Phyllodictyon robustum (Setchell et Gardner) comb. nov. (Siphonocladales, Chlorophyta), a morphologically variable species from the tropical Pacific coast of America

Abstract

Abstract - The systematic positions of Cladophoropsis robusta and Willeella mexicana, both occurring along the tropical Pacific coast of America, have long been uncertain. Willeella mexicana has been generally accepted as a synonym of C. robusta, which is currently placed in the genus Struveopsis. Reinvestigation of type material and recently collected specimens from Mexico and Panama, including culture observations, along with a molecular phylogenetic analysis inferred from rDNA ITS1-5.8S-ITS2 sequences, confirm that both taxa belong to the same, morphologically variable species. Thallus architecture ranges from stipitate, Struveopsis-like blades (produced by apical cell divisions and formation of opposite and flabellate branches) to Valoniopsis-like plants, composed of coarse, cylindrical filaments with lateral branches initiated by lenticular cells. Additionally, DNA sequence data and morphological features such as branching pattern of blade filaments, presence of tenacular cells and crystalline cell inclusions provide evidence that Struveopsis robusta falls within the Phyllodictyon clade and is most closely related with the Atlantic-Caribbean Phyllodictyon pulcherrimum, with the Indo-Pacific Phyllodictyon orientale forming a sister species. The binomial Phyllodictyon robustum (Setchell et Gardner) comb. nov. is thus proposed.