Molecular evidence for the coexistence of two sibling species in Pylaiella littoralis (Ectocarpales, Phaeophyceae) along the Brittany coast

Evolutionary Phycology: Toward a Macroalgal Species Conceptual Framework

Species concepts formalize evolutionary and ecological processes, but often conflict with one another when considering the mechanisms that ultimately lead to species delimitation. Evolutionary biologists are, however, recognizing that the conceptualization of a species is separate and distinct from the delimitation of species. Indeed, if species are generally defined as separately evolving metapopulation lineages, then characteristics, such as reproductive isolation or monophyly, can be used as evidence of lineage separation and no longer conflict with the conceptualization of a species. However, little of this discussion has addressed the formalization of this evolutionary conceptual framework for macroalgal species. This may be due to the complexity and variation found in macroalgal life cycles. While macroalgal mating system variation and patterns of hybridization and introgression have been identified, complex algal life cycles generate unique eco-evolutionary consequences. Moreover, the discovery of frequent macroalgal cryptic speciation has not been accompanied by the study of the evolutionary ecology of those lineages, and, thus, an understanding of the mechanisms underlying such rampant speciation remain elusive. In this perspective, we aim to further the discussion and interest in species concepts and speciation processes in macroalgae. We propose a conceptual framework to enable phycological researchers and students alike to portray these processes in a manner consistent with dialogue at the forefront of evolutionary biology. We define a macroalgal species as an independently evolving metapopulation lineage, whereby we can test for reproductive isolation or the occupation of distinct adaptive zones, among other mechanisms, as secondary lines of supporting evidence.

Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre- and postzygotic barriers in populations of individuals with varying ploidy levels

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid-diploid life cycles that include the principal genetic model organism for the brown algae. In haploid-diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E. siliculosus and E. crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly postzygotic and suggests that abnormal chromosome segregation during meiosis following hybridization of species with different genome sizes could be a major cause of interspecific incompatibility in this system.

Taxonomic revision of the Agaraceae with a description of Neoagarum gen. nov. and reinstatement of Thalassiophyllum

We confirmed the monophyly of the Agaraceae based on phylogenetic analyses of six mitochondrial and six chloroplast gene sequences from Agarum, Costaria, Dictyoneurum, and Thalassiophyllum species, as well as representative species from other laminarialean families. However, the genus Agarum was paraphyletic, comprising two independent clades, A. clathratum/A. turneri and A. fimbriatum/A. oharaense. The latter clade was genetically most closely related to Dictyoneurum spp., and morphologically, the species shared a flattened stipe bearing fimbriae (potential secondary haptera) in the mid- to upper portion. The phylogenetic position of Thalassiophyllum differed between the two datasets: in the chloroplast gene phylogeny, Thalassiophyllum was included in the A. clathratum/A. turneri clade, but in the mitochondrial gene phylogeny, it formed an independent clade at the base of the Agaraceae, the same position it took in the phylogeny when the data from both genomes were combined despite a larger number of bp being contributed by the chloroplast gene sequences. Considering the remarkable morphological differences between Thalassiophyllum and other Agaraceae, and the molecular support, we conclude that Thalassiophyllum should be reinstated as an independent genus. Dictyoneurum reticulatum was morphologically distinguishable from D. californicum due to its midrib, but because of their close genetic relationship, further investigations are needed to clarify species-level taxonomy. In summary, we propose the establishment of a new genus Neoagarum to accommodate A. fimbriatum and A. oharanese and the reinstatement of the genus Thalassiophyllum.

Species delimitation and phylogeographic analyses in the Ectocarpus subgroup siliculosi (Ectocarpales, Phaeophyceae)

The genus Ectocarpus (Ectocarpales, Phaeophyceae) contains filamentous algae widely distributed in marine and estuarine habitats of temperate regions in both hemispheres. While E. siliculosus has become a model organism for genomics and genetics of the brown macroalgae, accurate species delineation, distribution patterns and diversity for the genus Ectocarpus remain problematic. In this study, we used three independent species delimitation approaches to generate a robust species hypothesis for 729 Ectocarpus specimens collected mainly along the European and Chilean coasts. These approaches comprised phylogenetic reconstructions and two bioinformatics tools developed to objectively define species boundaries (General Mixed Yule Coalescence Method and Automatic Barcode Gap Discovery). Our analyses were based on DNA sequences of two loci: the mitochondrial cytochrome oxidase subunit 1 and the nuclear internal transcribed spacer 1 of the ribosomal DNA. Our analyses showed the presence of at least 15 cryptic species and suggest the existence of incomplete lineage sorting or introgression between five of them. These results suggested the possible existence of different levels of reproductive barriers within this species complex. We also detected differences among species in their phylogeographic patterns, range and depth distributions, which may suggest different biogeographic histories (e.g., endemic species or recent introductions).

Arctic marine phytobenthos of northern Baffin Island

Global climate change is expected to alter the polar bioregions faster than any other marine environment. This study assesses the biodiversity of seaweeds and associated eukaryotic pathogens of an established study site in northern Baffin Island (72° N), providing a baseline inventory for future work assessing impacts of the currently ongoing changes in the Arctic marine environment. A total of 33 Phaeophyceae, 24 Rhodophyceae, 2 Chlorophyceae, 12 Ulvophyceae, 1 Trebouxiophyceae, and 1 Dinophyceae are reported, based on collections of an expedition to the area in 2009, complemented by unpublished records of Robert T. Wilce and the first-ever photographic documentation of the phytobenthos of the American Arctic. Molecular barcoding of isolates raised from incubated substratum samples revealed the presence of 20 species of brown seaweeds, including gametophytes of kelp and of a previously unsequenced Desmarestia closely related to D. viridis, two species of Pylaiella, the kelp endophyte Laminariocolax aecidioides and 11 previously unsequenced species of the Ectocarpales, highlighting the necessity to include molecular techniques for fully unraveling cryptic algal diversity. This study also includes the first records of Eurychasma dicksonii, a eukaryotic pathogen affecting seaweeds, from the American Arctic. Overall, this study provides both the most accurate inventory of seaweed diversity of the northern Baffin Island region to date and can be used as an important basis to understand diversity changes with climate change.