A cycle of sexual reproduction in which particular species of organism reproduce in several phases (usually two but sometimes three). A given organism belongs to a particular phase, and produces reproductive cells of a certain type, from which grows an organism of the next phase. Some multicellular algae and all plant species exhibit alternation of generations. Fungi also alternate generations.
The phases of a two-phase (diplobiontic) alternation of generations are called sporophyte and gametophyte:
- The chromosomes of each cell in a sporophyte are diploid, that is, they come in pairs.
- The chromosomes of each cell of a gametophyte are haploid, that is, they are unpaired.
The cycle progresses as follows:
- Certain cells in a sporophyte undergo meiosis, producing haploid spores. The spores are often collected in a
capsule called a sporangium.
- The spores are dispersed. Algal spores often possess flagella by which they propel themselves through the
water. Plant spores are dispersed by wind and rain, or remain in place (more on that later).
- The spore develops via mitosis into a (haploid) gametophyte.
- Certain cells in the gametophyte develop (via mitosis) into (haploid) gametes, are differentiated by sex (i.e.
eggs and sperm). Similar to spores, gametes can be collected and protected in structures called
gametangia. Male gametangia are called antheridia and female gametangia are called
oogonia (algae) or archegonia (plants).
- Gametes travel to unite with each other (syngamy), producing a diploid zygote. There are three
variations:
- Eggs and sperm are equal-sized and motile (isogamous).
- Gametes are slightly different in size but both motile (anisogamous).
- The egg is much large than the sperm, and the sperm travels to the egg (oogamous).
- The zygote develops into the next generation of sporophyte.
In most species, one phase of the cycle dominates over the other. That is, the thallus (plant body) of one phase is
often much smaller than, and sometimes parasitic on, the thallus of the other phase. However, the thalli of some organisms' gametophytes and sporophytes are identical.
Algae
- Rhodophyta (Red Algae)
- Red Algae alternate generations in three phases:
- Diploid tetrasporophytes undergo meiosis to produce haploid tetraspores.
- The tetraspores develop into male and female gametophytes, which produce male and female gametes.
- The zygote produced after syngamy develops info a carposporophyte, which produces (via mitosis) carpospores, which develop into new tetrasporophytes.
- Chlorophyta (Green Algae) and Phaeophyta (Brown algae)
- Some types of green and brown algae alternate generation in two phases, as described at the beginning of the writeup.
Gametophytes and sporophytes are free-living. Many species' gametophytes are microscopic, but others are identical in
appearance to the sporophyte.
- Charophyta aka Streptophyta (Stoneworts and others)
- Charophytes are oogamous and the gameteophyte generation dominates the sporophyte. In fact, it could be said that the
sporophyte generation is entirely lacking, as the zygote, protected inside the oogonium, produces spores without dividing. This is rather peculiar, as a charophyte is supposed ot be the common ancestor of all land plants, some of which exhibit distinct sporophytes and gametophytes. The stonewort Chara carries both male and female gametangia
Plants
When certain types of oogamous algae grew onto land and developed into plants, their female gametangia developed
protective cases similar to those in the charophytes. With the cases, the gametangia are called archegonia.
All of the interesting developments in the evolution of land plant reproduction happened during the Devonian Period
418-360 mya.
- Bryophyta (mosses)
- Mosses are dominated by the gametophyte generation; the stuff called "moss" is all gametophyte. A heavy rainfall will
carry sperm from the antheridium of one moss plant to the archegonium of another, and a sporophyte (or "fruiting body")
will grow out of the archegonium. The sporophyte is parasitic on the gametophyte and consists of a stalk (seta) with a
capsule (sporangium) on the tip. The spores disperse, settle, and grow into new gametophytes.
- Hepaticophyta (liverworts)
- Liverworts are also dominated by the gametophyte generation, but come in two forms:
- Moss-like (Family Jungermanniidae)
- Moss-like liverworts resemble mosses and reproduce exactly like mosses. One of the few ways to tell them apart is the
color of the fruiting body: The seta of a liverwort is white with a black capsule. Liverwort sporophytes shrivel after
they release their spores, while moss sporophytes can live on.
- Leaf-like or thalloid (Class Marchantiopsida)
- After fertilization occurs in a leaf-like liverwort, a small umbrella-like structure grows up from the plant. The
sporophytes grow on the underside of the umbrella (which is part of the gametophyte).
- Anthocerophyta (hornworts)
- The dominant gametophyte resembles that of a liverwort, but the sporophyte is a horn-like stalk which grows up from the
plant. Unlike mosses and liverworts, hornwort sporophytes can be photosynthetic and can also outlive the gametophytes.
- The first vascular plants -- Rhyniophyta, Zosterophyllophyta, Trimerophyta
Rhyniophytes were the first land plants, and lived from the mid-Silurian to the early Devonian period.
