I have described

Pin and thrum eyed flowers
I has been suggested that the achievement of which Darwin was most proud was his discovery of heterostyly in flowers. His great insight was that it ensured the exchange of genes by cross fertilisation. 
All UK native primulas other than the Scots primrose have this characteristic.

Primula flowers exist in two forms, the pin and the thrum conditions.
A primula pin
Thrums on primroses
Pin is when the stigma is held at the top of the corolla on a long style and the anthers on the stamen are held halfway down the corolla tube. On the right is the stigma of a thrum flower
Pin primrose
The basic principle is that when a pollinating insect inserts its proboscis into a thrum the position of any sticky pin eyed pollen it might carry matches that of the thrum eyed stigma!  And vice versa when the cross is in the other direction the position of thrum pollen matches the pin position.
The result is that successful crosses will be thrum x pin or pin x thrum.

Pulmonarias are either thrum or pin eyed

Daffodils are a little more complicated 

In actual fact it is not quite so simple!
What puzzled me was that this much researched phenomenon is controlled by a large number of genes and yet the inheritance of pin and thrum follows simple Mendelian lines. Apparently the whole process is controlled by a single supergene that has three main components - for length of style, for position of stamen anthers and for size of pollen. For practical purposes they move together in sexual exchange and can be regarded as one!

The thrum condition is dominant and the pin is recessive. Apart from the relative positions of stamens and stigma a pin is physically unable to pollinate a pin and a thrum is physically unable to pollinate a thrum. 
Pin pollen is two thirds of the size of thrum pollen. It has sufficient resources to penetrate and grow the length of the short style of a thrum but not the long style of a pin.
Larger thrum pollen has in theory ample  resources to grow a pollen tube along the length of a thrum style but being larger the pollen tube is unable to penetrate the stigma.

In summary for primulas, a thrum can never fertilize a thrum and a pin only rarely fertilises a pin. Therefore in Mendelian terms double dominants are impossible for primula flowers and double recessives resulting from pin x pin are rare. An equal distribution of thrum and pin flowers is therefore maintained.
(If this last sentence hurts your head then recall;  double dominant thrums do not exist and all thrums are therefore a dominant/recessive combination; all pins are double recessive; ergo when genes are paired in meiosis half the plants are thrum and half are pin. Allowing for rare pin x pin crosses their will be just a few extra pins in a field of primroses!)

If anyone is pondering that their own primulas are all the same then remember if a named variety of primula is vegetatively propagated by division it will be!

And what about the Scots primrose? For plants that have evolved in harsh environments with few pollinators around self fertilisation is better than no pollination at all.

So what about introgression?

The pin and thrum condition occurs in thousands of different plant species. Common ones include pulmanaria, linum and purple loosestrife. Even daffodils have a very similar system. Not all examples are identical and for some plants there are three potential corolla positions. The worldwide family Rubiaceae which includes plants as diverse as coffee and madder is said to show the pin/thrum condition in four hundred species in eighty-odd genera.

The curious thing is that the normal evolutionary story is that the condition has separately evolved numerous times in the Rubiaceae. I can understand that if a common ancestor showed this condition that this would simply explain pin and thrum commonality in this family. The literature however seems adamant that it has evolved many times anew. It is quite beyond me and I would be foolish to speculate further!

A further word about tristyly
Lythrum, purple loosestrife can hold its stigma at three positions, low, intermediate and high. For each flower-condition the stamens are held at the two vacant levels. These are some pictures taken yesterday in my garden.

Although I have had some difficulty in interpreting this clearly is a pin

Here stamens appear to be held higher than the stigma
To me this is a bit of a jumble

Has the hover fly pollen on his head?
Going about his buzziness

Conclusion 
Fools jump in and I still wonder why introgression is not put forward as a possible explanation for the shared pin/thrum condition across so called species barriers. Evidence of ancient hybridity is very difficult to demonstrate from genomes, especially when nobody is looking. When you take a position as I do that the biological world overlooks the significance of hybridity as a fundamental force in evolution and generally sweeps it under the carpet then at least I want to ask the question.

These things make me wonder

Gene McCarthy writes about intergrading

Wikipedia defines introgression


2 comments:

  1. Absolutely fascinating Roger although your post has taken a while to digest, in short the answer is yes I agree and 42 according to Deep Thought.

    ReplyDelete
    Replies
    1. Thanks for putting your head above the parapet and making a comment Rick - where others fear to go!
      Your witty reply probably sums it all up

      Delete

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