Crossing direction–dependent lethality in the backcross hybrid embryos between Caenorhabditis briggsae and Caenorhabditis nigoni. (A) Schematic diagram illustrating crossing strategy and expected genome composition of the hybrid progeny between C. briggsae (Cbr, blue) and C. nigoni (Cni, green) as indicated, including their hybrid F1 embryos (F1 female), the backcrossing embryos, that is, bB2 and nB2, which are the crossing progeny between the F1 females and C. briggsae or C. nigoni males, respectively. The coloring scheme is used throughout the paper. (B) DIC (left) and fluorescent micrographs (right) showing the typical phenotypes of arrested hybrid embryos. The fluorescent embryo nuclei were ubiquitously labeled by a HIS-72::GFP marker. An unusually large nucleus in the bB2 embryo is indicated by arrowhead. Differentiated head is indicated with dashed line. Scale bar represents 20 µm. Note a more severe phenotype in the bB2 than in the F1 and nB2 embryos. (C) Quantification of the ratio of embryonic lethality. Shown are the data from five replicates with n = 1718, 1709, 1549, 2800, 2636 for Cbr, Cni, F1, bB2, and nB2, respectively. The error bar represents the standard error of mean. (***) Adjusted P < 0.001 (one-way ANOVA followed by multiple pairwise t-test, FDR adjusted). (D) Percentage of dead embryos with elongation. Shown are the data from five replicates with n = 483, 820, 521 for F1, bB2, and nB2, respectively. The error bar represents the standard error of mean. (****) Adjusted P < 0.0001 (one-way ANOVA followed by multiple pairwise t-test, FDR adjusted). Note that nearly all bB2 embryos arrested before the elongation stage without obvious morphogenesis.
