Effects of high temperature on the hemocyte cell cycle in silkworm larvae

To understand the inhibitory effects of high temperature on insect growth at the cellular level, we investigated the influence of high temperature on the proliferation and division of larval hemocytes in the silkworm, Bombyx mori. Although the total number of hemocytes in the larval body increased enormously over time at 26 °C, no increase was observed at 38 °C. The number of mitotic hemocytes in circulation increased between days 1 and 2 of the fourth larval stage at 26 °C, whereas fewer hemocytes were observed at 38 °C. Laser scanning cytometry revealed that the DNA content of hemocytes collected from the fourth-stadium larvae was predominantly 2C, 4C, and 8C, and the proportion of each type of hemocyte changed dynamically with development during the fourth instar. Specifically, the proportion of hemocytes with a higher DNA content increased gradually during the feeding phase then decreased during the molting phase at 26 °C; in contrast, no decrease was observed at 38 °C. The heat-induced accumulation of 8C hemocytes was mainly detected in granulocytes and plasmatocytes. These data suggest that high temperatures induce a G2 arrest in larval hemocytes.