Sex and stress may be more closely linked than you might think. Gerald LeBlanc of North Carolina State University is intrigued by the relationship between environmental stress responses and reproduction. Reproduction in the tiny freshwater crustacean Daphnia is normally an asexual all-female affair. But a stressful environment can trigger a switch to sexual reproduction in these creatures, producing freeze- and desiccation-tolerant eggs with better chances of coping with a hostile environment. Exploring this relationship between sex and stress, LeBlanc and colleagues found that the same hormonal pathway is responsible for two common environmental stress responses in Daphnia (p. 15).
LeBlanc already knew that a hormone called methyl farnesoate causes Daphnia eggs to develop into males. He had also noticed that Daphnia exposed to this hormone turned a copper colour. `Then I received a serendipitous phone call from Thomas Gorr of Harvard Medical School' LeBlanc recalls, `who suggested that the animals might be turning red because the hormone was triggering production of the oxygen-transporter haemoglobin.' Puzzled by this apparent link between haemoglobin accumulation and male offspring production, LeBlanc and Gorr began a joint venture to see if the methyl farnesoate hormone was regulating both of these stress responses in the little crustacean.
`If the same hormonal signalling pathway regulates haemoglobin production and male offspring production' LeBlanc reasoned, `exposing Daphnia to the hormone should trigger both haemoglobin and male offspring production.'The team decided to examine the response of 11 Daphnia clones to the hormone; five of these could produce male offspring, but the other six could not produce males. Sure enough, after the hormone treatment, haemoglobin levels in the five male-producing clones shot up and all but one of the clones also began producing male offspring. Clearly, the hormone was triggering both haemoglobin and male offspring production in these clones. But would the non-male-producing clones also increase their haemoglobin and male offspring production in response to the hormone? The team found that five of the six non-male-producing clones did not produce male offspring or haemoglobin after hormone treatment. This suggests that these clones lack a common component of the signalling pathway, most likely a receptor for the hormone. The co-responsiveness of haemoglobin accumulation and male offspring production in these 11 clones indicates that the same hormonal pathway regulates these two processes. `Discovering that haemoglobin is hormonally regulated is exciting'says LeBlanc, `because we already know a lot about the haemoglobin gene that responds to methyl farnesoate. So we now have an array of genetic tools to study this new stress signalling pathway.'
But why are haemoglobin and male offspring production linked in this way?`Stressful environmental signals that tell the animals to switch to sex might coincidentally be correlated with low environmental oxygen levels' LeBlanc suggests, `for example, overcrowding may result in a drop in oxygen levels. When the environment has suddenly turned hostile, increased haemoglobin production to help the animals transport oxygen and male offspring production resulting in eggs with better survival chances might both come in handy.'