Volvox 2015: development

Replica of Antonie van Leeuwenhoek's microscope.

Ray Goldstein‘s working (!) replica of Antonie van Leeuwenhoek’s microscope.

At the start of the Development session, I asked for a show of hands of people who self-identify as developmental biologists. About four went up. That’s not quite fair, since there’s some ambiguity in the question (primarily? exclusively?), but my point was that what all of us who are interested in the evolution of multicellularity study is the evolution of development. In fact, it might fairly be said that the origin of multicellularity is the origin of development.

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Volvox 2015: all about sex

I believe that sex is one of the most beautiful, natural, wholesome things that money can buy.

–Steve Martin

Volvox, and the volvocine algae in general, are well known as a model system for the evolution of multicellularity and cellular differentiation, but they are also an outstanding model for the evolution of sex and mating types. Volvocine algae are facultatively sexual, with haploid vegetative colonies reproducing asexually through mitosis but occasionally entering a sexual cycle that usually results in a diploid, desiccation-resistant zygote or ‘spore.’ Most of the small colonial species and unicellular relatives are isogamous, that is, the gametes are of equal size. Nevertheless, each species has two self-incompatible mating types, usually designated as ‘plus’ and ‘minus.’ In some of the larger species, the gametes have diverged into a small, motile form that we call sperm and a large, often immotile form that we call eggs. Across the eukaryotic domain, it is gamete size, not form of genitalia, fancy plumage, or receding hairline, that define males and females.

The volvocine algae span a wide range of mating systems, making them a useful (and I think underutilized) system for comparative studies of the evolution of sex. As I’ve already mentioned, both isogamous (equal-sized gametes) and oogamous (sperm and eggs) species exist, and there is good reason to suspect that oogamy has evolved independently in two separate lineages:

Isogamy and oogamy

Isogamy and oogamy (Kirk 2006. Curr. Biol., 16:R1028.)

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(Probably not) Precambrian Volvox

A new(ish) paper in National Science Review evaluates the evidence for various interpretations of Ediacaran microfossils from the Weng’an biota in South China (Xiao et al. 2014. The Weng’an biota and the Ediacaran radiation of multicellular eukaryotes. Natl. Sci. Rev., 1:498–520.). I recommend checking it out; it’s open access, and there’s a lot of interesting stuff in there that I’m not going to address.

These fossils are undoubtedly multicellular, probably eukaryotic, and extremely enigmatic. Their age (582-600 million years) means they could have important implications for the evolution of multicellularity, and their exceptional preservation in great numbers creates the potential for reconstructing their life cycles in great detail. Some of the Weng’an fossils have been interpreted as volvocine algae, an interpretation that I find highly unlikely.

Some of the Weng’an fossils are thought to represent red algae, and this would not be terribly surprising, since red algae have been around for at least 1.2 billion years. Others, for example the tubular fossils, are more problematic, with interpretations as diverse as cyanobacteria, eukaryotic algae, crinoids, and cnidarians.

Fig. 8 from Xiao et al. 2014

Figure 8 from Xiao et al. 2014: Schematic diagram showing diagnostic features of the five recognized species of tubular microfossils in the Weng’an biota.

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Evolution of eusociality

Last month, two papers on the evolution of eusociality were published in high-profile journals: one by Karen M. Kapheim and colleagues in Science, the other by Sandra M. Rehan and Amy L. Toth in Trends in Ecology & Evolution (TREE). Social and eusocial insects are an attractive system for studying major transitions, sharing some of the key features that make the volvocine algae so good for this purpose: multiple, independent origins of traits thought to be important to the transition and extant species with intermediate levels of sociality. These features make the social insects, like the volvocine algae, well-suited for comparative studies.
Figure 1 from Rehan & Toth: (A) Overview of phylogeny of aculeate Hymenoptera (with the nonhymenopteran but eusocial termites as an outgroup), highlighting independent origins of sociality (colored branches), groups with species ranging from solitary to primitively social (green), primitively social to advanced eusocial (orange), solitary to advanced eusocial (blue), and all species advanced eusocial (grey). (B) The full range of the solitary to eusocial spectrum (blue) and predictions of which genomic mechanisms are hypothesized to operate at different transitional stages of social evolution (broken arrows).

