Assisted suicide of a selfish gene

Medea (Rn) factors and the hybrid incompatibility factor (H) are involved i n two incompatibility systems in flour beetles that were previously thought to be independent. M factors are a novel class of selfish genes that act b y maternal lethality to nonself. The H factor causes the death of hybrids w ith a paternally derived H gene and previously uncharacterized maternal cof actors. We now find that M factors exhibit their selfish behavior only in t he absence of the H factor. Furthermore, we show that the previously unchar acterized maternal cofactors required for H-associated hybrid inviability a re identical to M factors. We propose that incompatibility between H strain s and WI strains is due to suppression by the H factor of the self-rescuing activity of the lethal M genes. This interaction has the effect of convert ing M elements from selfish into self-destructive or "suicidal" genes. M fa ctors are globally widespread, but are conspicuously absent from India, the only country where the H factor is known to occur. Such a mechanism could prevent the spread of selfish M elements by establishing an absolute barrie r to hybridization in the boundary between M and non-M zones.