Microsatellite-based Parentage analysis and male reproductive success in Nile Tilapia (Oreochromis niloticus)

Territorial behavior and reproductive competition among Oreochromis niloticus males for fertilizing a batch of eggs from a female may lead to a large skew in reproductive success among individuals. However, there are no studies that have analyzed the reproductive success of individual males in spawning aggregations of Nile tilapia. We analyzed parentage and estimated male reproductive success under commercial hatchery conditions in hapas. A broodstock of 50 females and 24 males of Nile tilapia were selected from an out-bred population maintained at the WorldFish Center station in Abbassa, Egypt. After collection of phenotypic data and blood sampling, were divided into two groups of 25 females and 12 males each, and stocked in two 8m x 2m (L x W) hapas. Broodstock were allowed to spawn naturally. After ten days of acclimatization and initiation of spawning, checking for incubating females was carried out on a weekly basis and fertilized eggs were collected and incubated until hatching. Egg collection continued for four weeks. In total two cycles of egg collection were performed with a reconditioning period of two weeks in between. A total of 76 spawns was obtained. All parental in addition to ten offspring from each spawn were analyzed using 11 microsatellite markers. In hapa 1, a total of 47 spawns were collected with 16 females spawning repeatedly. In hapa 2 a total of 29 spawns were collected from all except two females which did not spawn at all. The number of males participating at each collection was proportional to the number of spawns. Results of the parentage analysis showed that 99.2% of the offspring could be unambiguously allocated to their putative parental pairs. Multiple paternity (participation of 2 or 3 males in a family) occurred in 46.8% and 48.2% of the total number of families in the and second hapa respectively. In the hapa 89.4% of the offspring were sired by 6 of the 12 males; in the second hapa 75.2% of the offspring were sired by 4 out of the 12 males. There was a positive correlation (r = 0.62) between male body weight and its reproductive success, expressed as percent of offspring sired by that male, in both hapas. There was a signideviation from Hardy-Weinberg equilibrium at all eleven loci in the offspring from both hapas. This signideviation together with the large skew in reproductive success in males gives a clear evidence of non-random mating in the population under study. The high variance in reproductive success will affect the genetically effective population size and the increase of inbreeding. The results of this study are relevant for commercial farms that use mass spawning.