Synchronous nuclear division and the role of the cytoskeleton in the multinucleate red alga Griffithsia monilis

Most taxonomic groups of organisms harbor temporarily or permanently multinucleate cells in all or parts of their bodies. Each nucleus in the same cytoplasm responds almost identically to environmental cues, but little is known about the signals that mediate their coordinated division. In this study, we used Griffithsia monilis, a multinucleated giant cell, to investigate how its nuclear division occurs and the role of cytoskeleton in this process. Our results show that nuclear division is exquisitely coordinated and synchronized, but that nuclear division and chloroplast division are not coupled to each other. Microtubules are known to play an important role in synchronized nuclear division in some large multinucleate green algae, and microtubule arrangement is involved in shaping the cytoplasmic domains of each nucleus. However, we found no evidence for the involvement of the cytoskeleton in the synchronized nuclear division or regular nuclear arrangement in G. monilis. Although the nuclei were arranged at very regular intervals, these intervals became irregular during nuclear division, and there was no regular arrangement of actin or microtubules to maintain the spacing between the nuclei. Neither cortical microtubules nor spindle microtubules were physically connected to other neighboring nuclei during nuclear division, suggesting that microtubules are not involved in the coordination of nuclear division in G. monilis.