Figure 1. Meiotic drive arising from the SD complex in Drosophila melanogaster (not to scale). The primary Sd locus has two common alleles, the wild-type Sd⁺ and the driving Sd. For drive to occur, the Sd allele must be on the same chromosome as an insensitive responder allele, Rspⁱ, and the wild-type Sd⁺ must be linked to a sensitive responder, Rsp⁺. The effect is enhanced by linkage with, respectively, E(SD) and E(SD)⁺. Approximately 99% of the sperm generated by males that are heterozygous for the wild-type and driving chromosomes contain the latter.

Figure 2. Cytoplasmic incompatibility (CI) due to Wolbachia infection (represented by solid dots •) in the cytoplasm. Infected females pass on the infection via their eggs to their offspring, which are protected from any Wolbachia toxins (represented by the shaded cytoplasm) by an antidote (represented by solid crosses ×) also produced by the Wolbachia. Since the infection is not carried in the sperm, however, toxins produced early in the spermatogenesis of infected males kill offspring resulting from crosses with uninfected females.

Figure 3. The proportion of infected cytoplasm in a population with cost-free cytoplasmic incompatibility, according to eqn [2]. The reduction in fertility of the cross between infected males and uninfected females is given by s; curves for two values of s are shown, assuming the infection has an initial frequency of 1%.

Figure 4. The inheritance of the PSR B-chromosome in Nasonia vitripennis. Males arise from unfertilized eggs and are therefore haploid (and so have just one open bar representing the standard chromosomes), whereas fertilized eggs develop into diploid females (with two solid bars). When a female uses PSR-bearing sperm to fertilize an egg, the paternally derived standard chromosomes (open bar) are eliminated and the zygote is a haploid male carrying the PSR element. Over 90% of sperm of PSR males carry the PSR chromosome.