Fig. 1. Zygnema.
a, Three cells of a vegetative filament of SAG 698-1b (top) compared with one cell of a vegetative filament of SAG 698-1a (bottom, both samples of 1 month old cultures). Scale bar, 20 μm. C, chloroplast; N, nucleus; P, pyrenoid. One-cell filament contains two chloroplasts and one nucleus. b, Chromosome counting on light micrographs of SAG 698-1b fixed and stained with acetocarmine at prophase (0.5 months old); count was also performed in metaphase and telophase (Supplementary Fig. 1). The green dots represent the 20 chromosomes that were counted after rendering a stack of ~100 images. Scale bar, 10 μm. See Supplementary Fig. 1 for the original images. A minimum of ten cells each from three independent cell cultures were analyzed. c, A confocal laser scanning image of one SAG 698-1b cell (0.5 months). Scale bar, 20 μm. d, Transmission electron micrographs illustrating the filamentous nature of Z. circumcarinatum (SAG 698-1b). Left: overview showing that the cells are connected by extremely thin cross cell walls (cCW), while the outer cell wall (CW) is surrounded by a pectinous extracellular matrix (ECM); within the individual cells, pyrenoids (Py) and the nucleus (N) are clearly depictable. Scale bar, 5 µm. e, A detailed view of the cross wall separating two cells where chloroplast lobes are visible. Scale bar, 0.5 µm. Transmission electron micrographs (d and e) derived from the analysis of ≥15 algal filaments each for two independent cell cultures. f, Chromosome-level assembly of the SAG 698-1b genome. Concentric rings show chromosome (Chr) numbers, gene density (blue), repeat density (yellow), RNA-seq mapping density log10(fragments per kilobase of transcript per million mapped reads) (dark green) and guanine-cytosine content density (violet). The red and green links show respectively intra- and interchromosomal syntenic blocks. g, A comparison of genome properties for 13 algal and 3 land plant species. The time-calibrated species tree was built from 493 low-copy genes (all nodes supported by >97% nonparametric bootstrap; numbers at branches are estimated divergence times in million years (mean ± standard deviation) (see Supplementary Table 1f for details). Data for the bar plot can be found in Supplementary Table 1i,j.