Contrary to the haploid life histories, sporophyte is present in a diploid life history and the gametophyte is absent. Meiosis produces 4 gametes(1n), ie. isogamy, anisogamy and oogamy, which through mitosis develops into mature gametes. These mature gametes then fuse (syngamy) and developes into zygote (2n). The gametes in syngamy can come from just one individual or different individuals. Through mitosis, the zygote developes into the organism. The organism then produces what is called a meiocyte (sporocyte), or other structure depending on the type of the organism and then go through meiosis to produce the 4 gametes and the cycle repeats itself. The organism can also go through asexual reproduction.
Fucus (rockweed), a brown alga, displays a diploid life history. The zygote (2n) becomes an embryo (very young sporophyte) and developes into the mature Fucus with receptacles at the tip of the algae. The receptacles are reproductive branches and contain many cavities with external pores which contain antheridia (male) and oogonia (female), then through meiosis, sperm (1n) and egg(1n) are produced. Syngamy occurs when the two fuse and become a zygote (2n). (Note that only the gametes are 1n.)
In fungi, plasmodial slime mold (Myxomycota) displays a diploid (2n) life history. After meiosis, the developing sporangia become the mature sporangia which produce meiospores. The meiospores then becomes swarm cells (flagellate cells) and nonflagellate cells. Both of these cells functions as gametes and each fuse with its own kind during syngamy. After syngamy, the zygote developes into a young, and then an old 2n multinucleate plasmodia. Then the old 2n turns into a developing sporangia. The developing sporangia then goes through meiosis and developes into mature sporangia, and the cycle repeats again.
Meiosis and Syngamy |
| Evolution of Life Cycles |
| Haploid Life Cycle | Diploid Life Cycle | Haploid-Diploid Life Cycle |
| Ecology of Reproduction | Asexual Reproduction | Summary |
| Sources | G lossary |