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形態学的・生理学的・分子学的データに基づく謎の種複合体Saprolegnia diclina-Saprolegnia parasiticaの再評価
Re-evaluation of the enigmatic species complex Saprolegnia diclina-Saprolegnia parasitica based on morphological, physiological and molecular data.
PMID: 17446097 DOI: 10.1016/j.fgb.2007.02.010.
抄録
ITSのrDNA配列に基づいてSaprolegnia diclina-Saprolegnia parasitica複合体の分離株間の系統的関係を調べ、形態学的・生理的特徴との相関を調べた。本研究で得られた分離株は、系統的に5つのクレードに分類されていた。寄生虫と推定される分離株の大部分は，魚類から分離されたもので，S. diclina Type 1，S. parasitica，Saprolegnia salmonis，あるいはSaprolegnia spという名称で呼ばれてきたものからなる群に属していた。S. diclina 2型と同定された1株は，Saprolegnia ferax分離株とともにクラスター化していた。S. diclina s. str.（S. diclina Type.3）は2つのクラッドに分布し，Saprolegnia multisporaやSaprolegnia australisとして同定されたチリの分離株と密接に関連していた。その結果，本菌はチリから分離されたSaprolegnia multisporaと近縁であり，チリから分離されたSaprolegnia australisと近縁であることがわかった。このことから、本種は一見性交渉能力のある分離株でもクローン的に繁殖していることが示唆された。本種の寄生への適応は，胞子レベルでの宿主への付着を容易にするための長毛の発達や，引っ込み式の発芽を選択することによって生じたと考えられる。S. parasiticaという名称の使用は、長い巻き毛の束を持つ嚢胞を形成する単離株を含むクラッドIの単離株に割り当てられるべきである。
The phylogenetic relationships among isolates of the Saprolegnia diclina-Saprolegnia parasitica complex were investigated based on ITS rDNA sequences, and correlated with morphological and physiological characters. The isolates studied belong to five phylogenetically separate clades. The majority of presumed parasitic isolates, mostly isolated from fish lesions, fell within a clade that comprises isolates which has been variously named as S. diclina Type 1, S. parasitica, Saprolegnia salmonis or just as unnamed Saprolegnia sp. Presence of bundles of long-hooked hairs on secondary cysts, high frequency of retracted germination, and oogonia production at 7 degrees C (when occurring) were characteristic of this clade. A single isolate identified as S. diclina Type 2 clustered in a clade along with Saprolegnia ferax isolates. The isolates identified as S. diclina s. str. (S. diclina Type 3) distributed in two clades and appeared closely related to Saprolegnia multispora and to a number of Chilean isolates identified as Saprolegnia australis. The ITS sequences of clade I were almost identical even though the isolates were of diverse geographical origins and showed physiological and morphological differences and variations in their pathogenicity. This suggest these species reproduces clonally even in apparently sexually competent isolates. Adaptation to parasitism in Saprolegnia might have occurred at spore level by the development of long-hooked hairs to facilitate host attachment and selection of a retracting germination. The use of the name S. parasitica should be assigned to isolates of clade I that contained isolates forming cysts with bundles of long-hooked hairs.