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

A case for conifers, a conifer showcase

Mainly about conifers and birds

Not only do conifers enhance your garden they support wildlife

Some gardeners seem to consider conifers as alien invaders that have no place in a garden and believe them to be ecologically inferior. They are wrong.

Chaffinch nest in pine

Regular readers will know that since Harry died I have been doing blogmeister Cathi’s garden. They also know that my most popular posts are based on her pictures of animals. I have now realised why she has so many. Her garden is not only a haven for wildlife with  grass, trees, water and flowers, she has plenty of conifers.

Little and Large - not at all very special and taken for granted

Each Spring this cypress undergoes a transformation

It suddenly dawned on me that the new growth on her conifers looked rather pretty. I took it it into my head that it would make an ‘easy post’ if I just took a few pictures. I wandered back to my own garden thinking of course I don't have many and found I had fifteen! There is so much more to say about conifers than just pretty pictures.

This ‘volunteer’ is my number fifteen - albeit yew is not strictly a conifer

Spot three more conifers

What is lurking under Cathi’s juniper?

Oh it's Spike

Most conifers are evergreen and provide an all year round structure to the garden. The word ‘ever green’ is wrong in that conifers provide the most fantastic range of dramatic foliage colours. Not, as some claim static, but ever changing from season to season as they make young foliage and those wonderful cones.
No other group of plants provides such a wide range of well defined and dramatic shapes.

Lovely new foliage on Cathi’s thuja

Male and female cones on pine

See through the cedar

Not all conifers are evergreen. This metasequoia will eventually be too large

The Maiden hair tree is slow growing and deciduous

Most conifers provide cover and shelter. In design terms they provide a ‘permanent structure’ to the garden. They make very fine focal points too! As windbreaks they are second to none - although personally I prefer individual specimens to screens. They do actually make very fine hedges and a wider range respond to tight clipping than you might imagine. Even the dreaded Leyland makes a wonderful hedge if clipped six times a year!

The overgrown ‘hedge’ of leyland cypress is actually across the road

Conifer’s achilles heal is that many grow too large for the small garden. With some you need to be ruthless. If they are becoming too big cut them down and plant some new ones! Some so called ‘dwarf conifers’ might when established grow a foot a year. Other conifers much more! 

Three years ago under the then overgrown hedge these were invisible suppressed little runts. Look at them now

If carefully selected there are plenty of genuine dwarf ones. No matter, most conifers make fine plants lasting many years before becoming too big. Some eventually tall ones such as Picea breweriana have you wishing they might grow faster. Mine has taken ten years to make an elegant statement only now.

I wish more gardeners took the view that if a plant gives you fantastic value for say fifteen years and has achieved Its ‘use by date’ then it becomes expendable. It is not a crime to chop down!

I have chopped down more than fifty overgrown conifers over the years and have organised the felling of very many more! I cut as cleanly as possible close to the ground. I have never dug one out.

Cathi’s contribution

I e-mailed Cathi begging a few ornithological pictures and telling her of my intention to promote the values of conifers for wildlife. She replied…

Sure! Lots of finches nest in conifers. Lots of birds eat the insects that live in the bark. 

Thrushes, blackbirds like yew berries. Even sparrows

Hawks and buzzards nest in some conifers. You will have a lot of pics of these birds anyway. I’ll have a look for others. Got some of Harry’s finch pics - somewhere!

Siskin

Tawny owl

Blue tit

Thank you Cathi

Most gardens are ecolological hotspots of diversity. It is good to have a very wide range of plants that promote rich wildlife habitats for bees, butterflies, insects, amphibians and birds. Conifers make a valuable contribution.

Coniferous woodland

The tops of Cathi's pines are twenty foot high. The sparrow hawk likes to perch there

Conifers in Cathi’s adjacent small holding provides cover for corvids

Most research with regard to conifers and wildlife is carried out on dense woodland. Most woodland, whether deciduous

or coniferous, native or alien are valuable habitats but inferior to gardens in terms of diversity. Broadleaf forest has greater diversity than conifers.

But don’t damn coniferous forest. They provide a woodland home to very distinct residents. These include birds such as siskins, goldcrests, crested tit and crossbills. Raptors such as sparrowhawks and goshawks nest in conifers but prefer them in open woodlands or as individual specimens. They really like Cathi’s garden.

Many butterflies thrive and pupate in coniferous bark.

I wonder if the soft bark on my dawn redwood is suitable for butterfly pupation

Thank you Jim

Unashamed to gather information from knowledgable personal contacts I asked naturalist Jim. He is one of the Bolton Percy cemetery C team. By shear coincidence he lives in my old Bolton Percy home. Here is his e mail

Hi Roger - good to see you yesterday - I and my parents enjoyed looking around the cemetery on the Open day.

