Gametophyte

Several gametophytes growing in a terrarium

A gametophyte (/ɡəˈmiːtəfaɪt/) is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The sporophyte can produce haploid spores by meiosis that on germination produce a new generation of gametophytes.

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Plants! You're familiar with their work: They turn all that carbon dioxide that we don't want into the oxygen we do want, they're all around us, and they've been around for a lot longer than animals. The plants that we see today probably evolved from a single species of algae that noodged itself onshore about 1.2 billion years ago. And from that one little piece of algae, all of the half million or so species of plants that we have today evolved. But of course all this didn't happen overnight. It wasn't until about 475 million years ago that the first plants started to evolve. And they were very simple didn't have a lot of different tissue types, and the descendants of those plants still live among us today. They're the nonvascular plants: the liverworts, the hornworts and everybody's best friends, the mosses. Mmm, fuzzy! Now, yeah, it's clear that these guys are less complicated than an orchid or an oak tree, and if you said they were less beautiful, you probably wouldn't get that much argument from me. But by now I think you've learned enough about biology to know that when it comes to the simplest things: sometimes they're the craziest of all. Because they evolved early in the scheme of things they were sort of able to evolve their own set of rules. So, much like we saw with archaea, protists and bacteria, nonvascular plants have some bizarre features and some kooky habits that seem, to us, like, kind of, just, like, what? Especially when it comes to their sex lives. The main thing to know about nonvascular plants is their reproductive cycle, which they inherited from algae but perfected to the point where now it is used by all plants in one way or another, and there are even traces of it in our own reproductive systems. Usually when we're talking about plants, we're really talking about vascular plants, which have stuff like roots, stems, and leaves. Those roots, stems and leaves are actually tissues that transport water and nutrients from one part of the plant to another. As a result, vascular plants are able to go all giant sequoia. The main defining trait of nonvascular plants is that they don't have specialized conductive tissues. Since they don't have roots and stems, they can't reach down into the soil to get to water and nutrients. They have to take moisture in directly through their cell walls and move it around from cell to cell through osmosis, while they rely on diffusion to transport minerals. Another thing nonvascular plants have in common is limited growth potential. Largely because they don't have tissues to move the good stuff around, or woody tissue to support more mass, the way for them to win is to keep it simple and small. So small that when you look at one of these dudes, you sometimes might not know what you're looking at. And finally, nonvascular plants need water for reproduction This is kind of a bummer for them because it means they can't really survive in dry places like a lot of vascular plants can. But I'll get back to that in a minute. Other than that, nonvasculars are true plants: They're multicellular, they have cell walls made of cellulose, and they use photosynthesis to make their food. All the nonvascular plants are collectively referred to as bryophytes, and who knows how many different sorts there used to be back in the olden days, but we can currently meet three phyla of bryophytes in person: the mosses, in phylum Bryophyta, the liverworts, in phylum Hepatophyta, and the hornworts in phylum Anthocerophyta. Taken together, there are over 24,000 species of bryophytes out there: about 15,000 are mosses, 9,000 are liverworts and only only about 100 are hornworts. Hornworts and liverworts, funny names, but are named after the shape of their leaf-like structures horns for the hornworts and livers for the liverworts with "wort," stuck on the end there, which just means "herb." And you know what moss looks like, though some things that are called moss like "Spanish moss" in the southern United States, and "reindeer moss" up in the alpine tundra of Alaska, are imposters, they're actually lichens and lichens aren't even plants! The very oldest fossils of plant fragments look really similar to liverworts, but nobody really knows which of the bryophytes evolved first and which descended from which. We just know that something very bryophytic-looking was the first plant to rear its leafy head back in the Ordovician swamps. So, now we've got these ultra-old timey nonvascular plants to provide us with some clues as to how plants evolved. And like I mentioned, the most important contribution to the Kingdom Plantae, and everything that came after them, is their wonderfully complex reproductive cycle. See, plants, vascular and nonvascular, have a way more complicated sexual life cycle than animals do. With animals, it's pretty much a one-step process: two haploid gametes, one from the mom and one from the dad, come together to make a diploid cell that combines the genetic material from both parents. That diploid cell divides and divides and divides and divides until, voila! The world is one marmot or grasshopper richer. Plants, on the other hand, along with algae and a handful of invertebrate animal species, have evolved a cycle in which they take on two different forms over the course of their lives, one form giving rise to the other form. This type of reproductive cycle is called alternation of generations, and it evolved first in algae, and many of them still use it today. However, the difference between algae and plants here is that, in algae, both generations look pretty much the same, while in land plants, all land plants, the alternating generations are fundamentally different from each other. And by fundamental, I mean that the two don't even share the same basic reproductive strategy. One generation, called the gametophyte, reproduces sexually by producing gametes, eggs and sperm, which you know are haploid cells that only carry one set of chromosomes. And the bryophyte sperm is a lot like human sperm, except they have two flagella instead of one, and they're kind of coil shaped. When the sperm and egg fuse, they give rise to the second generation, called the sporophyte generation, which is asexual. The sporophyte itself is diploid, so it already has two sets of chromosomes in each cell. It has a little capsule called a sporangium, which produces haploid reproductive cells called spores. During its life, the sporophyte remains attached to its parent gametophyte, which it relies on for water and nutrients. Once its spores disperse and germinate, they in turn produce gametophytes, which turn around and produce another sporophyte generation. And so on. Weird, I know, but that's the fun of it. Life is peculiar and that's what makes it so great. This means that the nonvascular plants that we all recognize, the green, leafy, livery or horny parts of the moss, liverwort or hornwort, are actually gametophytes. Sporophytes are only found tucked inside the females, and they're super small and hard to see. So in this gametophyte generation, individuals are always either male or female. The male makes sperm through mitosis in a feature called the antheridia, the male reproductive structure. While the female gametophyte makes the egg, also through mitosis, inside the female reproductive structures, which are called the archegonia. These two gametophytes might be hanging out right next to each other, sperm and eggs totally ready to go, but they can't do anything until water is introduced to the situation. So let's just add a sprinkle of water and take a tour of the bryophytes' sex cycle, shall we? By way of the water, the sperm finds its way to the female and then into the egg, where the two gametes fuse to create a diploid zygote, which divides by mitosis and grows into a sporophyte. The sporophyte grows inside the mother, until one day it cracks open and the sporophyte sends up a long stalk with a little cap on top called the calyptra. This protective case is made out of the remaining piece of the mother gametophyte, and under it a capsule forms full of thousands of little diploid spores. When the capsule is mature, the lid falls off, and the spores are exposed to the air. If humidity levels are high enough, the capsule will let the spores go to meet their fate. Now, if one lands on a basketball court or something, it will just die if it doesn't get water. But if it lands on moist ground, it germinates, producing a little filament called the protonema, that gives rise to buds. These eventually grow into a patch of moss, which is just a colony of haploid gametophytes. That generation will mate, and make sporophytes, and the generations will continue their alternation indefinitely! Now because nonvascular plants are the least complex kind of plants, their alternation of generations process is about as simple as it gets. But with vascular plants, because they have all kinds of specialized tissues, things get a little more convoluted. For instance, plants that produce unprotected seeds, like conifers or gingko trees, are gymnosperms, and it's at this level that we start to see pollen, which is just a male gamete that can float through the air. The pollen thing is taken to the next level with angiosperms, or flowering plants, which are the most diverse group of land plants, and the most recently evolved. So the main difference between the alternation of generations in vascular and nonvascular plants is that in bryophytes you recognize the gametophyte as being the... you know, the plant part. The moss or the liverwort or whatever. While the sporophyte is less recognizable and smaller. But as plants get more complicated, like with vascular plants, the sporophytes become the dominant phase, more prominent or recognizable. Like the flower of an angiosperm, for instance, is, itself, actually the sporophyte. Now I maybe just stuck a spoon in all the stuff that you learned and stirred it up to confuse you more. But we'll get into this more when we talk about the reproduction of vascular plants. But whether they have a big showy sporophyte like a flower or a little, damp gametophyte like a moss, all land plants came from the same, tiny little ancient nonvascular plant who just put their sperm out there, hoping to find some lady gametophyte they could call their own. And I think that's kind of sweet. Thank you for watching this episode of Crash Course Biology. And thanks to all the people who helped put it together. There's a table of contents over there if you want to go review anything. And if you have any questions for us, we're on Facebook and Twitter and, of course, we're down in the comments below. Thanks a lot.

