Research

The class Oomycetes is one of the large phylogenetic groups in the stramenopiles. Oomycetes are known as a morphologically and ecologically diverged group, such as thallus morphology ranged from single-celled to highly developed hyphae, and ecological behavior as saprophytes, endo- or ecto-parasites in marine, freshwater and terrestrial environment. In spite of their large morphological and ecological diversity, the origin, evolutionary development and diversity process of oomycetes were still uncertain.

Recently, we investigated the molecular phylogeny and ultrastructural comparative morphology of some little-studied marine oomycetes, which include unicellular, holocarpic obligate endoparasites of marine algae. In our molecular trees based on both the SSU rRNA and cox2 genes, saprolegnian (mycelial group of freshwater saprophytes or fish pathogens) and peronosporalean oomycetes (mycelial group of land plant pathogens) each formed a monophyletic clade whilst our marine endoparasite species branched before the both clades. This indicates these marine parasites they occupied the most basal clades in the monophyletic oomycete lineage. Many of the morphological features of marine endoparasite species (e.g. direct release of zoospores from sporangia) were shared with the saprolegnian oomycetes rather than the peronosporalean species, suggesting that the peronosporalean oomycetes are the most derived and specialized group in the oomycetes. Our data indicated that oomycetes may have originated from marine environment as unicellular obligate endoparasites, both of algae and crustacea. Saprolegnian oomycetes then diverged in the freshwater environment as saprophytes and fish parasite, and peronosporalean oomycetes advanced into terrestrial environment predominantly as pathogens of land plants.

卵菌類は単細胞種から菌糸形成種までを含み，形態学的，生態学的に多様な鞭毛菌類である。しかしながら，その知見のほとんどは培養の容易な腐生性ミズカビ類や植物病理学上重要なツユカビ類に限られており，卵菌類の全体像をとらえきれていないのが現状である。

本研究では，純粋培養が困難なために詳細な研究がほとんどなされてこなかった海産の全実性卵菌類を材料とし，これらの分子系統解析及び微細構造比較解析を行うことで，卵菌類全体の系統関係，形態進化の過程を明らかにすることを試みた。SSU rRNA，cox2両遺伝子を用いた分子系統解析では海産全実性卵菌は二つのクレードに分けられ，それらは卵菌類内で最も初期に分岐した。微細構造学的形質はミズカビ類とツユカビ類のグループ内では比較的よく保存されているのに対し，海産全実性種は非常に多様性に富んでいた。また，海産全実性種における遊走子形成時の遊走子嚢内の液胞形成様式や鞭毛形成のタイミングはツユカビ類よりもむしろミズカビ類と共通していた。更に，ミズカビ類全般に共通して見られる二形性遊走子や遊走子鞭毛基部付近のK-小体様構造がいくつかの海産全実性種にも見られた。このことは，ミズカビ類で見られる特徴が卵菌類の原始的なものであり，ツユカビ類が卵菌類内で特殊化したグループであることを示唆している。