I guess I’m spiritual

Ethics for the New Millenium

I checked out Ethics for the New Millennium for my post on secular ethics, and as long as I’ve got it I figured I might as well read it. One thing that’s usually poorly defined is spirituality, as in “I’m spiritual but not religious”. I’ve never been clear exactly what that means, but I usually take it as something like “I don’t go to church, but I have some fuzzy idea that there’s something out there that cares about humans.”

In one of the few explicit definitions I’ve ever seen, the Dalai Lama makes it clear that that’s not what he means by spirituality (p. 22):

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Nicole Haloupek on germ-soma differentiation

Volvox carteri

Volvox carteri by Gavriel Matt & James Umen.

Back in December, I wrote about two studies that compared global patterns of gene expression between germ cells and somatic cells in Volvox carteri, one by Benjamin Klein, Daniel Wibberg and Armin Hallmann from the University of Bielefeld in Germany and one by Gavriel Matt and Jim Umen from Washington University in St. Louis and the Donald Danforth Plant Science Center, respectively. The Matt & Umen paper has also been highlighted on the Genetics Society of America blog, Genes to Genomes.

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Origins of the sexes: more on MID

Last week, I wrote about Takashi Hamaji’s new paper characterizing the mating-type/sex-determining loci in Eudorina and Yamagishiella. That paper showed that the sex-determining region of anisogamous Eudorina is, surprisingly, considerably smaller than the mating-type loci of isogamous ChlamydomonasGonium, or Yamagishiella. Because only one gene, MID, is present in the male version of the sex-determining region in Eudorina, Hamaji and colleagues concluded that

…the evolution of males in volvocine algae might have resulted from altered function of the sex-determining protein MID or its target genes.

I commented that

…we’re still left with two (non-mutually exclusive) possibilities: changes to the MID gene itself may have changed which genes it interacts with (or how it interacts), or there may have been changes in the genes whose expression is controlled by MID.

Now Sa Geng and colleagues have provided at least a partial answer. In a new paper in Development, they swapped versions of MID among different volvocine species (unfortunately, no unpaywalled version of the paper is currently available; I will add a link when I find one). We already knew that MID is necessary and sufficient for male development: genetically male Volvox carteri colonies that have MID expression turned off produce eggs, and genetically female colonies transformed with MID produce sperm packets (“Sex change (in Volvox)”). But that’s MID from the same species. It’s somewhat surprising that a single gene can cause Volvox to switch sexes, but at least Volvox MID evolved side-by-side with the genes whose expression it controls.

What would be really surprising is if MID from other species, species that diverged from the Volvox lineage ~200 million years ago, worked in Volvox. It would be extraordinarily surprising if MID from a species that doesn’t even have males  could control their development in Volvox. It won’t work. Waste of time; don’t bother trying.

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Origins of the sexes: Eudorina and Yamagishiella

Volvox and the volvocine algae have long been a model system for understanding the evolution of multicellularity and cellular differentiation, but more recently they’ve emerged as an important model for the evolution of males and females. Sperm-producing males and egg-producing females have evolved independently in most multicellular lineages, so understanding how and why this happens is crucial for understanding the evolution of complex life.

It’s a near certainty that the unicellular ancestors from which animals, plants, fungi, seaweeds, and other complex multicellular organisms evolved were isogamous. In other words, they were capable of sexual reproduction, but the gametes that fused to form a zygote were the same size (“iso” – equal; “gamous” – gametes). In each of these lineages, large and small gametes evolved, resulting in a condition referred to as anisogamy (unequal gametes).

One really interesting thing about anisogamy is that unlike other forms of cellular differentiation, which result from non-genetic differences, the differences between sperm-producing males and egg-producing females are often genetically controlled. The most familiar way this happens is through sex chromosomes, such as the XY system in most mammals and the ZW system in birds, but there are lots of variations on this theme (check out the duck-billed platypus for an odd example).

Last month Takashi Hamaji and colleagues reported new results related to the evolution of anisogamy in the volvocine algae. The article, in Communications Biology, describes the genetic basis of sex (or mating type) determination in two volvocine species, isogamous Yamagishiella and anisogamous Eudorina. Apart from this difference in gametes, Yamagishiella and Eudorina are otherwise very similar:

Hamaji et al. 2018 Fig. 3

Figure 3A&B from Hamaji et al. 2018. Sex induction and associated gene expression alternations in isogamous Y. unicocca and anisogamous Eudorina sp. a, b Asexual and sex-induced individuals of opposite sexes of Y. unicocca (plus/minus) (a) and Eudorina sp. (female/male) (b). Mating reactions (mixed, right panels) occurred after mixing induced cultures of the two sexes (middle panels). In Y. unicocca (a), clumping of the colonies and release of single-celled isogametes (arrowheads) were observed 1 h after mixing. In Eudorina sp. (b), sex induction treatment resulted in the formation of sperm packets and the packet dissociated into individual sperm that penetrated into a female colony (arrowheads) within 16 h after mixing. Scale bars, 20 µm.

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