Gonad Development in Triploid Ornamental Koi Carp and Results of Crossing Triploid Females with Diploid Males

Gonad development in 4‐year‐old triploid and diploid ornamental koi, a variant of Common Carp Cyprinus carpio, from corresponding heat‐shocked and control progenies was investigated. Diploid males were normally mature. Triploid males from heat‐shocked progeny demonstrated development of testes typical for triploid fish; triploid males did not release sperm and their testes had a pinkish color and were significantly reduced in size. Diploid females were normally mature and their gonadosomatic indices (GSIs) varied from 7.5% to 30.7% and the mean value was 21.3%. Triploid females had unexpectedly well‐developed ovaries, which were filled with fully grown oocytes; their GSIs varied from 4.2% to 30.1% and the the mean value was 17.0%. Four triploid koi females released large quantities (from 260,000 to 394,500 eggs per female) of ovulated eggs after hormonal injection. Eggs from triploid females were fertilized with sperm from normal diploid koi males. Mass mortality of hatched larvae occurred at the swim‐up stage, but about 32,000 swim‐up larvae were obtained and stocked for further rearing. A total of 248 juveniles (or less than 1% from the number of stocked larvae) were collected from outdoor tanks. Ploidy analysis of juveniles (n = 110) showed that most of them were aneuploid with ploidy ranging from 2.3n to 2.9n with a mean value of 2.6n; two juveniles were diploid (2n). This shows that triploid koi females produced aneuploid eggs with a ploidy range from haploid to diploid level with the modal ploidy level around 1.5n, similar to the production of aneuploid spermatozoa observed earlier for triploid males in fish.