An exuberant, undescribed colour polymorphism in Theridion californicum (Araneae, Theridiidae): implications for a theridiid pattern ground plan and the convergent evolution of visible morphs

An opisthosomal (abdominal) colour polymorphism is described in the North American spider, Theridion californicum, comprising a plain Yellow morph and (at least) ten patterned morphs, which exhibit areas of red or black pigments superimposed on the yellow background, or no pigment (white). The polymorphism appears to be present throughout the species' range. The Yellow morph is the most frequent in populations, with patterned morphs all, individually, being rather rare. Progeny from known mothers were reared and indicate that the polymorphism is genetic and that Yellow is probably recessive to patterned morphs. Similar to other theridiids with well-studied colour polymorphisms, T. californicum occupies an under-leaf habitat and the variation in all these cases might be maintained by sight-hunting predators exerting negative frequency-dependent (apostatic) selection. In T. californicum, blocks of guanine underlying the pigmented hypodermis indicate a segmental patterning, which is not usually apparent in adult spiders. These segments, plus dorso-lateral divisions, permit the dorsal surface of the opisthosoma to be divided up into two mirror-image halves, each comprising 12 compartments. Each compartment can either lack pigment (thus appearing white as a result of underlying guanine) or be yellow, red, or black. All patterns in T. californicum can be derived from this ground plan, as can the morphs of other colour-polymorphic theridiids. It is suggested that selection for polymorphism, combined with constraints imposed by this theridiid ground plan, may have led to the convergent evolution of colour patterns across the family. (C) 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 23-34.