Insect Science (2012) 19, 239–246, DOI 10.1111/j.1744-7917.2011.01441.x ORIGINAL ARTICLE Geographic expansion of the cabbage butterfly (Pieris rapae) and the evolution of highly UV-reflecting females Yuya Fukano 1 , Toshiyuki Satoh 1 , Tadao Hirota 2 , Yudai Nishide 1 and Yoshiaki Obara 1 1 Behavioural Biology, Department of Animal Life Science, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, 2 Department of Biology, Faculty of Science, Yamagata University, Kojirakawa-machi 1-4-2, Yamagata, Japan Abstract Reflection of ultraviolet (UV) light by the wings of the female Eurasian cabbage butterfly, Pieris rapae, shows a large geographic variation. The wings of the female of the European subspecies, P. rapae rapae, reflect little UV light, while butterflies of the Asian subspecies, P. rapae crucivora, may reflect it strongly or at only intermediate levels. The geographic region where P. rapae originated remains to be determined. Moreover, it is not clear if females with wings that reflect little UV light are ancestral to females with wings that reflect UV strongly or vice versa. In the present study, we aimed to determine the geographic origin and ancestral UV pattern of cabbage butterflies through mitochondrial DNA (mtDNA) sequence analysis and amplified fragment length polymorphism (AFLP) analysis. The results of these investigations suggest that P. rapae is of European origin and that it has expanded its distribution eastward to Asia. It follows that the ancestral subspecies is the type with UV-absorbing wings. Lower nucleotide diversities and haplotype network patterns of mtDNA derived from East Asian populations suggest that population expansion from Europe to East Asia probably occurred fairly recently and at a rapid rate. Key words dispersal, geographical origin, geographical variation, UV reflection Introduction Butterflies are among the species in which UV vision and its ecological functions have been studied extensively. It has been shown in several butterfly species that reflection of UV light by the wings has important roles in mate lo- cation, mate recognition and mate choice (Petersen et al., 1952; Obara, 1970; Silberglied & Taylor, 1973; Rutowski, 1977, 1981, 1982, 1985; Silberglied, 1979; Burkhardt et al., 2000; Knuettel & Fiedler, 2001; Robertson & Monteiro, 2005). In addition to behavioral studies, inves- tigations on the chemical mechanisms that underlie UV reflection or absorption revealed that pterins, which are harbored in ellipsoid granules found in the wing scales, are responsible for UV absorption in pierid butterflies Correspondence: Toshiyuki Satoh, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agri- culture and Technology, 3-5-8 Saiwai, Fuchu, Tokyo, Japan. 183-5893; email:tsatoh@cc.tuat.ac.jp (Makino et al., 1952; Yagi, 1954; Hidaka & Okada, 1970; Stavenga et al., 2004; Rutowski et al., 2005; More- house et al., 2006; Stavenga et al., 2006; Giraldo & Stavenga, 2007; Wijnen et al., 2007). With regard to structural UV color, which is fairly common in pierid but- terflies, anatomical investigations demonstrated the thin- layer elaboration of wing scales that causes UV reflection by interference (Ghiradella et al., 1972; Ghiradella, 1974, 1989; Rutowski et al., 2005). The cabbage butterfly, Pieris rapae, is one of the species in which UV vision and its ecological functions have been extensively studied. It was shown in the Japanese sub- species, Pieris rapae crucivora, that female (but not male) wings strongly reflect UV light, and that this is used by males as a basis for mate recognition (Obara, 1970). Vi- sual sensory mechanisms, including UV receptive visual cells that underlie UV perception in males, have been demonstrated through electrophysiological experiments (Arikawa et al., 2005). Unlike the female of the Japanese subspecies, the female of the British subspecies, Pieris rapae rapae, has strongly UV-absorbing wings (similar to C  2012 The Authors Journal compilation C  Institute of Zoology, Chinese Academy of Sciences 239