Rodriguez-Prieto, C. | bioRxiv Science

bioRxiv2025

Rare sex but a long life sustain seaweed populations at the warm edge of their range

AimThe life cycle of many organisms is all but stable across their distribution range. Most commonly, populations respond to environmental variation by shifting the timing of reproductive events (phenology). Or more profoundly, populations may (partly) shift their mode of reproduction from sexual to asexual. Life cycle variation can impact reproductive success, gene flow, genetic diversity, and, ultimately, the evolutionary trajectory of populations. Understanding the factors that influence life cycle variation is essential for grasping the biology and ecological roles of species. This study investigates the variation in life cycles and its effects on the genetic diversity of a brown seaweed, Dictyota, across its European range. LocationNorth-East Atlantic Ocean and the Mediterranean Sea. TaxonDictyota dichotoma (Hudson) J.V. Lamouroux (Phaeophyceae, Dictyotales) MethodsWe monitored phenology, fertility and lifespan in Atlantic and Mediterranean populations at its northern and southern boundaries, and used microsatellite markers to assess how these factors influence genetic and genotypic diversity. ResultsWe observed significant differences in phenology, reproductive strategies, and genetic diversity among northern and southern European populations. In Mediterranean populations, D. dichotoma exhibited sporophytic dominance with gametophytes being extremely rare, suggesting a shift towards asexual reproduction. In contrast, North-East Atlantic populations displayed more pronounced seasonal reproductive patterns with higher frequencies of gametophytes, indicating predominant sexual reproduction. However, genetic analysis showed lower allelic richness and unique alleles in northern populations, whereas southern populations were genetically more diverse, reflecting historical biogeographic processes. Clonal reproduction was more pronounced in the Mediterranean populations, influencing the spatial genetic substructure and contributing to lower genotypic diversity compared to Atlantic populations. Main conclusionsOur findings demonstrate that life cycle variation and phenology in D. dichotoma are closely tied to regional environmental conditions and have significant implications for population structure and genetic diversity. Results highlight how shifts in reproductive strategies contribute to the evolutionary and ecological dynamics of marine macroalgae across biogeographical gradients.

ecology↗

bioRxiv2023

Non-indigenous seaweeds in the Northeast Atlantic Ocean, the Mediterranean Sea and Macaronesia: a critical synthesis of diversity, spatial and temporal patterns

Effective monitoring and combatting the effect of non-indigenous seaweeds relies on a solid confirmation of the non-indigenous status of the species. We critically analysed the status of presumed non-indigenous seaweed species reported from the Mediterranean Sea, the Northeast Atlantic Ocean and Macaronesia, resulting in a list of 140 species whose non-indigenous nature is undisputed. For an additional 87 species it is unclear if they are native or non-indigenous (cryptogenic species) or their identity requires confirmation (data deficient species). We discuss the factors underlying both taxonomic and biogeographic uncertainties and outline recommendations to reduce uncertainty about the non-indigenous status of seaweeds. Our dataset consisted of over 19,000 distribution records, half of which can be attributed to only five species (Sargassum muticum, Bonnemaisonia hamifera, Asparagopsis armata, Caulerpa cylindracea and Colpomenia peregrina), while 56 species (40%) are recorded no more than once or twice. In addition, our analyses revealed considerable variation in the diversity of non-indigenous species between the geographic regions. The Eastern Mediterranean Sea is home to the largest fraction of non-indigenous seaweed species, the majority of which have a Red Sea or Indo-Pacific origin and have entered the Mediterranean Sea mostly via the Suez Canal. Non-indigenous seaweeds with native ranges situated in the Northwest Pacific make up a large fraction of the total in the Western Mediterranean Sea, Lusitania and Northern Europe, followed by non-indigenous species with a presumed Australasian origin. Uncertainty remains, however, regarding the native range of a substantial fraction of non-indigenous seaweeds in the study area. In so far as analyses of first detections can serve as a proxy for the introduction rate of non-indigenous seaweeds, these do not reveal a decrease in the introduction rate, indicating that the current measures and policies are insufficient to battle the introduction and spread of non-indigenous species in the study area. HighlightsO_LINon-indigenous seaweed species in the Northeast Atlantic Ocean, the Mediterranean Sea and Macaronesia are critically reanalysed. C_LIO_LI>19,000 distribution records revealed considerable variation in diversity of non-indigenous seaweed species in the study area. C_LIO_LITaxonomic and biogeographic uncertainties hamper a critical evaluation of the non-indigenous status of many seaweed species. C_LI

plant biology↗