Re birds, 3 pics attached:

Goldcrest - the tiny resident bird with the yellow stripe on its head - conifers are more or less essential for it, and this year especially it has been commonly heard singing round the village with a very high pitched repetitive song.

Coal tit is another fan of Conifers, but not exclusively so

Crossbill - another conifer specialist with crossed mandibles for prizing cones apart (cheating here - the pic was taken in Austria) - although it's not really a garden bird. There's a chance you'll get the occasional one passing through if you have a few mature pines around.

Re conifers in gardens - In principle I would say in a mixed garden, conifers are great, as they provide relatively secure nesting and roosting sites, and do attract spiders which spin webs in the branches, so the birds feed on those.

As a monocrop they are not so good (in my opinion) as they block out all the light from the ground, so there's no ground flora, and they can trash the soil pH with their acidic needles. 

Hope this is of interest

Regards

Jim

When Cathi’s fantastic pictures came in I had a dilemma. What with Jim’s great contribution perhaps I ought to make a post about birds rather than gardens. Bird posts are very popular… and my own snaps are somewhat blurred. No matter, I will go ahead and celebrate wildlife, conifers and gardens.

I love Cathi’s blue cones

Peak throughout the picea

Links
I wrote a series of posts about the fossil trees - all conifers

wollemi pine

dawn redwood (and swamp cypress)

ginkgo

The cemetery c team and basal pruning of chamaecyparis

Nearly a million

When I started blogging I dared dream that if I achieved a million readings in my lifetime I would be extremely old and my life would be over. 

So I am starting to worry!

Cajoled into blogging by Cathi and Harry four years ago I think their ambition was that an old man in his dotage might gain some pleasure by passing on his knowledge to a few dozen readers. Even that would be a huge advance on when he wrote monthly in Bolton Percy church magazine.

I used to find writing a chore. Harry despaired when I started to blog and he looked out of his window and saw me fumbling with my camera. I had barely clicked one in the last fifty years. I did not do pictures. As a child if an exam question said ‘with the aid of a diagram’ it filled me with dread and was always avoided.

Harry feared I would get depressed because no one would read me. Brenda suggested I would start to neglect my garden and talked about blogweeds. Blogweeds are things that grow when a blogger is writing when he should really be doing.

Harry would have been thrilled at my success, he was my greatest supporter. Brenda barely concedes that my garden is better when I claim that blogging has concentrated my mind. It has gone to my head and made me a bore.This did not deter her from marrying me last year!

I want to thank Harry, Cathi and Brenda. Without their contributions I would have never got started or ever continued. 

When I rationalise why I started blogging I think it was because as a former teacher I still had something to say. It was no good throwing my toys at the tele when a TV personality yet again said something stupid and wrong!  (This is a common failing of horticultural lecturers and ‘professionals’)

I have had some interesting gardening experiences and wanted to leave some kind of record out in the ether. I do garden in some unusual ways!

I wanted to

1. record the development of my cemetery gardens and describe how I gardened naturalistically
2. promote the use of glyphosate as an ecologically sound safe labour-saving land management tool
3. describe in detail how glyphosate can be effectively used
4. contribute to the now widespread understanding of the benefits of minimum cultivation to effective growing and preserving the environment
5. pass on knowledge of dicentras gained holding the National Collection for the past forty years
6. provide information to gardeners who wanted to get past the endless cycle of repetition of shallow generalised cliches in the gardening media
7. encourage gardeners to try new things and question much what they are told
8. declare my main gardening passion; love of the soil
9. persuade gardeners that sound maintenance and effective weed control is the key to gardening success - oh the chore of pulling out couch grass and ground elder every week because someone failed to comprehend it should be eliminated at the beginning!
10. reopen contact with former students

I think it is working and now am a bold blogging bore. Almost every morning I am up at about 6.30 and spend perhaps 90 minutes preparing and posting. Other than repeatedly checking my figures and responding to reader’s comments that’s it for the day. Other than whenever I see Cathi or Peter or indeed anyone who will listen I talk about blogs! Other than daydreaming and writing posts in my head. Other than wandering around with my camera. Other than...

Cathi advised me to make my posts short and snappy. Harry insisted I only use high quality pictures and generously shared his collection. Unfortunately not many were of flowers and he and Cathi were regularly in tucks watching me from their window. Brenda’s son Steven suggested I might give tips for the day. That’s definitely not me and it would be no original contribution. Cathi edited my first eighty seven posts and transformed them into  intelligible English. Poor girl my pictures were just add ons and came to her in no particular order!

