Variation in time to first cleavage in rainbow trout Oncorhynchus mykiss embryos:: A major factor in induction of tetraploids.

This study was undertaken to analyze the sources of variation in timing of first cleavage in rainbow trout Oncorhynchus mykiss embryos in order to adjust treatment protocols to maximize tetraploid induction. The first cleavage interval (FCI), or the time from fertilization to the mid-point of the appearance of first cleavage, was determined at several temperatures for eggs from individual females from each of four different strains. Statistical analyses of the data did not reveal any significant differences among samples taken on different dates or among females within populations, but there was a significant difference shown among the populations. Further analyses revealed this difference was due to a significantly shorter FCI in one population. Data on two of the populations incubated at elevated temperatures showed a decreased FCI within the populations, but the differences between the populations remained. The results suggested that modification of the treatment protocols for induction of tetraploidy (and, perhaps, gynogenesis and androgenesis) are needed to compensate for variation noted among populations if maximum induction and minimum mortality are to be achieved.