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Fig. 2 | EvoDevo

Fig. 2

From: Ectocarpus: an evo-devo model for the brown algae

Fig. 2

a Schematic view of the life cycle of Ectocarpus sp. Diploid sporophytes (ploidy 2n) produce plurilocular sporangia (Ps), where mito-spores are produced by mitosis. Mito-spores develop as clonal sporophytes. Sporophytes also produce unilocular sporangia (U), where meio-spores are produced via meiosis. Half of the meio-spores inherit a V sex chromosome and develop into male gametophytes, whereas the other half inherit a U sex chromosome and develop as female gametophytes. Male and female gametophytes (ploidy n) produce gametes at maturity in plurilocular gametangia (Pg). Male and female gametes are released into the surrounding seawater, where they fuse to produce zygotes that initiate the sporophyte generation. Gametophytes and sporophytes are subtly different in terms of cell types, cell size and angle of branching [16]. The Ectocarpus life cycle also includes several alternative pathways (dashed arrows). For example, if a gamete does not meet a gamete of the opposite sex, parthenogenesis may occur [17, 36, 74] and gametes develop into partheno-sporophytes, which are initially haploid (ploidy n) but may endoreduplicate to become diploid (ploidy 2n), allowing meiosis to occur [17]. In some Ectocarpus species, heteroblasty is common, and (haploid) meio-spores develop directly into haploid partheno-sporophytes. No difference has been observed between partheno-sporophytes arising from unfused gametes and partheno-sporophytes arising from heteroblasty (meio-spores). For simplicity, parthenogenesis and heteroblasty are illustrated for female gametes and meio-spores respectively, but in some strains male gametes and male meio-spores, respectively, may also go through these pathways. Details about the life cycle of Ectocarpus can be found in the literature [11, 15, 22, 74, 75]. Asterisk indicates cylindrical cells of the apical system. b Light micrographs of several stages of development of Ectocarpus sp. U: unilocular sporangium; Pg: plurilocular gametangium; Arrowhead: gametes

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