Co-operations between cells that do not stem from the same mother cell require a mutual recognition, i.e. an effector receptor system has to have evolved.
Effectors may be soluble and gradient-forming. Chemotactic substances: substances that are secreted into the medium, but they may also be molecules that are tightly anchored to either the cell membrane or the cell wall.
Signal recipients, i.e. receptors are usually components of the cell surface. After recognition, usually by binding of the effector, a piece of information is passed on to the cells interior where it is transformed into a physiological reaction. A specific cell-cell interaction is also the precondition of the sexual union of two cells. Sexual attractants that are secreted into the medium are called gamones. In the case of some species, their chemical nature has been analysed and some of their structures are given here.
When dealing with algae, you will see that the shape of the sexual cells, the gametes, is characteristic in each systematic group and that the male and female gametes do usually differ considerably, a condition called anisogamy. Only in the case of flagellates, flagellate-like species and some algae, the gametes of both sexes look the same, i.e. are isogamous, even though they do often differ in their biochemistry. In these cases, it is spoken of + and cells or + and stems or mating types, respectively. Mating occurs only between individuals of a + and a stem. Cells without the right type of signal are considered alien. In many species, the ability to mate, i.e. the production of gametes and the respective receptors is triggered by extern factors, like, for example, a lack in the supply of nitrate. The less favourable the living conditions are, the higher is the readiness to mate.
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