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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!

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Inside of a Seed: Gymnosperms

“Every tree has to stay where it put down roots as a seedling. However, it can reproduce, and in that brief moment when tree embryos are still packed into seeds, they are free. The moment they fall from the tree, the journey can begin.” — The Hidden Life of Trees by Peter Wohlleben

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Seed plants – also known as spermatophytes – make up the largest group of plants on earth. Seed plants consist of five divisions, and among them the angiosperm division (a.k.a. flowering plants) dominates in its number of species. The four remaining divisions are referred to collectively as gymnosperms. This incudes the cycads (Cycadophyta), Ginkgo biloba (the only living species in the division Ginkgophyta), gnetophytes (Gnetophyta), and the conifers (Coniferophyta). Conifers are by far the largest and most widespread gymnosperm division.

Angiosperms and gymnosperms have different evolutionary histories, resulting in their distinct genetic and morphological differences. That being said, an overly-simplistic way of differentiating the two groups is to say that, while both groups produce seeds, angiosperms produce flowers and fruits while gymnosperms produce pollen cones and seed cones. There are always exceptions (Ginkgo biloba, for example, doesn’t produce cones), but for the most part, this is the case.

Pollen cones (top) and seed cones (bottom) of mugo pine (Pinus mugo) via wikimedia commons

Sexual reproduction in gymnosperms follows a familiar pattern. Pollen, which contains the male sex cells, is produced in pollen cones, which are essentially miniature branches with modified leaves called scales that house the male reproductive organs. Mature pollen is shed and carried away by the wind. Lucky pollen grains make their way to the female cones, which are also modified branchlets, but are a bit more complex. Scales sit atop bracts, and on top of the scales are ovules – the female reproductive structures. During fertilization, the bracts open to collect pollen and then close as the seed develops.

When pollen lands on an ovule it forms pollen tubes that help direct the male sex cells to the egg cells inside. The process is similar to pollen tubes extending down the style of a flower. In flowering plants, additional pollen cells combine with cells in the ovule to produce endosperm, a storage tissue that feeds the growing embryo. This doesn’t happen in gymnosperms. Instead, haploid cells within the ovule develop into storage tissue and go on to serve the same role.

The ovule eventually matures into a seed, and the cone opens to release it. The seed sits atop the scale rather than enclosed within a fruit, as it would be in an angiosperm. For this reason gymnosperms are said to have naked seeds. The development of seeds can also be much slower in gymnosperms compared to angiosperms. In some species, seeds don’t reach maturity for as long as two years.

Seed cones and winged seeds of mugo pine (Pinus mugo) via wikimedia commons

Seeds in the genus Pinus are excellent representations of typical gymnosperm seeds. Their basic components are essentially identical to the seeds of angiosperms. The seed coat is also referred to as an integument. It was once the outer covering of the ovule and has developed into the seed covering. A micropyle is sometimes visible on the seed and is the location where the pollen cells entered the ovule. The storage tissue, as mentioned above, is composed of female haploid cells that matured into storage tissue in the ovule. Like angiosperms, the embryo is composed of the radicle (embryonic root), the hypocotyl (embryonic shoot), and cotyledons (embryonic leaves).

Angiosperms can be divided into monocotyledons and dicotyledons according to the number of cotyledons their embryos have (monocots have one, dicots have two). Gymnosperms are considered multi-cotyledonous because, depending on the species, they can have a few to many cotyledons.

Seedling of Swiss pine (Pinus cembra) showing multiple cotyledons via wikimedia commons

For the sake of this introduction to gymnosperm seeds, I have offered a simple overview of the production of seeds in the conifer division. Sexual reproduction and seed formation in the other three gymnosperm divisions is a similar story but varies according to species. Even within the conifers there are differences. For example, the “seed cones” of several gymnosperm species can actually be quite fruit-like, which serves to attract animals to aid in seed dispersal. Also, the pollen of gymnosperms is often thought of as being wind dispersed (and occasionally water dispersed in the case of Ginkgo biloba and some cycads); however, researchers are continuing to discover the pivotal role that insects play in the transfer of pollen for many cycad species, just as they do for so many species of angiosperms.

