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From Wikipedia, the free encyclopedia

 Several gametophytes growing in a terrarium.
Several gametophytes growing in a terrarium.
 Pine gametophyte (outside) surrounding the embryo (inside)
Pine gametophyte (outside) surrounding the embryo (inside)

A gametophyte is a stage in the life cycle of plants and algae that undergo alternation of generations. 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 in which each cell has two sets 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 function of which is to produce haploid spores by meiosis.

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  • The Sex Lives of Nonvascular Plants: Alternation of Generations - Crash Course Biology #36
  • Sporophytes and Gametophytes
  • Biology Of Plants | Learn About Ovule and Gametophyte
  • Plants
  • On Alternation of Generations

Transcription

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.

Contents

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. Either the sporophyte or the gametophyte may be reduced (heteromorphic).[2]

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 typically attached to the gametophytes and dependent on them.[3] 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.[4]

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

Ferns

In most ferns, gametophyte is a photosynthetic free living organism called a prothallus. For example, in the leptosporangiate fern Dryopteris, the gametophyte is a free-living, autotrophic 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.

Lycophytes

Extant lycophytes produce several different types of gametophytes. In the families Lycopodiaceae and Huperziaceae, gametophytes are subterranean and mycotrophic, deriving nutrients from symbiosis with fungi. Isoetes and Selaginella, which are heterosporous, megagametophytes develop inside the megaspores, which crack 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 giant arborescent clubmosses, Lepidodendron and Lepidostrobus.[5]

Seed plants

By contrast, in seed plants (gymnosperms and angiosperms), gametophytes develop into multicellular organisms while still enclosed within the sporangium.[6]

Vascular plants 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 are normally bisexual, capable of producing both sperm and eggs. In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophyte develops endosporically, within the spore wall. These gametophytes are unisexual, producing either sperm or eggs but not both.

All vascular plants are sporophyte dominant, and a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved towards reproduction by seeds.[7]

Heteromorphy

In plants with heteromorphic gametophytes, 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. Gametophytes which produce egg and sperm on separate plants are termed Dioicous.

In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct sporangia, each of which produces a single kind of spore and 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 the seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of two or three 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 grain) 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.[8]

In angiosperms, the megagametophyte is reduced to only a few nuclei and 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 endosperm, which becomes the food storage tissue in the seed.

See also

References

  1. ^ Sadava, David; Hillis, David; Heller, H. Craig; Berenbaum, May (2012). Life: The Science of Biology, Volume 1 (10th ed.). Macmillan. ISBN 978-1464141225. 
  2. ^ Bennici, Andrea (2008). "Origin and early evolution of land plants". Communicative & Integrative Biology. 1 (2): 212–218. ISSN 1942-0889. PMC 2686025Freely accessible. PMID 19513262. 
  3. ^ Budke, J.M.; Goffinet, B.; Jones, C.S. (2013). "Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica". Annals of Botany. 111: 781–789. PMC 3631323Freely accessible. PMID 23471009. doi:10.1093/aob/mct033. 
  4. ^ Ralf Reski (1998): Development, genetics and molecular biology of mosses. In: Botanica Acta 111, pp 1-15.
  5. ^ Brack-Hanes, S.D. (1978). "On the megagametophytes of two Lepidodendracean cones.". Botanical Gazette. 139: 140–146. doi:10.1086/336979. 
  6. ^ C.Michael Hogan (2010): Fern. Encyclopedia of Earth. National council for Science and the Environment Archived November 9, 2011, at the Wayback Machine.. Washington, DC
  7. ^ Stewart, W.N.; Rothwell, G.W. Palaeobotany and the evolution of plants, second edition. Cambridge, U.K.: Cambridge University press. ISBN 0521382947. 
  8. ^ "Vascular Plants :: Description". Digimuse.nmns.edu.tw. Archived from the original on 2014-05-22. Retrieved 2014-07-13. 
This page was last edited on 30 September 2017, at 19:49.
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