Geographic parthenogenesis in the brown alga Scytosiphon lomentaria (Scytosiphonaceae): Sexuals in warm waters and parthenogens in cold waters

Geographic parthenogenesis, a phenomenon where parthenogens and their
close sexual relatives inhabit distinct geographic areas, has been
considered an interesting topic in evolutionary biology. Reports of
geographic parthenogenesis from land and freshwater are numerous, however,
this occurrence has been rarely reported from the sea. Brown algae are
mostly marine and are thought to include numerous obligate parthenogens;
still, little is known about the distribution, origin, and evolution of
parthenogens in this group. Here we report a novel pattern of geographic
parthenogenesis in the isogamous brown alga Scytosiphon lomentaria. Sex
ratio investigation demonstrated that, in Japan, sexual populations grew
in the coast along warm ocean currents, whereas female-dominant
parthenogenetic populations grew mainly in the coast along a cold ocean
current. In the two localities where sexual and parthenogenetic
populations were parapatric, parthenogens grew in more wave-exposed areas
than sexuals. Population genetic and phylogenetic analyses, including
those based on genome-wide single nucleotide polymorphism data, indicated
that parthenogens have initially evolved at least twice and subsequent
hybridizations between the parthenogens and sexuals have generated
multiple new parthenogenetic lineages. The origin of the initial
parthenogens is not clear, except that it would not be inter-species
hybridization. Interestingly, we found that the production of sex
pheromones, which attract male gametes, has been independently lost in the
initial two parthenogenetic lineages. This parallel loss of the sexual
trait may represent the direct origin of parthenogens, or the regressive
evolution of a useless trait under asexuality.