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.