development
pharyngula | November 19, 2007Scooped! I've long adored vulvas, having written a few things on how to develop a vulva and how to evolve a vulva, so I'm a little peeved that this upstart at scienceblogs, Greg Laden, has written up a recent story on nematode vulva evolution before me. Aaargh, all vulvas must be mine! I've just got too much other work stacked in front of me.
I may have to revisit this story later, though. I made a quick skim of the paper and don't see quite how they arrived at their conclusion, that vulva evolution is dominated by selective events rather than chance. I can't say they're wrong, but I'm going…
pharyngula | November 6, 2007
What do you do when a child is born with ischiopagus?
Sell her to the circus?
Turn her into an object of religious veneration?
Try surgery to correct the condition as much as possible?
This little girl born with six legs and two arms had the option of all three; she's currently being operated on to remove four of the limbs. I don't think it is an easy decision, except for the fact that her condition is messed up enough that she's not likely to survive to adulthood without the surgery. On the complicating side, the operation costs £100,000, has substantial risk of death or paralysis,…
pharyngula | November 5, 2007Perhaps you haven't noticed, but we've got a serial spammer in the comments. This twit, calling himself Peter Moore (also known as Ken DeMyer, or Kdbuffalo, as he was known on Wikipedia before being banned there), is repeating himself over and over again, asking the same stupid question, never satisfied with any answer anyone gives him. Forty nine insipid comments in three days is enough.
I will answer him one last time. Any further attempt to spam multiple comment threads with his demands (and this alone makes him an ass: an incompetent, unqualified hack like Moore is in no position to make…
pharyngula | October 18, 2007My latest column for Seed is now available online. It's an abbreviated summary of how vertebrates make segments (so it's illustrated with a fly…), with special emphasis on the global and clocklike mechanism we use.
pharyngula | October 8, 2007I've known for years that this was going to happen: Mario Capecchi, Oliver Smithies and Briton Martin Evans have won the Nobel Prize in Medicine for their work on targeted gene mutations. If you're interested in what kinds of work they've done, I described one paper on Hox regulatory evolution, and this work on the evolution of the Hox code wouldn't have been possible without their knockout techniques.
purepedantry | October 8, 2007Functional MRI (fMRI) is a very useful technique, but it lacks in resolution making some systems difficult to study. Adams et al. show in a study of ocular dominance columns in humans why good old staining is still useful when we reach the limits of imaging.
Ocular Dominance Columns
Ocular dominance columns are areas in the primary visual cortex (V1) of mammals that show a preference in activity to one eye over the other. (I think this applies to reptiles and amphibians as well, but I don't know that for sure).
Here is a brief schematic of the visual system. The primary visual cortex is…
pharyngula | October 7, 2007
I've written a long introduction to the work I'm about to describe, but here's the short summary: the parts of organisms are interlinked by what has historically been called laws of correlation, which are basically sets of rules that define the relationship between different characters. An individual attribute is not independent of all others: vary one feature, and as Darwin said, "other modifications, often of the most unexpected nature, will ensue".
Now here's a beautiful example: the regulation of the growth of mammalian molars. Teeth have long been a useful tool in systematics—especially…
pharyngula | October 6, 2007Cuvier, and his British counterpart, Richard Owen, had an argument against evolution that you don't hear very often anymore. Cuvier called it the laws of correlation, and it was the idea that organisms were fixed and integrated wholes in which every character had a predetermined value set by all the other characters present.
In a word, the form of the tooth involves that of the condyle; that of the shoulder-blade; that of the claws: just as the equation of a curve involves all its properties. And just as by taking each property separately, and making it the base of a separate equation, we…
pharyngula | October 1, 2007
All of us mammals have pretty much the same set of genes, yet obviously there have to be some significant differences to differentiate a man from a mouse. What we currently think is a major source of morphological diversity is in the cis regulatory regions; that is, stretches of DNA outside the actual coding region of the gene that are responsible for switching the gene on and off. We might all have hair, but where we differ is when and where mice and men grow it on their bodies, and that is under the control of these regulatory elements.
A new paper by Fondon and Garner suggests that there…
pharyngula | September 23, 2007
One concept that is sometimes used in developmental biology is the idea of the "master control gene" or "master switch" — a single gene whose expression is both necessary and sufficient to trigger activation of many other genes in a coordinated fashion, leading to the development of a specific tissue or organ. It's a handy concept on which to hang a discussion of transcription factors, but it may actually be of rather limited utility in the real world of molecular genetics: there don't seem to be a lot of examples of master control genes out there! Pax-6 is the obvious one, a gene that…
pharyngula | September 17, 2007
The Hox genes are a set of transcription factors that exhibit an unusual property: they provide a glimpse of one way that gene expression is translated into metazoan morphology. For the most part, the genome seems to be a welter of various genes scattered about almost randomly, with no order present in their arrangement on a chromosome — the order only becomes apparent in their expression through the process of development. The Hox genes, on the other hand, seem like an island of comprehensible structure. These are all genes that specify segment identity — whether a segment of the embryo…
pharyngula | September 8, 2007I got some email today with lots of constructive suggestions (See? Not all my email is evil!) for how we ought to change the education of biology students — such as by giving them a foundation in the history and philosophy of our science, using creationist arguments as bad examples so the students can see the errors for themselves, etc. — and it was absolutely brilliant, even the parts where he disagreed with some things I'd written before. Best email ever!
