More evidence for co-option in the evolution of soma

One of the reasons Volvox was developed as a model organism was that it has the minimum number of cell types something with cellular differentiation can have: two. This property focuses investigations of cellular differentiation in a way that an organism with many cell types could not. In describing their move from studying avian and mammalian models to studying Volvox, Marilyn and David Kirk said,

The thing that appealed to us most about V. carteri – in addition to the genetic accessibility that Starr (1970) had already demonstrated – was the fact that it presented the germ-soma dichotomy in such a clear and simple form. Each asexual adult (or “spheroid”) of V. carteri contains only two cell types: small, biflagellate somatic cells, and large asexual reproductive cells, called gonidia (figure 1). The somatic cells are mortal; once they have provided the organism with motility for a few days they die. The gonidia, in contrast, are potentially immortal; each mature gonidium acts as a stem cell, dividing to produce a juvenile organism containing a new cohort of gonidia and somatic cells. No one has ever found a way to make wild-type somatic cells divide, but the only way to prevent gonidia from dividing is by withholding energy or poisoning them. Who could ask for a clearer presentation of one of the central issues of developmental biology: how are cells with extremely different phenotypes produced from the progeny of a single cell?

Kirk & Kirk 2004 Fig. 1

Figure 1 from Kirk & Kirk 2004. A young adult spheroid of V. carteri consists of thousands of small, biflagellate somatic cells that are embedded at the surface of a transparent sphere of extracellular matrix, and about 16 large asexual reproductive cells, called gonidia, that are located just internal to the somatic cells.

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Another step toward understanding sex determination in Volvox

Volvox and its relatives are a great model system for understanding the evolution of multicellularity. Their simplicity (relative to most other multicellular groups) and the variety of ‘intermediate’ species (‘intermediate’ in terms of size and complexity) make them especially suitable for comparative studies of their morphology, development, genetics, genomics, and so on. David Kirk’s book on the topic thoroughly reviews the work done up through the late ’90s, and advances since then have only increased the pace of discovery.

But in the last ten years or so, I would argue that the volvocine algae have emerged as a leading model system for an entirely different set of questions related to the evolution of the sexes. Males and females are defined by the gametes they produce, and the sexes came into existence when their gametes diverged into two different types. The existence of different male and female gametes (sperm and eggs, in most cases) is called anisogamy, and the ancestral condition of similar gametes is isogamy.

In 2006, Hisayoshi Nozaki and colleagues reported that volvocine males evolved from the minus (isogamous) mating type. To the best of my knowledge, this is the only group for which we know this. Since then, more clues have been forthcoming, and these were competently reviewed last year by Takashi Hamaji and colleagues. A new paper in PLoS ONE, by Kayoko Yamamoto and colleagues, adds another piece to the puzzle.

Figure S2 from Yamamoto et al. 2017. Light microscopic images of Volvox africanus (homothallic, monoecious with males type) and V. reticuliferus (heterothallic, dioecious type). Scale bars = 50 μm. sp: sperm packet, e: egg. A-C. V. africanus strain 2013-0703-VO4. A. Asexual spheroid. B. Monoecious spheroid. C. Male spheroid. D, E. V. reticuliferus. D. Male spheroid in male strain VO123-F1-7. E. Female spheroid in female strain VO123-F1-6.

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Volvox 2017: David Kirk will be there

David Kirk

Dr. David Kirk, Professor Emeritus at Washington University in St. Louis.

I just found out from Jim Umen, who’s organizing the Fourth International Volvox Conference, that David Kirk is planning to attend. This is great news; we’ve been wanting Dr. Kirk to come since the first meeting in 2011, but it hasn’t previously worked out.

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Volvox 2017: early registration extended

Volvox 2017

Discounted registration for the Volvox 2017 meeting has been extended to June 16th. This is a pretty good deal as scientific meetings go: $550 for faculty includes registration, most meals, and a shared room. Registration for postdocs and students is $100 less, and there are travel grants available. If you’ve been debating whether or not to go, it’s decision time: prices will go up $100 after the 16th.

New review of green algal sex

Hiroyuki Sekimoto from Japan Women’s University has published a review of sexual reproduction in the volvocine algae and in the Charophyte Closterium in the Journal of Plant Research. In addition to a brief description of the Chlamydomonas sexual cycle, it includes a succinct review of the genetics of sex and sex determination. Unfortunately, the article is paywalled, and my inquiry to the author has so far gone unanswered.

Figure 1 from Sekimoto 2017. The life cycle of Chlamydomonas reinhardtii. Vegetative cells (V) di erentiate into mt+ and mt− gametes (G) during nitrogen starvation (−N). Mating types are restricted by mating-type loci (+ and −). When gametes are mixed, the plus and minus agglutinin mol- ecules on their agellar surfaces adhere to each other, and this adhe- sion results in increased intracellular cAMP levels. The signal trig- gers gamete cell wall release and mating-structure activation. Cells then fuse to form binucleate quadri agellated cells. Zygotes with thick cell walls germinate in response to light and nitrogen supple- mentation, and undergo meiosis to release four haploid vegetative cells

Figure 1 from Sekimoto 2017. The life cycle of Chlamydomonas reinhardtii. Vegetative cells (V) differentiate into mt+ and mt− gametes (G) during nitrogen starvation (−N). Mating types are restricted by mating-type loci (+ and −). When gametes are mixed, the plus and minus agglutinin molecules on their flagellar surfaces adhere to each other, and this adhesion results in increased intracellular cAMP levels. The signal triggers gamete cell wall release and mating-structure activation. Cells then fuse to form binucleate quadriflagellated cells. Zygotes with thick cell walls germinate in response to light and nitrogen supplementation, and undergo meiosis to release four haploid vegetative cells.

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Volvox 2017: one week left for early registration

Volvox 2017 LogoJust what the headline says: early registration for The Fourth International Volvox Conference ends May 19th. After that, prices go up $100 for everybody. The registration fees sound a bit steep (up to $650), but when you consider that they include meals, lodging, and transportation between the hotel and the conference, they’re not bad at all:

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Sex change (in Volvox)

Alexey Desnitskiy from Saint Petersburg State University has published a new review of sexual development in the genus Volvox in the International Journal of Plant Reproductive Biology. 

The article includes an up-to-date review of Professor Desnitskiy’s own work describing four developmental “programs” in the various species of Volvox:

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The Volvox 2017 website is live

volvoxwebheader_3_orig

The website for the Volvox 2017 conference is up at www.volvox2017.org. Registration isn’t open yet, but there’s some information about the venue, the Donald Danforth Plant Science Center in St. Louis. The meeting is set for August 16-19, 2017.

The goal of the International Volvox Conference is to bring together international scientists working with Volvox and its relatives (aka Volvocales or volvocine algae). We cordially invite experimentalists and theorists interested in these fascinating organisms.

I’ll keep you posted!

Evolution of microRNAs in the volvocine algae

The following guest post was kindly provided by Dr. Kimberly Chen. I have edited only for formatting.

MicroRNAs (miRNAs) are a class of non-coding small RNAs that regulate numerous developmental processes in plants and animals and are generally associated with the evolution of multicellularity and cellular differentiation. They are processed from long hairpin precursors to mature forms and subsequently loaded into a multi-protein complex, of which the Argonaute (AGO) family protein is the core component. The small RNAs then guide the protein complex to recognize complementary mRNA transcripts and conduct post-transcriptional gene silencing.

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