4.1 Geographic parthenogenesis in S. lomentaria
We showed that, along the coast of Sea of Japan, sexuals were distributed from Kyushu to the west coast of Hokkaido and overlapped with parthenogens along the west coast of Hokkaido. In contrast, along the Pacific coast, sexuals were distributed south of Choshi (the sampling locality of p24 and p25) and parthenogens were distributed north of Choshi. Choshi, close to where the Kuroshio current and Oyashio current meet, is known as one of the boundaries of seaweed flora in the Pacific coast; it is the southern limit of cold waters species (Okamura, 1931; Yoshizaki, 1979). The northern limit of subtropical species, the Cape Taito (35°18’N, 140°24’E), is also near Choshi (Okamura, 1931; Yoshizaki, 1979). On the other hand, the seaweed flora along the coast of the Sea of Japan is generally warm temperate being affected by the Tsushima Current, a relatively smaller warm current that bifurcates from the Kuroshio Current. Furthermore, the Tsugaru Strait has been suggested as a boundary of seaweed floras (Okamura, 1931; Yoshizaki & Tanaka, 1986). Considering the above, the distribution of sexuals and parthenogens of Japanese S. lomentaria seems to be heavily associated with ocean currents.
In Esashi (p4 and p5) and Choshi (p24 and p25), where sexual and parthenogens are parapatric, parthenogens grew in the upper more wave-exposed intertidal zones compared with sexuals. Intertidal seaweed species often grow in specific vertical or horizontal “zones” or “bands” that respond to environmental gradients caused by wave exposure and abiotic conditions resulting from tidal levels (e.g., temperature, salinity, dehydration, light intensity), partly due to interspecific differences in the tolerance to the environmental stress (Hurd et al., 2014). The above habitat segregation between parthenogenetic and sexual populations provides evidence for niche differentiation. Our SNP-based analyses suggested that the two sexual populations near Esashi and Choshi (p6 and p25) included parthenogenetic lineages that have probably never hybridized with sexuals, however, it is unknown if sexuals and parthenogens had habitat segregation, also in these localities.
In Japan, It has been often suggested that parthenogenetic organisms tend to be biased towards marginal and disturbed environments, when compared with their close sexual relatives (Tilquin & Kokko, 2016). Lynch (1984) hypothesize that the association of parthenogens with marginal habitats may be a consequence of selection pressure over parthenogens to use habitat that is not occupied by the parental sexuals. Lynch (1984) mentioned “since one of the most stringent requirements for the establishment and maintenance of a parthenogen will be the avoidance of backcrosses and competition with parental sexuals, incipient clones that tend to use habitats not normally occupied by the parental sexuals will have an initial selective advantage over other clone.” In S. lomentaria , parthenogens and sexuals clearly occupied different ecological niches. However, it is unclear whether the cold waters and the wave-exposed areas, where parthenogens existed, are the marginal/disturbed habitats for sexuals. Although we have performed culture experiments at 10°C, 15°C, and 20°C, at least for the gametophytic phase, no difference was observed in the growth and maturity between the culture isolates from parthenogenetic and sexual populations (data not shown). Further study is necessary to examine whether sporophytes of sexuals can mature (i.e., whether sexuals complete their life cycle) in low temperature. Wave-exposed areas, on the other hand, are possibly unsuitable habitats for sexuals since waves can disturb interaction between male and female gametes. In some oogamous brown algae, laboratory experiments have shown that gamete release is enhanced in calm/no-flow conditions and is inhibited in shaken/high-flow conditions (Gordon & Brawley, 2004; Pearson & Serrão, 2006). However, it is unclear if S. lomentaria sexual populations are distributed in wave-exposed areas in warm regions where parthenogenetic populations are not distributed.