Pergiliran keturunan: Perbedaan antara revisi
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[[Image:Alternation of generations simpler.svg|thumb|360px|Diagram pergiliran keturunan antara sporofit dan gametofit.]]
'''Pergiliran keturunan''' atau '''metagenesis''' adalah [[daur hidup]] yang dialami oleh organisme, yang untuk setiap fase/tahapnya melibatkan individu dengan kandungan genetik berbeda: biasanya tahap haploid (''n'') dan tahap diploid (''2n''). Istilah ini umumnya dipakai pada konteks organisme yang termasuk [[Archaeplastida]] ([[tumbuhan hijau]], [[alga merah]], dan [[Glaucophyta]].)
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'''Alternation of generations''' (also known as '''alternation of phases''' or '''metagenesis''') is a term primarily used to describe the [[Biological life cycle|life cycle]] of [[plant]]s (taken here to mean the [[Archaeplastida]]). A multicellular [[gametophyte]], which is [[haploid]] with ''n'' [[chromosome]]s, alternates with a multicellular [[sporophyte]], which is [[diploid]] with 2''n'' chromosomes, made up of ''n'' pairs. A mature sporophyte produces [[spore]]s by [[meiosis]], a process which reduces the number of chromosomes to half, from 2''n'' to ''n''. Because meiosis is a key step in the alternation of generations, it is likely that meiosis has a fundamental adaptive function. The nature of this function is still unresolved (see [[Meiosis]]), but the two main ideas are that meiosis is adaptive because it facilitates repair of DNA damages and/or that it generates genetic variation.
The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces [[gamete]]s by [[mitosis]], which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same [[species]] or from the same organism) [[fertilisation|fuse]] to produce a [[zygote]], which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many [[algae]] undergo [[sexual reproduction]].
The relationship between the sporophyte and gametophyte varies among different groups of plants. In those [[alga]]e which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In [[liverwort]]s, [[moss]]es and [[hornwort]]s, the sporophyte is less well developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen.<ref name=Thomas1978/><ref name=Glime2007/> By contrast, in all modern [[vascular plants]] the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity.<ref name=Kerp/> In [[fern]]s the gametophyte is a small flattened [[autotrophic]] [[prothallium|prothallus]] on which the young sporophyte is briefly dependent for its nutrition. In [[flowering plant]]s, the reduction of the gametophyte is much more extreme; it consists of just a few cells which grow entirely inside the sporophyte.
All animals develop differently. A mature animal is diploid and so is, in one sense, equivalent to a sporophyte. However, an animal ''directly'' produces haploid gametes by meiosis. No haploid spores capable of dividing are produced, so neither is a haploid gametophyte. There is no alternation between diploid and haploid forms.
Other organisms, such as fungi, can have life cycles in which different kinds of organism alternate. The term 'alternation of generations' has also been applied to these cases.{{Citation needed|date=January 2011}}
Life cycles of plants and algae with alternating haploid and diploid phases are referred to as '''diplohaplontic''' (the equivalent terms '''haplodiplontic''', '''diplobiontic''' or '''dibiontic''' are also in use). Life cycles, such as those of animals, in which there is only a diploid multicellular phase are referred to as '''diplontic'''. (Life cycles in which there is only a haploid multicellular phase are referred to as '''haplontic'''.)
▲'''Metagenesis''' atau '''pergiliran keturunan''' adalah [[daur hidup]] yang dialami oleh organisme, yang untuk setiap fase melibatkan individu dengan kandungan genetik berbeda. Metagenesis dibedakan dari [[metamorfosis]] berdasarkan kandungan genetiknya ini: metamorfosis hanya melibatkan perubahan bentuk yang disebabkan perubahan pada ekspresi gen-gen, semetara individunya sendiri tidak berubah identitas [[genetika|genetik]]nya. Metagenesis melibatkan dua individu yang berbeda karena identitas genetiknya tidak sama antara individu pada satu fase dengan fase yang lain.
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