Contributors

We have bees in our bonnet and a bird on our head

I am ever grateful to those of you who use the comments column. It would be impossible to mention all your names and I will confine myself to thank the two of you who I have met. Blogger Sue Garett can be relied on for cheeky pertinent comment! Rick always adds thoughtful additions and generally agrees with me.

In point of fact I am happy to take objection and would welcome more debate than actually happens. I fear in the past I have myself not always been delicate when commenting in other places.

I would also like to thank biologist and gardener Peter Williams who lives in the village and has provided articles, advice and ideas. When we forged our friendship ten years ago I am told he said I was a ‘good find’. Ditto Peter and Julie.

I never expected the blog to change my belief system! ‘Fool on a Hill’ blundered in and sparked my intense interest in hybridization and my suppressed childhood thought that it must play a significant role in evolution.To me now hybridity as a principal component of evolution stands out a mile and I find there is evidence. I never imagined I would have the temerity as a layman to enter the fray.

Pondering Past Posts

Now past three hundred I approach each new one with naive enthusiasm. I can never predict which ones will be damp squibs or others more successful. Success bears little correlation to effort.

Half way to the cloud forest in Costa Rica

Temperate and tropical plants overlap

I will mention a few that I particularly remember. I wonder why Mission Unaccomplished was so successful when my other posts about my holiday in Costa Rica were relatively unread? Perhaps googlers were searching for something different.

Achimenes in Peter’s greenhouse

The unsexy titled ‘Her Indoors, Part 3’ about a fairly obscure house plant has been read nearly three thousand times!  I have now done several posts that have been much more successful but that was in early days. Where did that come from?

Calm and defiant 

Harry always said that posts about birds would always be popular. Shame I know so little about them! It was a godsend when he plucked a wayward sparrow hawk that had wandered into my garage and cupped in his hands brought it to our door. Brenda and I snapped away. I now realise the morning light was quite perfect. A modern camera  works wonders....

Those pictures really got my blog underway

It now astounds me that when I check the table of ten day top hits few are about gardening!

When I wrote in 2013 about a scorching phenomenon that occurs in some seasons on Garrya elliptica it hit a raw nerve and has now been read 6500 times! It thrills me that today when I searched for the link the quickest way to find it was to put Garrya elliptica in google and find myself on page one! 

It gave me an even greater thrill when my son Ben reported that his friend searching for garrya had found his father!

Natural browning and May leaf fall on ceder. There is nothing wrong!

I am frequently surprised. Sudden shrub death was one of the first posts when I started to include more detail and became rather verbose. I regarded the post as a failure as it had had little attention. I stumbled on it today and found it has made 5000. Whoops, I found a major typo. My big picture of Betula jaquemontii was labelled Acer jaquemontii - oh the shame. Why did no one tell me… obviously nobody was looking!

No fears of ‘damping off’ in my greenhouse

The posts I enjoy writing the most are those with serious intent but are rather ‘off the wall’.

I was particularly pleased with my effort about not washing pots and why ‘damping off’ is not a problem for me.

This might be your pot’s water profile

Many gardeners do not understand water/soil relationships. I was very serious about my naughty innuendos when trying to explain field capacity with a story about using a sponge in my bath.

It was early days and Cathi feared for my image and muttering ‘shades of grey’ refused to accept it. I stumbled on it a year later and bravely posted. No one took umbrage and hardly anyone read it.

Harry described this as ‘bodge extraordinaire

My post with the obscure name Part BPTA… was an early attempt at humour. It focussed on Harry’s genius as an engineer - and my own incompetence. I include it today remembering Harry who died three years ago

No one wanted to read about the crocodile plant and its curious pollination

No one wanted to read about myrmecochory 

What happened to no digging?

It’s all still there. I have exhausted the main arguments for not digging and discussed all the reasons why gardeners dig. I don’t wish to repeat myself. (Well not very much). I of course continue to incorporate my minimum cultivation philosophy in posts but I do urge gardeners who wish to seriously stop digging to read my previous articles linked in the themes column. Similarly those gardeners who are persuaded that their gardens can be improved by using glyphosate should study my series of glyphosate posts. That will be hard work and some of my methods are unorthodox - and horticulturally dangerous in novice hands. I can say without much contradiction that they are fairly unique!

Polygon

The week I started blogging we bought a lovebird. Poppy changed our life even more than blogging.  Poppy got several mentions. He was named after early reader Poppy Murray. She mildly protested that he was a male!

Last year our pet died. We were distraught. I could not bring myself to tell you.

Against our better judgement we are grateful to Po Simpson and Cathi who brought us Sparkle, a conure. Last week was his first hatch day. I am told they live for thirty years….

Do you think Poppy was trying to warn me about the referendum?

A serene celebratory dance