THE HAPLOID LIFE CYCLE

Haploid life cycle is the most common in algae (without dikaryotic phase) and most fungi (with dikaryotic phase). Colonial alga such as red alga, brown alga, and green alga and filamentous alga such as red, green, and brown alga all exhibit haploid life cycle. A general life history includes both a 1n stage and a 2n stage separated by meiosis and syngamy. Before meiosis, the individual is diploidzygote. After the zygote goes through meiosis, it develops into a haploid (1n) spore or some other 1n structure. Meiosis produces four cells from each zygote and these four cells can be spores or other structure, depending on the organism. The four cells would then go through mitosis and become the organism. The organism has two options but not in all cases. It can either start asexual reproduction or it can produce gametes through mitosis. Isogamy and anisogamy, and sometimes oogamy may occur. The gametes then fuse in a process called syngamy, or fertilization. The fused gametes either all come from one single individual or from more than one individuals. After syngamy, the fused gametes become the zygote and become diploid again, the process repeats again. One important aspect about haploid life cycle is that only the gametophyte phase is present, the sporophyte does not exist in a haploid life cycle.

Haploid life cycle occurs in green algae. Volvox, for example is a colonial green algae in which both male gametes and egg are produced in the 1n stage, which then fuse together to form a zygospore, an encysted zygote that is protected from the harsh conditions of the environment. Another green algae that exhibits 1n life cycle is the Oedogonium, which is filamentous, or a chain of cells formed in one plane. Oedogonium grows in two different ways, through zoospores, or through syngamy of sperm and egg.

Volvox

In the first process, zoospores (mitospores) escape from the zoosporangium which is located in the parent algae and they develope into filaments. The parent also contains antheridia which produce sperm(1n) and an oogonium which produces the egg(1n). Syngamy occurs when the sperm and egg fuse and forms the zygote (2n). The 2n zygote then developes into the filamentous green algae.

Another example is found in the fungi kingdom. Rhizopus nigricans , or black bread mold, also has a haploid life history. R. nigricans is a coenocytic, or tubular fungus (zygomycota). After the zygote undergoes meiosis, germination occurs forming mycelia and sporangia with opposite mating strains, (+) and (-). The sproangia produces mitospores which through mitosis developes into gametangia, or sex organs. When the gametangia fuse, syngamy occurs and produces a zygote (2n). The zygote then goes through meiosis to form new 1n spores and the life cycle repeats.

A fungus, Stinkhorn



LIFE CYCLES

Meiosis and Syngamy | Gametes | Evolution of Life Cycles |
| Haploid Life Cycle | Diploid Life Cycle | Haploid-Diploid Life Cycle |
| Ecology of Reproduction | Asexual Reproduction | Summary |
| Sources | G lossary |