Genetic parameters for black spot disease (diplopstomiasis) caused by Uvulifer sp. infection in Nile tilapia (Oreochromis niloticus L.)

Abstract

Black spot disease (diplopstomiasis), an external melanized host inflammatory response caused by a number of digenetic trematode parasites, results in slow growth, deformities and increased mortality among many freshwater fish species globally. We investigated the severity of infection, and genetic parameters for black spot disease among 150 families of Nile tilapia (Oreochromis niloticus) of the Abbassa strain in ponds in Egypt. The causative agent for the black spot disease was identified molecularly as a Uvulifer species using 28S gene. Although 27% of the families remained completely unaffected, the others exhibited signs of black spot infestations randomly distributed in the skin and fins at different levels of severity. Histological examination revealed multiple parasitic cysts surrounded by fibrous capsules with melanin deposits embedded in the muscles. Females were significantly more susceptible than males (P < 0.001) representing 65% of all infected fish. Females had significantly more black spot (predicted value 0.16 ± 0.01) than males (0.09 ± 0.01). Least square means for harvest weight of fish with no black spot (240.0 ± 2.2 g) was higher than fish with black spot (210.0 ± 3.5 g) with stronger reduction of weight in males. The heritability estimate (h2) for infection with blackspot disease was 0.29 ± 0.04 indicating additive genetic variance for the trait. The heritability estimate for harvest weight was 0.41 ± 0.05, but the genetic correlation (rg) between black spot and harvest weight was not significantly different from zero (0.02 ± 0.10) indicating that both traits could be selected for independently without negative effects on each other.