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Gynogenesis: shortcut to superior catfish.

Imagine being able to accomplish 6 to 9 years of research in a half or a third the time.

That's what scientists in the ARS Catfish Genetic Research Unit at Stoneville, Mississippi, are doing.

Using a process called gynogenesis--for the first time in catfish-they're speeding up development of stocks of fish with an assortment of desirable traits. What they do is collect semen from selected male catfish and expose it to ultraviolet light, which destroys the DNA and sterilizes the spermatozoa but does not affect their motility.

Then, they obtain eggs from females that have been stimulated to spawn by behavioral interactions with nonsterile males cohabiting their fish tank. Fertilization of the eggs is performed externally by the scientists.

Since the spermatozoa used are sterile, they pass along no genetic information to the offspring.

But eggs fertilized with sterile sperm contain only half the genetic information, stored in chromosomes, that is necessary for normal development. So the embryos do not survive.

To restore a full complement of chromosomes, the Stoneville scientists treat the fertilized eggs with pressure shocks or exposure to hot or cold water temperatures. These treatments prevent the loss of a set of female chromosomes which would otherwise be eliminated during cell division to make room for insertion of the male chromosomes. Thus, the gynogenetic eggs acquire the normal amount of genetic material, but all of it is contributed by the female.

"This combination of genes," says reproductive physiologist Cheryl Goudie, "allows the expression of many harmful or lethal recessive genes that would normally be hidden, and it greatly reduces offspring survival. But the gynogenetic fertilization process also decreases genetic variability within those offspring that do survive, enabling us to more easily select and enhance a particular trait. So we can use gynogenesis to inbreed fish with desirable features faster than when we mate siblings generation after generation."

Although the techniques of gynogenetic reproduction take just a few minutes to perform, the fertilized eggs must still be allowed to hatch and the hatchlings to grow for 3 to 4 years, to reach sexual maturity and reproduce.

Even so, the combination of traits achieved in a single generation of gynogenetic offspring is comparable to the result of two or three generations of conventional inbreeding of mated siblings. Thus, researchers can, in effect, accomplish in one spawning season what would have taken as many as 9 years of breeding studies (three fish generations, each taking at least 3 years to mature and mate).

"When grown, the gynogenetic fish can be used as a brood stock to produce second-generation gynogenetic offspring that are all clones of the mother, and the process can be continued for as many generations as you want," Goudie says. "This exact genetic replication allows us to study environmental effects on catfish without having to factor in genetic variability."

Goudie and colleagues are using gynogenesis to produce families of catfish with selected desirable traits that include tolerance to poor water quality, disease resistance, more efficient feed conversion, and improved growth rate and body composition.

Once the researchers successfully reproduce selected traits in subsequent gynogenetic generations, they cross the inbred lines with other catfish strains that show additional desirable traits. They know the importance of restoring genetic variability--or hybrid vigor--within the species, to reduce its vulnerability to disease or other problems.

One serious drawback of gynogenetic catfish reproduction has been that all of the offspring produced are females. Until recently, the only way to propagate gynogenetic lines has been by crossing them with nongynogenetic males.

Now, Goudie and the other Stoneville scientists researching sex-control mechanisms in catfish have developed females with a male sex genotype. These females produce both male and female gynogenetic offspring, thus allowing crossbreeding to occur within and among gynogenetic lines.

An added bonus is that gynogenetic males, when mated with normal females, produce only male offspring. Since male catfish grow faster than females, commercial farms using these gynogenetic "supermales" for broodstock would have an economic advantage over farms using males that produce mixed-sex offspring.

New, improved catfish lines developed through gynogenesis should eventually be available to aquaculturists--and sooner than if conventional breeding programs were used. "But we're still talking years down the road," says Goudie.

"We are currently conducting studies to optimize sperm irradiation and the timing of egg shock treatments, to improve our yields of gynogenetic offspring," she says. "We hope to have, within a few years, the knowledge needed about induced gynogenesis to develop domesticated catfish lines with the traits most desired by commercial producers."--By Marcie Gerrietts, formerly with ARS.

Cheryl Goudie is in the USDAARS Catfish Genetics Research Unit, Jamie Whirten Delta States Research Center, Stonerifle, MS 38776; phone (601) 686-5460, fax (601) 686-9406.
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Author:Gerrietts, Marcie
Publication:Agricultural Research
Date:Nov 1, 1993
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