A species dilemma: utilizing biological species concept to reconcile DNA barcode and morphological divergences for Hypnea pseudomusciformis (Gigartinales, Rhodophyta)

Taxonomic studies based on molecular markers led to the description of a new species, Hypnea pseudomusciformis, to accommodate three morphologically divergent species that occur on the coast of Brazil and have identical DNA barcode sequences (H. musciformis, H. nigrescens, and Hypnea valentiae). These three previous species were then identified as morphological variants of H. pseudomusciformis. Later, phylogeographic studies revealed that H. pseudomusciformis is subdivided in three distinct haplotype groups with significant intraspecific divergence for the COI-5P marker. Moreover, each haplogroup presents physiological and morphological differences related to each of the three floristic provinces along the Brazilian coast: tropical, transition, and warm temperate. Therefore, these haplogroups were denominated as ecotypes. In this work, we tested the biological species concept (BSC) in two crossing experiments: (i) between specimens belonging to different haplogroups (obtained from tropical and warm temperate provinces) and (ii) between different morphological variants (musciformis and nigrescens) belonging to the same haplogroup. As a result, we observed that all crosses performed were positive and the reach of fertility in all tetrasporophytic plants of the F1 and F2 generations. Therefore, we concluded that (i) specimens from tropical and warm temperate provinces and (ii) specimens identified as musciformis and nigrescens belong to the same biological entity, despite their intraspecific molecular divergences and physiological differences and their differences in morphology, respectively. When testing and confirming the biological concept of species, we also validate previous works or confirm that the species has great phenotypic and physiological plasticity, which allows its occurrence in different niches and in provinces with different abiotic characteristics. The BSC proved to be a valuable tool for responding to conflicts raised by the DNA barcode technique.

Automated summary

Taxonomic studies based on molecular markers led to the description of a new species, Hypnea pseudomusciformis, which accommodates three morphologically divergent species with identical DNA barcode sequences found on the coast of Brazil. Further phylogeographic studies showed that H. pseudomusciformis is divided into three distinct haplotype groups related to different floristic provinces along the Brazilian coast.