Christmas has arrived and around the world nativity scenes depict the birth of Christ. According to the tale, Mary gives birth as a virgin; a marvel of asexual reproduction. The ‘immaculate conception’ remains a compelling fable, whatever we think about the truth of this young couples love-life (or lack of it). But this yuletide miracle of creation isn’t exclusive to the Christian faith; plants can do it too thanks to a process called apomixis.
Nurserymen and women have known for millennia how to produce plants for free by vegetative propagation, and home gardeners do this all the time by splitting their herbaceous perennials and taking cuttings. Such methods are collectively known as vegetative propagation because we are creating new plants from the parent’s living material, whether that’s leaf, stem, bud or root. We can reproduce plants this way that are exact genetic copies of the original. Every time we ‘chip’ a bulb, layer a stem, strike a cutting or divide a plant we are producing a clone with the same DNA as the parent. Modern techniques of micro-propagation using cultured tissue in lab-like growing units do the same, albeit at an expanded scale and over a radically reduced time-frame, allowing for mass production and distribution of new varieties.
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But even though humans have harnessed this plant power, the ability is an evolved capacity utilised by some wild species. Take the Crack Willow (Salix fragilis) for instance, whose twigs and branches break free from the tree and float down stream where they lodge into the riverbank and root, creating a new tree that will develop into an adult. We have seen this process in action when revisiting a Hebe shrub trimmed over for a customer. Apparently we had tidied the clippings slightly haphazardly leaving some to fall onto the soil…we were amazed to find these <1cm ‘tip-cuttings’ had since rooted, creating a handful of healthy, if diminutive, individuals that could be potted up!
Vegetative propagation is one of the key horticultural techniques employed by gardeners both amateur and professional, and accounts for the production of the vast majority of plants we buy today. However, whilst modern techniques can induce this astounding ability in almost every plant, most wild species aren’t specially adapted to use it as their primary method of reproduction. Instead, plants produce seed.
Seed production is the opposite of clonal duplication; the aim is to produce a plant with a new and unique set of DNA, not a copy of the parent’s. Male organs produce pollen which is transferred to the female, with the help of insects, the wind and even birds and bats, exchanging chromosomes in the process. Although this procedure relies on a complex and delicate chain of interactions to be successful, the advantages are multifarious as genetic variation enables adaptation to local conditions and resistance to pests and disease.
In clonal populations every individual is genetically identical and will interact with a disease or stress in a similar way; in a genetically varied population, some individuals may exhibit signs of natural resilience. Sexual variation and hybridisation are key mechanisms in plant speciation (the emergence of new species). So whilst asexual vegetative propagation allows for rapid production of indistinguishable stock, it is sexual reproduction by seed that proffers us the ever expanding horizon of plant cultivars available to buy, and the breath-taking botanical treasury we’ve inherited from aeons of evolution.
Until recently we thought these two forms of reproduction (taught to us from infant school to horticultural college) encompassed the full range of options available to plants but we were wrong. Enter a third and distinct type of reproduction: apomixis.
A Christmas Miracle?
Apomixis is an ability inherent in some plant species to produce seed without fertilisation. This isn’t vegetative reproduction, or even self-fertilisation. Without so much as a visit from a passing bumblebee, fruits form containing viable seed. These seeds are independent of the parent and can grow into healthy and (sometimes) fertile adults…an ‘immaculate conception’ indeed!
Well, not quite. In apomixis, there is no transfer of DNA between male and female plants or flowers, which makes the seed a clone of the female parent; in apomixis, sisters are doing it for themselves! And to stretch the nativity analogy, if vegetative reproduction would be taking a hair from Mary and growing a copy of her, apomixis is closer to Mary giving birth to a baby Mary! Once distributed, apomictic seed goes through all the same life-stages as if sexually fertilised, whereas vegetative reproduction simply carves a piece off the parent to avoid the faff of germination and juvenility.
Apomixis has some interesting consequences. Brambles (Rubus fruticosus) and Dandelions (Taraxacum officinale) are both known for their prodigious production of seed, entrenching their reputation as both a pleasure and pain for the gardener (can you ever be too old for blowing Dandelion clocks?). Their commonalities continue in that they can be apomicts, i.e. plants that exhibit apomixis, but can also reproduce sexually too. This means that slight variations in offspring that come from natural variation in sexual reproduction can be reproduced apomictically (and widely distributed by wind and birds) creating colonies of clones.