All of this to say that Botany 101 is simply a window into what is undoubtedly an incredibly diverse and endlessly fascinating group of organisms, and that, as with all branches of science, there is still so much to discover.

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Inside of a Seed: Two Dicots

“A seed is a living thing that embodies roots, stems, leaves, and fruit in an embryonic state and retains the ability to convert the sun’s energy into a source of food.” — Seedtime by Scott Chaskey

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Few things are more miraculous than seeds. Within them is a living plant in embryonic form. Under the right conditions, these tiny objects expand, pushing out the beginnings of the most minuscule weed to the most humongous tree. Looking at these otherwise unassuming specks, you would hardly guess that they held such potential.

Housed in a seed is the genetic material necessary for growth and reproduction, along with some stored sugars to get the plant started. All of this is enclosed in a protective case. It is a rare moment in a plant’s life – a time when it isn’t rooted in place and can, for a brief period, move around. With the help of agents like wind, water, and animals it can travel anywhere in the world, venturing as far as inches or miles from its parent plant. As long as it finds a suitable place to grow, its voyage is not in vain.

Seeds are the result of sexual reproduction in plants (with rare exceptions, which we will cover in a future post). After pollination, a pollen grain sends three haploid cells into the ovule of a flower. These cells unite with the haploid cells found within. One germ cell from the pollen grain goes to the formation of an embryo, while the other two cells help form endosperm, the food source for the developing embryo. The wall of the ovule becomes the outer layer of the seed, known as the seed coat or testa. The seed matures as the fruit it is nested in ripens. Eventually, the fetal plant within the seed is ready to find a new home.

Seed heads of rubber rabbitbrush (Ericameria nauseosa) – the fuzzy pappus attached to the fruits allows seeds to float in the breeze and travel away from their parent plant.

As with so many things in biology, there is no single type of seed. When it comes to seed anatomy, most seeds consist of the same basic components, but each species of plant has its own unique seed. In fact, a well-trained taxonomist can identify plants simply by observing their seeds. With such a wide variety of seeds, it is difficult to organize them into discrete categories, but we still try. What follows is an introduction to two types of seeds – endospermic and non-endospermic – using two basic examples.

The first thing you should know about these two examples is that both species are dicotyledons (or dicots, for short). This means that when the baby plant emerges, it has two cotyledons, which are also called embryonic leaves because they look like little leaves. All flowering plants have been divided into two groups based on the number of cotyledons they have, the second group being the monocotolydons (or monocots) which have only one cotyledon. This is an old-fashioned way to classify plants, but it is still useful in some instances.

Endospermic Seeds

The seeds of the castor bean plant (Ricinus communis) are endospermic seeds. This means that they retain the endosperm that was formed when two pollen grain cells joined up with the haploid cell in the ovule. The endosperm will help feed the growing embryo as it germinates. The two cotyledons are visible within the seed, but they are thin and broad, leaving plenty of space in the seed for the endosperm. The cotyledons are part of the embryo and are attached to the radicle, which is the embryonic root. The radicle is the first thing to emerge from the seed upon germination. The area between the radicle and the cotyledon is known as the hypocotyl. It becomes the stem of the germinating seedling.

An elaisome is attached to the outside of the seed coat of castor bean seeds. This fleshy, nutrient-rich appendage is particularly attractive to ants. They carry the seeds back to their colony and feed the elaisome to their young. The seeds, however, remain unconsumed. In this way, the ants aid in the seeds’ dispersal.

seeds of castor beans (Ricinus communis)

Non-endospermic Seeds

The seeds of plants in the bean family (Fabaceae) are non-endospermic seeds. This means that as the embryo develops, it uses up the majority of the endosperm within the seed. The food necessary for the seedling to get its start is all stored in its cotyledons. The common pea (Pisum sativum) is a good example of this. The embryo – which consists of the cotyledons, plumula (or plumule), hypocotyl, and radicle – takes up all available space inside of the seed coat. After germination, as the seedling develops, the plumule appears above the cotyledons and is the growing point for the first true leaves and stems.

seeds of the common pea (Pisum sativum)

In future posts, we will look at a few other types of seeds, as well as discuss various other seed-related topics. If you have a story to share about seeds, please do so in the comment section below.

