4.2 Do parthenogens have larger distributions than sexuals?
Parthenogens often occupy larger distribution areas than their sexual
relatives (Bierzychudek, 1985; Hörandl, 2009). This could be also the
case of S. lomentaria . We found that populations from Argentina
and Europe included no males and they were genetically close/identical
to parthenogenetic populations in Japan. Regarding populations in North
America, although only cox 1 data was available, most of them had
haplotypes which were close/identical to those from the parthenogenetic
populations in Japan. Furthermore, sexual reproduction (gamete fusions)
has not been reported from S. lomentaria in North America (Wynne,
1969). These facts suggest that the populations of S. lomentariafrom Argentina, Europe, and North America are parthenogenetic and that
parthenogens of this species are more widely distributed than sexuals.
To explain a larger distribution of parthenogens, two hypotheses
focusing on a selection among genetically diverse parthenogenetic clones
(clonal selection) are often cited: “general-purpose genotype” (GPG)
and “frozen niche-variation” (FNV) hypotheses. The GPG hypothesis
supposes that clonal selection promotes the evolution of highly
generalized (or general-purpose) genotypes, which are characterized by
both broad tolerance ranges and low fitness variance for relevant
environmental gradients; it expects that a widespread distribution of a
parthenogen has resulted from the dispersal of a single (several) highly
generalized clone(s) (Lynch, 1984). The FNV hypothesis, on the other
hand, expects that a widespread distribution of a parthenogen has
resulted from the joint distribution of many locally adapted
(specialist) clones (Vrijenhoek, 1984; Hörandl, 2009). In S.
lomentaria , several parthenogenetic clones (or clonal lineages) were
found in multiple populations and had wide distributions, as the GPG
hypothesis expects. However, the whole parthenogenetic populations
consisted of about 30 polyphyletic clones and the major clone in each
parthenogenetic population was often unique to the population. Although
it is unknown if each clone is locally adapted or not, this high clonal
diversity seems to fit the FNV hypothesis.
Another important factor to explain a wide distribution of parthenogens
is a superior colonization ability. Parthenogens should have a
significant advantage over bi-sexual parents in colonization, since they
can initiate a new colony by a single individual and their potential
increase in numbers per generation is double that of a bi-sexual parent
(Cuellar, 1977). Parthenogens are known to be commonly biased towards
previously glaciated areas, and it has been speculated that it is partly
because of that parthenogens have occupied these areas after the
glaciation as a consequence of their superior colonization ability
(Kearney, 2005). The last glacial maximum impacted on the distribution
of seaweeds and some seaweeds show shallow population structure probably
due to postglacial northward recolonization from southern glacial
refugia (e.g., Provan et al., 2005; Hoarau et al., 2007; Coyer et al.,
2011; Neiva et al., 2014). Judging from our cox 1-haplotype
network, the population structure of Japanese parthenogenetic
populations was not so shallow, but populations from North America,
which are probably parthenogenetic, showed a clear star-like network
pattern. It may suggest the recent population expansion (i.e.,
postglacial recolonization) of S. lomentaria parthenogens in
North America. The superior colonization ability of parthenogens should
also be advantageous in expanding the distribution as invasive species.
In the brown algae Cutleria multifida and Mutimo
cylindrica , populations that seems to have been introduced are usually
female-dominant (probably parthenogenetic populations; Kogishi et al.,
2010; Kawai et al., 2016). Our SNPs-based analyses showed that most
Argentinean samples were genetically identical to those from the
Japanese parthenogenetic population p2. These Argentinean
parthenogenetic populations may have been introduced from Japan , as has
been hypothesized for other seaweed that are considered to be introduced
from Asia to Argentina (Hoshino et. al., 2020a).