In exclusively sexual reproduction, a particular variation might occur in the offspring of one generation (a slightly different leaf shape for example) but then be lost by the next, as the plant with the variation in turn breeds with other nearby plants that don’t have the same DNA. Variation in the progeny will depending on which bits of DNA from the male or female plants happen to get written into the DNA of the seed (a brunette and a redhead might have red-head children, then again, they might not). Sometimes variations occur over and over again, becoming so common they are transmitted down the generations. However, more often than not, slight variations ‘come out in the wash’; they don’t lead to the creation of sub-species but remain an interesting and transient anomaly.
Apomicts have a trick up their sleeve here. Variation arises from sexual reproduction but can then be reinforced by the production of clonal seed. A slight variation in leaf shape here, or flower colour there can be replicated across thousands of individuals in short order; a single apomictic dandelion plant can produce up to 15,000 seeds! In contrast to natural vegetative reproduction (like in the Crack Willow) which requires individual pieces breaking off from the parent, apomixis distributes clones in orders of magnitude further and faster. An additional advantage is that the spatial distribution of seed can counter vulnerability to local pests and diseases; a bird eaten Blackberry from Dorset, deposited later in Durham, doesn’t suffer so much from being genetically identical to the parent if a pathogen is only present in the former area (although pathogens can travel fast too, particularly as humans transport plants around).
One question that arises is whether large communities of plants with identical minor variations count as a sub-species. Generally botanists are hesitant to allow this and instead use the term ‘microspecies’, with plants labelled agg. for ‘aggregates’, such as in the Hawkweeds, Hieracium agg. (411 microspecies). In total there are 334 microspecies of Bramble, 232 of Dandelion and around 30 species of apomictic Whitebeam (Sorbus sp.). In the list of native British flora, just the addition of the microspecies from these four plants would almost double it’s length and only a handful of botanists worldwide can correctly identify them by their minute differences.
A final interesting outcome of apomixis is the ability for communities of otherwise sterile plants to produce seed. The stunning Tea Crab Apple (Malus hupehensis) is triploid meaning it has an extra set of chromosomes (three sets in total). In general, plants with odd numbers of chromosomes (tetraploids, pentaploids etc.) produce sterile pollen, whereas plants with even numbers (diploids, tetraploids etc.) are fertile. But as apomictic propagation is asexual, there is no need for fertilisation, meaning a plant like the Tea Crab Apple can produce seed even though it is sterile!
The natural world continues to amaze us and we have barely scratched the surface of the complexity of life. Perhaps all of us should be a little kinder to the Dandelions in our lawns…you never know if they are a unique microspecies never before known to science! Then again, you might not care as there are 232 others knocking about. Either way, we think the process of apomixis is every bit as miraculous as the Christmas story. Whether it’s sterile Crab Apples setting seed or Brambles producing berries without the help of bees, your garden might be engaged in a (not quite) ‘immaculate conception’ under your very eyes.
Further info:
The number of microspecies in Taraxacum, Rubus, Sorbus and Hieracium was gleaned from Harrap’s Wild Flowers, a brilliant photo book for wild flower ID. We recommend it highly! Buy it from one of these Summerfield Books or NHBS Field Guides & Natural History
Find more detail on rare and endangered microspecies in the latest edition of the Vascular Plant Red List for England, which can be downloaded from the Botanical Society of Britain and Ireland here England_Red_List_1.pdf (bsbi.org)
Malus hupehensis was identified as triploid thanks to a brilliant article on the Trees and Shrubs Online tool of the International Dendrology Society. The tool is an amazing, searchable encyclopedia with information about every tree or shrub you could think of! Just be careful to spell the plant name you are interested in correctly, it doesn’t supply a ‘did you mean this’ suggestion. Find it here Trees and Shrubs Online – A modern reference to temperate woody plants and the IDO website here International Dendrology Society
Finally, if any of the information here is incorrect, we would like to know about it! Drop us a private message or post to our Facebook page Ellie’s Wellies – Organic Gardening with your comments.
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