Algae

In some multicellular green algae (Ulva lactuca is one example), red algae and brown algae, sporophytes and gametophytes may be externally indistinguishable (isomorphic). In Ulva the gametes are isogamous, all of one size, shape and general morphology.^[1]

Land plants

In land plants, anisogamy is universal. As in animals, female and male gametes are called, respectively, eggs and sperm. In extant land plants, either the sporophyte or the gametophyte may be reduced (heteromorphic).^[2] No extant gametophytes have stomata, but they have been found on fossil species like the early Devonian Aglaophyton from the Rhynie chert.^[3] Other fossil gametophytes found in the Rhynie chert shows they were much more developed than present forms, resembling the sporophyte in having a well-developed conducting strand, a cortex, an epidermis and a cuticle with stomata, but were much smaller.^[4]

Bryophytes

In bryophytes (mosses, liverworts, and hornworts), the gametophyte is the most visible stage of the life cycle. The bryophyte gametophyte is longer lived, nutritionally independent, and the sporophytes are attached to the gametophytes and dependent on them.^[5] When a moss spore germinates it grows to produce a filament of cells (called the protonema). The mature gametophyte of mosses develops into leafy shoots that produce sex organs (gametangia) that produce gametes. Eggs develop in archegonia and sperm in antheridia.^[6]

In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores (or gametangiophores).

