Allotriploidy increases sterility in the hybrid between Clarias macrocephalus and Clarias gariepinus

Abstract

Hybrids between female Thai walking catfish ( Clarias macrocephalus) and male African catfish ( Clarias gariepinus) are capable of reproducing, and this has led to introgression of genes from the paternal species into natural populations of the native C. macrocephalus. Therefore this study was conducted aiming at producing sterile triploid hybrids by means of cold shock. Eggs from C. macrocephalus were fertilized with sperm from C. gariepinus and were subsequently immersed in cold water (7°C) starting 4.5 min after water activation of the eggs, and lasting for 14 min. The cold shock induced 72%, 74% and 88% triploid fish with 44–49% hatching success compared to the control value of 69–78%. At 28 weeks old body weight of the triploid fish was smaller than the diploid counterparts. The frequencies of retarded gonads and ovotestes were smaller in diploid as compared to the triploid hybrids. The average gonado-somatic index (GSI) of female diploid F 1 hybrids was significantly higher than those of the triploids whereas male GSI of the two groups were not statistically different. Fifty percents of the diploid testes produced a few spermatozoa while only 15.8% of the triploid testes did. Likewise larger percentage of the female diploid hybrids (64.3%) had well-developed ovaries marked with vitellogenic oocytes as compared to the triploids (21%). At 44-week-old, although the triploid and diploid females showed similar response to hormone injection, the triploids produced significantly less eggs than the diploids. Both diploid and triploid eggs were capable to fertilize sperm of C. macrocephalus. However, all fry from the triploid F 1 hybrids died at yolk absorption, whereas approximately 80% of the fry from diploid F 1 hybrids survived. No survived fry at yolk absorption were obtained from fertilization between C. macrocephalus eggs and sperm obtained from both triploid and diploid F 1 hybrids.