The life cycles can be haplobiontes or diplobiontes.
In the cycles haplobiontes (from the Greek haplós , unique, bónon , life, and óntos , to be), there is only one type of organism as to the ploidy of the cells (number of sets of chromosomes ). If this organism is haploid, the life cycle will be called haplobionte haplonte ; if it is diploid, it will be called haplobionte diplonte .
In the diplobionte cycles (from the Greek diploos , double), there are two types of organism, a haploid and a diploid, which alternate, so these cycles are also known as alternation of generations or metagenesis.
Life cycle haplobionte haplonte
The haplobionte cycle haplonte , previously denominated haplobionte, presents the haploid phase (n) predominant and represented by the adult organism. This, upon reaching maturity, forms gametes by mitotic division of some of its cells.
The diploid phase (2n), considered transitory and of short life, is represented by the zygote, formed by the union of two haploid gametes.
In this life cycle, meiosis occurs in the zygote, soon after fertilization, and is therefore called zygote or early meiosis . It is the life cycle of most fungi , some protozoa and many green algae , such as Chlamydomonas , Spirogyra , Zygnema and Ulothrix .
Haplobionte diploma Life Cycle
In the diplobionte haplobionte cycle , the diploid phase (2n), represented by the adult organism, is the predominant phase. The transient and short-lived haploid phase (n) is represented by the gametes.
Meiosis is called gametic or final and occurs at the time of formation of gametes by the adult organism.
In this type of cycle, a diploid individual is originated from the fusion of two haploid gametes. Successive mitotic divisions of the zygote originate the adult individual. This is the life cycle of animals and some green and brown algae.
Diplobionte life cycle
The diplobionte cycle , previously called haplodiplobionte, presents metagenesis or alternation of generations between the diploid (sporophytic and asexual) and haploid (gametophytic and sexed) phases.
In this cycle, meiosis occurs at the time of formation of spores in the sporophyte, being called meiosis sporadic or intermediate . It is the cycle of green algae of the genus Ulva and plants, such as bryophytes , pteridophytes , gymnosperms and angiosperms .
Considering only plants, sporophytic (2n) generation is predominant in the cycles of pteridophytes, gymnosperms and angiosperms. The gametophytic (n) generation is predominant only in the bryophyte cycle.