Mating Systems in Unicellular Algae

Sexual processes in unicellular algae provide outstanding opportunities for the study of various aspects of cell interaction and intercellular communication, such as unilateral or mutual inductions, signalling and signal perception, chemotaxis, membrane adhesion and confluence, cell recognition and cell and nuclear fusion. Because the continuity of their existence has been protected by asexual reproduction, these algae have been able to explore their full evolutionary potential with respect to sexual differentiation and reproduction, leading to a great variety in their sex phenomena and to the evolution of highly efficient interrelations. The different modes of gamete interactions they reveal illustrate component mechanisms in model-like simplicity, and offer the opportunity for experimental analysis of their physiological, genetic, molecular and ultrastructural basis. The variation in sex phenomena arises from the type of fertilization (iso-, aniso- and oogamy; gametangiogamy; potential for parthenogenesis), of sex distribution (monoecy vs. dioecy), of sex determination (genetic vs. modificatory), and of meiosis (gametic, zygotic or intermediate). Additional variation depends on the types of gametes (motile vs. immotile) and on special features of the particular organism. A normal sex act involves an ordered sequence of interrelated steps between and within the partners, the harmonious coordination of which is assured by the principles mentioned at the outset. Any environmental, experimental or genetic disturbance may lead to a developmental block, sterility, sexual isolation, or, occasionally, a shunt into another reproduction mode such as parthenogenesis or automixis, in which the activated cell somehow compensates for the absence of a partner.