THE EFFECTS OF GENETIC BACKGROUND ON THE ISLAND-SPECIFIC CONTROL OF ACOLOR POLYMORPHISM IN THERIDION GRALLATOR (ARANEAE, THERIDIIDAE), THEHAWAIIAN HAPPY-FACE SPIDER

The Hawaiian happy-face spider, Theridion grallator, is endemic to fou r islands in the Hawaiian archipelago. In all populations it exhibits an exuberant colour and pattern polymorphism involving both the carapa ce and opisthosoma (abdomen). Previous work has demonstrated that four of the opisthosomal colour morphs (Yellow, Red front, Red blob and Re d ring) are found in both sexes on Maui but on Hawai'i Yellow and Red blob are limited to females with the alleles controlling them producin g morphs Red front and Red ring, respectively, in males. In addition, there is evidence for two unlinked loci controlling colour on Hawai'i, but only one on Maul. Here we use crosses between spiders of Maui and Hawai'i origin to examine further the differences in genetic structur e between populations on the different islands. They also allow an ass essment of the influence of a disrupted genetic background on the expr ession of colour morphs. Our results confirm the presence of two unlin ked loci in Hawai'i. In addition, the control of expression of colour morphs in males and females is shown to be a property of the colour al leles themselves or of closely linked, cis-acting regulatory sequences . In all cases, the sex-limited morphs derived from Hawai'i behave on a mixed-island genetic background exactly as they do in pure Hawai'i c rosses. Similarly, a mixed genetic background has no effect on the nor mal expression of the non-sex-limited morphs from both Hawai'i and Mau l. The full expression of one male-limited morph from Hawai'i (Red fro nt) is dependent on the presence of an X chromosome of Hawai'i origin. With one possible exception, dominance hierarchies are maintained on a mixed genetic background, possibly because for most morphs, dominanc e is determined by the superimposition of one pattern upon another. A preliminary model is developed to explain the quantum shift in the gen etic control of the colour polymorphism in T. grallator on Hawai'i dur ing its colonization from Maul.