ALT Figure 1. Classes of killer meiotic drive systems. (a) In killer/target systems, the driving locus (red) produces a toxin that targets the nondriving locus (green). This may be at an idiomorphic, syntenic location, or in heteromorphic genomic regions, such as sex chromosomes. During or after meiosis, all nuclei or cells carrying the nondriving allele are destroyed or inactivated. In poison/antidote systems, the driving locus produces a toxin and an antitoxin. All meiotic products are exposed to the toxin, but only the driving locus is immune to the effect, thus only cells carrying the driving locus survive. (b) Fungi and fruit flies differ in whether the haploid or diploid phase is dominant in their life cycles. In fungi, haploid cells or gametes fuse to produce a diploid zygote, followed by meiosis to produce sexual spores that will form the progeny. Spores may have a single, haploid nuclear type (monokaryotic, n), or possess two nuclei of different mating types (dikaryotic, n + n).