Once scientists began studying the impact of global warming on everything
from tourism to asthma, it was only a matter of time before they got around to
sex. Now two biologists at Spain's Higher Council for Scientific Research (CSIC)
have done just that, at least when it comes to fish.
You may have missed it in biology class, but in some finned species like the
Atlantic silverside — as well as in many reptiles — sex is determined not by
genetics, but by temperature: the undifferentiated embryo develops testes or
ovaries on the basis of whichever option conveys evolutionary advantages for
that particular environment. Now, in a study published in the July 30 edition of
the scientific journal Public Library of Science (PLoS), Natalia Ospina-Alvarez
and Francesc Piferrer have gone a little further toward explaining how that
mechanism works. In laboratory tests, they have demonstrated that higher water
temperatures result in more male fish.
"We found that in fish that do have temperature-dependent sex determination ["or
TSD"], a rise in water temperature of just 1.5 degrees C can change the male to
female ratio from 1:1 to 3:1," says Piferrer, the study's co-author. In
especially sensitive fish, a greater increase can throw the balance even further
out of whack. Ospina-Alvarez and Piferrer have found that in the South American
pejerrey, for example, an increase of 4 degrees C can result in a population
that is 98% male.
What makes these findings especially troubling, of course, is that the
International Panel on Climate Change predicts that ocean water temperatures are
likely to rise by 1.5 degrees over the course of this century, and may even go
up a few degrees more. "If climate change really does result in a rise of 4
degrees, which is the maximum the IPCC predicts, and if species can't adapt in
time or migrate, then in the most sensitive cases of TSD, we're looking at
extinction," says Piferrer.
Most research into fish sex determination has been done in the lab (for obvious
reasons), but the pejerrey is one of the few species that scientists have been
able to study in the field. And those studies have revealed that already in the
species, the proportion of males to females is skewed. "It could be because of
chemical pollution, or it could be because of climate change. We don't know,"
cautions Piferrer. "But the field data matches our predictions."
At this stage, it is hard to tell what these results bode for already declining
fish populations around the world. Of an estimated 33,000 piscatorial species,
only 5,000 have had their sex-determination mechanism affirmed. But the study by
the two CSIC scientists also suggests that the percentage of TSD fish is lower
than previously believed. In tests of 59 species believed to be reproductively
sensitive to temperature, only 40 proved to be true TSDs.
That would be good news in this grim era of climate change if it weren't for one
factor: even genotypic sex determination can be affected by anomalous
conditions, including anomalous temperature. "Basically, if you freeze it or
cook it enough," says Piferrer, "you can get whatever sex you want."