Fig. 1. GFP-PCNA is a single-cell marker of S phase in Fucus serratus zygotes. (A) Eggs were biolistically co-loaded with Texas Red dextran (red images) and GFP-PCNA (green images), and visualized using 2-photon microscopy. Fertilization was confirmed using Calcofluor white (CFW), shown as the white edge in the red images. The same representative zygote is shown 0.5 hours (left) after fertilization (AF) and 3.5 hours after fertilization (right). (B) Nuclear GFP-PCNA/Texas Red dextran ratios (R) were plotted relative to their starting values (R₀) for zygotes (white circles) and unfertilized eggs (black circles). Zygotes displayed significant nuclear GFP-PCNA increases between 3 and 5 hours after fertilization. Data are mean±s.e.m.; ^*P<0.03. (C) The nuclear GFP-PCNA increase seen 4 hours after fertilization (controls are white bars; `Fert', zygotes; `Un', unfertilized eggs) is completely abolished following treatment with the DNA polymerase inhibitor aphidicolin (20 µM; black bars). Data are mean±s.e.m.; ^*P<0.05.