One of my favorite signal transduction pathways (what? You didn’t know that true nerds had favorite molecular pathways?) is the one mediated by the receptor Notch. Notch is one of those genes in the metazoan toolkit that keeps popping up in all kinds of different contexts—it’s the adjustable wrench of the toolbox, something that handles a general problem very well and therefore gets reused over and over again, and the list of places where it is expressed in Drosophila is impressive.
Oh happy day, the Sea Urchin Genome Project has reached fruition with the publication of the full sequence in last week’s issue of Science. This news has been all over the web, I know, so I’m late in getting my two cents in, but hey, I had a busy weekend, and and I had to spend a fair amount of time actually reading the papers. They didn’t just publish one mega-paper, but they had a whole section on Strongylocentrotus purpuratus, with a genomics mega-paper and articles on ecology and paleogenomics and the immune system and the transcriptome, and even a big poster of highlights of sea urchin research (but strangely, very little on echinoderm development). It was a good soaking in echinodermiana.
This is a gross violation of my expectations: the Halloween edition of The Synapse doesn’t even once crack a zombie joke. I was terribly disappointed.
At least it does serve up a nice platter of brains.
How we sense the world has, ultimately, a cellular and molecular basis. We have these big brains that do amazingly sophisticated processing to interpret the flood of sensory information pouring in through our eyes, our skin, our ears, our noses…but when it gets right down to it, the proximate cause is the arrival of some chemical or mechanical or energetic stimulus at a cell, which then transforms the impact of the external world into ionic and electrical and chemical changes. This is a process called sensory signaling, or sensory signal transduction.
While we have multiple sensory modalities, with thousands of different specificities, many of them have a common core. We detect both light and odor (and our cells also sense neurotransmitters) with similar proteins: they use a family of G-protein-linked receptors. What that means is that the sensory stimulus is received by a receptor molecule specific for that stimulus, which then actives a G-protein on the intracellular side of the cell membrane, which in turn activates an effector enzyme that modifies the concentration of second messenger molecules in the cell. Receptors vary—you have a different receptor for each molecule you can smell. The effector enzymes vary—it can be adenylate cyclase, which changes the levels of cyclic AMP, or it can be phospholipase C, which generates other signalling molecules, DAG and IP3. The G-protein that links receptor and effector is the common element that unites a whole battery of senses. The evolutionary roots of our ability to see light and taste sugar are all tied together.
We are all familiar with the idea that there are strikingly different kinds of eyes in animals: insects have compound eyes with multiple facets, while we vertebrates have simple lens eyes. It seems like a simple evolutionary distinction, with arthropods exhibiting one pattern and vertebrates another, but the story isn’t as clean and simple as all that. Protostomes exhibit a variety of different kinds of eyes, leading to the suggestion that eyes have evolved independently many times; in addition, eyes differ in more than just their apparent organization, and there are some significant differences at the molecular level between our photoreceptors and arthropod photoreceptors. It’s all very confusing.
There has been some recent press (see also this press release from the EMBL) about research on a particular animal model, the polychaete marine worm, Platynereis dumerilii, that is resolving the confusion. The short answer is that there are fundamentally two different kinds of eyes based on the biology of the cell types, and our common bilaterian ancestor had both—and the diversity arose in elaborations on those two types.
Everyone knows the story of Konrad Lorenz and his goslings, right? It was a demonstration of imprinting: when young animals are exposed to a stimulus at a critical time, they can fix on it; Lorenz studied this phenomenon in geese, which if they saw him shortly after hatching, would treat him like their mother, following him around on his walks. Similarly, many animals seem to experience sexual imprinting, where they acquire the sexual preferences that will be expressed later on.
I just ran across a charming short letter about imprinting in cephalopods, and somehow the story seems so appropriate. Imprint a young, freshly hatched cuttlefish on something, and they don’t treat it like Mom, and they don’t later want to mate with it—they want to eat it. Lorenz was lucky he was working on birds rather than cephalopods.
The experiment is straightforward. Cuttlefish normally prefer to eat shrimp over crab. If, the day after hatching, small crab are put in the tank with the hatchlings for at least two hours, and then removed (the crabs are not eaten), then 3 days later when tested again, the cuttlefish will prefer to kill and eat crabs over shrimp. The procedure is very specific: they have to be exposed to crab for at least two hours, within 2 hours after sunrise on their first day after hatching.
The paper has a good, succinct description of why many animals would have this mechanism:
Precocial animals, like domestic
chicks and cuttlefish, which are
independent within hours of hatch
or birth and which receive no
posthatch parental care have
two options for acquisition of
information: bring it into the world
with you (unlearned preferences
for food, sexual partners and so on)
or pick up the information as you go
(trial and error learning). Imprinting
allows something in between:
a certain degree of flexibility in
response, useful for learning
information for which the timing is
likely to be predictable—food
seen in first few hours of life,
sibling/parents seen during
juvenile stages—but in which
specifying the exact details of
the experience is not useful.
