Principles of Genetics and Breeding

4.1.1 Sex determination

• The sexuality of fish has great significance in fish breeding because there are differences in growth rate, behaviour pattern, breeding time, body colour, shape, or size between male and female in each cultured species.
• The expression of sexuality in fishes is greatly diversified, including various types of sex chromosomal mechanisms, gynogenetic reproduction, differentiated and undifferentiated gonochorism,synchronous, protandrous and protogynous hermaphroditism, species specific external diversified ethological sexes.
• However, methods of sex determination are known for relatively few species of fishes.
• From simple hermaphrodites to species with marked sexual dimorphism and complex mating and parental care behaviour reported in fishes imply an evolutionary adaptive radiation of sex control.
• Sexuality in fishes is under low grade of differentiation. The authors studying fish chromosomes were unable to find heterochromosomes in them for quite a long time. This is explained by the fact that many fishes possess a primitive mechanism of sex determination as compared to higher vertebrates.