The effects of genetic background on the island-specific control of a colour polymorphism in Theirdion grallator (Araneae: Theridiidae), the Hawaiian happy-face spider

The Hawaiian happy-face spider, Theridion grallator, is endemic to four islands in the Hawaiian archipelago. In all populations it exhibits an exuberant colour and pattern polymorphism involving both the carapace and opisthosoma (abdomen). Previous work has demonstrated that four of the opisthosomal colour morphs (Yellow, Red front, Red blob and Red 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 producing 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 Maui. Here we use crosses between spiders of Maui and Hawai'i origin to examine further the differences in genetic structure between pcpulations on the different islands. They also allow an assessment of the influence of a disrupted genetic background on the expression of colour morphs. Our results confirm the presence of two unlinked 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 alleles 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 crosses. Similarly, a mixed genetic background has no effect on the normal expression of the non-sex-limited morphs from both Hawai'i and Maui. The full expression of one male-limited morph from Hawai'i (Red front) 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, dominance is determined by the superimposition of one pattern upon another. A preliminary model is developed to explain the quantum shift in the genetic control of the colour polymorphism in T. grallator on Hawai'i during its colonization from Maui.