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Life cycles of sexually reproducing organisms

Fertilization and meiosis alternate in sexual life cycles . What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant    , in which the multicellular diploid stage is the most obvious life stage, such as with most animals including humans; haploid-dominant    , in which the multicellular haploid stage is the most obvious life stage, such as with all fungi and some algae; and alternation of generations    , in which the two stages are apparent to different degrees depending on the group, as with plants and some algae.

Diploid-dominant life cycle

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells    , are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state ( [link] ).

This illustration shows the life cycle of animals. Through meiosis, adult males produce haploid (1n) sperm, and adult females produce haploid eggs. Upon fertilization, a diploid (2n) zygote forms, which, through mitosis and cell division, grows into an adult.
In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.

Haploid-dominant life cycle

Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals, designated the (+) and (−) mating types, join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the “parents,” these spores contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are conducive, the spores form multicellular haploid structures by many rounds of mitosis ( [link] ).

Art connection

This illustration shows the life cycle of fungi. In fungi, the diploid (2n) zygospore undergoes meiosis to form haploid (1n) spores. Mitosis of the spores occurs to form hyphae. Hyphae can undergo asexual reproduction to form more spores, or they form plus and minus mating types that undergo nuclear fusion to form a zygospore.
Fungi, such as black bread mold ( Rhizopus nigricans ), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. (credit “zygomycota” micrograph: modification of work by “Fanaberka”/Wikimedia Commons)

If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

Questions & Answers

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Siyanbola Reply
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R0se
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Explain the function of nematocysts in cnidarians?
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The nemotocyst is used by Cnidarians (hydra, jellyfish, sea anemones) to sting their prey and any threatening enemy.
Lee
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If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
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Which of the following organisms is most likely to be a diploblast?
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weight
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Oyekemi
You serves as an example of matter Because matter is anything that has mass and occupy space e.g man and every other things that exist on earth.. So think of every other things around you ...
Biola
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Biola
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the action of making a person immune to infections ,for immunisation
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Phathu Reply
biology is the study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behavior, origin, and distribution.
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The study of all aspects of life. The study of all living organisms (such as animal cells and plant cells) in greater detail (their structure and how they function). It's a very broad science.
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pathogens are a bacterium, virus, or other microorganism that can cause disease.
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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