TRIPLOIDY IN CHANNEL CATFISH, ICTALURUS PUNCTATUS
Abstract
Channel catfish, Ictalurus punctatus, are one of the most widely cultured fish species for sport fishing and food purposes. Much research has been conducted on rearing, stocking, nutrition, and feeding, but very little research has been directed toward genetic improvements. This study describes a lymphocyte culture method for chromosome analyses and the experimental induction of triploidy by cold-shocking fertilized eggs. Eggs were handstripped from female channel catfish that had been induced to ovulate with injections of carp pituitary extract (4.4 mg per kilogram of body weight). Eggs were fertilized with sperm from mascerated testes of donor male and cold-shocked at 5 C for 1.0, 1.5, 2.0, and 3.0 hours beginning 5 minutes after fertilization. All cold shocks longer than 1.0 hour resulted in 100% mortality, while control eggs and 1.0 hour cold-shocked eggs had 89% and 79% hatching success, respectively. Counts of chromosomes from cultured lymphocytes and mascerated kidney tissue showed all control fish to be diploid (2n = 58). In the 1.0 hour cold-shock group, the incidence of triploidy (3n = 87) was 100%. Minor axis, major axis, and volume measurements of erythrocyte nuclei were compared by discriminant analysis to determine which variable best predicted ploidy levels. Predicted correct classification percentages for minor axis, volume, major axis, and all avariables combined were 81.33%, 86.21%, 92.36%, and 92.65%, respectively. Minor axis and volume measurements were not as accurate in predicting ploidy levels because nuclear shape changed as the ploidy level changed from the diploid to triploid state. Mean major axis measurements used alone classified 92.36% of the fish correctly with a discriminant level of 4.726 (mu)m. Surviving diploid and triploid channel catfish were reared indoors in circular fiberglass tanks connected to a common revolving plate biofilter. Triploids were found to be significantly larger than diploids at 8 months of age and older. Triploid male and female channel catfish were also found to have small gonads with altered histological features indicating sterility. The sterile condition in triploids may be responsible for better feed efficiency, and may provide greater profits in commercial catfish culture.
Degree
Ph.D.
Subject Area
Forestry
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