The Beauty of Muties part 1: a symphony in white.

In the world of science fiction, mutants are generally ugly and often evil – indeed, the very first Dalek story for Doctor Who was originally titled “The Mutants”.  In the real world, however, mutations happen in all organisms, and in pretty much every individual. A mutation is simply an accidental change in the sequence of the DNA, a copying error that occurs as the DNA replicates during cell division, a sort of biological typo.  This blog post is about some real mutants that can be seen when out and about looking at plants, and will focus on white flowers in species that are normally other colours.

The most famous white mutant in the British flora has to be the lucky white heather (Calluna vulgaris), which gypsies supposedly collect and sell as a good luck charm.  These mutants occur at frequencies of probably well under one in a million plants, but because heather is so abundant and the mutants so striking, they are not that hard to find.  In fact white mutants occur in many heather relatives, and I’ve seen them in Erica cineria, and Daboecia cantabrica (in Spain); they have also been reported for Rhododendron ponticum in Turkey.

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Rare white form of the Lesser Celandine Ficaria verna

Many mutations have no effect at all on the organism because, for various reasons, they don’t change the instructions the DNA provides for building proteins.  Other mutations may have an effect that is not immediately visible but may reduce the fitness of an individual, for example the sickle cell allele in human beings.  Others may have a visible effect on how an organism looks.  One way this can happen is through colour.  Colours in flower petals come from chemical pigments which are made by enzymes, coded through the DNA.  A mutation in the relevant part of the DNA can stop one of these enzymes from working, and as a result one can find a flower that is not its usual colour.  Most commonly, these mutant flowers are white when they’d normally be pink, purple, blue or yellow.

For me the most memorable example I have seen is a white form of lesser celandine, Ficaria verna.  The petals, instead of bright yellow, were shiny white, and there were no intermediate forms visible.  I’ve been lucky enough to see this twice, once on a dark woodland path in Surrey, and once by a path near Dartmouth.  Given how abundant this species is, the frequency of these mutants must be well under one in a million, but they are still worth keeping an eye out for in the lean botanical months of early spring!

Other very rare white forms I’ve seen include Impatiens glandulifera and Vinca minor near Loanhead, Verbascum phlomoides on waste ground at Millerhill, Lathyrus latifolius in London, Dactylorhiza praetermissa, D. incarnata and Acinos arvensis all in Norfolk, Ophrys apifera in Bristol, Centaurium erythraea in Devon, Thymus polytrichus on Lindisfarne, and Lamium purpureum in Craigillar Park (Edinburgh) and Romulea bulbocodium in Jersey.  White forms also occur in several Campanula species (perhaps in all of them); I’ve seen white versions of C. latifolia, C. rotundifolia and C. persicifolia.

Very rare mutants like these which may turn up for one generation and then vanish.  If you know your genetics, these will be double-recessives: most organisms have two copies of every gene, one from each parent, and if one copy is mutant and does not work, then it will usually have no effect because the other continues to do the work, like a wife holding together a household where the husband is drunk and lazy.  It is only when an offspring happens to inherit a mutant gene from both parents, that the mutant trait becomes visible.  So in populations where a mutant allele (i.e. the mutated form of the gene) which disables pigment production is present,  some individuals will be “carriers” that have one copy of the gene but show no mutation, and visible mutants will appear in ¼ of the seed produced when two of these cross, or one of them self-pollinates.

That the white mutants are so rare in the above species suggests that they are at a selective disadvantage.  This is difficult to prove, however.  If a white mutant for example receives 10% fewer insect visits than a purple one, a scientist will need a very large sample size in order to prove an effect, and the mutants are too rare in nature to allow a statistically meaningful survey.  Hence large numbers would need to be planted in order to test this.  Curiously Warren and Mackenzie (2001), found that the absence of anthocyanin pigments actually reduced drought tolerance in some species, but this is about why some plants have reddish colouring in their stems and leaves.

A few of our wild species have white variants at higher frequency, so much so that many or most populations will have more than one colour present.  A familiar British example of this is the common milkwort, Polygala vulgaris, where purple-blue, pink and white forms occur intermixed at often similar frequencies.  Darwin suspected that such regular colour polymorphisms are maintained by selection, which may make whichever colour form is rarer more successful, hence maintaining a long-term balance between them.  White forms are also seen quite commonly in Cirsium palustre and Galeopsis tetrahit, for instance, whereas large Centranthus ruber populations typically comprise a mix of white, pink and red morphs.  The huge foxglove population on Arthur’s Seat includes a substantial white minority.  How selection plays into these cases is less clear.

Many readers will have seen white or pink bluebells in the wild, but these are usually garden escapes, belonging to the hybrid Hyacinthoides x massartiana.  (Let me correct a common misconception here: the pure Spanish bluebell H. hispanicus rarely or never escapes into the wild in Britain; cultivated bluebells are almost always the hybrid).  In wild populations of the British bluebell, natural white mutants do occasionally arise, and pink ones are even rarer.  I have seen both in riverside woods south of Boarhills in Fife.  I suspect that the same will be true of many related blue-flowered species, and I once saw a pink mutant of Scilla verna on the north coast of Scotland.  The simplest explanation for how these pink and white mutations come about is that there are two pigments involved in producing bluebell flowers: a blue one and a pink one.  If only the pink one is missing we don’t see much difference, but if only the blue is missing, they come out pink, whereas without either they look white.  This may also apply to Centaurea scabiosa and Succisa pratensis, each of which very rarely has both pink and white forms.

Artificial selection among the cultivated bluebell hybrids has favoured the pink and white forms, making them nearly as common in gardens (and hence among escaped populations) as the blue form.  The same has happened for some other cultivated species that occur as escapes, e.g. Lychnis coronaria and Lunaria annua, and I suspect that white forms of Malva moschata are often or usually escapes.  For some white variants of non-native plants, like the magnificent white form of Ribes sanguinem I found in an old quarry in the Pentlands, it can be hard to know if it descends from a white garden planting, or if the mutation arose during the naturalisation process.

Colour mutants don’t have to be white, where one pigment is missing but another still present, as in the pink bluebell.  I’ve seen mutants of the common red poppy Papaver rhoeas with pink flowers, and with deep wine-coloured petals that looked as though they were already decaying, even when young.  This latter was probably not cause by a missing pigment but some other mutation, possibly accelerating chemical aspects of the ageing process in the petals.

The great thing about these mutants is that they can appear absolutely anywhere – one is just as likely to find one in an unpromising field edge or pathside close to home as in some remote biodiversity hotspot.  So keep your eyes peeled for something that’s both ordinary and extraordinary!

https://nph.onlinelibrary.wiley.com/doi/full/10.1046/j.1469-8137.2001.00159.x

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