We’ve been talking about flies nonstop for the last month — it’s been nothing but developmental genetics and epistasis and gene regulation in weird ol’ Drosophila — so I’m changing things up a bit, starting today. We talked about vertebrates in a general way, giving an overview of major landmarks in embryology, and a little historical perspective.
We take a very bottom-up approach to studying fly development: typically, fly freaks start with genes, modifying and mutating them and then looking at phenotype. Historically, vertebrate embryology goes the other way, starting with variations in the phenotype and inferring mechanisms (this has been changing for the last decade or two; we often start with a gene, sometimes from a fly, and use that as a probe to hook into the genetic mechanisms driving developmental processes). What that means is the 19th and early 20th century literature on embryology is often comparative morphology, looking at different species or different stages and trying to extract the commonalities or differences, or it’s experimental morphology, making modifications (usually not genetic) to the embryo and asking what happens next. Genes were not hot topics of discussion until the last half of the 20th century, and even then it took a few decades for the tools to percolate into the developmental biologists’ armory.
And much of 19th century embryology went lurching down a dead end. We talked about Haeckel, the grand sidetracker of the age. There was a deep desire to integrate development and evolution, but they lacked the necessary bridge of genetics, so Haeckel borrowed one, his theory of ontogenetic recapitulation. A theory that quickly went down in flames in the scientific community (jebus, Karl Ernst von Baer had eviscerated it 50 years before Haeckel resurrected it). We actually spent a fair amount of class time going over arguments for and against, and modern interpretations of phylotypy — it isn’t recapitulation, it’s convergence on a conserved network of global spatial genes that define the rough outlines of the vertebrate body plan.
Finally, I gave them a whirlwind tour of basic developmental stages of a few common vertebrate models: frog, fish, chick, and mouse. We’re going to talk quite a bit about early axis specification events in vertebrates (next week), and gastrulation (probably the week after), so I had to introduce them to the essential terminology and events. I think they can see the fundamental morphological events now — next, β-catenin and nodal and Nieuwkoop centers and all that fun stuff!