Alternation of generations
In sexual reproduction, organisms have a haploid phase, with one set of chromosomes, alternating with a diploid phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is restricted to the gametes. In other eukaryotes alternation of generations may occur. The classic example is the mosses, where the green plant is a haploid gametophyte, and the reproductive phase is the diploid sporophyte.p18
The term alternation of generations refers only to the sexual cycle; organisms often have asexual reproduction as well.
The alternation of generations is an important concept in the evolution of plants.
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, which has a double set of chromosomes. In mosses, the gametophyte is the dominant generation, while the sporophytes consist of sporangium-bearing stalks growing from the tips of the gametophytes. For flowering plants (Angiosperms), the sporophyte generation comprises almost their whole life cycle (the whole green plant, roots etc.), except phases of small reproductive structures (pollen and ovule).
The sporophyte produces spores (hence the name), by meiosis. These meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes. Later, the mature gametophyte produces male or female gametes (or both) by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte. This is the cycle which is known as alternation of generations or alternation of phases.
Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic).
Bryophytes (mosses, liverworts and hornworts) have a dominant gametophyte stage on which the adult sporophyte is dependent on the gametophyte for nutrition. The embryo of the sporophyte develops from the zygote within the female sex organ, and in its early development is therefore nurtured by the gametophyte.
[change] Vascular plants
[change] Earlier evolution
Early land plants had sporophytes that produced identical spores (isosporous or homosporous) but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, and fewer in number, than the male microspores.
During the Devonian period several plant groups independently evolved heterospory and subsequently the habit of endospory, in which single megaspores were retained within the sporangia of the parent sporophyte. These endosporic megaspores contained within them a miniature multicellular female gametophyte complete with female sex organs containing egg cells which were fertilised by free-swimming sperm produced by windborne miniatuarised male gametophytes in the form of pre-pollen.
The resulting zygote developed into the next sporophyte generation while still retained within the pre-ovule, the single large female meiospore or megaspore contained in the modified sporangium of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms today.
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