3.1 Sex ratio and geographic distribution of sexual and
parthenogenetic populations
Results of sex check by sex-check PCR and crossing experiments were
concordant, except for some samples from p24, p27, and Argentina which
had both female and male markers in sex-check PCR (Figure 1; Table 1;
Table S1). In crossing experiments, samples from p27 released gametes
fused with male gametes of p22 (Table S1) and did not function as male
gametes. Therefore, the samples having both markers were considered
female (at least, phenotypically).
Populations were considered sexual when at least one male was
identified, or parthenogenic when no males could be identified in
samples of ten or more individuals (Table 1). For p1, although the sex
of only two individuals were examined, this population was considered
parthenogenetic (Table 1), since sexual reproduction was not observed in
this population in a previous study (Kogame et al., 2005). Along the Sea
of Japan coast, sexual populations were distributed from Kyushu to
Hokkaido and overlapped with parthenogenetic populations on the west
coast of Hokkaido (Figure 1). Along the Pacific coast, sexual
populations were limited to the South of Choshi, Chiba Prefecture (the
sampling locality of p24 and p25) and parthenogenetic populations were
found North of Choshi (Figure 1). Sexual and parthenogenetic populations
were parapatric in two localities: Esashi, the west coast of Hokkaido
(p4 and p5) and Choshi (p24 and p25). At these localities, the two types
of populations were 1 km and 0.3 km apart along the coast, respectively
(Figure S1). In both localities, parthenogenetic populations were found
in more upper part of intertidal zone and more wave-exposed areas,
compared with sexual populations. No males were found among European and
Argentinean samples (four and 17 samples, respectively; Table S1).