Asporophyte (/ˈspɔːr.əˌfaɪt/) is thediploidmulticellular stage in thelife cycle of aplant oralga which produces asexualspores. This stagealternates with a multicellular haploidgametophyte phase.
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The sporophyte develops from thezygote produced when ahaploid egg cell is fertilized by a haploidsperm and each sporophyte cell therefore has a double set ofchromosomes, one set from each parent. Allland plants, and most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploidgametophyte phase.[citation needed] In theseed plants, the largest groups of which are thegymnosperms andflowering plants (angiosperms), the sporophyte phase is more prominent than the gametophyte, and is the familiar green plant with its roots, stem, leaves and cones or flowers. In flowering plants, the gametophytes are very reduced in size, and are represented by the germinatedpollen and theembryo sac.
The sporophyte producesspores (hence the name) bymeiosis, a process also known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set ofchromosomes.Meiosis in the diploid sporophyte provides a directDNA repair capability for dealing withDNA damages, including oxidative DNA damages, ingermline reproductive tissues.[1]
The mature gametophyte produces male or femalegametes (or both) bymitosis.The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte. This cycle is known asalternation of generations or alternation of phases.
Bryophytes (mosses,liverworts andhornworts) have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition. Theembryo sporophyte develops bycell division of the zygote within the female sex organ orarchegonium, and in its early development is therefore nurtured by the gametophyte.[2]Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as theembryophytes.[citation needed]
Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes aremorphologically similar (isomorphic). An independent sporophyte is the dominant form in allclubmosses,horsetails,ferns, gymnosperms, and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores (isosporous orhomosporous) but the ancestors of the gymnosperms evolved complexheterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the femalemegaspores tending to be larger, and fewer in number, than the malemicrospores.[3]
During theDevonian period several plant groups independently evolvedheterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall. By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it. Themegagametophytes of endosporic plants such as theseed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia. Theoocytes were fertilized in the archegonia by free-swimmingflagellate sperm produced by windborne miniaturized 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 modifiedsporangium ornucellus of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution ofseeds of the kind produced by gymnosperms and angiosperms today. The rRNA genes seems to escape global methylation machinery in bryophytes, unlike seed plants.[4]
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