Gametophyte

• 1. Introduction

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  A gametophyte carries just one set of chromosomes, and yet it is one of two alternating multicellular phases that organize the entire life cycle of plants. It is the sexual phase, the part of the cycle where sex organs form and gametes are made. Those gametes, haploid sex cells, meet in fertilization to produce a diploid zygote with a double set of chromosomes. From that single fused cell grows the other phase entirely, a multicellular organism called the sporophyte. The sporophyte returns the favor by making haploid spores through meiosis, and each spore that germinates becomes a fresh gametophyte. So two organisms, one with a single chromosome set and one with a double set, take turns building each other across the generations. The questions that follow run through this alternation. Why do mosses live most of their lives as gametophytes while a fern's gametophyte is a small green flap on the soil? Why has the gametophyte shrunk almost to nothing inside a grain of pollen? And how far has it been reduced, in some plants, to a structure you could barely find under a microscope?

• 2. Twin Phases In Algae

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  Ulva lactuca shows how hard it can be to tell the two phases apart. In some multicellular green algae like this one, along with certain red algae and brown algae, the sporophytes and gametophytes look externally identical, a condition called isomorphic. In Ulva, the gametes are isogamous, meaning they are all of one size, one shape, and one general form. There is no division into a larger egg and a smaller sperm. That symmetry sets up a sharp contrast with what happens once plants moved onto land, where the two kinds of gametes parted ways completely.

• 3. Footprints In Stone

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  No gametophyte alive today has stomata, the tiny pores that plants use to exchange gas. That makes a set of fossils in the Rhynie chert all the more striking, because they preserve gametophytes that broke this rule. The early Devonian Aglaophyton is one such fossil species, found with stomata that no living gametophyte possesses. Other fossil gametophytes from the same Rhynie chert were far more developed than present forms. They resembled the sporophyte, carrying a well-developed conducting strand, a cortex, an epidermis, and a cuticle with stomata, yet they remained much smaller. In land plants generally, anisogamy is universal, with female gametes called eggs and male gametes called sperm. Either phase can be the reduced one, a heteromorphic arrangement, and the rest of the story is largely about which phase wins out.

• 4. The Leafy Generation

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  In bryophytes, the mosses, liverworts, and hornworts, the gametophyte is the most visible stage of the whole life cycle. It lives longer than the sporophyte and feeds itself, while the sporophyte stays attached to it and depends on it for nutrition. When a moss spore germinates, it first grows a filament of cells called the protonema. From there the mature moss gametophyte develops into leafy shoots that bear the sex organs, the gametangia. Eggs form inside archegonia and sperm inside antheridia. Some bryophytes go further with dedicated mounts for these organs. In many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores, also known as gametangiophores.

• 5. Free-Living Prothalli

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  Dryopteris, a leptosporangiate fern, grows its gametophyte as a free living photosynthetic organism called a prothallus. This small autotroph produces the gametes and sustains the young sporophyte through its early multicellular development. Not every fern follows that sunlit path. In the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes live underground and survive by forming mycotrophic relationships with fungi. Homosporous ferns also wage a kind of chemical communication, secreting a substance called antheridiogen. The free-living gametophyte is a feature of vascular plants that make only one type of spore, the homosporous ones such as clubmosses and many ferns. Their gametophytes are exosporic, developing outside the spore wall, and they can be either monoicous, making both sperm and eggs in one thallus, or dioicous, split into separate male and female plants. Where a plant instead makes both microspores and megaspores, the gametophytes develop endosporically, inside the spore wall, and stay dioicous.

• 6. Underground And Cracked Open

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  Lycopodiaceae and Huperziaceae send their spores down into the dark. These homosporous lycophyte families germinate into bisexual, free-living, subterranean gametophytes that are mycotrophic, drawing nutrients from symbiosis with fungi. Isoetes and Selaginella take a different route, being heterosporous. Their microspores and megaspores leave the sporangia either passively or by active ejection. Microspores grow into microgametophytes that make sperm, while megaspores grow into reduced megagametophytes held inside the spore wall. At maturity the megaspore splits open along the trilete suture, letting the male gametes reach the egg cells in the archegonia within. The gametophytes of Isoetes appear similar in this respect to those of two extinct Carboniferous arborescent lycophytes, Lepidodendron and Lepidostrobus.

• 7. Pollen And The Hidden Egg

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  Among gymnosperms, a single pollen grain can hold as few as one cell or as many as forty. Each male gametophyte forms inside a microspore within the microsporangia of male cones, beginning as four haploid cells from the meiotic division of a diploid microspore mother cell. The cell count of the mature grain splits by order. Cycadophyta carry 3 celled pollen grains and Ginkgophyta 4 celled grains. Gnetophyta range from 2 to 3 celled depending on the species, while Coniferophyta vary greatly from single celled up to 40 celled. After pollination, the male gametophyte keeps developing, with a tube cell growing into the diploid tissue of the female cone toward the egg. In Cycadophyta and Ginkgophyta the germ cell is mobile, carrying flagella, so no direct tube path is needed. In some Gnetophyta the germ cell releases two sperm nuclei in a rare double fertilization done with nuclei alone. The female gymnosperm gametophyte spends its whole life in one organ, the ovule inside the female cone, normally fully dependent on the sporophyte. Ginkgo biloba is the exception that proves the rule, since its female gametophytes contain chlorophyll and make some of their own energy, though never enough to support themselves. Forming from a diploid megaspore that undergoes meiosis, the female gametophyte starts single celled. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta it then divides through many cycles of mitosis into thousands of cells. In select Gnetophyta it stays single celled, doing mitosis without ever splitting, ending as one cell crowded with free nuclei and 90% smaller than the female gametophytes of other orders.

• 8. Seven Cells, Eight Nuclei

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  Three of every four haploid cells made by an angiosperm megaspore simply die. The female gametophyte of angiosperms develops in the ovule, beginning with a diploid megaspore that meiosis splits into four haploid daughter cells. Three degenerate, and the survivor, the gametophyte mother cell, normally holds a single nucleus. It then divides by mitosis into 8 nuclei arranged as 1 egg cell, 3 antipodal cells, 2 synergid cells, and a central cell carrying two nuclei. Special cases bend even this. Some species mature with only 4 cells and 4 nuclei, while others reach 10 cells holding 16 total nuclei. The male side is just as compressed, starting from a diploid microspore mother cell in the anther. After meiosis makes four haploid cells, one or two rounds of mitosis build a 2 or 3 celled grain, which becomes pollen once dehiscing occurs. The three celled form before dehiscing has evolved multiple times and appears in about a third of angiosperm species, speeding fertilization. This whole life cycle is even more reduced than in ferns and lycophytes, and seed plant gametophytes cannot live on their own. With the exception of mature pollen, gametophyte tissue separated from the sporophyte will not survive. Scholars still disagree on whether the fertilized central cell counts as gametophyte tissue at all. Some call this endosperm gametophyte tissue, roughly 2/3 female and 1/3 male, with fertilized central cells ranging from 2n to 9n. Others count it as sporophyte tissue, and some believe it is neither.