4 DISCUSSION
This study shed light on
biogeographic and evolutionary questions involving the parthenogens of
brown algae by focusing on a species that is widely distributed – a
topic that has been rarely studied within this group of organisms.
Firstly, we found a novel pattern of geographic parthenogenesis in the
brown alga S. lomentaria . Across various sampling sites in Japan,
sexuals were found in localities influenced by warm ocean currents,
whereas parthenogens were mainly found in localities influenced by a
cold ocean current. When parthenogens and sexuals were parapatric,
parthenogens were biased to wave-exposed areas. This type of
distributional pattern of parthenogens had probably not been described
until now so far, at least for brown algae. Secondly, we demonstrated
that parthenogens have evolved at least twice in S. lomentaria .
Although the origin of these parthenogens was unclear, our data
indicated that they did not originate from inter-species hybridization,
as is common in animals and land plants. Thirdly, we found that new
parthenogenetic lineages arose through hybridization between the
parthenogens and their parental sexuals. Finally, we also showed that
the sexual trait (sex pheromone production) has been independently
lost/suppressed in the two parthenogenetic lineages.