Vascular plants

All vascular plants are sporophyte dominant, and a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved reproduction by seeds.^[7] Those vascular plants, such as clubmosses and many ferns, that produce only one type of spore are said to be homosporous. They have exosporic gametophytes — that is, the gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus (monoicous), or specialized into separate male and female organisms (dioicous).

In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophytes develop endosporically (within the spore wall). These gametophytes are dioicous, producing either sperm or eggs but not both.^{[citation needed]}

Ferns

In most ferns, for example, in the leptosporangiate fern Dryopteris, the gametophyte is a photosynthetic free living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. Homosporous ferns secrete a chemical called antheridiogen.

Lycophytes

Extant lycophytes produce two different types of gametophytes. In the homosporous families Lycopodiaceae and Huperziaceae, spores germinate into bisexual free-living, subterranean and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella, which are heterosporous, microspores and megaspores are dispersed from sporangia either passively or by active ejection.^[8] Microspores produce microgametophytes which produce sperm. Megaspores produce reduced megagametophytes inside the spore wall. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside. The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus.^[9]

Seed plants

The seed plant gametophyte life cycle is even more reduced than in basal taxa (ferns and lycophytes). Seed plant gametophytes are not independent organisms and depend upon the dominant sporophyte tissue for nutrients and water. With the exception of mature pollen, if the gametophyte tissue is separated from the sporophyte tissue it will not survive. Due to this complex relationship and the small size of the gametophyte tissue—in some situations single celled—differentiating with the human eye or even a microscope between seed plant gametophyte tissue and sporophyte tissue can be a challenge. While seed plant gametophyte tissue is typically composed of mononucleate haploid cells (1 x n), specific circumstances can occur in which the ploidy does vary widely despite still being considered part of the gametophyte.

In gymnosperms, the male gametophytes are produced inside microspores within the microsporangia located inside male cones or microstrobili. In each microspore, a single gametophyte is produced, consisting of four haploid cells produced by meiotic division of a diploid microspore mother cell.^[10] At maturity, each microspore-derived gametophyte becomes a pollen grain. During its development, the water and nutrients that the male gametophyte requires are provided by the sporophyte tissue until they are released for pollination. The cell number of each mature pollen grain varies between the gymnosperm orders. Cycadophyta have 3 celled pollen grains while Ginkgophyta have 4 celled pollen grains.^[10] Gnetophyta may have 2 or 3 celled pollen grains depending on the species, and Coniferophyta pollen grains vary greatly ranging from single celled to 40 celled.^[11]^[10] One of these cells is typically a germ cell and other cells may consist of a single tube cell which grows to form the pollen tube, sterile cells, and/or prothallial cells which are both vegetative cells without an essential reproductive function.^[10] After pollination is successful, the male gametophyte continues to develop. If a tube cell was not developed in the microstrobilus, one is created after pollination via mitosis.^[10] The tube cell grows into the diploid tissue of the female cone and may branch out into the megastrobilus tissue or grow straight towards the egg cell.^[12] The megastrobilus sporophytic tissue provides nutrients for the male gametophyte at this stage.^[12] In some gymnosperms, the tube cell will create a direct channel from the site of pollination to the egg cell, in other gymnosperms, the tube cell will rupture in the middle of the megastrobilus sporophyte tissue.^[12] This occurs because in some gymnosperm orders, the germ cell is nonmobile and a direct pathway is needed, however, in Cycadophyta and Ginkgophyta, the germ cell is mobile due to flagella being present and a direct tube cell path from the pollination site to the egg is not needed.^[12] In most species the germ cell can be more specifically described as a sperm cell which mates with the egg cell during fertilization, though that is not always the case. In some Gnetophyta species, the germ cell will release two sperm nuclei that undergo a rare gymnosperm double fertilization process occurring solely with sperm nuclei and not with the fusion of developed cells.^[10]^[13] After fertilization is complete in all orders, the remaining male gametophyte tissue will deteriorate.^[11]

