Seed Oddities: Apomixis and Polyembryony

Plants have uncanny ways of reproducing themselves that are unparalleled by most other living things. Offshoots of themselves can be made by sending out modified stems above or beneath the ground which develop roots and shoots (new plants) at various points along the way. Various other underground stem and root structures can also give rise to new plants. Small sections of root, stem, or leaf can, under the right conditions, push out new plantlets in a fashion that seems otherworldly. (Picture chopping off a bit of your finger and growing a whole new you from it.)

These are some of the ways in which plants reproduce asexually, and it’s kind of freaky if you think about it. Plants can clone themselves. But one major disadvantage of reproducing this way is that clonal offspring are genetically identical to the parent plant, which truncates any advantage that might be gained by genetic mixing between two separate plants. For one, it means that a plant population composed of all clones is at risk of being wiped out if something in the environment comes along (such as a disease or change in climate) and none of the plants in the population have adapted any sort of resistance to it.

New plants forming along the lateral stems of Ranunculus flammula – via wikimedia commons

That’s where seeds come in. Seeds are produced sexually, when the gametes of one plant fuse with the gametes of another. Genetic recombination occurs, and a genetically unique individual is born, housed within a seed. Unless, of course, that seed is produced asexually. Then the seed is a clone, and we’re back to where we started.

Apomixis is the process by which seeds are produced asexually. In flowering plants, this means that viable seeds are formed even when flowers haven’t been pollinated. In some cases, pollination stimulates apomixis or is required to produce endosperm; but either way, the result is the same: an embryo containing an exact copy of the genes of its single parent plant.

To understand this process, it’s important to familiarize yourself with the basic anatomy of an ovule, the part of a plant where embryos are formed and which ultimately becomes a seed. In gymnosperms, ovules sit inside cones; in angiosperms, they are surrounded by an ovary. The wall of the ovule is called an integument. A small opening at the top of the ovule, known as a micropyle, is where the pollen tube enters. Diploid cells of the nucellus compose the interior of the ovule, and within the nucellus resides the megasporocyte, which is where meiosis occurs and egg cells are produced. In sexual reproduction, a germ cell introduced through the pollen tube fuses with the egg cell to form a zygote and eventually an embryo. In the case of apomixis, the fusion of germ cells isn’t necessary for an embryo to form.

ovule anatomy via wikimedia commons

There are three main types of apomixis: diplospory, apospory, and adventitious embryony. In diplospory, the megasporocyte skips meiosis and produces diploid cells instead of haploid cells (germ cells). These unreduced cells go on to form an embryo inside of the embryo sac, just like an egg cell would if it had been fertilized with a pollen cell. Additional unreduced cells go on to form endosperm, and the ovule then matures into a seed. This type of apomixis is common in dandelions (Taraxacum officinale). As much as bees love visiting dandelion flowers, their pollination services are not required for the production of viable seeds. Yet another reason you are stuck with dandelions in your yard whether you like it or not.

In apospory, an embryo develops inside of an embryo sac that has been formed from cells in the nucellus. Embryo development within the megasporocyte is bypassed; however, pollination is usually necessary for endosperm to form. Plant species in the grass family commonly produce seeds using this type of apomixis.

Adventitous embryony is also known as sporophytic apomixis because an embryo is formed outside of an embryo sac. Cells from either the integument or the nucellus produce an embryo vegetatively. In this case, a sexually produced embryo can form along with several vegetatively produced embryos. Extra embryos die off and a single, surviving embryo is left inside the mature seed. But not always. Two or more embryos occasionally survive, including the sexually produced one. The mature seed then consists of multiple embryos. This phenomenon is called polyembryony and is common in citrus and mangoes. When it comes to plant breeding, polyembryony is incredibly useful because the asexually derived seedlings are exact copies of their parent, which means the desirable traits of a specific cultivar are retained.

Depiction of seed with three viable embryos after germination.

Polyembryony can occur in a number of ways, and not always as a result of apomixis. In some species, additional embryos “bud off” from the sexually produced embryo. This is called cleavage polyembryony and is known to happen frequently in the pine family (Pinaceae), as well as other plant families. Another common form of polyembryony in gymnosperms is simple polyembryony, in which several egg cells in a single ovule are fertilized resulting in the development of multiple embryos. This doesn’t always mean there will be multiple seedlings sprouting from a single seed. Most embryos usually fail to mature, and only one prevails. However, sometimes more than one survives, and if you’re lucky, you’ll find a seed with multiple plant babies pushing out from the seed coat.

