Thursday, June 9, 2011
This is a story about sleep. It is also a story about blindness. Do the blind sleep more or less than the sighted? Is that a result of having no sight, a result of the environment, a genetically controlled characteristic, or a little bit of everything? Today's post involves a study about sleep in fish.
Astyanax mexicanus is a characin fish commonly known as the Mexican cave fish or Mexican tetra, and it occupies a wide variety of freshwater habitats within both it's native range and naturalized range. They are typically carnivorous, but has also been reported to be omnivorous in some parts of it's range. The species has a characteristic adipose fin, a strongly compressed body, a relatively small number of scales on the lateral line, and is approximately 12 cm (4.7in) long. Most notably, it has two forms: the normal form and the blind cave form. The blind cave form is albino with no eyes. The embryos are sightless and as the animal ages the organs decay and the eyes are scaled over. They live in the freshwater pools or streams deep within caves where no light penetrates. As such, they have lost their eyes, but are able to navigate using their lateral lines, which are highly sensitive to fluctuating water pressure. This is a pretty common pattern that you see in cave dwellers worldwide. They tend to converge on a suite of traits including eyelessness, loss of pigmentation, metabolic changes, and changes in feeding behavior. This type of convergence on traits in unrelated lineages is called convergent evolution.
A new study published in Current Biology takes a look at both blind and normal A. mexicanus populations. There are numerous cave populations known that are largely independent in their origins. This makes this fish ideal for studying the genetic basis of convergent evolution. Additionally, ecological conditions change as you go from surface to cave and this change is likely to have an impact on the fishes' sleep. This makes the fish good for studying the variability in patterns of sleep.
The researchers reared fish collected from isolated, blind cave populations in Pachon, Tinaja, and Molino as well as surface sighted fish. The Pachon and Tinaja populations are derived from the same ancestral stock but are geographically distant and hydrologically isolated by surface and subsurface drainage divides. The Molino population is derived from a different stock than the Pachon and Tinaja and is also isolated. Hybridization experiments were conducted where each population of fish was mated to each of the other populations of fish to create F1 hybrids (first cross/generation). They also reared an F2 (the following generation, mated F1's), and backcross (a hybrid to the original/wildtype) hybrids. Then the researchers tested whether or not a fish could see (just having eyes doesn't mean you can see out of them). To do this they immobilized a fry (recently hatched fish), placed them in a cylinder that flashed alternating patterns of black and white stripes, and watched to see if their eyes moved according to the color divisions. Eye movement equals sight. Gene assays were run to assess the complemation of the genes between populations. Then they developed an assay to characterize sleep in the F1, F2, and backcross hybrids.
In the inital crosses they found that, in some cases, the F1's could see. They found that the genetic mutations causing blindness are different in different lineages of fishes. So, why the restoration of sight as quickly as one generation? That is because populations from different caves are blind for different reasons. That means that in each population a different set of genes is nonfunctional, causing the blindness. Even though the fish were blind they still had basic functional visual systems, they had just been deactivated. The activated genes in the normal fish were able to overcome the inactivated genes of the blind fish. Interestingly, this relationship was stronger in populations that were more geographically distant from one another. The more distant the populations the less overlap in blindness-causing genes. The genetics results showed that the three cave populations also converged on the phenotype of reduced sleep.
So they then conducted a few sleep experiments:
1. They tested the threshold of arousal after inactivity they measure the responsiveness of a fish to repeated mechanical stimulus. Basically, they wait until the fish falls asleep (they stop moving, sink to the bottom of the tank, and drop their tail), wait a certain period of time, and they try to wake them up by tapping on the tank.
2. They tested if the fish were really sleeping by depriving them of sleep for one night and measuring their level of activity the next day.
3. They tested the day and night cycles by simulating them in alternating 12-hour light and dark periods.
4. From their breeding experiments, they looked at the sleep duration and cycles of the hybrids. This would give a genetic link to the sleep habits.
In the first, wake-the-fish-up, experiment they found that both surface and cave fry transitioned toward a higher wake up threshold as the period of inactivity increased. Once the fish had been inactive for 60 seconds it was in a different state entirely (it was sleeping) and it took more taps on the tank to wake them up. The second, keep-the-fish-awake, experiment found that all of the sleep deprived fish were less active the next day, and that depriving a fish of sleep is a good way to induce subsequent sleep behavior. A sleep rebound, if you will. The third, lights-on-lights-off-cycle, experiment found that the three populations of cave fish slept significantly less (110-250 min/day) than the surface fish (800 min/day). This is probably because the cave fish are in dark environments all the time while on the surface the light and dark cycles are tied to food availability and predator activity. So surface fish have a greater need to be attuned to light/dark cycles than do cave fish. The fourth, hybrid-sleep, experiment showed that the F1's slept like the cave fish. This indicates that the gene for less sleep is dominant. The F2 generation showed an intermediate sleep behaviors. Put these two results together and it suggests that sleep is controlled by a few specific genes.
Overall, a pretty cool experiment. Kinda reminds me of how I sleep. Which, ultimately was the purpose of the study - to equate fish sleep patterns and genetic control to human patterns and genes.
Yawn. Kinda makes me want to take a nap.
Here is the paper:
Duboué, Eric R., Alex C. Keene, Richard L. Borowsky. (2011) Evolutionary Convergence on Sleep Loss in Cavefish Populations. Current Biology: 21(8), 671-676. (DOI: 10.1016/j.cub.2011.03.020)
And here are a few story links:
(image via the Wired story linked above)