RESUMEN
Two molecular techniques which reveal highly variable DNA polymorphisms, RAPD and multilocus DNA fingerprinting, were used to evaluate genetic diversity between six aquacultural strains of Oreochromis niloticus (tilapia) from the Philippines. The results using both techniques were in close agreement. Within-strain heterozygosity values were similar and were correlated between the two data sets, but statistical errors associated with the RAPD data set were lower. Although genetic distances between strains were greater using DNA fingerprinting, the distances measured using both methods were significantly correlated. Both methods were useful in estimating variation between strains, but they offered different advantages. RAPD was technically easier to perform and produced results with low statistical error, whereas DNA fingerprinting detected greater genetic differentiation between strains. The theoretical basis for using RAPD and multilocus minisatellite markers for population studies is discussed.
Asunto(s)
Dermatoglifia del ADN , Técnicas de Amplificación de Ácido Nucleico , Tilapia/genética , Animales , Marcadores Genéticos , Variación Genética , Heterocigoto , Modelos Teóricos , Polimorfismo GenéticoRESUMEN
Established techniques of genetic manipulation were used to elucidate sex-determining mechanisms in the commercially important tilapia, Oreochromis niloticus. Analysis of sex ratios from single-pair matings of normal broodstock showed these to be heterogeneous, with an asymmetrical frequency distribution. Data were homogeneous, with the exclusion of a number of broods with sex ratios not significantly different from 3â¶1 (male: female), and further progeny testing revealed atypical female heterogamety in the parents of these broods. Analysis of sex ratios from complete diallele-type crosses using five males and five females demonstrated no association between male parent, female parent and progeny sex ratio. Sex ratios of gynogens (0â¶1) and triploids (1â¶1), and from progeny testing of sex-reversed males (0â¶1) and sex-reversed females (3â¶1), provide evidence for female homogamety in this species. Progeny testing of male gynogens derived from sex-reversed females demonstrated recombination between the centromere and the sex-determining locus (68.9%). Novel YY "supermales" were shown to be viable and produced all-male offspring. It was concluded that this species exhibits monofactorial, genotypic sex determination with male heterogamety. However, rare autosomal or environmental sex-modifying factors may account for occasional deviations from expected sex ratios.
RESUMEN
Sex ratios from 62 single-pair matings of normal broodstock O. aureus were highly heterogeneous with an overall deficit of males (41.4%). Peaks in the sex ratio frequency distribution occurred at 1â¶1, 3â¶5 and 1â¶3 (maleâ¶female). Hybridisation of O. aureus with O. mossambicus, O. spilums and O. niloticus produced highly variable sex ratios, suggesting a complexity of hybrid sex determination. Few valid inferences could be made regarding intraspecific sex determination from these hybrid data. Sex ratios from progeny testing of sex-reversed males (1â¶3) and most sex-reversed females (1â¶0) provide evidence for female heterogamety in O. aureus. Several aberrant ratios observed suggest Mendelian inheritance of an autosomal recessive gene (F,f), epistatic to the major sex-determining gene (W,Z). Sex ratios of triploids and gynogens support the hypothesis of recombination between the centromere and the major sex-determining locus. Progeny testing of a female mitogyne demonstrated the viability of a novel WW "superfemale", which gave only female offspring. Not all data could be explained by a two-factor model of sex determination. Further exceptional sex ratios may be accounted for by rare autosomal or environmental sex-modifying factors. It is concluded that O. aureus has a multifactorial mechanism of sex determination with the underlying primary mechanism of female heterogamety.