Maternal and paternal hybrid triploids of tetras
Hybrid maternal triploids were generated by crossing grey-body and red or yellow-tailed Buenos Aires tetra Hemigrammus caudovittatus (BT) males with albino or black Gymnocorymbus ternetzi [widow tetra (WT)] females and retaining the second polar body by heat shock (HS) or cold shock (CS). Paternal triploids were also generated using 2·5% polyethylene glycol (PEG) incubated BT semen to facilitate the entry of two sperm into an egg of WT. Optimum temperature for CS was identified as 6° C and the optimum age for thermal shocking the zygote as 3 min after fertilization. At hatching, survival was 25, 17 and 5% for the HS, CS and paternal triploids, respectively. At maturity, it was further reduced to >2% and the maturity inordinately delayed. With the presence of undifferentiated ‘steriles’ and almost total absence of females, the expected sex ratio was distorted. Triploid males produced spermatocytes, spermatids but not spermatozoa. Triploidy was confirmed by phenotypic markers, karyotyping, erythrocyte measurement and molecular markers. The fusiform body shape of WT was a dominant phenotypic trait over the typical piscine slender body shape of BT. Polymerase chain reaction products of the genomic DNA of the triploids amplified by OPF6 primer were 300, 450 and 1000 bp length, characteristic of BT, and 500 and 800 bp, typical of WT and thereby confirmed the biparental genomic contribution to the triploids. Analyses of genomic DNA of selected progenies using DMRT-1 marker showed that (1) like the diploid BT males, the hybrid triploid males were also true genetic males, and amplified 237 and 300 bp products but (2) the triploid steriles amplified all the three products of 100, 237 and 300 bp indicating that they were mosaics but the diploid steriles failed to amplify.
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