Cytological Evaluation of the Likelihood that Triploid Grass Carp Will Reproduce

When injected with carp pituitary, triploid male grass carp Ctenopharyngodon idella produced milt that contained sperm of abnormal shape and variable size. Flow cytometry of this milt revealed three types of cells based on DNA content: 1.5n “haploid,” triploid, and hexaploid. Hexaploid cells were interpreted to be spermatogonia in which premeiotic chromosome doubling had occurred; the coefficient of variation (CV = 100˙SD/mean) of DNA content in hexaploids was not different from the CV of DNA in other somatic cells. Meiotic triploid and 1.5n haploid cells from milt had CVs for DNA about 6–7% higher than those of somatic cell populations. This increase was consonant with the theoretical distribution expected from random segregation of the extra chromosomes of triploids. Therefore, both mean and relative variation of DNA content in aneuploid cells support the hypothesis that the third homologous chromosome segregates randomly during anaphase I of meiosis. Because the segregation of chromosomes follows the expected theoretical distribution, it is possible to estimate the probability that competent sperm types will be produced. This estimate can, in turn, be used as one component in estimating the likelihood that triploids will produce diploid offspring, which themselves will reproduce. The probability that a competent sperm from a single triploid male will unite with a normal egg and produce a reproductively capable offspring is conservatively estimated to be 4 × 10⁻¹¹ for every meiotic reduction of hexaploid spermatogonia.