Animal-like multicellularity: only once?

Plants, fungi, slime molds: lots of times

The accumulation of small heritable changes within populations over time.

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Animal-like multicellularity: only once?

#1  Postby lpetrich » May 12, 2017 2:58 am

Origin of animal multicellularity: precursors, causes, consequences—the choanoflagellate/sponge transition, neurogenesis and the Cambrian explosion | Philosophical Transactions of the Royal Society B: Biological Sciences by Thomas Cavalier-Smith

Evolving multicellularity is easy, especially in phototrophs and osmotrophs whose multicells feed like unicells. Evolving animals was much harder and unique; probably only one pathway via benthic ‘zoophytes’ with pelagic ciliated larvae allowed trophic continuity from phagocytic protozoa to gut-endowed animals. Choanoflagellate protozoa produced sponges. Converting sponge flask cells mediating larval settling to synaptically controlled nematocysts arguably made Cnidaria.

Phototrophs -- organisms that get energy by capturing light -- photosynthesizers -- "algae" and land plants.

Multicellularity evolved several times among photosynthesizers -- green algae, red algae, kelp, and among prokaryotes, cyanobacteria ("blue-green algae").

Osmotrophs -- organisms that absorb their food from their environment -- fungi in the traditional sense.

Multicellularity evolved at least three times there, among the "true" fungi, the oomycetes, relatives of kelp, and among prokaryotes, the actinobacteria or actinomycetes. These organisms take the form of thin strands, though they can make fruiting bodies, like mushrooms. These structures make spores to disperse the organisms.

There are also slime-mold-like multicellular organisms. These are only part-time multicellular, living much of the time as separate one-celled organisms. They get together only to make a fruiting body for dispersing themselves. That has evolved several times, not only in Amoebozoa (the slime molds proper), but also in Excavata (acrasids), Rhizaria (Guttulinopsis vulgaris), Alveolata (Sorogena stoianovitchae), Stramenopiles (Sorodiplophrys stercorea, related to kelp and oomycetes), Opistokonta (Fonticula alba), and some prokaryotes, the myxobacteria.

Multicellularity is sometimes reversed into a one-celled state, as with yeasts.

Animal-like multicellularity evolved only once, and TCS asks why that might be the case. That is because it involves a lot of cooperation between the organism's cells, cooperation involving mechanisms that require a lot of evolution.

He proposes that that is because it evolved from convenient precursors. The closest relatives of the animals are some protists called choanoflagellates or collar flagellates. They use their flagella to move water past them, and they feed on bacteria and the like that get caught in their collars.

Multicelled choanoflagellates can have their cells cooperate to make water currents past them, and this can be elaborated into what sea sponges have. These animals make water currents through their bodies, and their cells filter out food -- cells that look much like choanoflagellates.

As part of their adaptation for their size, sponges developed oogamy, reproduction with egg and sperm cells. They release sperm cells that swim to egg cells, and when they meet, the fertilized egg cells develop into a small ball that uses its cells' flagella to swim away and find a home for itself.

Also part of their adaptation was the development of an interior layer or mesenchyme, underneath their outer layer or epithelium. The interior-layer cells do construction and other such tasks.

Most present-day sponges have a difficulty with further progress. They take in water over most of their bodies and release it in large openings. TCS's scenario requires a sponge that goes in reverse, taking water in its large openings and releasing it in the rest of its surface.

But once a sponge can do that, it can eat relatively large prey, like sponge larvae. TCS proposes that this predatory sponge may then develop feeding tentacles for capturing prey. These tentacles require coordination, and that would be handled by some cells specializing into neurons (nerve cells). Eventually, one gets to where coelenterates (cnidarians and ctenophores) are at.

An alternative route would be for sponge larvae to start growing some adult features before settling down. More specifically, folding inward in preparation for their adult shape. Thus doing gastrulation. The larva may continue with growing mesenchyme -- and eventually skip the adult-sponge phase. Six major steps in animal evolution: are we derived sponge larvae? also proposes this scenario.

I've also found Evolutionary origin of gastrulation: insights from sponge development | BMC Biology | Full Text

Our results are compatible with sponge cell layers not undergoing progressive fate determination and thus not being homologous to eumetazoan germ layers. Nonetheless, the expression of GATA in the sponge inner cell layer suggests a shared ancestry with the eumetazoan endomesoderm, and that the ancestral role of GATA in specifying internalised cells may antedate the origin of germ layers. Together, these results support germ layers and gastrulation evolving early in eumetazoan evolution from pre-existing developmental programs used for the simple patterning of cells in the first multicellular animals.

This single evolution of animals has some interesting astrobiological consequences. It could be that animals are difficult to evolve, unlike plants, fungi, and slime molds. So there might be planets with big forests and lots of mushrooms, but no animals.
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