An evil man could think of many nefarious things to do with this bit of information, I think.
Healy SD (2006)Imprinting: seeing food and eating it. Curr Biol.16(13):R501-502.
I’m a little surprised at the convergence of interest in this news report of a conserved mechanism of organizing the nervous system—I’ve gotten a half-dozen requests to explain what it all means. Is there a rising consciousness about evo-devo issues? What’s caused the sudden focus on this one paper?
It doesn’t really matter, I suppose. It’s an interesting observation about how both arthropods and vertebrates seem to partition regions along the dorso-ventral axis of the nervous system using exactly the same set of molecules, a remarkable degree of similarity that supports the idea of a common origin. Gradients of a molecule called Bmp may be the primitive mechanism for establishing dorso-ventral polarity in animals.
I was reading a review paper that was frustrating because I wanted to know more—it’s on the evolution of complex brains, and briefly summarizes some of the current confusion about what, exactly, is involved in building a brain with complex problem solving ability. It’s not as simple as “size matters”—we have to jigger the formulae a fair bit to take into account brain:body size ratios, for instance, to get humans to come out on top, and maybe bulk is an inaccurate proxy for more significant matters, such as the number of synapses and nerve conduction velocities.
There’s also a growing amount of literature that takes genomic approaches, searching for sequences that show the signatures of selection, and plucking those out for analysis. There have been some provocative results from that kind of work, finding some candidate genes like ASPM, but another of the lessons of that kind of work seems to be that evolution has been working harder on our testis-specific genes than on our brains.
The encouraging part of the paper is that the authors advocate expanding our search for the correlates of intelligence with another group of organisms with a reputation for big brains, but brains that have evolved independently of vertebrates’. You know what I’m talking about: cephalopods!
The Cephalopoda are an ancient group of mollusks originating in the late Cambrian. Ancestors of modern coleoid cephalopods (octopus and squid) diverged from the externally-shelled nautiloids in the Ordovician, with approximately 600 million years of separate evolution between the cephalopod and the vertebrate lineages. The evolution of modern coleoids has been strongly influenced by competition and predatory pressures from fish, to a degree that the behavior of squid and octopus are more akin to that of fast-moving aquatic vertebrates than to other mollusks. Squid and octopuses are agile and active animals with sophisticated sensory and motor capabilities. Their central nervous systems are much larger than those of other mollusks, with the main ganglia fused into a brain that surrounds the esophagus with additional lateral optic lobes. The number of neurons in an adult cephalopod brain can reach 200 million, approximately four orders of magnitude higher than the 20-30,000 neurons found in model mollusks such as Aplysia or Lymnaea. Cephalopods exhibit sophisticated behaviors a number of studies have presented evidence for diverse modes of learning and memory in Octopus and cuttlefish models. This learning capacity is reflected in a sophisticated circuitry of neural networks in the cephalopod nervous system. Moreover, electrophysiological studies have revealed vertebrate-like properties in the cephalopod brain, such as compound field potentials and long-term potentiation. Thus cephalopods exhibit all the attributes of complex nervous systems on the anatomical, cellular, functional and behavioral levels.
Unfortunately, the purpose of the paper is to highlight an unfortunate deficiency in our modern research program: there is no cephalopod genome project. The closest thing to it is an effort to sequence the genome of another mollusc, Aplysia, which is a very good thing—Aplysia is a famous and indispensable subject of much research in learning and memory—but it’s no squid. The authors are advocating additional work on another animal, one with a more elaborate brain.
A parallel effort on a well-studied octopus or squid should provide insights on the evolutionary processes that allowed development of the sophisticated cephalopod nervous system. For example, have cephalopods undergone accelerated evolution in specific nervous system genes, as has been suggested for primates? Have specific gene families undergone expansion in the cephalopod lineage and are these expressed in the nervous system? Are there clear parallels in accelerated evolution, gene family expansion, and other evolutionary processes between cephalopods and vertebrates? Answers to these and related questions will provide useful perspectives for evaluation of the processes thought to be involved in the evolution of the vertebrate brain.
I’m all for it—let’s see a Euprymna genome project!
Jaaro H, Fainzilber M (2006) Building complex brains—missing pieces in an evolutionary puzzle. Brain Behav Evol 68(3):191-195.
These things are just sneaking up on me today: the Synapse #5 is out.
Are there any more carnivals lurking out there today?
Let’s start the week with another open thread, and a few carnivals.
