Elena Varela-Álvarez

The genusPorphyra sensu lato(Bangiaceae, Rhodophyta), an important seaweed grown in aquaculture, is the most genetically diverse group of the Class Bangiophyceae, but has poorly understood genetic variability linked to complex evolutionary processes. Genetic studies in the last decades have largely focused on resolving gene phylogenies; however, there is little information on historical population biogeography, structure and gene flow in the Bangiaceae, probably due to their cryptic nature, chimerism and polyploidy, which render analyses challenging. This study aims to understand biogeographic population structure in the two abundantPorphyraspecies in the Northeast Atlantic:Porphyra dioica(a dioecious annual) andPorphyra linearis(protandrous hermaphroditic winter annual), occupying distinct niches (seasonality and position on the shore). Here, we present a large-scale biogeographic genetic analysis across their distribution in the Northeast Atlantic, using 10 microsatellites and cpDNA as genetic markers and integrating chimerism and polyploidy, including simulations considering alleles derived from different ploidy levels and/or from different genotypes within the chimeric blade. ForP. linearis, both markers revealed strong genetic differentiation of north-central eastern Atlantic populations (from Iceland to the Basque region of Northeast Iberia) vs. southern populations (Galicia in Northwest Iberia, and Portugal), with higher genetic diversity in the south vs. a northern homogenous low diversity. For.P. dioica, microsatellite analyses also revealed two genetic regions, but with weaker differentiation, and cpDNA revealed little structure with all the haplotypes mixed across its distribution. The southern cluster inP. linearisalso included introgressed individuals with cpDNA fromP. dioicaand a winter form ofP. dioicaoccurred spatially intermixed withP. linearis. This third entity had a similar morphology and seasonality asP. linearisbut genomes (either nuclear or chloroplast) fromP. dioica. We hypothesize a northward colonization from southern Europe (where the ancestral populations reside and host most of the gene pool of these species). InP. linearisrecently established populations colonized the north resulting in homogeneous low diversity, whereas forP. dioicathe signature of this colonization is not as obvious due to hypothetical higher gene flow among populations, possibly linked to its reproductive biology and annual life history.

Genome size variation is of crucial biological importance, however variation in genome sizes within a single individual/organism is rarely reported except for some species groups such as algae where polygenomy, endopolyploidy and mixopolyploidy have previously been reported. The red algal genus Porphyra forms part of very profitable marine food products commonly known as ‘Nori’. Farming of these valuable marine crops was revolutionized by the discovery of their life cycle in the 40’s. One of the most remarkable characteristics of these taxa is the formation of chimeric gametophytic thalli. After meiosis, the four meiotic products are not released as individuals spores, but instead develop together into a single leafy thallus through successive mitotic divisions. In this study, we used flow cytometry to estimate genome sizes in 670 vegetative thallus sections from 195 blades from three Porphyra species, to determine if this chimerism could be related to the presence of multiple genome sizes and mixoploidy within thalli. Our results show a wide variety of genome sizes both within and between thalli. We interpreted these results as the presence of two different genome types of different sizes (a and b) with separate rounds of genome duplications within the vegetative thalli. By analyzing several sections per thallus, we were able to show that the different genome types and ploidy levels are not distributed haphazardly through the thallus, but are distributed along the thallus in a sectorial way in mosaics. In some individuals, the 2C genome size can either be interpreted as diploids or alternatively as haploid cells that are arrested at the G₂-stage of the mitotic cycle, acting as diploid with two copies of their genome during most of their life-time. We conclude that Porphyra species belong to an aneuploid/euploid system, where genome duplications, mixoploidy, chromosomal dynamics and the presence of different genome types in the chimeric thalli play a role in shaping the genetic diversity of these taxa. Our results may have important implications to understand red algae biology and evolution and raise further questions on concepts of what constitutes an individual.

Whole genome duplication is now accepted as an important evolutionary force, but the genetic factors and the life history implications affecting the existence and abundance of polyploid lineages within species are still poorly known. Polyploidy has been mainly studied in plant model species in which the sporophyte is the dominant phase in their life history. In this study, we address such questions in a novel system (Porphyra, red algae) where the gametophyte is the dominant phase in the life history. ThreePorphyraspecies (P.dioica,P.umbilicalis, andP.linearis) were used in comparisons of ploidy levels, genome sizes and genetic differentiation using flow cytometry and 11 microsatellite markers among putative polyploid lineages. Multiple ploidy levels and genome sizes were found inPorphyraspecies, representing different cell lines and comprising several cytotype combinations among the same and different individuals. InP.linearis, genetic differentiation was found among three polyploid lineages: triploid, tetraploid and mixoploids, representing different evolutionary units. We conclude that the gametophytic phase (n) inPorphyraspecies is not haploid, contradicting previous theories. New hypotheses for the life histories ofPorphyraspecies are discussed.

The red algal genus Porphyra is sister to the genus Pyropia, the single most valuable marine crop in the Orient. We developed microsatellite loci for the red alga Porphyra umbilicalis, a widespread species in the Northern Atlantic. Enriched DNA libraries were constructed and 68 loci were screened for amplification and polymorphism. Seven polymorphic microsatellite markers were isolated using 44 individuals collected from four natural populations. The number of alleles per locus ranged from two to 12. Null alleles were detected in three loci. Among the markers reported, we tested also cross-amplification with two other Porphyra spp. These polymorphic microsatellite markers should be useful for investigating population genetic structure of P. umbilicalis in the North East Atlantic.

The genus Porphyra (and its sister genus Pyropia) contains important red algal species that are cultivated and/or harvested for human consumption, sustaining a billion-dollar aquaculture industry. A vast amount of research has been focused on species of this genus, including studies on genetics and genomics among other areas. Twelve novel microsatellite markers were developed here for Porphyra linearis. Markers were characterized using 32 individuals collected from four natural populations of P. linearis with total heterozygosity varying from 0.098 to 0.916. The number of alleles per locus ranged from 2 to 18. All markers showed cross amplification with Porphyra umbilicalis and/or Porphyra dioica. These polymorphic microsatellite markers are useful for investigating population genetic diversity and differentiation in P. linearis and may become useful for other genetic research on the reproductive biology of this important species.

Caulerpa species are marine green algae, which often act as invasive species with rapid clonal proliferation when growing outside their native biogeographical borders. Despite many publications on the genetics and ecology of Caulerpa species, their life history and ploidy levels are still to be resolved and are the subject of large controversy. While some authors claimed that the thallus found in nature has a haplodiplobiontic life cycle with heteromorphic alternation of generations, other authors claimed a diploid or haploid life cycle with only one generation involved. DAPI-staining with image analysis and microspectrophotometry were used to estimate relative nuclear DNA contents in three species of Caulerpa from the Mediterranean, at individual, population and species levels. Results show that ploidy levels and genome size vary in these three Caulerpa species, with a reduction in genome size for the invasive ones. Caulerpa species in the Mediterranean are polyploids in different life history phases; all sampled C. taxifolia and C. racemosa var. cylindracea were in haplophasic phase, but in C. prolifera, the native species, individuals were found in both diplophasic and haplophasic phases. Different levels of endopolyploidy were found in both C. prolifera and C. racemosa var. cylindracea. Life history is elucidated for the Mediterranean C. prolifera and it is hypothesized that haplophasic dominance in C. racemosa var. cylindracea and C. taxifolia is a beneficial trait for their invasive strategies.