Fig. 2.

Dynamics of CBA daisy-chain gene-drive systems. (A) After being cut by an upstream daisy allele, a wild-type allele is repaired either by homologous recombination (HR), creating a second copy of the other allele at the locus, or by nonhomologous repair (e.g., NHEJ), leading to generation of a resistant allele. This process occurs in the germline and is independent at each locus. We assume that resistance at the cargo locus, A, is dominant lethal if inherited. (B) A highly efficient daisy drive ($95\%$ homing efficiency) with an $8\%$ fitness cost for the cargo element seeded at $2\%$ spreads the cargo nearly to fixation (Left). (B, Center) A low-efficiency drive ($60\%$) with the same initial release size no longer allows drive spread. (B, Right) Increasing the release size of the inefficient drive to $15\%$ again allows cargo spread to near fixation. (C) The maximum (Max) frequency achieved by cargo alleles as a function of the homing efficiency and the cargo fitness cost, for release sizes of $1\%$ (Left), $5\%$ (Center), and $10\%$ (Right). Throughout, we assume a $0.01\%$ fitness cost for C and B elements and neutral resistant alleles at the C and B loci.