Filling in the gaps

I'm going to continue on with the algal theme here, because I keep getting reminded lately of neat examples. However, I'm going to take a great leap sideways and deal with a different group from rhodophytes. I'm moving towards the brown algae (sort of...)

It is universally accepted these days that the algae are a polyphyletic grouping, at least from the viewpoint of nuclear and cytoplasmic ancestry. Chlorophyll originally developed within the blue-green algae, actually a clade of bacteria (Cyanobacteria). Chloroplasts in eukaryotes then arose through endosymbiosis between a non-photosynthetic protist and a cyanobacterium. However, many authors now agree that there was probably only one such primary endosymbiosis event that led to the majority of modern chloroplasts (there is a lonely cercozoan*, Paulinella, that appears to have derived its chloroplast independently). The direct descendants of this lucky protist today are the green plants and algae (Viridiplantae), the red algae (Rhodophyta) and a small group of unicells called the blue-green algae (Glaucophyta). Red and blue-green algae both have only a single chlorophyll type, chlorophyll a, while green algae possess a second form as well, chlorophyll b (interestingly, a small handful of cyanobacteria also possess chlorophyll b, and molecular phylogenies show that these oddjobs are not closely related within the cyanobacteria). The remaining algae are derived from secondary symbioses, where a eukaryotic alga has become an endosymbiont of another eukaryote followed by loss of the endosymbiont's independence and genetic material. This is rather spectacularly demonstrated by two secondary algal groups, the chlorarachneans and cryptophytes, whose chloroplasts retain a highly-reduced eukaryotic nucleus between the membranes surrounding the chloroplast. Two groups, the amoeboid chlorarachneans and flagellate euglenoids, have chloroplasts derived from green algae as shown by their possession of chlorophyll b. Four groups, the dinoflagellates, cryptophytes, haptophytes and ochrophytes, have chloroplasts seemingly derived from red algae. These four groups also share a third chlorophyll type, chlorophyll c, as well as the ancestral chlorophyll a, and on this basis it has been suggested that they all derive from a single endosymbiotic ancestor (though this seems likely, the case is not airtight as there are a number of non-photosynthetic protists without chloroplasts that seem to be closely related to one or another of the chlorophyll c groups). Some dinoflagellates have replaced their ancestral chloroplasts with chloroplasts derived from haptophytes in a tertiary endosymbiosis.

*Originally I had identified Paulinella in this post as an amoebozoan, but it belongs to a different amoeboid group, the Cercozoa.

Multicellularity has evolved a number of times within algae. Viridiplantae became multicellular multiple times, while red algae probably evolved multicellularity once at the base of the Bangiophyceae + Florideophyceae clade (see the previous post). Ochrophytes include two groups of multicellular algae, the brown algae (Phaeophyceae) and some members of the yellow-brown algae (Xanthophyceae).

Ochrophytes are the clade of photosynthetic heterokonts. Heterokonts are a well-supported clade of protists distinguished in most members by, among other features, a shorter posterior flagellum and a longer anterior flagellum with numerous side bristles (mastigonemes). At cell division, the anterior flagellum moves backwards and loses the mastigonemes to become the posterior flagellum, while a new anterior flagellum is generated (Andersen, 2004). As well as the two above-mentioned classes, ochrophytes include diatoms and a whole bunch of unicellular algae previously united as the golden algae (chrysophytes). The chrysophytes have proven to be paraphyletic with regard to the other ochrophytes, and have been divided into a whole host of smaller classes.

Now we've gotten through all that, I'll finally introduce the star of today's post, Schizocladia ischiensis Henry, Okuda & Kawai in Kawai, Maeba et al., 2003. The position of the brown algae in relation to other ochrophytes has been obscured by the absence of clear connecting features between the multicellular brown algae and the various unicellular golden algae. The significance of Schizocladia is that it goes some way towards filling that gap. Schizocladia is a small marine ochrophyte that grows as filaments of cells in single file. Like phaeophytes, Schizocladia has cell walls impregnated with alginates. Unlike brown algae, Schizocladia lacks cellulose or plasmodesmata (cytoplasmic connections between cells). Propagation in Schizocladia was via zooids produced in individual compartments in swollen cells at the end of the filaments.

The molecular phylogenies presented in the original description of Schizocladia agreed in positioning it as the sister group of Phaeophyceae. They also agreed with the result found by other studies that there is a clade composed of Phaeophyceae (+ Schizocladia), Xanthophyceae and the unicellular Phaeothamniophyceae (the unicellular Chrysomeridales may also belong to this clade, but do not appear to have been investigated molecularly). While the Xanthophyceae do include some multicellular members, it also includes unicellular forms, and multicellularity was probably evolved independently of Phaeophyceae. Xanthophyceae do possess cellulose in the cell walls, and the presence of alginates has also been demonstrated in some species.

Due to the absence of some supposed key phaeophyte characters, Schizocladia was not included in Phaeophyceae but placed in its own independent class Schizocladiophyceae. Nevertheless, its simple morphology provides a nice connection between the unicellular ochrophytes and multicellular phaeophytes.

REFERENCES

Andersen, R. A. 2004. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany 91 (10): 1508-1522.

Kawai, H., S. Maeba, H. Sasaki, K. Okuda & E. C. Henry. 2003. Schizocladia ischiensis: a new filamentous marine chromophyte belonging to a new class, Schizocladiophyceae. Protist 154: 211-228.