The XX Factor
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'The XX Factor': A fellowship enables a doctoral student's research to create a predominantly female --- and thus more lucrative --- flounder stock. --- By Dee Shore
In May, Luckenbach (below, at N.C. State's Fish Barn) won the World Aquaculture Society's award for best student oral presentation, when he demonstrated his Southern flounder research at the society's annual conference in Brazil. (Photo by Sheri D. Thomas)

ornate letter I t was a tale of sex, warm water, Southern flounder – and a remarkable development for U.S. aquaculture.

Speaking at the world’s largest annual aquaculture conference in May in Salvador, Brazil, doctoral student Adam Luckenbach explained how he and three College of Agriculture and Life Sciences scientists had developed a technique for manipulating water temperature to create all-female stocks of Southern flounder.

Because female flounder grow up to be three times larger than males, the breakthrough brings the team significantly closer to its ultimate goal — developing an economically viable flounder industry in the United States.

Given the high demand for flounder and the declining wild stocks, College researchers believe that flounder could become a high-value crop for North Carolina. Some say that within 10 years, flounder farming could generate five to 10 times the $3.5 million in annual income now produced by the state’s hybrid striped bass industry.

Working with Department of Zoology faculty members Russell Borski, Harry Daniels and John Godwin, Luckenbach learned how to influence the sex of Southern flounder by changing water temperature during the fish’s early development.

Whether a flounder becomes a male or a female is determined partly as it is in humans, with females having two X chromosomes and males having an X and a Y. But, at particular stages of the fish’s development, cooler or warmer water can turn an otherwise female flounder into a male, with functioning testes.

Knowing this, Luckenbach and his team set out to develop a method of ensuring all-female production. Luckenbach started by filling in gaps in the scientific literature, determining what flounder gonads look like microscopically, at what stage in life temperature would influence sexual development and the precise temperatures at which sex changes occur.

Over three years, he gained insights that led to a breakthrough in February. Within the span of a few minutes, Luckenbach irradiated mullet sperm, destroying the sex chromosome it would pass along, then used the sperm to fertilize a flounder egg. Then, he put the fertilized egg into ice-cold water for an hour – a process that “tricked” the offspring into keeping a second X chromosome present in the egg.

The process worked, producing offspring that will be raised in heated water so that the juvenile fish’s gonads will turn into testes. The sexually mature XX males will then fertilize eggs guaranteed to yield high-value, all XX females.

The research is one piece of a larger puzzle that the N.C. State University team has spent nearly a decade trying to piece together. Already, the team has figured out how to get the finicky Southern flounder to thrive and reproduce in captivity through the fingerling stage.

But the scientists still need to figure out the best ways to keep larger fish healthy through the grow-out stage, which lasts two years.

If they are successful, their findings could have significant economic and environmental implications, Luckenbach said. Because of overfishing in the 1980s, flounder populations along the East Coast are at historic lows. In North Carolina’s waters, the population dropped an estimated 32 percent in one decade.

Down the road, Luckenbach said, cultured flounder might be used to enhance wild stocks.

Perhaps more significantly, if the fish can be grown consistently on farms, then the pressure on wild stocks could be lessened. Meanwhile, farmers could gain a viable alternative to tobacco and other traditional agricultural commodities.

“ The state is interested in another agricultural business that’s high value, high demand,” he said. “And given the worldwide demand and the premium price paid for flounder, this could be one option.”

Related: Fellowship enhances
environmental studies

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