Choanoflagellates with inversion

Salpingoeca rosetta

Figure 1A from Dayel et al. 2011. Spherical colony of Salpingoeca rosetta. Scale bar = 5 μm.

The closest (known) living relatives of animals are a group of unicellular or colonial filter-feeders known as choanoflagellates. Much of what we know about the evolution of multicellularity in animals comes from comparisons with choanoflagellates. For example, many of the gene families involved in multicellular development in animals, and previously thought to be unique to animals, have turned out to be present in choanoflagellates as well, suggesting that these gene families were present in animal ancestors before they evolved multicellularity. Some multicellular choanoflagellates have even been shown to have differentiated cell types (Laundon et al. 2019):

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The rock, the clock, and organismal complexity

Honeybees, photo by Will Ratcliff

Recently reproduced (swarmed) honeybees.
Photo by Will Ratcliff

Scientific articles can be dry, technical, and, yes, boring. They aren’t always, though. Now and then you come across a gem, as I did this morning while searching for some background for a manuscript I’m working on. In 2007, Joan Strassmann and David Queller wrote, in a section titled “The rock, the clock, and organismal complexity,”

Darwin built his theory of descent with modifications from many quarters. He took uniformitarianism from the geologist Charles Lyell, the struggle for existence from the economist Thomas Malthus, and homology from a number of continental biologists. Perhaps most surprising is his debt to a theologian, William Paley. At university, Darwin had Paley’s Natural Theology almost by heart. Paley pointed to the complexity of organisms and claimed that such complexity required a supernatural intelligence. Darwin’s chief achievement was to provide a scientific explanation for adaptive complexity.

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Debunked by the Institute for Creation Research

Folks, it’s been fun. I feel like I had a pretty good run as a scientist. I met some amazing people, went to beautiful places, and learned things I never would have imagined (Hodgkinia, WTF?!). With all my frustrations and failures, I’ve never once regretted going back to school and becoming a biologist. But now I need to close the door on all of that and find a new way to make a living.

See, the main project I’ve been working on for the last six years, the one that was supported by a NASA postdoctoral fellowship, and that just came out in Scientific Reports, has been debunked:

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Ulvophyte multicellularity: the sea lettuce genome

Ulva

Sea lettuce (Ulva sp.), Jericho Beach, Vancouver, BC, February 28, 2011.

David Kirk called the Chlorophyte green algae “master colony-formers” because multicellularity has evolved so many times within this class:

Although members of most chlorophycean genera and species are unicellular flagellates, multicellular forms are present in 9 of the 11 chlorophycean orders (Melkonian 1990). Multicellularity is believed to have arisen independently in each of these orders, and in some orders more than once.

In contrast, multicellularity has probably only evolved once or twice in the probable sister group of the Chlorophyceae, the Ulvophyceae. So when numbers like 25 get thrown around for the number of times multicellularity has evolved, something like half of those times were in the green algae.

We know a lot less about how multicellularity evolved in the Ulvophyceae than we do in the volvocine algae within the Chlorophyceae. A big step forward in understanding ulvophyte multicellularity happened last week, though, with the publication of the Ulva mutabilis genome.

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Life cycles in major transitions, and some clueless critique

Jordi van Gestel and Corina Tarnita have published a ‘Perspective’ in PNAS, “On the origin of biological construction, with a focus on multicellularity“:

…we propose an integrative bottom-up approach for studying the dynamics underlying hierarchical evolutionary transitions, which builds on and synthesizes existing knowledge. This approach highlights the crucial role of the ecology and development of the solitary ancestor in the emergence and subsequent evolution of groups, and it stresses the paramount importance of the life cycle: only by evaluating groups in the context of their life cycle can we unravel the evolutionary trajectory of hierarchical transitions.