The female gametophyte in gymnosperms differs from the male gametophyte as it spends its whole life cycle in one organ, the ovule located inside the megastrobilus or female cone.^[14] Similar to the male gametophyte, the female gametophyte normally is fully dependent on the surrounding sporophytic tissue for nutrients and the two organisms cannot be separated. However, the female gametophytes of Ginkgo biloba do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte.^[15] The female gametophyte forms from a diploid megaspore that undergoes meiosis and starts being singled celled.^[16] The size of the mature female gametophyte varies drastically between gymnosperm orders. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta, the single celled female gametophyte undergoes many cycles of mitosis ending up consisting of thousands of cells once mature. At a minimum, two of these cells are egg cells and the rest are haploid somatic cells, but more egg cells may be present and their ploidy, though typically haploid, may vary.^[14]^[17] In select Gnetophyta, the female gametophyte stays singled celled. Mitosis does occur, but no cell divisions are ever made.^[13] This results in the mature female gametophyte in some Gnetophyta having many free nuclei in one cell. Once mature, this single celled gametophyte is 90% smaller than the female gametophytes in other gymnosperm orders.^[14] After fertilization, the remaining female gametophyte tissue in gymnosperms serves as the nutrient source for the developing zygote (even in Gnetophyta where the diploid zygote cell is much smaller at that stage, and for a while lives within the single celled gametophyte).^[14]

The precursor to the male angiosperm gametophyte is a diploid microspore mother cell located inside the anther. Once the microspore undergoes meiosis, 4 haploid cells are formed, each of which is a singled celled male gametophyte. The male gametophyte will develop via one or two rounds of mitosis inside the anther. This creates a 2 or 3 celled male gametophyte which becomes known as the pollen grain once dehiscing occurs.^[18] One cell is the tube cell, and the remaining cell/cells are the sperm cells.^[19] The development of the three celled male gametophyte prior to dehiscing has evolved multiple times and is present in about a third of angiosperm species allowing for faster fertilization after pollination.^[20] Once pollination occurs, the tube cell grows in size and if the male gametophyte is only 2 cells at this stage, the single sperm cell undergoes mitosis to create a second sperm cell.^[21] Just like in gymnosperms, the tube cell in angiosperms obtains nutrients from the sporophytic tissue, and may branch out into the pistil tissue or grow directly towards the ovule.^[22]^[23] Once double fertilization is completed, the tube cell and other vegetative cells, if present, are all that remains of the male gametophyte and soon degrade.^[23]

The female gametophyte of angiosperms develops in the ovule (located inside the female or hermaphrodite flower). Its precursor is a diploid megaspore that undergoes meiosis which produces four haploid daughter cells. Three of these independent gametophyte cells degenerate and the one that remains is the gametophyte mother cell which normally contains one nucleus.^[24] In general, it will then divide by mitosis until it consists of 8 nuclei separated into 1 egg cell, 3 antipodal cells, 2 synergid cells, and a central cell that contains two nuclei.^[24]^[21] In select angiosperms, special cases occur in which the female gametophyte is not 7 celled with 8 nuclei.^{[clarification needed]}^[17] On the small end of the spectrum,^{[clarification needed]} some species have mature female gametophytes with only 4 cells, each with one nuclei.^[25] Conversely, some species have 10-celled mature female gametophytes consisting of 16 total nuclei.^[26] Once double fertilization occurs, the egg cell becomes the zygote which is then considered sporophyte tissue. Scholars still disagree on whether the fertilized central cell is considered gametophyte tissue. Some botanists consider this endospore as gametophyte tissue with typically 2/3 being female and 1/3 being male, but as the central cell before double fertilization can range from 1n to 8n in special cases, the fertilized central cells range from 2n (50% male/female) to 9n (1/9 male, 8/9th female).^[21] However, other botanists consider the fertilized endospore as sporophyte tissue. Some believe it is neither.^[21]

Heterospory

In heterosporic plants, there are two distinct kinds of gametophytes. Because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg-producing gametophyte is known as a megagametophyte, because it is typically larger, and the sperm producing gametophyte is known as a microgametophyte. Species which produce egg and sperm on separate gametophytes plants are termed dioicous, while those that produce both eggs and sperm on the same gametophyte are termed monoicous.

In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct types of sporangia, each of which produces a single kind of spore that germinates to produce a single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants. That is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium; Sphaerocarpos is an example of such a plant.

In seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of several (typically two to five) cells when the pollen grains exit the sporangium. The megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carried by a physical or animal vector) and produces two sperm by mitosis.

In gymnosperms, the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell. The gametophyte becomes a food storage tissue in the seed.^[27]

In angiosperms, the megagametophyte is reduced to only a few cells, and is sometimes called the embryo sac. A typical embryo sac contains seven cells and eight nuclei, one of which is the egg cell. Two nuclei fuse with a sperm nucleus to form the primary endospermic nucleus which develops to form triploid endosperm, which becomes the food storage tissue in the seed.

References

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