Abstract:

Background: Cephaleuros, a genus within the family Trentepohliaceae (Ulvophyceae, Chlorophyta), is a significant plant-pathogenic alga. It infects a range of economically important plants, causing diseases commonly known as “algal spot disease” or “red rust”. Additionally, certain species of Cephaleuros act as photosynthetic partners in lichen symbioses. 

Progress: This article provides a systematic review of the advances, challenges, and future directions in the taxonomy of the genus Cephaleuros. Early classification of this genus relied on morphological traits such as thallus structure, sporangial characteristics, and depth of host tissue invasion. However, taxonomic confusion and species misidentification have arisen due to issues such as the loss of type specimens, high phenotypic plasticity, and overlapping morphological features among species. Recent phylogenetic studies employing molecular markers including 18S rDNA, ITS, and rbcL have confirmed the monophyly of Cephaleuros and clarified its phylogenetic position within the order Trentepohliales. These studies also revealed the polyphyletic nature of species such as C. virescens, suggesting the presence of numerous cryptic species within the genus. Nonetheless, such research has highlighted persistent challenges, including conflicts between morphological and molecular data, incomplete species sampling, and insufficient resolution of conventional molecular markers. 

Prospects: Future research should focus on: (1) expanding global sampling efforts and molecular data acquisition; (2) re-evaluating the identities of type species to establish a reliable taxonomic baseline; and (3) developing an integrated taxonomic framework that combines morphological, molecular, host-specificity, and multi-omics data, while exploring the application of artificial intelligence in feature recognition and data integration. Through interdisciplinary collaboration, a more stable and objective classification system for Cephaleuros can be established. This will provide a solid scientific foundation for controlling associated plant diseases, promoting sustainable use of biological resources, and advancing research on algal terrestrialization.

Key words: green algae, Trentepohliaceae, Cephaleuros, algal spot disease, phycobiont