Chicks are the model system of choice for researchers interested in the effects of low oxygen, or hypoxia, on embryo development. But since birds'nests are usually above ground, getting enough oxygen is not high on a developing chick's list of concerns. Young American alligators, on the other hand, are much more likely to find themselves in an oxygen-starved nest, since crocodilian mothers lay their eggs underground. Dane Crossley at the University of California Irvine decided to see how developing alligators cope when oxygen levels plummet (p. 31).
Teaming up with Jordi Altimiras at Linköpings University in Sweden,Crossley began tracking down alligator eggs. Collecting eggs with wary mothers protecting their nests is a hazardous task, so Crossley got in touch with alligator expert Ruth Elsey at the Rockefeller Wildlife Refuge in Louisiana,who agreed to collect eggs from field nests for him. To ensure that his fragile cargo emerged unscathed from its journey to California, Crossley decided to travel with the eggs. He admits that he was so concerned for the youngsters' wellbeing that he even took some eggs on board as hand luggage, to the bemusement of other passengers.
Once the eggs had arrived safely at the Californian lab, the team incubated the eggs at three different oxygen levels: a normal 21% oxygen level and hypoxic 15% and 10% oxygen levels. They exposed the alligator embryos to a sudden drop in oxygen to see how the youngsters coped at four stages of development - at 60%, 70%, 80% and 90% of the normal incubation period of 72 days. Plunging the eggs into a low-oxygen chamber, the team measured the embryos' blood pressure and heart rate by making a small hole in the eggshell and placing a catheter into an artery. Between 70% and 80% of incubation, the normal-oxygen animals responded to plummeting oxygen levels with a dramatic drop in heart rate, and an increase in blood pressure once they were recovering back in comfortable oxygen levels. But the team were surprised to find that the heart rate and blood pressure of the embryos reared at hypoxic 10% oxygen levels were unaffected by a sudden exposure to low oxygen. So low-oxygen reared embryos were less responsive to sudden low-oxygen stress than normal-oxygen reared reptiles. `It was as if the hypoxic-reared alligators had got used to low oxygen levels, so their bodies just didn't respond to a sudden drop in oxygen' says Crossley. At 90% of incubation the team noticed two profound morphological changes in the embryos. Those reared in low oxygen were miniature versions of the normal-oxygen embryos. Even stranger was that the oxygen-starved alligators had enlarged hearts. `They hatched at the same time and were at the same developmental stage as normal-oxygen alligators, but they were just incredibly tiny with very big hearts' says Crossley.
Crossley suspects that a bigger heart and a blunted cardiovascular response to a sudden drop in oxygen levels might represent a repertoire enabling the youngsters to survive in hypoxic nests. But the mechanisms and genes involved in this response remain elusive for now.