Heterozygotes are WT and segregate WT, dead eggs and Uncs. At 25C the Unc Spds are Sterile; at 16C the Unc Spds are fertile but produce mostly dead eggs. Unc Spd animals will exhibit a fully penetrant maternal-effect embryonic lethal phenotype if shifted to 25C at the L4 stage. unc-13 spd-2 homozygotes may be propagated at 16C but may become sick causing immense frustration! See also WBPaper00004200.