REPRODUCTIVE ABILITY OF TRIPLOID KOI × GOLDFISH HYBRID FEMALES AND RESULTS OF THEIR CROSSES WITH GOLDFISH AND KOI MALES

Previous studies have shown that F₁ hybrid females between koi carp (Cyprinus carpio) and goldfish (Carassius auratus) produce diploid eggs due to a transformation of meiosis. Crossing of F₁ hybrid females with males of parental species resulted in appearance of triploid progeny without application of any physical treatment to early embryos. The goal of this study was to evaluate the reproductive ability of backcross triploid koi x goldfish hybrid females, which were obtained by crossing F₁ hybrid females with koi males. These triploids have in their genome two haploid sets of koi and one haploid set of goldfish.

In spring 2017, five triploid hybrid females were individually crossed with goldfish males; two of these females were also crossed with koi males. Ploidy of juveniles in obtained progenies was determined by flow cytometric analysis of DNA content. Pooled data on ploidy distribution of fish from crosses triploid hybrid females with goldfish males are shown in Fig. 1A. Most of the fish were aneuploid with ploidy range from 2.3n to 3.0n with a mean value about 2.5n; one fish had a ploidy of 3.6n. Ploidy distribution of fish from crosses triploid hybrid females with koi males are shown in Fig. 1B. Fish from both progenies had ploidy distributions similar to those observed in crosses with goldfish; one cross had more fish with higher values of ploidy (3.6n-4.0n). Since aneuploid fish have in their genomes one haploid set from parental males, the data obtained indicate that triploid hybrid females produced aneuploid eggs with ploidy range from 1.3n to 2.0n, and a modal ploidy level around 1.5n. Apparently fish with ploidy range 3.6n-4.0n resulted from spontaneous suppression of 2^nd meiotic division in aneuploid eggs.