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Year of Pollination: Botanical Terms for Pollination, part two

“The stage is set for reproduction when, by one means or another, compatible pollen comes to rest on a flower’s stigma. Of the two cells within a pollen grain, one is destined to grow into a long tube, a pollen tube, that penetrates the pistil’s tissues in search of a microscopic opening in one of the ovules, located in the ovary. … The second of a pollen grain’s cells divides to become two sperm that move through the pollen tube and enter the ovule.” – Brian Capon, Botany for Gardeners

“Once pollination occurs, the next step is fertilization. Pollen deposited on the sticky stigma generates a fine pollen tube that conveys the sperm through the style to the ovary, where the ovules, or eggs, have developed. After fertilization, the rest of the flower parts wither and are shed as the ovary swells with seed development.” – Rick Imes, The Practical Botanist

Pollination tells the story of a pollen grain leaving an anther by some means – be it wind, water, or animal – and finding itself deposited atop a stigma. As long as the pollen and stigma are compatible, the sex act proceeds. In other words, the pollen grain germinates. One of the pollen grain’s cells – the tube nucleus – grows down the length of the style, forming a tube through which two sperm nuclei can travel. The sperm nuclei enter the ovary and then, by way of a micropyle, enter an ovule. Inside the ovule is the female gametophyte (also referred to as the embryo sac). One sperm nucleus unites with the egg nucleus to form a zygote. The remaining sperm nucleus unites with two polar nuclei to form a triploid cell which becomes the endosperm. The sex act is complete.

The illustration on the left includes the cross-section of a pistil showing the inside the ovary where pollen tubes have made their way to the ovules. The illustration on the right shows pollen grains germinating on a stigma and their pollen tubes begining to work their way down the style. (photo credit: wikimedia commons)

The illustration on the left includes the cross section of a pistil showing the inside of the ovary where pollen tubes have made their way to the ovules. The illustration on the right shows pollen grains germinating on a stigma and pollen tubes as they work their way down the style. (image credit: wikimedia commons)

The zygote divides by mitosis to become an embryo. The endosperm nourishes the development of the embryo. The ovule matures into a seed, and the ovary develops into a fruit. During this process, the remaining parts of the flower wither and fall away. In some cases, certain flower parts remain attached to the fruit or become part of the fruit. The flesh of an apple, for example, is formed from the carpels and the receptacle (the thickened end of a flower stem – peduncle – to which the parts of a flower are attached).

As the seed matures, the endosperm is either used up or persists to help nourish the embryonic plant after germination. Mature seeds that are abundant in endosperm are called albuminous. Examples include wheat, corn, and other grasses and grains. Mature seeds with endosperm that is either highly reduced or absent are called exalbuminous – beans and peas, for example. Certain species – like orchids – do not produce endosperm at all.

The cross section of a corn kernel showing the endosperm and the embryo (image credit: Encyclopedia Britannica Kids)

The cross section of a corn kernel showing the endosperm and the embryo (image credit: Encyclopedia Britannica Kids)

It is fascinating to consider that virtually every seed we encounter is the result of a single pollen grain making its way from an anther to a stigma, growing a narrow tube down a style, and fertilizing a single ovule. [Of course there are always exceptions. Some plants can produce seeds asexually. See apomixis.] Think of this the next time you are eating corn on the cob or popcorn – each kernel is a single seed – or slicing open a pomegranate to reveal the hundreds of juicy seeds inside. Or better yet, when you are eating the flesh or drinking the milk of a coconut. You are enjoying the solid and liquid endosperm of one very large seed.

Much more can be said about pollination and the events surrounding it, but we’ll save that for future posts. The “Year of Pollination” may be coming to an end, but there remains much to discover and report concerning the subject. For now, here is a fun video to help us review what we’ve learned so far:

 

Also, take a look at this TED talk: The Hidden Beauty of Pollination by Louie Schwartzberg

And finally, just as the “Year of Pollination” was coming to an end I was introduced to a superb blog called The Amateur Anthecologist. Not only did it teach me that “anthecology” is a term synonymous with pollination biology, it has a great series of posts called “A Year of Pollinators” that showcases photographs and information that the author has collected for various groups of pollinators over the past year. The series includes posts about Bees, Wasps, Moths and Butterflies, Flies, and Beetles, Bugs, and Spiders.