Introduction

In the unicellular green alga Chlamydomonas the two flagella are used not only for locomotion but also for sexual cell–cell interactions. In a compatible combination of mating-type plus and minus (mt+ and mt−) partners, the cells adhere to each other by their flagella. This results not only in a close proximity of the cell bodies that are going to fuse with each other, but also in the generation of a signal, telling the cells to prepare for fusion. This implies at least the total or partial hydrolysis of the cell wall, and the activation of a specialised zone at the anterior part of the cell surface. Thus, Chlamydomonas is an example in which signalling is generated by physical cell-to-cell contact. This allows us to investigate the nature and behaviour of the surface receptors involved in this process and the mechanism of signal transduction over the surface membrane and inside the cells. Despite the clarity in which these processes in this simple eukaryotic system are exhibited, and the considerable progress that has been made in a number of laboratories, several enigmas remain, which I address in this chapter.

Strategies of gametic approach

Most of the research concerning sexual interactions has been carried out with the heterothallic Chlamydomonas eugametos and Chlamydomonas reinhardtii. In these species, cells within one clone all have the same mating type. So when mt+ and mt− clonal populations are mixed, the cells exhibit sexual conjugation when they are mating-competent.