Historical biogeography of the widespread macroalga Sargassum (Fucales, Phaeophyceae)

Abstract

Sargassum is a cosmopolitan brown algal genus spanning the three ocean basins of the Atlantic, Pacific and Indian Oceans, inhabiting temperate, subtropical and tropical habitats. Sargassum has been postulated to have originated in the Oligocene epoch approximately 30 mya according to a broad phylogenetic analysis of brown macroalgae, but its diversification to become one of the most widespread and speciose macroalgal genera remains unclear. Here, we present a Bayesian molecular clock study, which analyzed data from the order Fucales of the brown algal crown radiation (BACR) group to reconstruct a time‐calibrated phylogeny of the Sargassum clade. Our phylogeny included a total of 120 taxa with 99 Sargassum species sampled for three molecular markers – ITS‐2, cox3 and rbcLS – calibrated with an unambiguous Sargassaceae fossil from between the lower and middle Miocene. The analysis revealed a much later origin of Sargassum than expected at about 6.7 mya, with the genus diversifying since approximately 4.3 mya. Current geographic distributions of Sargassum species were then analyzed in conjunction with the time‐calibrated phylogeny using the dispersal‐extinction‐cladogenesis (DEC) model to estimate ancestral ranges of clades in the genus. Results strongly support origination of Sargassum in the Central Indo‐Pacific (CIP) region with subsequent independent dispersal events into other marine realms. The longer history of diversification in the ancestral CIP range could explain the much greater diversity there relative to other marine areas today. Analyses of these dynamic processes, when fine‐tuned to a higher spatial resolution, enable the identification of evolutionary hotspots and provide insights into long‐term dispersal patterns.

Phylogeny and Evolution of the Brown Algae

“…This is presumably the dominant process in lineages such as Lessonia or Durvillaea, where species rarely overlap in range and patterns of speciation tend to represent lineage dispersal pathways (Fraser et al, 2010;Zuccarello and Martin, 2016). Similarly, recent phylogeographic analyses of both the Laminariales (Starko et al, 2019) and the genus Sargassum (Yip et al, 2020) indicate that regionspecific diversification is likely a common process. The high number of regionally endemic large brown algae in each of the Southern Hemisphere temperate regions (S. America, S. Africa and Australia/New Zealand) suggests that trans-oceanic distances remain effective barriers to dispersal (Peters et al, 1997;Phillips, 2001;Bolton, 2010), contrasting with the comparatively contiguous landmasses in the Northern Hemisphere.…”

“…The earliest phylogenetic studies were limited by the coarse resolution of chosen markers (i.e. 18S; Tan and Druehl, 1993), but further work has dramatically enhanced our knowledge of brown algal systematics by including multiple markers, time calibrated phylogenies (Silberfeld et al, 2010;Martin and Zuccarello, 2012;Starko et al, 2019;Yip et al, 2020), and, more recently, genome-scale datasets for some brown algal groups (Jackson et al, 2017;Starko et al, 2019).…”

“…The phylogeny and diversification of Fucales has been the subject of several studies (e.g. Serrão et al, 1999;Coyer et al, 2006;Fraser et al, 2010;Draisma, Ballesteros, et al, 2010;C anovas et al, 2011;Bruno de Sousa et al, 2019;Yip et al, 2020).…”

“…Brown algae are less prevalent in the tropics, with most tropical diversity stemming from Dictyotales (particularly Dictyota, Lobophora, and Padina; Silberfeld et al, 2014;Vieira et al, 2017) and Fucales (primarily Sargassum). Although most lineages of large habitat forming algae (e.g., Fucaceae, Laminariales, Durvilliaceae) are generally absent from tropical regions, the genus Sargassum is a notable exception with more than 350 currently recognized species (Yip et al, 2020). Despite the warm oligotrophic waters of the tropics, Sargassum manages to form analogous habitats to kelp forests in some areas (Coleman and Wernberg, 2017;Fulton et al, 2019), indicating that large brown algae are capable of evolving tolerance to low nitrogen and high temperatures.…”

Takeaways from Mobile DNA Barcoding with BentoLab and MinION

“…The practice of DNA barcoding—involving the generation of standardized genetic markers that, when matched to databases, allow for species identification—was first popularized by Hebert et al (2003) [ 1 ]. Since then, the field of DNA barcoding has evolved and expanded considerably beyond just species identification [ 2 , 3 , 4 , 5 ] to include species discovery, population genetics, and phylogenetics [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. This rapid growth in DNA barcoding capabilities has occurred as a result of advancements in sequencing technologies.…”