| Summary: | Genetic effects influencing salinity tolerance (ST) and cold tolerance (CT) were evaluated in two full diallel mating designs using six tilapia varieties: Oreochromis aureus (BL), O. mossambicus (MO), O. niloticus (NI), O. niloticus crossbreds (RE), Mississippi commercial strain (MC) and Florida red tilapia (FL). Statistical analyses provided estimates of direct heterosis (h<sub>i</sub>), cross heterosis (h<sub>ij</sub>), maternal effects (m<sub>i</sub>), line effects (l<sub>i</sub>), reciprocal and specific reciprocal effects (r<sub>ij</sub> and r**<sub>ij</sub>), and general and specific combining abilities (GCA and S<sub>ij</sub>).
Analysis of genetic effects for ST indicated that FL exhibited significant GCA (P < 0.01). BL, FL, and MO exhibited highly significant l<sub>i</sub> (P < 0.01). Highly significant m<sub>i</sub> (P < 0.01) was apparent for FL and RE. Highly significant S<sub>ij</sub> and h<sub>ij</sub>% (P < 0.01) were exhibited in two and eight variety combinations (VCs), respectively. Highly significant r<sub>ij</sub> (P < 0.01) was observed in BL-MO, and MC-RE. In addition, highly significant r**<sub>ij</sub> (P < 0.05) was noted in BL-MO and FL-RE.
Analysis of genetic effects for CT indicated that BL, MC and RE exhibited significant GCA's (P < 0.05). Highly significant l<sub>i</sub> and m<sub>i</sub> (P < 0.01) were apparent in BL and RE, respectively. Significantly negative S<sub>ij</sub> (P < 0.05) was exhibited only in BLxMC, while negative and significant h<sub>ij</sub>% (P < 0.05) was apparent in BLxMC, FLxMO and FLxRE. Highly significant r<sub>ij</sub> (P < 0.01) was apparent in FL-MC and MC-RE, while negative and significant r**<sub>ij</sub> (P < 0.05) was exhibited only in FL-MC. No significant direct heterosis (h<sub>i</sub>) was apparent in ST or CT.
Improvement in ST in could be accomplished by developing a breeding program combining selection, hybridization and backcrossing in MO, BL and FL, while improvement of CT may be accomplished by selection and hybridization in BL. The potential environmental and commercial implications of developing salinity-tolerant and cold-tolerant tilapia varieties and crosses are discussed.
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