1 INTRODUCTION
Although sexual reproduction is the predominant mode of reproduction in nearly all multicellular organisms, organisms reproducing only by parthenogenesis (obligate parthenogens) are numerous. An important ecological pattern is geographic parthenogenesis, a phenomenon where parthenogens and their close sexual relatives occupy distinct geographic areas. Parthenogens often have a biased distribution towards particular environmental settings (e.g., high latitude, high altitude, and deserts) when compared with close sexual relatives (Kearney, 2005). The loss of sex (or success of parthenogens) in these particular environmental settings has been an topic of interests in evolutionary biology, shedding light on the cost and benefit of sex, adaptation to marginal habitats, and the role of hybridization and polyploidy (e.g., Lynch, 1984; Bierzychudek, 1985; Kearney, 2005; Kawecki, 2008; Hörandl, 2009). Although reports of geographic parthenogenesis from land and freshwater are numerous, this occurrence has been rarely reported on from the sea.
Brown algae (Class Phaeophyceae, Stramenopiles) are one of few lineages to have evolved complex multicellularity (Cock et al., 2010) and most of them are exclusively marine. In brown algae, parthenogenesis, the development of a new individual from an unfertilized gamete, is a common phenomenon. Even in sexual lineages, parthenogenetic development of unfused gametes is common in laboratory cultures (Luthringer et al., 2014), and has been detected in sexual field populations at low frequency (e.g., Oppliger et al., 2007; Klochkova et al., 2017; Hoshino & Kogame, 2019). There are numerous reports of brown algal populations which are speculated to have an obligate parthenogenetic life cycle (i.e., populations in which sexual reproduction cannot be observed; examples include Wynne, 1969; Müller, 1977; Peters, 1987; Deshmukhe & Tatewaki, 1993; Hoshino et al., 2020a). However, there are only a few reports in which these populations were compared with their close sexual relatives and it remains largely unclear in what kind of environment parthenogenesis is favored over sexual reproduction and how parthenogens arise.
To the best of our knowledge, there are three cases in which brown algal asexuals have been compared with close sexual relatives in detail. One example is Fucus radicans , a species endemic to the brackish water of the Baltic Sea (Bergström et al., 2005). Although parthenogenesis is not known in this species, it has asexual populations that are maintained by clonal reproduction using adventitious branches (as dwarf morphotype of F. vesiculosus in Tatarenkov et al., 2005). In this species, asexual populations tend to be distributed in lower salinity areas than sexual populations (Ardehed et al., 2015). It has been posited that low salinity restricts sexual reproduction due to lysis of the egg cell or polyspermy (Serrão et al., 1999), which could favor a switch to asexual reproduction (Tatarenkov et al., 2005; Ardehed et al., 2015). Although this is a convincing and interesting hypothesis for the evolutionary process of asexual lineages, it cannot be applied to asexuals living in saline environments where the majority of brown algal asexuals occur. A second example is Mutimo cylindricus in the Japanese Islands, where female-dominant parthenogenetic populations are parapatric with sexual populations in the Tsugaru Strait (Figure 1; the strait between Hokkaido and Honshu), the northern limit of this species (as Cutleria cylindrica in Kitayama et al., 1992). However, since sex ratio investigations over the entire distributional area in Japan has not been conducted (Kitayama et al., 1992; Kogishi et al., 2010), it is unclear if the parthenogens are geographically limited to areas in the North.
The third known example is Scytosiphon lomentaria (Family Scytosiphonaceae). This algae is distributed worldwide in warm and cold temperate waters. In Japan, it has been reported from Hokkaido to Kyushu (Figure 1; Hoshino et al, 2020c). It has a heteromorphic life history, where generations of macroscopic dioicous isomorphic gametophytes alternate with generations of microscopic discoid sporophytes (Nakamura & Tatewaki, 1975). Its sexual reproduction is isogamous; female gametes settle on the substratum sooner than male gametes and secrete sex pheromones that attract male gametes (Fu et al., 2014b). In culture condition, both female and male gametes develop parthenogenetically (Nakamura & Tatewaki, 1975). In these Japanese populations, in addition to the sexual populations that included both female and male gametophytes, parthenogenetic populations consisting of only females are known (Hoshino et al., 2019). To date, parthenogenetic populations have been found only in Hokkaido, and sexual populations have been reported from southern Honshu (Hoshino et al., 2019). Females in the parthenogenetic populations release gametes that are larger in size, produce lesser sex pheromones, and their parthenogenetic development is rapid, compared with females in the sexual populations (Hoshino et al., 2019).
In the present study, we focused on S. lomentaria , aiming to reveal what kind of environments parthenogenesis is favored over sexual reproduction and how parthenogens have arisen. To achieve this aim, we used samples from a wide geographical range (33 localities across Japan, three localities from Europe, and two localities from Argentina), we conducted (1) sex ratio investigations, (2) phylogenetic and population genetic analyses based on mitochondrial cox 1, nuclear cetn -int2, and genome-wide single nucleotide polymorphism (SNP) markers, and (3) crossing experiments and analyses of sex pheromone production.