Map showing collection sites in North America (A) and in Asia (B). Abbreviations are as follows: LCA, Little Campbell River anadromous form; LCS, Little Campbell stream-resident form; LCH, Little Campbell River hybrid form; CL, Crystal Lake population; HPA, Lake Harutori Pacific Ocean anadromous form; APA, Akkeshi Pacific Ocean anadromous form; HP, Hyotan Pond-resident form; NS, Nishikitappu stream-resident form; AJS, Akkeshi Japan Sea population; SJS, Sakhalin Japan Sea population.

(A) Representative images of male and female dorsal spines in the Akkeshi Japan Sea population. (B) Means ± SD of serration indexes. Fifteen males and 15 females of the Akkeshi Pacific anadromous form (APA), 17 males and 13 females of the Akkeshi Japan Sea population (AJS), 15 males and 15 females of the Hyotan pond-resident form (HP), ten males and ten females of the Little Campbell River anadromous form (LCA), and ten males and ten females of the Crystal Lake-resident form (CL) were analyzed. Abbreviations: M, males; F, females. In all populations, males have a significantly larger serration index than females (Mann–Whitney U-test; **, P < 0.001, *, P < 0.01).

(A) Representative images of male and female anadromous forms of the Akkeshi Pacific Ocean population (APA). Scale bar = 1 cm. (B) Location of landmarks used for geometric morphometrics. Numbers in the figure indicate the eleven landmarks: (1) anterior tip of upper lip, (2) anterior base of the first dorsal spine, (3) anterior base of the third dorsal spine, (4) caudal end of caudal keel, (5) base of anal spine, (6) anterior base of pelvic spine, (7) ventral border of operculum, (8) posterior edge of anglular, (9) center of eye, (10) upper end of pectoral-fin base, and (11) lower end of pectoral-fin base. (C) Scatter plot of first and second relative warp scores (RW1 and RW2, respectively) for APA males (n = 17; open square), APA non-gravid females (n = 12; open circle), and APA gravid females (n = 5; black circles). (D) Thin-plate spline deformation grids visualizing the body shape with RW1 of −0.07 and RW1 of 0.07, which reflect the male- and female-specific body shapes, respectively.

(A) Summary of sexual dimorphism in ten populations of wild-caught threespine sticklebacks. The differences between male and female means are expressed as a percentage of the female mean for each population, where a negative value indicates that females are larger and a positive value indicates that males are larger. All traits except standard length are corrected for body size. In each panel, bars are arranged from left to right in the order of LCA, LCS, LCH, CL, APA, HPA, HP, NS, AJS, and SJS populations. Black bars indicate that the sexual difference is significant, even after sequential Bonferroni correction. Sexual dimorphism that is significant at P < 0.05, but not significant after sequential Bonferroni correction is indicated by a above or below each bar. The traits for which sexual dimorphism was not tested due to slope heterogeneity are indicated by b. (B) Morphological traits that show sexual dimorphism in most populations are shown in black, while other traits are depicted in gray. The upper panel is a schematic figure of a left side of a threespine stickleback, while the lower panel is a schematic figure of a ventral side of a threespine stickleback.