Ectocarpus: an evo-devo model for the brown algae

Abstract

Ectocarpus is a genus of filamentous, marine brown algae. Brown algae belong to the stramenopiles, a large supergroup of organisms that are only distantly related to animals, land plants and fungi. Brown algae are also one of only a small number of eukaryotic lineages that have evolved complex multicellularity. For many years, little information was available concerning the molecular mechanisms underlying multicellular development in the brown algae, but this situation has changed with the emergence of Ectocarpus as a model brown alga. Here we summarise some of the main questions that are being addressed and areas of study using Ectocarpus as a model organism and discuss how the genomic information, genetic tools and molecular approaches available for this organism are being employed to explore developmental questions in an evolutionary context.

Keywords: Brown algae; Complex multicellularity; Ectocarpus; Gametophyte; Life-cycle; Marine; Phaeoviruses; Sex determination; Sporophyte.

Fig. 1

a Schematic view of the Eukaryotic tree, redrawn from [73]. The coloured groupings correspond to the currently recognised ‘supergroups’. Unresolved branching orders among lineages are shown as multifurcations. Broken lines reflect lesser uncertainties about the monophyly of certain groups. Asterisks represent lineages where complex multicellularity emerged. Ectocarpus is a brown alga, belonging to the Stramenopila (indicated with an arrowhead). b Ectocarpus sp. gametophyte (asterisk) in the field growing on the brown alga Scytosiphon lomentaria (arrowhead) (https://www.algaebase.org/search/species/detail/?tc=accept&species_id=76)

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