If coral reefs are the rain forests of the tropical oceans, kelp forests are the woodlands of the Northern seas. Kelp is one of the algal species that can survive the harsh conditions of the North Sea that I know and love, together with other hardy seaweeds like bladder wrack. All these seaweeds are part of the larger family of the brown algae, which are generally good at dealing with unfavorable conditions, such as large fluctuations in light, temperature and salinity. The evolutionary past of Brown Algae is particularly interesting, as it is assumed that they arose via the fusion of two eukaryotes (their chloroplasts have four membranes)!
The first brown algal genome sequences will be entered in sequence databases soon, since scientists published the Ectocarpus genome in Nature a few weeks ago. As the first representative of brown algae, Ectocarpus has the honour of joining the ranks of giant pandas and body lice in having its genome sequenced, promising exciting insights in how multicellularity can evolve.
Kelp blowing in the "wind" in Diamond Bay. Kelp forests are one of the most productive ecosystems of temperate and cooler seas. Source: saspotato on Flickr
The genome is of course rich in interesting nuggets of molecular insights into biological observations. For example, brown algae are known to have some strange polysacharides in their cell walls, such as alginates, which give seaweeds their gummy feeling. The genes involved in the biosynthesis of alginates were first described in bacteria, but the brown algal genes have were never identified. Now with the Ectocarpus genome in hand, the researchers still couldn’t find any homologs of the bacterial genes. This probably means that brown algae independently evolved enzymes to synthesize these funny polysacharides. Whatever biological problem arises, evolution will find a (different) way!
The Ectocarpus genome is also rich in genes dedicated to harvesting light in fluctuating conditions, containing 53 light harvesting complex genes. What’s more, the team found an enzyme (DPOR) that can synthesize chlorophyll in dim light or in the dark. The DPOR enzyme cannot be find in many terrestrial plants, and seems to be more commonly found in green algae.
On to the real interesting stuff. Brown algae are one of the few clades that can lay claim to inventing multicellularity. Because contrary to what simple overviews of evolution tell you, the emergence of multicellular life wasn’t a single ‘big leap’ in the evolution of life on earth. ‘Multicellular leaps’ occurred in at least five different branches of the tree of life: the metazoans (animals), fungi, green algae / plants, red algae and brown algae.
The place of brown algae and Ectocarpus within eukaryotes. All five multicellular lineages have been coloured, with brown algae unsurprisingly in brown and us metazoans in funky blue.
In an attempt to explain why certain species became multicellular, the authors analyzed the total gains and losses of gene families in separate lineages. I’m not a big fan of this approach, it reeks a bit of the ‘the deflated ego problem’ where we assume that ‘complexer organisms’ must have more genes than ‘less complex’ lineages. To support their statement that ‘multicellular organisms have lost fewer gene families and evolved more new gene families than unicellular lineages’, they predicted the gains and losses of different gene families in some eukaryotic lineages (below). I am not really convinced: Ectocarpus and Laccaria seem to fall within the range of half of the unicellular lineages analyzed. The authors themselves have to admit that they fail to detect significant trends.
The gains and losses of gene families in different lineages of eukaryotes.
The authors however do find several gene families that could have contributed to developing multicellularity. Of particular interest are the integrins, that are not found in other stramenopile genomes (Oomycetes and Diatoms in the phylogeny above). In animals integrins are vital for ‘sticking’ cells together, so it’s easy to see why they would be important for other multicellular organisms. The team also found many ion channels that are unique to animals, including the IP3 receptor. This receptor plays a central role in the very fast calcium signaling, which animal cells use to quickly react to stimuli (it is used in muscle contraction, for example).
This typical combination of metazoan and algal genes in the Ectocarpus genome reflects its interesting evolutionary past, where two eukaryotes fused to give rise to a successful lineage. Considering the many genes derived from algae , the chimera maybe arose from a photosynthetic algal-like eukaryote and a (heterotrophic?) eukaryote that is closer to the metazoan lineage. Consider the identity crisis the poor guy must have faced! Luckily, by keeping the right sets of genes and ditching the obsolete ones, the brown algae overcame this crisis and blossomed into a rich and diverse branch on the tree of life.
Cock, J., Sterck, L., Rouzé, P., Scornet, D., Allen, A., Amoutzias, G., Anthouard, V., Artiguenave, F., Aury, J., Badger, J., Beszteri, B., Billiau, K., Bonnet, E., Bothwell, J., Bowler, C., Boyen, C., Brownlee, C., Carrano, C., Charrier, B., Cho, G., Coelho, S., Collén, J., Corre, E., Da Silva, C., Delage, L., Delaroque, N., Dittami, S., Doulbeau, S., Elias, M., Farnham, G., Gachon, C., Gschloessl, B., Heesch, S., Jabbari, K., Jubin, C., Kawai, H., Kimura, K., Kloareg, B., Küpper, F., Lang, D., Le Bail, A., Leblanc, C., Lerouge, P., Lohr, M., Lopez, P., Martens, C., Maumus, F., Michel, G., Miranda-Saavedra, D., Morales, J., Moreau, H., Motomura, T., Nagasato, C., Napoli, C., Nelson, D., Nyvall-Collén, P., Peters, A., Pommier, C., Potin, P., Poulain, J., Quesneville, H., Read, B., Rensing, S., Ritter, A., Rousvoal, S., Samanta, M., Samson, G., Schroeder, D., Ségurens, B., Strittmatter, M., Tonon, T., Tregear, J., Valentin, K., von Dassow, P., Yamagishi, T., Van de Peer, Y., & Wincker, P. (2010). The Ectocarpus genome and the independent evolution of multicellularity in brown algae Nature, 465 (7298), 617-621 DOI: 10.1038/nature09016
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