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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. [ABSTRACT FROM AUTHOR]

The red alga Gracilaria caudata is one of the main agarophytes exploited on the Brazilian coast. A greenish-brown mutant was found growing side-by-side with the common red wild type in a natural population. In this work, we tested the hypotheses that the greenish-brown phenotype has a recessive nuclear inheritance and less fitness (growth, photosynthesis, number of apices, secondary attachment percentage, and number of cystocarps) under laboratory conditions when compared with the red phenotype. Crosses between plants of the same color (red or greenish-brown) showed tetrasporophytic and subsequent gametophytic generations with parental phenotype. However, heterozygous tetrasporophytes, obtained from crosses between plants with different colors, showed a brown phenotype. From their progeny, we concluded that the brown phenotype is due to a monolocus Mendelian genetic system with two codominant alleles. Bisexual gametophytes were found in the progeny of tetrasporophytic color mutants, which is an unusual and poorly known characteristic in the life cycle of Gracilariales. Red female gametophytes (rd genotype) presented better performance in relation to the greenish-brown female gametophytes (gb genotype): growth rates (rd = 17.14 ± 1.36, gb = 13.81 ± 0.95); number of apices (rd = 41.05 ± 6.35, gb = 16.00 ± 5.47); secondary attachment percentage (rd = 68.75 ± 23.93, gb = 25 ± 0.00); and number of cystocarps (rd = 38.61, gb = 18.81). Our results explain the maintenance of the greenish-brown mutant and the predominance of red specimens in natural population. The life cycle was completed in 5 months, one of the shortest times reported for Gracilaria, highlighting the importance of the species as a possible model organism for reproductive studies and in selecting strains for use in mariculture. [ABSTRACT FROM AUTHOR]