Scubaism

Gender-bending chemicals could provide a new way to combat invasive species, say researchers. Originally conceived as a cure for the enormous populations of Asian carp and tilapia plaguing the Mississippi River, scientists now think the approach could be used to battle unwelcome crustaceans, molluscs, fish, amphibians and reptiles around the world.

Invasions of exotic species are thought to be second only to habitat destruction as a threat to global biodiversity. The traditional approach to dealing with these interlopers has been to introduce a known predator and let nature take its course. But this has led to numerous disasters — for example, cane toads swamped Australia after being introduced to control the cane beetles blighting the country’s sugar crop.

In Florida, tilapia were deliberately introduced to control an aquatic weed, Hydrilla, that has been choking US rivers since the 1960s. Two species of snail were also introduced at a later date by the authorities, says Gutierrez, but neither they nor the tilapia chose to feed on Hydrilla, both preferring native species to the invader.

By exposing genetic males to female hormones, or vice versa, it is therefore possible to create a male that is genetically XX, or a female that is XY or even YY. Such individuals, with the genetics of one sex but the physical characteristics of the other, are referred to as carriers of ‘Trojan sex chromosomes’.

Repeated introduction of YY females would result in an extremely male-dominated population, as all offspring produced from meetings between males (XY) and YY females are male, and many more males are born in subsequent generations. With fewer and fewer females around, the birth rate declined and finally ground to a halt, and the population was extinct within just a few decades.

The idea of controlling an invasive population by manipulating its sex ratio is nothing new, says Juan Gutierrez, a bio-mathematician at Florida State University. However, previous plans required using transgenic organisms, which is undesirable as it carries the risk that the genes will ‘escape’ into the wild. Using hormones gets round this problem. “With our technique the animals are not genetically modified,” explains Gutierrez. “We’re not introducing new genes — it’s very different.”