Egg activation and actin-myosin dependent elongation of the Dictyota zygote

Cellular morphogenesis in land plants and brown algae is typically a slow process. However, elongation of the zygote of the brown alga Dictyota dichotoma changes its shape in a 90 second burst from a sphere to an elongated spheroid.

TEM pictures of Dictyota dichotoma eggs before and after fertilization. — TEM pictures of *Dictyota dichotoma* eggs before and after fertilization. (A, B) TEM section of an egg (<15 min after release) (A) and zygote (B). The arrowhead in (B) shows the cell wall, which is absent in the eggs. Scale bars are 5μm. (C) Detail of a cortical region of a zygote with a cell wall (arrowhead). Scale bar is 1μm. ch, chloroplast; cw, cell wall; mi, mitochondrion; nu, nucleus; pm, plasma membrane. (D) Confocal images (upper row) of a representative egg or zygote visualizing Calcofluor white (CFW) staining and respective bright field images (lower row) during elongation. Scale bar is 50μm. (E) Elongation and cell wall synthesis of four representative elongating zygotes as a function of time after addition of male gametes. Red spheres and the line denote the length (μm) every 15s of a single cell (left y-axes). Black triangles and the line denote the CFW staining intensity relative to the highest intensity measured (right y-axes). Imaging started 15s after addition of male gametes. Note: CFW intensity starts increasing the moment the elongation can be detected. The elongation process takes about 90s.

The elongation can be parthenogenetically triggered by inducing membrane depolarisation (high potassium seawater) and calcium influx (ionomycin). Bogaert et al. show that the fast elongation is accomplished by forces generated by F-actin and myosin, regulated by cytoplasmic calcium concentrations, and by secretion during elongation lowering the antagonistic force.