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).