Twin malformation induced in chum salmon eggs by elevated water temperature, with a suggestion as to its mechanism

Inseminated eggs of chum salmon, Oncorhynchus keta, were incubated at 18 °C. In batches of eggs from different females, we regularly observed twinning in a proportion of the eggs (0.5–4%) continuously incubated at this temperature, although no twins were obtained at 8 °C. Twinning was, however, observed at 8 °C when inseminated eggs had been previously treated at 18 °C until the 2- to 4-cell stage. In contrast, eggs attaining telophase in the second meiosis at 8 °C did not show the twin malformation, even when they continued embryonic development at 18 °C. In sections of eggs developed as twins, we detected accumulations of PAS-positive vesicular bodies in the ooplasm between the two embryos. A small number of eggs showing impaired exocytosis of cortical vesicles (alveoli) during egg activation developed into twins. A similar malformation was also induced after part of the ooplasm was dislodged in activated eggs. We propose that vesicular bodies interfere with the convergent migration of the axial determinant during the early phase of embryonic development, which leads to the formation of multiple morphogenetic centers in the eggs incubated at 18 °C.