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In the following life cycles, organisms alternate between haploid and diploid phases, through the processes of meiosis and fertilization. The differences between these lifecycles relate to whether these haploid and/or diploid phases are multicellular.
There are three different life cycles that eukaryotic organism can exhibit: Diplontic, Haplontic, or Diplo-haplontic
Sexual Reproduction
Sexual Reproduction
Eukaryotic organisms with a sexual lifecycle alternate between meiosis and fertilization
Meiosis
Meiosis
A cellular cycle in which a cell's chromosomes are halved. The result is gametes in some organisms and spores in others
Fertilization
Fertilization
A process in which two haploid gametes fuse through plasmogamy and karyogamy to become a diploid cell, sometimes called a zygote
Gametes
Gametes
Specialized haploid cells that are produced to recombine with another gamete during fertilization (e.g., eggs, sperm)
Sexual Life Cycles
Sexual Life Cycles
Within the eukaryotes, there are three types of sexual life cycles:
This is the life cycle seen in animals, where almost all cells are diploid, but sexual cells are haploid (see below)
This is the life cycle seen in fungi and some algae, where almost all cells are haploid, but during reproduction diploid cells are briefly made (see below)
This is the life cycle seen in plants, where there are two separate multicellular phases: diploid and haploid. The diploid phase creates spores, and the haploid phase creates gametes (see below)
Diplontic Life Cycle
Diplontic Life Cycle
Which phase is multicellular in this life cycle?
Which phase is multicellular in this life cycle?
The diploid (2n) phase is multicellular and dominant.
What happens with meiosis?
What happens with meiosis?
This life cycle is also called "gametic meiosis" because gametes (eggs/sperm) are created by meiosis
The diploid phase produces reproductive cells through the meiotic division of a diploid gametocyte to create four sex cells, such as eggs or sperm
What happens with fertilization?
What happens with fertilization?
Sperm and egg (gametes) fuse during fertilization to create a young zygote
Life cycle synopsis
Life cycle synopsis
Mature diploid → meiosis → sex cells → fertilization → zygote → mitosis → mature diploid
Which groups exhibit this life cycle?
Which groups exhibit this life cycle?
Water molds, Diatoms, some Brown algae, all Animals
Haplontic Life Cycle
Haplontic Life Cycle
Which phase is multicellular in this life cycle?
Which phase is multicellular in this life cycle?
The haploid (1n) phase is multicellular and dominant.
What happens with meiosis?
What happens with meiosis?
This life cycle is also called "zygotic meiosis" because the zygote goes through meiosis to create spores
A diploid cell that forms from fertilization called the zygote, goes through meiosis to create spores, which are dispersed into the environment
Spores germinate into new haploid organisms
What happens with fertilization?
What happens with fertilization?
The haploid phase produces reproductive cells, such as sperm and eggs, which fuse during fertilization to create a diploid cell.
Life cycle synopsis
Life cycle synopsis
Mature haploid → sex cells → fertilization → zygote → meiosis → spores → mitosis → mature haploid
Which groups exhibit this life cycle?
Which groups exhibit this life cycle?
Most Fungi (mucoromycetes, ascomycetes, basidiomycetes); some green algae such as the charophytes
Diplo-haplontic (or Haplo-diplontic) Life Cycle
Diplo-haplontic (or Haplo-diplontic) Life Cycle
Which phases are multicellular in this life cycle?
Which phases are multicellular in this life cycle?
A life cycle in which both the diploid (2n) and haploid (1n) phases are multicellular.
Since this is a life cycle found in plants and algae, we use the following terms:
- "Sporophyte" is the diploid phase
- "Gametophyte" is the haploid phase
What happens with meiosis?
What happens with meiosis?
This life cycle is also called "Alternation of Generations" or "sporic meiosis" because spores are created by meiosis
The sporophyte produces diploid sporocytes in a sporangium, which are cells dedicated to making spores
Each sporocyte goes through meiosis to create four haploid spores, and multiple sporocytes will create spores in multiples of 4s
These spores are dispersed and germinate into gametophytes
What happens with fertilization?
What happens with fertilization?
Gametophytes mature and produce sex cells through mitosis: male (e.g., sperm) and female (e.g., eggs) sex cells in gametangia.
Sex cells fuse, during fertilization, to create a young sporophyte or zygote
Life cycle synopsis
Life cycle synopsis
Mature diploid → meiosis → spores → mitosis → mature haploid → sex cells → fertilization → zygote → mitosis → mature diploid
Which groups exhibit this life cycle?
Which groups exhibit this life cycle?
All Land plants*, Chytridiomycetes, Red algae, some Brown algae, some Green algae
Most land plants (except bryophytes) have a dominant sporophyte phase with a reduced gametophyte. This can make land plants seem like they are diplontic, but they are not.
Every land plant group has a multicellular gametophyte, although hidden, which makes them diplo-haplontic.
Additional Resources
Additional Resources
Plant genome evolution shows both episodic and gradual diploidization patterns (Phys.org 27Jun2025)
└Wang et al. (2025) Diploidization in a wild rice allopolyploid is both episodic and gradual
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