van Gestel 2017 Fig. 2

Figure 2 from van Gestel and Tarnita, 2017. Relationship between life stages in hypothesized life cycles of solitary ancestors and group formation in derived group life cycles. (Upper) Simplified depiction of hypothesized ancestral solitary life cycles of the green alga Volvox carteri, the cellular slime mold Dictyostelium discoideum, and the wasp Polistes metricus. Life cycles here consist of a life stage expressed under good conditions (black) and a life stage expressed under adverse conditions (green). For the latter life stage, we show an environmental signal that might trigger it and some phenotypic consequences. (Lower) Simplified depiction of group life cycles of: V. carteri, D. discoideum, and P. metricus. Developmental program underlying life stages in solitary ancestor is co-opted for group formation (shown in green): differentiation of somatic cells (V. carteri), fruiting body formation (D. discoideum), and appearance of foundress phenotype (P. metricus).

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Dissent on Bangiomorpha

Bangiomorpha pubescens

Figure 5 from Butterfield 1990. Bangiomorpha pubescens.

In my post on Bangiomorpha, I said

…Bangiomorpha was probably a red alga. This conclusion seems to be accepted by most everyone in the field. In fact, I don’t know of any dissenters, and that kind of consensus is rare for fossils this old.

I guess I didn’t look hard enough, because reader not the FTB Stewart commented

Cavalier-Smith dissents (dissented?) from the consensus
https://books.google.co.uk/books?id=lE6r5q5op94C&pg=PA63&lpg=PA63#v=onepage&q&f=false

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Origins of the sexes: isogamy and anisogamy

Sex didn’t always involve males and females. I know it still isn’t always between males and females, but that’s not what I mean. I mean that there was a time when sex was happening, but there were no males and females. Sex existed before males and females, and many species are still doing it without them.

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Multicellularity rundown

Too many papers, not enough time: each of these deserves a deep dive, but my list just keeps getting longer, so I’m going to have to settle for a quick survey instead. To give you an idea of what I’m up against, these papers were all published (or posted to bioRxiv) in July and August, 2016. By the time I could possibly write full-length posts about them all, there would probably be ten more!

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Heads I win; tails you lose: Evolution News & Views on Gonium, part 1

Figure 6 from Hanschen et al. 2016. Multicellularity hinges on the evolution of cell cycle regulation in a multicellular context with subsequent evolution of cellular differentiation (here, cell size-based) and increased body size.

Figure 6 from Hanschen et al. 2016. Multicellularity hinges on the evolution of cell cycle regulation in a multicellular context with subsequent evolution of cellular differentiation (here, cell size-based) and increased body size.

Remember how I said they’re prolific? Before I’ve even had a chance to write up my thoughts on the Gonium genome paperEvolution News & Views has already published theirs. The story has also been picked up by the Washington PostNew HistorianGenNews, and ScienceDaily (that last one looks like just a reprint of the press release from University of the Witwatersrand). By the way, the genome paper is open access, so you don’t need a subscription to see it for yourself.

We already know that cdesign proponentsists are not fans of research into the evolution of multicellularity, and that they have trouble understanding it. In an unsigned article on the Gonium genome at ENV, they admit that

After reading this paper, we’re none the wiser.

That’s too bad. I’m here to help.

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Time for a revision? Maureen O’Malley and Russell Powell on Major Transitions, part 1

The so-called ‘Major Transitions’ framework is an attempt to explain the hierarchical structure of life on Earth: genes within chromosomes, chromosomes within cells, cells within cells (eukaryotic cells), individuals within sexual partnerships, cells within multicellular organisms, and organisms within societies. The best-known attempt to unify the origins of these relationships is a book by John Maynard Smith* and Eörs SzathmáryThe Major Transitions in Evolution.

MajorTransitionsCover

First published in 1995, the book focused on the origins of these hierarchical levels, connecting them with the unifying theme that

…entities that were capable of independent replication before the transition can replicate only as part of a larger whole after it.

For example, after a transition from unicellular to multicellular organisms (there were several), cellular reproduction either contributes to the growth of the organism or to production of new multicellular organisms.

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