Phylogeography of Colpomenia sinuosa (Ectocarpales, Phaeophyceae) along the Brazilian coast.

Colpomenia sinuosa is a cosmopolitan brown macroalgal species complex and hence a great candidate for evolutionary studies in the marine environment. Since 2009, three major C. sinuosa phylogenetic lineages, subdivided into eight subgroups, have been identified based on cox3 DNA sequences from worldwide collections. However, worldwide sampling remains limited and spotty. To date molecular data from Brazilian C. sinuosa populations have been limited to 10 specimens collected in a single locality. Nonetheless, C. sinuosa populations occur along the entire ~8,000 km Brazilian coast. Consequently, knowledge on population genetic diversity and spatial genetic structuring along most of the Brazilian coastline is nonexistent. To fulfill this gap in knowledge, we performed a phylogeographic analysis of C. sinuosa populations in Brazil. The highly variable cox3 marker was sequenced for 148 individuals collected in 12 localities in Brazil. Results identified two genetically distinct population groups (north vs. south) separated at 20.5° S latitude. Genetic diversity in northern populations is 14.6 and 15.5 times greater than southern populations in terms of haplotype and nucleotide diversity, respectively. Among northern populations, the Bahia state holds the largest genetic diversity. The southern populations had lower genetic diversity and no internal genetic sub-structure suggesting past bottlenecks followed by recent colonization from northern haplotypes. Our results do not indicate recent introductions of foreign haplotypes in Brazil and reinforce the crucial importance of historical and extant allopatric, parapatric, and sympatric processes driving marine macroalgal evolution in the Southwestern Atlantic Ocean.

Marine Heatwave Drives Cryptic Loss of Genetic Diversity in Underwater Forests.

Extreme events have profound ecological impacts on species and ecosystems, including range contractions and collapse of entire ecosystems. Although theory predicts that extreme events cause loss of genetic diversity, empirical demonstrations are rare, obscuring implications for future adaptive capacity of species and populations. Here, we use rare genetic data from before an extreme event to empirically demonstrate massive and cryptic loss of genetic diversity across ∼800 km of underwater forests following the most severe marine heatwave on record. Two forest-forming seaweeds (Sargassum fallax and Scytothalia dorycarpa) lost ∼30%-65% of average genetic diversity within the 800-km footprint of the heatwave and up to 100% of diversity at some sites. Populations became dominated by single haplotypes that were often not dominant or present prior to the heatwave. Strikingly, these impacts were cryptic and not reflected in measures of forest cover used to determine ecological impact of the heatwave. Our results show that marine heatwaves can drive strong loss of genetic diversity, which may compromise adaptability to future climatic change.