In algae, there are two basic types of asexual reproduction:
- binary division: Common in unicellular forms that occur to mitosis to effect cell division.
- zoosporia: common in aquatic multicellular algae. Each zoospore, dispersing in the medium, is capable of generating new algae.
Gametes and reproductive cycles:
In many aquatic algae there is the production of gametes that, if merged, will give rise to zygotes. These zygotes, after a short period of dormancy, undergo meiosis with production of four cells (zoospores). Each of these cells will give rise to new algae, necessarily haploid. Note that in this case we have a reproductive cycle in which the adult organism is haploid.
The cycle is called haplobionte (or haplon). Meiosis occurs in the zygote phase, being called zygotic. It is also called initial meiosis, as each cell will begin the formation of a new adult organism.
In other algae, the adult generation is diploid and produces gametes by meiosis. From the meeting of gametes, during fertilization, a zygote emerges that ends up producing a diploid adult. The reproductive cycle is diplobionte (or diplonte). Meiosis is gametic because it served to form gametes. It is also called final meiosis because it occurs at the end of the diploid adult's developmental period.
Most multicellular algae show alternation of generations, that is, in their life cycle alternate generations of haploid and diploid individuals.
Ex: Talosa green seaweed of the genus Ulva
O cycle haplodiplobionte It also occurs in algae and can be seen on the page dealing with Gymnosperms.
As for gametes produced by algae, there are cases of:
- Isogamy - equal male and female gametes;
- Heterogamy - Male and female mobile gametes, flagellated, but the male much smaller in size than the female.
- Oogamy- Male gamete is small and mobile and female gamete is large and immobile.
In some freshwater filamentous algae pairing of two individuals occurs with the passage through a communication channel of whole cells from one filament to another. The cells are haploid and upon joining give rise to zygotes. The zygotes are divided by meiosis and each formed cell will be able to give new haploid filament. Note that this conjugation is part of the haplobionte cycle and zygote meiosis contributes to the emergence of variability.