EGGS SUFFER IN HEAT

Abstract

Art Woods is fascinated by tobacco hornworms. And he's luckier than most,he's seen them in their natural environment, the North American Sonoran desert, where they are far more intriguing than the dull insects reared in laboratories. Wondering how the insects extract all of the essential elements they require from their harsh environment, Woods has turned his attention to their eggs. He was puzzled by the problems insect eggs face supplying the developing embryo with adequate oxygen, while remaining sufficiently watertight to protect the egg from dehydration. Woods wondered whether the environmental temperature might affect oxygen diffusion into the egg and limit the developing embryo's access to oxygen at temperatures experienced by the embryos on a typical Sonoran day(p. 2267).But first, Woods needed to discover whether the developing embryos were sensitive to oxygen availability, as adults seem unaffected by varying oxygen levels. Maybe the embryos' development would be unaffected. Working with the team in the local chemistry workshop, Woods designed a Plexiglas block where he could control the partial pressure of oxygen in various egg-filled chambers while filming their development. Monitoring the eggs' development at oxygen concentrations ranging from 21% down to 9%, and temperatures from 22 up to 32°C, the results were dramatic. Woods watched the hatching events move through his Plexiglas box like a wave, with the warmest and most highly oxygenated eggs hatching first, while eggs incubated at lower oxygen concentrations and temperatures hatched later. But when he looked closely to see if there was an interaction between temperature and oxygen that affected the insects' development, he failed to find any. Would he see a strong interaction by looking at other metabolic indicators over a shorter period?Woods decided to measure the eggs' carbon dioxide production rates to see if temperature and oxygen supply might affect their metabolic rate. But it took another trip to the chemistry workshop to crack the problem of measuring the tiny amounts of carbon dioxide produced by clutches of 40 eggs. Woods was now ready measure the eggs' carbon dioxide production rates over oxygen concentrations ranging from 5–50%.This time he found a strong interaction between temperature and oxygen levels; the eggs' carbon dioxide production rates were far more sensitive to oxygen availability at higher temperatures. And when Woods raised the oxygen level to 50%, the egg's metabolic rate rose still further. It seemed that the oxygen supply within the egg was limited even at naturally occurring temperatures, and as the temperature rose, the limitation became worse.But would this be reflected in the amount of oxygen inside the egg's tissues? Woods measured the oxygen gradient within the eggs by gently inserting an oxygen electrode, and found that the oxygen concentrations dropped dramatically, plummeting to 7% of air saturated levels at the egg's core. `This is really neat, that such a small piece of tissue could have problems with oxygen supply' says Woods.Having found that tobacco hornworm's eggs appear to be oxygen limited,Woods is keen to investigate how the eggs respond to the natural temperature fluctuations they experience while incubating on leaves in the desert, and the challenges they face supplying sufficient oxygen to the embryo within.