Induction of gynogenesis in muskellunge (Esox masquinongy)
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Muskellunge females grow faster and live longer than males. Females reach the desired size of 760 mm in 3 years, while males require 4 years. Only females ever reach the “trophy size” (1016 mm). Thus, stocking of all female muskellunge will have great potential for increasing the efficiency of stocking and benefit the sport fish hatchery. The goal of this study is to induce gynogenetic muskellunge by UV irradiation and heat shock. Semen was diluted and irradiated with UV for different time periods. A typical “Hertwig effect” was found. Irradiation at 1440 to 2160 J per m2 proved to be an appropriate dose to inactivate sperm DNA. Eggs fertilized with sperm irradiated at these doses yielded 100 % abnormal embryos. Heat shock was applied for the retention of the second polar body. Eggs fertilized with irradiated sperm were heat shocked at 26 and 28°C for 10 and 20 min starting at different times after fertilization. The effect of heat shock duration was analyzed at three temperatures (26, 28, and 30°C) with five different time periods, starting at 20 min after fertilization. There were no significant differences in survival rate at the eyed-stage among the treatment groups. However previous results showed heat shock at 32°C was lethal for all eggs. Gynogens were induced in most of the groups with hatching rates at 0.2 % to 3.3 % . The highest yield of gynogens was 6.0 % (hatched gynogens/eyed-stage eggs). Haploid muskellunge can be easily identified morphologically from eyed stage onward. Most of the haploids hatched, but died within a week. Haploid larvae were curled and shorter than normal larvae. Ploidy was verified by chromosome preparation, silver staining of nuclear organizer region (NOR's) and flow cytometry measurement of DNA content of the eyed-stage eggs and larvae. Cells from suspected haploids showed only one (89.9 % ) or no NOR's (10.3 % ), while cells from diploids showed two (51.3 % ), one (41.5 % ), or no NOR's (7.2 % ). The DNA content of haploid cells was half (0.507) that of diploid cells. Cell cycle analysis by a computer program, Multicycle, indicated that the haploid cells were actively dividing and had a complete chromosome set of maternal origin (G1 % = 88.6, G2 % = 4.2, S % = 11.9, = 1.979).