Sex change wipes out invasive species

Louis Buckley

Altering their chromosomes could help deal with fishy pests. NOAA

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.

Hormone-determined gender

In 2004, alerted to Florida's problems with invasive species, Juan Gutierrez, a bio-mathematician at Florida State University, constructed a mathematical model of a population in which males carry two different sex chromosomes (XY) and females are XX. In many species of fish, amphibians, and other animals, gender is determined not only by sex chromosomes, as it is in humans, but also by environmental conditions such as the presence of hormones, explains Gutierrez.

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'.

In Gutierrez's model, repeated introduction of YY females resulted 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. His calculations have been published in the Journal of Theoretical Biology.

But Gutierrez is cautious about trying out his ideas in the real world just yet. "More research is needed to better understand how sex reversal can be efficiently achieved in various potential target species," he says. "The approach is not a quick fix for invasive species, but rather a long-term solution that will require a committed effort over many years."

Non-GM

The idea of controlling an invasive population by manipulating its sex ratio is nothing new, says Gutierrez. 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."

The approach has numerous advantages, says Samuel Cotton of the University of Lausanne, Switzerland, who has reviewed Gutierrez's theory. "The method only affects the target species, so there is no probability of collateral ecological damage," he notes. "Also, it is reversible - if there are any unwelcome side-effects you can just stop adding YY females."

Gutierrez has joined forces with John Teem of the Florida Department of Agriculture to examine how the strategy might stop the spread of another invasive species - the apple snail - throughout the state's waterways. They are aiming to publish an advanced version of the model later this year, which will allow scientists and policy-makers to assess the suitability of the technique for specific cases.

But whether the issue will receive adequate support is far from certain. "The problem is big, expensive, and the government agencies in charge of it are under-funded," Gutierrez says. "The Program of Research on the Economics of Invasive Species Management had only US$4.9 million to spend between 2003 and 2006. One can only wonder why the agency in charge spends only $4.9 million in four years for a problem that according to conservative estimates costs the United States $200 billion per year."

1. Gutierrez, J. B. & Teem, J. L. J. Theor. Biol. 241, 333-341 (2006).
2. Cotton, S. & Wedekind, C. Trends Ecol. Evol. doi:10.1016/j.tree.2007.06.010 (2007).