Up Next: Vivipary!


Poisonous Plants: Buttercups

Hold a buttercup flower under your chin. If your chin glows yellow, you love butter. That is according to a classic childhood game anyway. Recent research explored the cellular structure of buttercup petals and revealed the anatomical reason behind their yellow glow. Apart from helping to warm the flower’s sex organs, this glow is thought to draw in pollinating insects to ensure proper pollination.

Now take the fresh green leaves of buttercups, crush them up, and rub them against your skin. On second thought, DON’T DO THAT! This is not a childhood game and should absolutely be avoided…unless, of course, you derive some sort of pleasure from painful blisters.

Buttercups, also commonly known as crowfoots, are in the genus Ranunculus and the family Ranunculaceae. Ranunculus consists of a few hundred species and is a common group of annual and perennial herbaceous plants with alternately arranged, palmately veined leaves that are either entire, lobed, or finely divided. Buttercup flowers are usually yellow (sometimes white) with 5 petals (sometimes 3 or 7) that are either borne singly or in loose clusters. The flowers are complete, having both male and female reproductive structures that are easily identifiable. Flowering usually occurs in the spring.

bulbous buttercup (Ranunculus bulbosus) – photo credit: wikimedia commons

Ranunculus species are found throughout the world. Common habitats include moist woods, meadows, open fields, wetlands and other riparian areas, as well as drier sites like roadsides and neglected, urban lots. Several species are commonly grown as ornamentals, and others are common weeds in natural areas, urban landscapes, and agricultural fields.

All buttercups contain a compound called ranunculin. When the leaves are crushed or bruised, ranunculin breaks down to form an acrid, toxic oil called protoanemonin. Contact with this oil causes dermatitis. Symptoms occur within an hour of contact and include burning and itching along with rashes and blisters. When the leaves are chewed, blisters can form on the lips and face. If swallowed, severe gastrointestinal irritation can follow, accompanied by dizziness, spasms, and paralysis. The toxic oil is also irritating to the eyes.

Ranunculus species vary in their levels of this toxic compound, and individual plants are said to be more toxic in the spring when they are actively growing and flowering. Protoanemonin breaks down further into an innocuous compound called anemonin, so dead and dried out plants are generally safe. Commonly encountered (and particularly toxic) species in North America include tall buttercup (R. acris), cursed buttercup (R. sceleratus), creeping buttercup (R. repens), littleleaf buttercup (R. arbortivus), and sagebrush buttercup (R. glaberrimus). Bulbous buttercup (R. bulbosus) has bulbous roots that are toxic when fresh but are said to be edible after they are well boiled or completely dried.

cursed buttercup (Ranunculus sceleratus)

The toxicity of Ranunculus species seems to be more of an issue for livestock than for humans. Grazing animals tend to avoid it since it tastes so bad. Those that do eat it exhibit responses similar to humans – blistering around the mouth, gastrointestinal issues, etc. In The Book of Field and Roadside, John Eastman writes about Ranunculus acris: “Cattle usually avoid the plant – its acrid juices can blister their mouths – though they can also develop something like an addiction to it, consuming it until it kills them.” Buttercups becoming dominant in pastures and rangelands is often a sign of overgrazing.

Despite – and likely due to – their toxicity, buttercups have a long history of medicinal uses. Civilizations in many parts of the world have used the leaves and roots of the plant to treat numerous ailments including rheumatism, arthritis, cuts, bruises, and even hemorrhoids. A report published in 2011 describes three patients in Turkey that had applied poultices of corn buttercup (R. arvensis) to parts of their body to treat rheumatism. The patients were treated for chemical burns caused by the applications. The report concludes by advising against treatments “whose therapeutic effects have not been proven yet by scientific studies.”

In The North American Guide to Common Poisonous Plants and Mushrooms, buttercups are listed among plant species that are skin and eye irritants, honey poisons, and milk poisons (see Appendices 3, 4, and 5). Other genera in the buttercup family may also contain high levels of protoanemonin, including popular ornamentals like Clematis, Helleborus, Anemone, and Pulsatilla. Thus, the moral of this story: handle these plants with care.

sagebrush buttercup (Ranunculus glaberrimus)

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