Pd/a Crsp Sixteenth Annual Technical Report

Effective and practical control of reproduction is the major constraint in tilapia culture. Uncontrolled reproduction can result in less than 25% of the adults being greater than 250 g after a six-month culture period, with the majority of the population being progeny less than 10 g each. Intraspecific breeding programs have been developed to exploit the sex inheritance mechanism in the tilapia Oreochromis niloticus. Females are said to be homogametic (XX) and males heterogametic (XY), but the sex inheritance of the progeny from a single pair often does not conform to the expected 50:50 ratio. This lack of conformity to a simple XX:XY sex inheritance pattern complicates the intraspecific breeding approach of developing YY males that would give all-male progeny. The identification of tilapia populations with minimal variation in progeny sex ratios from individual spawns would be a significant contribution to the development of a YY male breeding program. Three strains of Oreochromis niloticus—Egypt, Ghana, and Ivory Coast—were spawned in outdoor hapas at 28 to 32°C. A total of 44, 34, and 52 spawns from the Egypt, Ghana, and Ivory Coast strains, respectively, were successfully reared to a sexable size and the sex ratio of each spawn established. The mean percentage of males, females, or intersex fish did not differ among the three strains evaluated. A given male did not give consistent sex ratios when mated with different females. Multiple spawns from a given female also had variable progeny sex ratios. SIXTEENTH ANNUAL TECHNICAL REPORT 66 crushed by placing an additional slide on top and pressing the two together. Visual inspection was then made at 100X magnification along the length of each gonad, which was determined to be teste, ovary, or intersex with the % ovarian tissue recorded. RESULTS AND DISCUSSION A total of 44, 34, and 52 spawns from the Egypt, Ghana, and Ivory Coast strains, respectively, were successfully reared to a sexable size and the sex ratio of each spawn established. The mean percentage of males, females, or intersex fish did not differ among the three strains evaluated (Table 1). There was considerable variation in the sex ratios of individual spawns, with the Ivory Coast strain the most variable, followed by Ghana and Egypt (CVs of 33.2, 21.2, and 19.4%, respectively). The frequency distribution of percent males per spawn for each strain is given in Figure 1. Multiple spawns by a given male or female mated to a new fish were often highly variable. Seventeen males spawned with three or more females, with one individual producing nine spawns. Of these males, only 3 of 17 produced progeny where the percentage of males was between 40 and 60% in all sets. Seventy-six percent of the males gave one or more spawns with > 60% males, and 18% gave one or more spawns with < 40% males. A given male did not give consistent sex ratios when mated with different females. Only 3 of 17 males with three or more spawns gave progeny where the CV in percent males among spawns was 10% or less. The one male that sired nine sets of progeny gave one that was 16.3% males, four between 40 and 60%, and one that was 100% male. Multiple spawns from a given female also had variable progeny sex ratios. Seven females gave three or more sets of progeny. Only two females consistently gave progeny where the frequency of males was between 40 and 60%. Three of seven gave one or more spawns with < 40% males, and three gave spawns with > 60% males. The CV in percent males among sets of progeny was 10% or less for only one female. No one sex could be said to determine sex inheritance based on the observations to date. Where two or more spawns were obtained from a given pair, there were differences of 20% or more in the percentage of males among spawns from three of eight such pairs. Nine spawns were successfully reared for 45 days at two temperatures (two Ghana, four Egypt, and three Ivory Coast). There was no difference in the percent males obtained when reared at 28.3°C and 35.3°C (Table 2). No one strain was more sensitive to the effect of temperature on sex ratios. Individual spawns did respond differently to temperature. In four spawns, siblings reared at the higher temperature had an increase in percent males of more than 10%, while the percent males from siblings from four spawns differed 10% or less at the two temperatures. The percent males in one set of siblings at 35.3°C was 20% less than the siblings at 28.3°C. The variation in sex ratios among individual spawns was not unexpected from the Ivory Coast strain. Variation from 1:1 ratios of individual spawns has also been reported by Calhoun and Shelton (1983) from Ivory Coast strain Nile tilapia. The Ivory Coast strain of O. niloticus used in this study is highly inbred. The founder stock introduced in 1974 consisted of 100 to 200 juveniles from five to ten pairs of brooders at the Centro de Pesquisas Ictiologicas, Pentecoste, Ceara, Brazil, whose stock was founded from 50 to 100 fish introduced from the Station de Recherches Piscicoles, Bouake, Ivory Coast, in 1972 (Tave and Smitherman, 1980). Additional bottlenecks have occurred since that time. Abdelhamid (1988) compared the Auburn Ivory Coast strain of O. niloticus to six others and found 13.2% of the 38 loci to be polymorphic while the other strains were > 26% polymorphic. He found a mean heterozygosity of 0.018 for the Ivory Coast strain; the others ranged from 0.037 to 0.069. Tave and Smitherman (1980) established a predicted heritability of 0.04 ± 0.14 for the Auburn Ivory Coast strain. Teichert-Coddington and Smitherman (1988) found the realized heritability to be -0.10 ± 0.02 for the strain. These low levels of heritability reflect the significant level of inbreeding that has occurred in this line. Such an inbred line should also have a very inbred sex inheritance mechanism with little variation in observed ratios, but this was not the case. The high variability in sex ratios also occurred in the Egypt and Ghana lines. The high degree of variation in individual spawns within a population and among repeat spawns of the same individual does not support a simple XX:XY sex-determining mechanism in any of the strains examined. Such lack of conformity has been encountered in other tilapia studies and explained as autosomal influence (Avtalion and Hammerman, 1978), Strain Number of spawns % Males % Females % Intersex Range in % males Egypt 44 52.9 46.3 0.8 26-72 Ghana 34 54.7 44.9 0.4 16-77 Ivory Coast 53 50.6 49.0 0.4 7-100 Table 1. Mean and range of sex ratios of progeny from pair spawns of Egypt, Ghana, and Ivory Coast strains of Oreochromis niloticus.