Fig. 1. Germline progenitor zone size varies among natural Caenorhabditis populations.

a The germline progenitor zone (PZ), located at the distal end of each gonad arm, contains mitotically dividing stem and progenitor cells. The germ stem cell (GSC) pool is maintained through Delta/Notch signalling by the somatic Distal Tip Cell (DTC), which enwraps distal PZ cells. The Delta/Serrate/LAG-2 (DSL-family) ligands, LAG-2 and APX-1, activate Notch (GLP-1) in germ cells to promote proliferation while also preventing meiotic entry. Germ cells differentiate into gamete progenitors through meiotic stages as they progress toward the proximal end of the arm. b PZ size was used as a proxy for PZ cell number. Whole-mount worms stained with DAPI were imaged under epifluorescence. The PZ boundary (dotted line) was identified as previously described based on germ cell nuclei shape³⁸. b’ The PZ region was manually cropped away from other tissues in ImageJ. b” The PZ region was segmented from the background using a fluorescence threshold. The PZ area was measured in pixels. Scale bars 20μm. c PZ area (scaled to PZ size—μm²/1000) measured this way correlates well with hand-counted PZ cell number. Data were obtained from measurements of the two isolates JU1200 and JU751 at two adult stages: mid-L4 + 24 h and young adult + 24 h. Each data point represents an individual (linear model adj R² = 0.758, p < 2.2E-16, n = 87); the experiment was repeated twice with similar results. d PZ size (μm²/1000) in young adult hermaphrodites (1-10 eggs in utero) of select wild isolates of C. elegans and C. briggsae. Cross bars and error bars represent estimated marginal means ± standard errors derived from a generalized linear model. Lowercase letters indicate significant (p < 0.05) Tukey-adjusted pairwise contrasts: isolates that share the same letters do not exhibit a significant difference in PZ size. n-values across two blocks are indicated above the x-axis. e Geographical origins of examined wild isolates; maps produced using the R packages rnaturalearth and rnaturalearthdata v. 0.1.0¹¹² and rgeos v. 0.5-9¹¹³. For data and statistical results, see Supplementary Notes 1 and 2 and the Source Data file.