Of course, what helped is that I spent my summer "vacation" putting together a new freshman first semester course for biology majors that…
pharyngula | September 6, 2007
When we look at the face of another person, we can recognize specific features that have familial resemblances. In my family, for instance, I can recognize a "Myers nose" that my grandmother and my father and some of my siblings and kids have, and it's different than my wife's or my mother's nose. These are subtle differences in shape—a bit of a curve, a knob, a seam—and their inheritance suggests that these differences are specified somehow in the DNA. If you think about it, though…how can whether the profile of a nose is straight or curved be encoded in a linear stretch of nucleotides? The…
pharyngula | August 24, 2007
Some of you may have never seen an arthropod embryo (or any embryo, for that matter). You're missing something: embryos are gorgeous and dynamic and just all around wonderful, so let's correct that lack. Here are two photographs of an insect and a spider embryo. The one on the left is a grasshopper, Schistocerca nitens at about a third of the way through development; the one on the right is Achaearanea tepidariorum. Both are lying on their backs, or dorsal side, with their legs wiggling up towards you.
There are differences in the photographic technique — one is an SEM, the other is a DAPI-…
pharyngula | August 21, 2007Yesterday's discussion of future biological advances that will piss off the religious right had me thinking about other innovations that I expect will happen within a few decades that might just cause wingnuts to freak out. First thing to come to mind is that it will be something to do with reproduction, of course, and it will scramble gender roles and expectations…so, how about modifying men to bear children? It sounds feasible to me. Zygotes are aggressive little parasites that will implant just about anywhere in the coelom — it's why ectopic pregnancies are a serious problem — so all we…
pharyngula | August 17, 2007
I just finished Sean B. Carroll's Endless Forms Most Beautiful: The New Science of Evo-Devo the other day, and I must confess: I was initially a bit disappointed. It has a few weaknesses. For one, I didn't learn anything new from it; I had already read just about everything mentioned in the book in the original papers. It also takes a very conservative view of evolutionary theory, and doesn't mention any of the more radical ideas that you find bubbling up on just about every page of Mary Jane West-Eberhard's big book. One chapter, the tenth, really didn't fit in well with the rest—the whole…
pharyngula | August 10, 2007Here is an excellent article on the biology of sexual orientation. We all know this is a contentious issue — are we born with an orientation, or is it a 'choice' that people make? — and the article just lays the facts out for us and points out some of the lacunae in our knowledge.
First, I'll confess to my own position on that nature-nurture debate: it's both and it's neither, and the argument is misplaced. There is no template on the Y chromosome that triggers a sexual response when Pamela Anderson enters the visual field, but there almost certainly are general predispositions that are a…
purepedantry | August 6, 2007It's mad, I tell you, madddd! Mad scientists these days. Always going around saying, "Hey, you know how that animal could be better? If it had another head. Muahahaha!"
Anyway, the (possibly mad) scientists Wolfgang Jakob and Bernd Schierwater wanted to know more about the genes that determine the body plan of multicellular organisms. In mammals, these genes are called Hox genes, and in organisms that have circular symmetry like jellyfish they are called Cnox genes. We know that these genes broadly pattern the body plan of multicellular organisms because in a variety of settings if you…
pharyngula | August 1, 2007Since I was asked what a cnidarian "head" is in reference to this work on multi-headed cnidarians, I'll answer. In short, they don't have one.
Longer answer: the paper in PLoS describes a procedure for generating homeotic mutations in cnidarians by manipulating the expression of Cnox genes in hydrozoans. Knock outs of various cnidarian Hox-like genes and the medusae develop extra manubriae, or the tentacled part at the oral end of the animal, which the authors colloquially call "heads" (and they do usually put the word in quotes). These structures aren't homologous to the things we consider…
pharyngula | July 31, 2007This is a short video clip of myotome formation in a zebrafish embryo — it's the subject of an upcoming column in Seed, so I'm putting a short visual aid here.
You can Download the Quicktime movie (620K), or you can watch it via YouTube. Watch closely, it's short and it flies by!
If you're totally mystified still, let me orient you. Here's a whole zebrafish embryo. There's a large yolk filling most of the center of the image, and the embryo itself arcs along the dorsal side, stretching from where I've labeled the eye to the tailbud.
Along the length of the trunk, you might be able to see…