Rhyniophytes are named for the Rhynie Chert formation near Aberdeen in Scotland, where they were preserved by
siliceous sinter from hot springs at the beginning of the Devonian period. They were later replaced by the more complex
zosterophylls and trimerophytes (from which more complex plants are believed to have evolved).
No gametophytes are found in the fossil record for any of these plants, but their sporophytes consisted of leafless
green stems bearing sporangia. Rhyniophytes and trimerophytes bear sporangia at the tips of stalks, and the zosterophylls
bear the multiple sporangia distributed up and down and around the stalks.
- Filicophyta (ferns)
- Ferns have separate gametophyte and sporophyte thalli. The gametophyte is a tiny structure that resembles a liverwort.
Again, sperm are carried into the archegonia by rain, and the sporophyte grows and overwhelms the gametophyte, developing
into the "fern" proper. Spores develop on the underside of the ferns' leaves, and are dispersed by the wind.
- Psilophyta (whisk ferns)
- The sporophytes of whisk ferns resemble trimerophytes, three-dimensionally branching stems without leaves or roots.
The tips of the stalks bear protruberances called erations, below which are structures called synangia, which appear to consist of several fused sporangia. Unlike their extinct relatives, we can find separate gametophytes for whisk ferns. The gametophytes are very small, live undergound and are parasitic on nearby fungi.
- Lycopodiophyta (club mosses and extinct Devonian trees)
- Lycopod sporangia grow against the stems, protected by leaflike sporophylls. In many lycopods, multiple sporophylls
and sporangia develop a strobilus at the tips of a stalk. Some gametophytes are photosynthetic, resembling little
bundles of algae, others are like the whisk ferns, undergrouns and parasitic.
- Sphenophyta (horsetails and extinct Devonian-Permian relatives)
- Horsetails grow artichoke-like strobili at the tips of their bamboo-like stalks. Each scale on a strobilus contains a
groups of sporangia called sporangiophores. Some sphenophytes are heterosporous (explained below). The gametophytes
grow from the spores as tiny stalks.
- Seed Plants: seed ferns, conifers, cycadeoids,
cycads, ginkgoes, Gnetophytes, and flowering plants
At the end of the Devonian Period, certain plants evolved a heterosporous "seed" habit, analagous to
the oogamous habit of the gametes:
- The female spores very large, and certain sporangia (called macrosporangia) develop a single female spore (called a
macrospore) inside a structure called an ovule. The female gametophyte spends its entire life inside its ovule, and
consists of the archegonium and a single egg.
- In contrast, the male spores are small, and a large number of "microspores" develop inside each "microsporangium".
Each microspore developes into a male gametophyte, which is reduced to a sperm-containing structure, a grain of
pollen.
- Pollen grains are carried through a pollen tube into the ovule and the archegonium, where one of the sperm fertilizes
the egg.
- The zygote develops into a sporophyte embryo, and this gets packaged inside the ovule's outer covering (which becomes the seed case) with some food for development.
All of these structures are carried on strobili, which can contain both male and female strctures ("Perfect"
"synoecious", "hermaphroditic" strobili), or just one type of structure. If only one, a given sporophyte can have one type
of strobilus (monoecious) or both tyes (dioecious).
The only further developments to be made involved the dispersal of pollen and modification of strobili to facilitate it:
- Some pollen is carried by wind or rain.
- In some conifers, pollen is captured by moisture secreted from the ovule.
- Some pollen is carried by insects from pollen-bearing structures to the ovules of other plants. This is the most
effective method of pollen distribution, and after the Devonian, plants evolved all sorts of ways of enticing insects to pollinate them. During the Mesozoic Era, the strobili developed into flowers, designed to visually entice the insects with a reward of food for spreading pollen.
Fungi
Fungi have a three-part alternation:
- Fungi spend most of their time as mycelia underground or inside whatever they're digesting. Mycelia are composed of hyphae which are filaments of connected single-cell haploid cells.
- From time to time, compatible hyphae line up and the cells fuse (plasmogamy).
- The fused hyphae are composed of dikaryotic cells; that is, each cell has two haploid nuclei.
- The dikaryotic mycelia extrude basidiocarps ("fruting bodies" such as mushrooms) into the atmosphere.
- Certain cells in the gills of the fruiting bodies fertilize to form a (diploid) zygote.
- The zygote immediately undergoes meiosis to form spores.
- The spores disperse, eventually settle and develop into mew mycelia.
Biology 1: Chapter 27: ALGAE AND FUNGUSLIKE PROTISTS
http://www.sirinet.net/~jgjohnso/algae.html
Natural Perspective
http://www.perspective.com/nature
Monterey Bay Aquarium Research Institute
http://www.mbari.org
University of California, Berkeley - Museum of Paleontology
http://www.ucmp.berkeley.edu
Palaeos: The Trace of Life on Earth
http://www.palaeos.com