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In regard to changes of ploidy, there are three types of cycles: haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic". diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic".
Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic. The equivalent terms haplodiplontic, diplobiontic and dibiontic are also in use, as is describing such an organism as having a diphasic ontogeny. [5]
Pediastrum possesses a haplontic life cycle, and can be divided into three types. [12] The most commonly seen is the first kind of asexual life cycle (ALC1), which generates autocolonies. In ALC1 the zoospores aggregate into an orderly flat disk, with concentric rings (central cell, six cells, and then nine cells.)
The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to produce a new diploid organism. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. [50] [citation needed]
Dinoflagellates have a haplontic life cycle, with the possible exception of Noctiluca and its relatives. [5] The life cycle usually involves asexual reproduction by means of mitosis, either through desmoschisis or eleuteroschisis. More complex life cycles occur, more particularly with parasitic dinoflagellates.
The life cycle of Oedogonium is haplontic. The egg from the oogonia and the sperm from the antheridia fuse and form a zygote which is diploid (2n). The zygote then undergoes meiosis and reproduces asexually to form the filamentous green alga which is haploid (1n).
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes.Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitosis.
The alga would have had a haplontic life cycle. It would only very briefly have had paired chromosomes (the diploid condition) when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes (the haploid condition).