According to the nature of the thallus, the sexual reproduction process of algae can be divided into two types.
1. Homothelic: The process of sexual reproduction in which sperm and egg produced in the same thallus combine to form a zygote is called homothelic sexual reproduction. E.g. Spirogyra, Chlamydomonas etc.
2. Heterothelic: The process of sexual reproduction in which sperm and egg produced in different thallus combine to form zygote is called heterothelic sexual reproduction. eg Ulothrix.
According to the nature of the reproductive cells or gametes, the process of sexual reproduction of algae (heterothallic) can be divided into three types.
1. Isogamy: The union of two gametes of the same nature in terms of size, shape and structure is called isogamy. Gametes that participate in sexual reproduction are called isogametes. It can be flagellated and motile or non-flagellated and immobile. Such as Ulothrix, Chlamydomonas, Spirogyra etc.
2. Anisogamy: The union of two gametes of different nature in terms of size, shape and structure is called anisogamy. Gametes that participate in sexual reproduction are called anisogametes. It can be flagellated and motile or non-flagellated and immobile. One is large in size and the other is small in size. Larger gametes are called macrogametes or female gametes and smaller gametes are called microgametes or pungametes. Such as Pandorina, Chlamydomonas, Caulerpa, Ectocarpus, Ulothrix etc.
3. Oogamy: The process of sexual reproduction in which the male gamete is small and motile and the female gamete is large and stationary is called oogamy. A single ovule is produced in the female genitalia. A zygote unites with an egg to form a zygote. Such as Fucus, Oedogonium, Chlamydomonas, Vaucheria, Volvox, Chara, Polysiphonia, Sargassum etc.