Induction of gynogenesis and gonad development in the muskellunge

Muskellunge Esox masquinongy eggs activated with UV-irradiated yellow perch Perca flavescens sperm were subjected to heat shock at 31 ± 1°C for 6 min, 20 min after fertilization in three experiments. Survival at eyed stage was 1.7 ± 1.6, 6.8 ± 4.8 and 2.3 ± 0.5% in experiments I, II and III, respectively. After rearing the gynogenetic muskellunge in troughs and then in ponds, the sex ratio of gynogens in experiment I did not significantly (P>0.05) differ from the expected ratio 0:1 (male:female), however, one male and one intersex were observed. In experiments II and III, the sex ratio of gynogens differed significantly from expected (0:1). Three months after hatching, the growth of females did not significantly (P>0.05) differ among control and gynogenetic groups, whereas male growth was significantly (P<0.01) higher in the control v. gynogenetic group. The histological structures of the gynogenetic fish gonads in both sexes were similar to those described in the gonads of control muskellunge. After the overwintering period, signs of active spermatogenesis were observed in the testis of 1 + year old gynogenetic fish, whereas ovaries contained only oocytes at the perinucleolar stage. At this stage, plasma sex steroids testosterone (T) and oestradiol-17β (E2) cannot be used to discriminate the sex of gynogenetic muskellunge, as intersex, male and female fish had similar levels of T and E2.