Clock Zoo
clock | September 6, 2006
Second post in a series of five (from April 05, 2006):
In the previous two posts, here and here, I have mentioned how the discovery of circadian clocks in Cyanobacteria changed the way we think about the origin and evolution of circadian clocks. Quite soon after the initial discovery, the team from Carl Johnson's laboratory published two papers [1,2] describing a more direct test of adaptive function of circadian clocks in the Synechococcus elongatus.
Wild-type and various clock-mutants in Synechoccocus, when raised in isolation in light-dark cycles, have comparable reproductive rates.…
clock | September 5, 2006
First in a series of five posts on clocks in bacteria (from March 08, 2006)...
As I stated in the introductory post on this topic, it was thought for a long time that prokaryotes were incapable of generating circadian rhythms. When it was discovered, in 1994 [1], that one group of prokaryotes, the cyanobacteria, possess a circadian clock, the news was greeted with great excitement. This was the first definitive demonstration of a circadian clock in a bacterium (I intend to revisit the E.coli saga in a later post).
All three hypotheses for the origin of the circadian clock suppose that it…
clock | September 4, 2006The first in a series of posts on circadian clocks in microorganisms (from February 23, 2006)...
Many papers in chronobiology state that circadian clocks are ubiqutous. That has been a mantra since at least 1960. This suggests that most or all organisms on Earth possess biological clocks.
In the pioneering days of chronobiology, it was a common practice to go out in the woods and collect as many species as possible and document the existence of circadian rhythms. Technical limitations certainly influenced what kinds of organisms were usually tested.
Rhythms of locomotor activity are the…
clock | September 1, 2006I rarely write about biological rhythms outside of circadian range (e.g., circannual, circalunar, circatidal rhythms etc.), but if you liked this post on lunar rhythms in antlions, you will probably also like this little review of lunar rhythms in today's Nature:
Pull of the Moon:
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Studies of fiddler crabs, for example, have shown that even when kept in the lab under constant light and temperature, the animals are still most active at the times that the tide would be out. A similar internal 'circalunar' clock is thought to tick inside many animals, running in synchrony…
clock | August 18, 2006Chossat's Effect in humans and other animalsThis April 09, 2006 post places another paper of ours (Reference #17) within a broader context of physiology, behavior, ecology and evolution.
The paper was a result of a "communal" experiment in the lab, i.e., it was not included in anyone's Thesis. My advisor designed it and started the experiment with the first couple of birds. When I joined the lab, I did the experiment in an additional number of animals. When Chris joined the lab, he took over the project and did the rest of the lab work, including bringin in the idea for an additional…
clock | August 17, 2006
This is a summary of my 1999 paper, following in the footsteps of the work I described here two days ago. The work described in that earlier post was done surprisingly quickly - in about a year - so I decided to do some more for my Masters Thesis.
The obvious next thing to do was to expose the quail to T-cycles, i.e., non-24h cycles. This is some arcane circadiana, so please refer to the series of posts on entrainment from yesterday and the two posts on seasonality and photoperiodism posted this morning so you can follow the discussion below:
There were three big reasons for me to attempt…
clock | August 15, 2006
This post from March 27, 2006 starts with some of my old research and poses a new hypothesis.
The question of animal models
There are some very good reasons why much of biology is performed in just a handful of model organisms. Techniques get refined and the knowledge can grow incrementally until we can know quite a lot of nitty-gritty details about a lot of bioloigcal processes. One need not start from Square One with every new experiment with every new species. One should, of course, occasionally test how generalizable such findings are to other organisms, but the value of models is…
clock | August 15, 2006
One of the assumptions in the study of circadian organization is that, at the level of molecules and cells, all vertebrate (and perhaps all animal) clocks work in roughly the same way. The diversity of circadian properties is understood to be a higher-level property of interacting multicelular and multi-organ circadian systems: how the clocks receive environmental information, how the multiple pacemakers communicate and synchronize with each other, how they convey the temporal information to the peripheral clocks in all the other cells in the body, and how perpheral clocks generate…
clock | August 15, 2006
This post, from January 25, 2006, describes part of the Doctoral work of my lab-buddy Chris.
Mammals have only one circadian pacemaker - the suprachiasmatic nucleus (SCN). Apparently all the other cells in the body contain circadian clocks, too, but only the SCN drives all the overt rhythms. Without the SCN, there are no rhythms - the peripheral clocks either get out of phase with each other, or their clocks stop ticking altogether.
If you place various tissues in a dish, the SCN cycles indefinitely. All other tissues are capable of only a few oscillations in the absence of a daily signal…
clock | August 15, 2006
One of the important questions in the study of circadian organization is the way multiple clocks in the body communicate with each other in order to produce unified rhythmic output.
In the case of mammals, the two pacemakers are the left and the right suprachiasmatic nucleus (SCN). The tow nuclei are anatomically close to each other and have direct nerve connections between them, so it is not difficult to imagine how the two clocks manage to remain continuously coupled (syncronized) to each other and, together, produce a single output, thus synchronizing all the rhythms in the body.
