Figure 1: Schematic representation of algal life cycles. Diploid and haploid stages are marked by white and black arrows, respectively. (A) Brown algal life cycle. Fucales are characterized by a diplontic life cycle. Meiosis in the reproductive tissue is immediately followed by gametogenesis and syngamy producing a diploid zygote. Most algae such as Dictyota, Scytosiphon and Laminaria exhibit a diplohaplontic life cycle where a haploid gametophyte alternates with a diploid sporophyte. Here following meiosis the resulting spore develops into a multicellular organism. Both haploid and diploid phases may be of identical morphology (isomorphic) (Dictyota), or the two phases may develop differently (heteromorphic) with either the gametophyte (Laminaria) or the sporophyte (Scytosiphon) being microscopic. Many unicellular algae (Chlamydomonas) are characterized by a haplontic life cycle where the formation of a zygote is immediately followed by a meiotic division. (B) Simplified diplohaplontic life cycle of Ectocarpus siliculosus. Meiosis (a) takes place in the sporophyte (diploid) to produce haploid spores. First cell division in germinating spores is asymmetric (b) and they grow into multicellular gametophytes. Gametophytes produce morphologically identical but physiologically differentiated male and female gametes (c), which fuse to form a zygote. After a symmetrical first cell division (d) the zygote grows into a diploid sporophyte. Alternatively, gametes that do not meet a partner of the opposite sex grow into diploid parthenosporophytes by means of parthenogenesis combined with endoreduplication (e) or into a haploid parthenosporophyte (f). The latter produces meiospores via a nonreductive apomeiotic event (g). (C) Life cycle of the red alga Chondrus showing three distinct stages: the gametophyte, the sporophyte and the carposporophyte, which develops parasitically on the gametophyte after fertilization. Loops connecting a generation with itself denote asexual reproduction mediated by vegetative reproduction (e.g. fragmentation, propagule formation) (Fucus, Dictyota), the formation of asexual spores (mitospores) (Dictyota, Ectocarpus, Laminaria), mitosis (Chlamydomonas) or parthenogenetic development of unfertilized (female) gametes (Scytosiphon, Laminaria, Ectocarpus). Figure adapted from Bogaert et al. (2013).