Life cycles in the Archaeplastida: chlorophytes

The dominant vegetative phase of members of the chlorophytes can be haploid or diploid. Vegetative growth is associated with mitotic cell division. In species with a haploid life history, meiosis occurs at zygote germination and the cells remain haploid for the whole of their vegetative life. Diploid formation occurs only in the zygote.  Species with a diploid life history are only haploid at gamete formation and after zygote formation continue their life cycles as diploids.

Motile compatible gametes first entangle flagella, cells then conjugate and nuclei fuse to form a zygote.  The zygote may remain motile or it may form a thick-walled resting cyst. Meiosis occurs within the zygote and haploid, flagellate cells are released. Similar events are seen in the colonial forms; all cells of a colony may develop into free- swimming gametes after breakdown of the colony structure.

Filamentous species may reproduce sexually by conjugation. In this process, two vegetative cells form a conjugation tube between them and fuse.  The cellular content from one cell then moves into the other, where nuclear fusion and zygote formation occur.

                           Figure:  Sexual reproduction in the unicellular Archaeplastida.

The zygote encysts and meiosis occurs before the emergence of a haploid new filament. Other filamentous species produce motile gametes of two mating types from different vegetative cells. Often one gamete is considerably larger (macrogamete) than the other (microgamete). In some species, only one motile microgamete is produced and it fuses with a nonmotile gamete cell called an oogonium to form the zygote.

                                 Figure .  Conjugation between two filamentous green algal cells.