In the…
clock | August 14, 2006Going into more and more detail, here is a February 11, 2005 post about the current knowledge about the circadian organization in my favourite animal - the Japanese quail.
Japanese quail (Coturnix coturnix japonica), also known as the Asian Migratory Quail, are gallinaceous birds from the family Phasianidae, until 1960s thought to be a subspecies of European migratory quail (Coturnix coturnix coturnix), but now considered to be a separate species, designated as Coturnix japonica. The breeding range of the wild population encompasses Siberia, Mongolia, northeastern China and Japan, while the…
clock | August 14, 2006
This post was originally written on February 11, 2005. Moving from relatively simple mammalian model to more complex systems.
I have previously described the basic properties of the circadian organization in mammals. Non-mammalian vertebrates (fish, amphibians, reptiles and birds) have more complex circadian systems than mammals. While the suprachiasmatic area remains a site of circadian pacemakers, it is, unlike in mammals, not the only such site.
The pineal organ, which in mammals is a purely secretory organ, is directly photosensitive in other vertebrates (with the exception of snakes)…
clock | August 14, 2006
This February 06, 2005 post describes the basic elements of the circadian system in mammals.
The principal mammalian circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The general area was first discovered in 1948 by Curt Richter who systematically lesioned a number of endocrine glands and brain areas in rats. The only time he saw an effect on circadian rhythms was when he lesioned a frontal part of hypothalamus (which is at the base of the brain) immediatelly above the optic chiasm (the spot where two optic nerves cross). Later studies in the 1970s…
clock | August 10, 2006A nice new study on ecological aspects of circadian rhythms:
To a tiny tadpole, life boils down to two basic missions: eat, and avoid being eaten. But there's a trade-off. The more a tadpole eats, the faster it grows big enough to transform into a frog; yet finding food requires being active, which ups the odds of becoming someone else's dinner.
Scientists have known that prey adjust their activity levels in response to predation risk, but new research by a University of Michigan graduate student shows that internal factors, such as biorhythms, temper their responses.
Michael Fraker, a…
clock | August 4, 2006We have recently covered interesting reproductive adaptations in mammals, birds, insects, flatworms, plants and protists. For the time being (until I lose inspiration) I'll try to leave cephalopod sex to the experts and the pretty flower sex to the chimp crew.
In the meantime, I want to cover another Kingdom - the mysterious world of Fungi. And what follows is not just a cute example of a wonderfully evolved reproductive strategy, and not just a way to couple together my two passions - clocks and sex - but also (at the very end), an opportunity to post some of my own hypotheses online.…
clock | July 17, 2006This post about the origin, evolution and adaptive fucntion of biological clocks originated as a paper for a class, in 1999 I believe. I reprinted it here in December 2004, as a third part of a four-part post. Later, I reposted it here.
III. Whence Clocks?
Origin, Evolution, and Adaptive Function of Biological Clocks
The old saw about the early bird just goes to show that the worm should have stayed in bed. (Heinlein 1973)
Now darkness falls.
Quail chirps.
What use Hawk eyes?
(Basho)
Local/temporary and global/universal environments. In the study of adaptive functions, usually the question…
clock | July 10, 2006This post, originally published on January 16, 2005, was modified from one of my written prelims questions from early 2000.
EVOLUTIONARY PHYSIOLOGY OF BIOLOGICAL CLOCKS
"Circadian clocks allow organisms to predict, instead of merely react to, cyclic (predictable) changes in the environment". A sentence similar to this one is the opening phrase of many a paper in the field of chronobiology. Besides becoming a truth by virtue of frequent repetition, such a statement appeals to common sense. It is difficult to imagine a universe in which it was not true. Yet, the data supporting the above…
clock | July 4, 2006Writing a chronobiology blog for a year and a half now has been quite a learning experience for me. I did not know how much I did not know (I am aware that most of my readers know even less, but still....). Thus, when I wrote about clocks in birds I was on my territory - this is the stuff I know first-hand and have probably read every paper in the field. The same goes for topics touching on seasonality and photoperiodism as my MS Thesis was on this topic. I feel equally at home when discussing evolution of clocks. I am also familiar with the clocks in some, but not all, arthropods. And…
clock | June 29, 2006This is in the bread-mold Neurospora crassa. It is unlikely to be universal. I expect to see the connection in some protists and fungi, perhaps in some animals. I am not so sure about plants, and I am pretty sure it is not like this in Cyanobacteria in which the cycle of cell division is independent from circadian timing:
Novel connection found between biological clock and cancer
Hanover, NH--Dartmouth Medical School geneticists have discovered that DNA damage resets the cellular circadian clock, suggesting links among circadian timing, the cycle of cell division, and the propensity for…
clock | June 27, 2006
The origin and early evolution of circadian clocks are far from clear. It is now widely believed that the clocks in cyanobacteria and the clocks in Eukarya evolved independently from each other. It is also possible that some Archaea possess clock - at least they have clock genes, thought to have arived there by lateral transfer from cyanobacteria.[continued under the fold]
It is not well known, though, if the clocks in major groups of Eukarya - Protista, Plants, Fungi and Animals - originated independently or out of a common ancestral clock. On one hand, the internal logic of the clock…