PIGMENT CELL STUDIES AND GENETICS OF DOMESTICATED COLOUR VARIETIES OF THE PLATYFISH, XIPHOPHORUS MACULATUS

Abstract

The platyfish, Xiphophorus maculatus, is an introduced species in Singapore but thrives wider the local culture environment. A conmon aquarium fish, it is a favourite among hobbyists because of its varied colours and ability to co-exist with other species in a community tank. A farm survey was carried out on the culture of domesticated stocks of platyfish in Singapore. The aim was to determine prevailing fanning practices. All the farmers surveyed cultured colour and finnage varieties separately as the demand is for purity of the strains. An analysis of the farm-made feed in these farms to determine the nutritional content as compared to commercial pellets used in one farm was carried out. All formers also supplemented these feeds with live Tubifieiids for brooders and Moina spp. for fry. Brooders and fry were cultured in concrete aquaria while grow-out was done in cage-nets in ponds. A chromatophore study was then initiated as one of the major attractive features or this species was its intense and vibrant colouration. Scales from various colour varieties were studied under transmitted light, confocal laser scanning and transmission electron microscopes. The basis of colouration in the platy was established was combinations or 5 chromatophore types; melanophore, xanthophore, xanthocrythrophore, crythrophore and guanophore. Each of these was studied in detail using the 3 forms of microscopy. Chromatophore combinations resulting in colouration of 5 popular varieties were also determined. In addition, chromoatophore types in fry could birth up to 30 days or age were studied in 6 colour varieties to determine if fry could be distinguished according to their colour varieties within one month or birth. Since it was determined that at the forms. colour varieties were cultured separately, genetic crosses were carried out to determine the inheritance of background colouration. Intra-variety crosses were initially carried out to confirm the hue-breeding colour varieties. Four true-breeding colour varieties (Green, Yellow Comet, Red and Sunset) were selected and reciprocal inter-variety crosses were set up. Green and yellow background colouration were determined to be controlled autosomally while red and sunset colouration were not only sex-linked but probably sex-limited and controlled by multiple loci. Genetic variation in the sarcoplasmic proteins of the platyfish was analysed by isoelectric focusing (IEF) on pre-cast, thin, flat-bed polyacrylamide gels with pH ranging from 3.5 to 9.3. Although several Poeciliid species closely related to the platyfish could be easily distinguished using IEF, platy colour varieties however could not be distinguished by this electrophoretic method. Three polymorphic sarcoplasmic protein loci (Sp-1, Sp-2, Sp-3) however were discovered in the 4 colour varieties (Green, Red, Yellow Comet and Sunset) studied. These loci were all found to be in Hardy-Weinberg equilibrium except for Green variety Sp-1 locus and Red variety Sp-2 and Sp-3 loci.