Chromosome elimination in the interspecific hybrid medaka between Oryzias latipes and O. hubbsi.

An interspecific hybrid medaka (rice fish) between Oryzias latipes and O. hubbsi is embryonically lethal. To gain an insight into the cellular and molecular mechanisms that cause the abnormalities occurring in the hybrid medaka, we investigated the behavior of chromosomes and the expression patterns of proteins responsible for the chromosome behavior. The number of chromosomes in the hybrid embryos gradually decreased to nearly half, since abnormal cell division with lagging chromosomes at anaphase eliminated the chromosomes from the cells. The chromosome lagging occurred at the first cleavage and continued throughout embryogenesis even after the midblastula transition. Fluorescent in-situ hybridization analyses revealed that the chromosomes derived from O. hubbsi are preferentially eliminated in both O. latipes-hubbsi and O. hubbsi-latipes embryos. Whole-mount immunocytochemical analyses using antibodies against alpha-tubulin, gamma-tubulin, inner centromere protein, Cdc20, Mad2, phospho-histone H3 and cohesin subunits (SMC1alpha, SMC3 and Rad21) showed that the expression patterns of these proteins in the hybrid embryos are similar to those in the wild-type embryos, except for phospho-histone H3. Phospho-histone H3 present on chromosomes at metaphase was lost from normally separated chromosomes at anaphase, whereas it still existed on lagging chromosomes at anaphase, indicating that the lagging chromosomes remain in the metaphase state even when the cell has proceeded to the anaphase state. On the basis of these findings, we discuss the cellular and molecular mechanisms of chromosome elimination in the hybrid medaka.