Cytological Observations On Induction Of Mitogynogenesis By Heat-shocking Of The Eggs In Grass Carp, Ctenopharyngodon Idellus

The developmental procedure of first cleavage of grass carp eggs activated by UV-irradiated common carp sperm were systematically observed by method of histological section. The optimum parameters, such as the exact time for the activated eggs finishing the first chromosome sets copying, the optimum period for artificial chromosome set duplication, the optimum temperature and intensity of heat shocking etc, were determined. These parameters provide scientific basis for developing the technique of producing high rate of homozygous diploid mitogynogenetic grass carp. Indeed, groups of homozygous diploid mitogynogenetic fry of grass carp, which is very important for both the theoretical and practical research of genetic breeding, have been produced with this techniques based on the parameters.Results show that when incubated at 24, the activated eggs entered the S phase after activation for 21 min, the prophase for 24 min, the metaphase for 27-30 min, the anaphase for 33 min and the telophase for 36 min. Obviously, the optimum period for chromosome set duplication is from 27 to 30 min after the eggs were activated, since the chromosome set of the activated grass carp eggs was copied completely at that time.In order to define the optimum temperature for inhibiting the eggs' first cleavage, different temperatures were tested to heat shock the activated eggs that had finished the chromosomeset copying completely. Results showed that the treatment is invalid by the heat shocking below 39 for 2 min, because no spindles of the activated eggs were destroyed by these treatments. When the activated eggs were treated by the heat shock at 40 for 2min, the spindle was partly destroyed. Only by over 41 ,the spindles were destroyed completely. But the mortality rate of embryos with the treatment of 42 was too high. So the optimum temperature for chromosome set duplication is 41 癈 and the effective intensity of treatment of heat shocking is 41 for 2 min.After the heat shocking was released, the spindles reformed in about 72. 8% of the observed activated eggs and did not in the others. After the treatment was removed for 6 min, the nucleus began to reform in those eggs that the spindle did not recover. After 9 min, new nuclei had been reformed completely. The activated eggs entered the prophase of the next cleavage cycle in about 14 min, the metaphase in 19min; the anaphase in 24min and the telophase in 29min.Cytological genetic analysis revealed that the chromosome number of gynogenetic grass carp is 48 and the karyotype is 18m+24sm+6st, fundamental arm number (NF) is 90, just as those of the common grass carp. No additional chromosomes or chromosomal fragment were observed. This result mean that the fish obtained by the above technique treatments is real diploid mito-gynogenetic grass carp.