Preliminary study on the cytological mechanism of triploidy and tetraploidy induced by hydrostatic pressure shock in transparent colored crucian carp

The cytological mechanism of triploidy and tetraploidy induced by hydrostatic pressure shock was analysed through histological section techniques in transparent colored variety, Crucian carp the major effects of hydrostatic pressure treatment are to destruct spindle and interfere chromosome movement so that triploids and tetraploids are induced because of the retention of the second polar body and blocking of the first cleavage. The sensitive period for inducing second polar body retention is short and is at the metaphase and anaphase of meiotic division II, i.e. 4-5 min after sperm-egg fertilization. Before this period, the fertilized eggs are in the initializing stage. Hydrostatic pressure treatment of this stage would disrupt the exciting and decorating process of fertilized eggs, hindering embryo development. After the sensitive period the tendency of second meiotic division is so strong that application of hydrostatic pressure fails to control and retain second polar body. The stage is the nonsensitive period. Fertilized eggs of transparent colored crucian carp were shown to develop into metaphase of first mitosis at 50-52 min after fertilization, into anaphase at 52-57 min and into telophase at 57 min (water temperature 14-16 degree C). The effective period for inhibiting first mitosis for inducing tetraploidy by hydrostatic pressure was found to be during the metaphase and anaphase of first cleavage. Hydrostatic pressure treatment not only produced reversible microtubule depolymerization, damaged the spindle and arrested the process of first mitosis, but also resulted in chromosome condensation to various extents. Tripolar spindle bodies were observed in some treated fertilized eggs. These phenomena, especially formation of tripolar spindle body, might be related to chromosome breakage and loss in the induction of tetraploidy. It is these changes of mitotic apparatus that resulted in a large number of aneuploids. Based on the cytological mechanism of triploidy and tetraploidy induction, a discussion was made on the conditions for the prevention of second body extrusion for producing triploids, and on the conditions for the inhibition of first cleavage for inducing tetraploidy.