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RESEARCH PRODUCT

The genetic basis of balanced polymorphism in Philaenus (Homoptera)

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In Philaenus spumarius (Homoptera), colour polymorphism is mainly determined by a series of allelic genes. The dominance and co-dominance relationships of the following seven alleles were analysed: T (trilineatus), M (marginellus), F (flavicollis-F), L (lateralis), C (flavicollis-C, gibbus and leucocephalus), O (quadrimaculatus, albomaculatus and leucophthalmus) and t (typicus). The colour phenotypes are indicated in brackets. On the basis of partly direct and partly indirect evidence, T was found to be the top dominant and t the bottom recessive allele in the female sex. In the males, T is also the top dominant, but t comes second in the hierarchy. Conclusive evidence of the dominance of T over L, T over O and T over t was obtained, as well as of dominance of M over O. Codominance was found between L and F, L and C, and M and F. Three different allele combinations, M/t, L/F and L/C, were found to be expressed as marginellus. Modifiers non-allelic with the genes mentioned above influence the expression of the genes C and O. The presence and manifestation of these modifiers is basic to the existence of the colour forms flavicollis-C, gibbus, quadrimaculatus, and albomaculatus. Evidence is presented for the hypothesis, here advanced explicitly for the first time, that the colour genes are components of supergenes. Certain assumptions are made to support the hypothesis that alleles high in the dominance hierarchy are of more recent origin than alleles at or near the bottom. This hypothesis is discussed in connexion with Sheppard's well-known theory of the co-evolution of dominance and balanced polymorphism.