GONAD DEVELOPMENT IN ANEUPLOID ORNAMENTAL KOI CARP OBTAINED BY CROSSING TRIPLOID FEMALES WITH DIPLOID MALES

Recent studies showed that some triploid ornamental koi carp (Cyprinus carpio) females were able to develop large ovaries and produce mass aneuploid progenies when crossed with diploid koi males. Aneuploid fish had very low survival; only 248 4-month-old juveniles were collected from outdoor tanks in which about 32,000 swim-up larvae obtained from triploid females were stocked. For further rearing, fish were stocked in 400-l tanks in indoor recirculating systems with water temperature maintained at 24-26°C. Under these rearing conditions, normal diploid koi males and females reach maturity at 2 years of age.

Reproductive ability and gonad development of aneuploid fish obtained from triploid females were investigated at fish age of 28 months; only 33 fish survived to this age. Preliminarily, fish were individually marked with PIT tags and their ploidy determined by flow cytometric analysis. All investigated fish were injected with carp pituitary extract (CPE) at a dose of 3 mg/kg. No fish released sperm or eggs upon stripping after hormonal injection. Fish were then dissected and their gonads were investigated.

The data on fish ploidy and gonad development are presented in Table 1. All investigated fish were aneuploid with ploidy range from 2.31n to 2.78n. No dependence of sex or stage of gonad development on fish ploidy was detected. Similar to triploid males, aneuploid males had pinkish (brownish) testes which did not contain sperm. Ovaries of most aneuploid females were undeveloped and contained oocytes at the beginning of vitellogenic growth. However, one investigated female (with gonadosomatic index 9.6%) had relatively large ovaries filled with fully grown oocytes. In general, results of this study showed that reproductive ability of aneuploid koi was similar to that of triploid koi investigated earlier. Males are functionally sterile while some potentially fertile aneuploid females can be found.