Geographic parthenogenesis (GP), a phenomenon where parthenogens and
their close sexual relatives inhabit distinct geographic areas, has been
considered an interesting topic to understand the adaptation to marginal
habitats and the role of hybridization in evolution. Reports of GP from
land and freshwater are numerous, however, this occurrence has been
rarely reported on 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 GP 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, suggested that: (1) parthenogens
evolved at least twice in S. lomentaria, (2) parthenogens did not
originate from inter-species hybridization, (3) new parthenogenetic
lineages have arisen from hybridizations between parthenogens and
sexuals, and (4) parthenogens have a wider distribution than sexuals. We
also showed that the production of sex pheromones, which attract male
gametes, has been independently suppressed/lost in two parthenogenetic
lineages. This parallel suppression/loss of the sexual trait may
represent the direct origin of parthenogens, or the regressive evolution
of a useless trait under asexuality.
The complete plastid genome of the brown alga Scytosiphon lomentaria (scytosiphonaceae, phaeophyceae)
