Diversity of the green algal genus Spirogyra (Conjugatophyceae) in the Hawaiian Islands

Abstract

:Identification of species in the green algal genus Spirogyra is complicated by cryptic morphological diversity, seasonality of critical features of sexual reproduction necessary for morphological identification, the presence of polyploid species groups and a lack of molecular sequence data tied to authentic specimens. Yet the prevalence and importance of these species in freshwater ecosystems drives researchers to advance knowledge of this lineage in various regions around the world. In this study, 180 specimens of Spirogyra from five different Hawaiian Islands were collected and analysed using both morphological and molecular approaches. Comparisons to published data on the genus were also performed to determine the species identification and to estimate the number of unique species in the Hawaiian Islands. Phylogenetic analyses of the rbcL marker revealed 16 clusters of Hawaiian Spirogyra specimens, of which eight were unique to the Hawaiian Islands and recognised as putative endemics, and five were confidently named to species with taxonomic names from other parts of the world (S. communis, S. fluviatilis, S. maxima, S. setiformis and S. submaxima). Ten clusters of Hawaiian Spirogyra were revealed in the SSU phylogenetic analyses, of which only one was unique to the Hawaiian Islands; interpretation of the SSU results was limited to assessment of molecular diversity due to a paucity of named reference sequences. A DNA bar-coding approach was used to examine species boundaries and identification for the Hawaiian Spirogyra specimens with the UPA marker, resulting in 27 unique haplotypes, the vast majority of which were from Hawai'i. Five single-locus species delimitation methods were compared within the different marker data sets to explore congruence in interpretation of species boundaries, and although the number of estimated species varied by method, clusters of specimens were identified based on consensus of these results as well as tree topology and identification criteria used in previous publications. This study revealed a remarkable level of genus-level diversity for Hawaiian Spirogyra, which may be interpreted cautiously as potential endemism for this widespread genus of common freshwater green algae, and which emphasises the importance of continued support for survey-style research into biological diversity of distinct regions.