Steen, F. | 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↗

bioRxiv2019

Population structure and geographically structured reproductive strategies of the haplodiplontic seaweed Dictyota dichotoma

Both mating system variation and the propensity of many seaweeds to reproduce both sexually and asexually, leave a strong imprint in the genetic structure of species. In this respect, we study the population genetic structure of Dictyota dichotoma, a common haplodiplont brown subtidal seaweed. This benthic species is widespread in the NE-Atlantic, from the Canary Islands and Mediterranean Sea to southern Norway, but lately populations have been reported from Argentina and South Africa. Phenology and reproduction of D. dichotoma was monitored year-round in four populations to investigate how the species has adapted to the steep thermal gradient in southern and northern ranges of its distribution. Thirteen microsatellites are developed in order to assess patterns of population diversity and structure across the biogeographic range, as shaped by past and present processes. Last, we assess the genetic structure of South African and South American populations and their relationship to the northern hemisphere populations.\n\nThroughout its range, D. dichotoma shows a varying reproductive effort, with sexual reproduction being more abundant in the northern range. In contrast, the Mediterranean populations show a clear sporophyte dominance, suggesting that sexual reproduction is not the prime mode of reproduction, and indicating that the species potentially resorts to other modes of propagation as for instance fragmentation or apospory.\n\nGenetic diversity is highest in the southern population decreasing gradually northward, indicative for a recolonization pattern after the demise of the last glacial maximum where these areas served as glacial refugia. European mainland populations show an isolation by distance pattern, while the population in the Canary Islands has its own genetic identity, being significantly diverged from the mainland population. Populations in South Africa and Argentina are seemingly introduced from mainland Europe, but no conclusion can be made on the exact timing of these introductions.

ecology↗