Figure 1 from Rehan & Toth: (A) Overview of phylogeny of aculeate Hymenoptera (with the nonhymenopteran but eusocial termites as an outgroup), highlighting independent origins of sociality (colored branches), groups with species ranging from solitary to primitively social (green), primitively social to advanced eusocial (orange), solitary to advanced eusocial (blue), and all species advanced eusocial (grey). (B) The full range of the solitary to eusocial spectrum (blue) and predictions of which genomic mechanisms are hypothesized to operate at different transitional stages of social evolution (broken arrows).

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Actin evolution in the Volvocales

Kato-Minoura Figure 1

Fig. 1 from Kato-Minoura et al. 2015: Genomic structure of volvocine actin and NAP genes. For comparison, previously identified sequences are also shown. Filled boxes, putative coding exons; open boxes, putative 5′ and 3′ untranslated regions. Intervening sequences are shown by solid lines. Intron positions are indicated by codon and phase numbers with reference to the three alpha-actins of vertebrates (377 amino acids) (Weber and Kabsch 1994). The conserved intron positions are linked with dotted lines. ATG, translation start codon; TAA or TGA, stop codon.

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New Scientist article on experimental evolution of multicellularity

On the second day of AbSciCon, members of the Ratcliff lab and I met with a reporter, Bob Holmes, from New Scientist. We had all given our talks on the first day of the meeting. The resulting article came out yesterday.

I’ve dealt with New Scientist before, and I find them among the better science news outlets. They make a real effort to understand the science behind their stories, a refreshing change from sites that slap misleading headlines onto barely reworded university press releases. Aaaand I’m going to wrap this up before it turns into a rant.

Peter Conlin, Jennifer Pentz, Bob Holmes, and Will Ratcliff

Peter Conlin, Jennifer Pentz, Bob Holmes, and Will Ratcliff enjoying some sushi in a Chicago park.

AbSciCon day 4: Mars, life, and Mars life

marvinmartian

I stepped out of my comfort zone a bit this morning and went to a session on Mars (okay, there weren’t any biology talks). This is far outside of my expertise, so if I say something outrageously wrong, feel free to set me straight in the comments (actually, you can always do that). I’ve never really given up on the idea of life on Mars. I remember the Viking missions and the ambiguous* results of their biological experiments, and I’m still surprised that none of the subsequent robotic missions have followed them up. I think there are better candidates further out in the Solar System, but Mars is a lot easier to get to.

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AbSciCon day 3: the tape of life

When I was in my 20’s, out of college and largely floundering, my dad lent me a paperback copy of Stephen Jay Gould’s Wonderful Life. I had occasionally enjoyed Gould’s column in Natural History, but I lacked the background to understand much beyond that (my undergrad was in political science). Wonderful Life got me interested in evolution, and I started reading other popular books, including more of Gould’s, Dawkins, Simon Conway Morris (the anti-Gould), etc., and pretty soon I found myself in Chris Parkinson‘s lab working on a master’s degree. This is all just to say that this particular book had a big influence on my life, and my first year in Montana was punctuated (see what I did there?) by a trip to Yoho National Park to see the Burgess Shale, the nominal topic of Wonderful Life.

One of the central arguments of Wonderful Life (and others of Gould’s works) was that the outcome of evolution is inherently unpredictable. Contingency, which can be interpreted as true randomness, stochasticity, or sensitive dependence on initial conditions, plays a large role in Gould’s view. Rarely at a loss for a good metaphor, Gould claimed that if “life’s tape” were rewound to some arbitrary time in the past and played again, the outcome would almost certainly be different from the first run.

This idea of ‘rewinding the tape of life’ has become a mainstay of discussions about evolutionary processes. Many of these discussions revolve around the question of how contingent and how deterministic are evolutionary outcomes, and this was the topic of two AbSciCon sessions chaired by Betul Kacar and Rika Anderson.

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