Abstract
BackgroundScleractinian corals and their algal endosymbionts (genus Symbiodinium) exhibit distinct bathymetric distributions on coral reefs. Yet, few studies have assessed the evolutionary context of these ecological distributions by exploring the genetic diversity of closely related coral species and their associated Symbiodinium over large depth ranges. Here we assess the distribution and genetic diversity of five agariciid coral species (Agaricia humilis, A. agaricites, A. lamarcki, A. grahamae, and Helioseris cucullata) and their algal endosymbionts (Symbiodinium) across a large depth gradient (2-60 m) covering shallow to mesophotic depths on a Caribbean reef.ResultsThe five agariciid species exhibited distinct depth distributions, and dominant Symbiodinium associations were found to be species-specific, with each of the agariciid species harbouring a distinct ITS2-DGGE profile (except for a shared profile between A. lamarcki and A. grahamae). Only A. lamarcki harboured different Symbiodinium types across its depth distribution (i.e. exhibited symbiont zonation). Phylogenetic analysis (atp6) of the coral hosts demonstrated a division of the Agaricia genus into two major lineages that correspond to their bathymetric distribution (“shallow”: A. humilis / A. agaricites and “deep”: A. lamarcki / A. grahamae), highlighting the role of depth-related factors in the diversification of these congeneric agariciid species. The divergence between “shallow” and “deep” host species was reflected in the relatedness of the associated Symbiodinium (with A. lamarcki and A. grahamae sharing an identical Symbiodinium profile, and A. humilis and A. agaricites harbouring a related ITS2 sequence in their Symbiodinium profiles), corroborating the notion that brooding corals and their Symbiodinium are engaged in coevolutionary processes.ConclusionsOur findings support the hypothesis that the depth-related environmental gradient on reefs has played an important role in the diversification of the genus Agaricia and their associated Symbiodinium, resulting in a genetic segregation between coral host-symbiont communities at shallow and mesophotic depths.
Highlights
Scleractinian corals and their algal endosymbionts exhibit distinct bathymetric distributions on coral reefs
We know that Symbiodinium diversity in tropical scleractinian corals consists of several different phylogenetic clades [13,14,15], species [16,17] and hundreds of different subclades based on the ITS2 region of the ribosomal DNA [14,16]
At 10-15 m, A. agaricites becomes the most dominant species, with colonies of A. humilis and A. lamarcki being present in lower abundances
Summary
Scleractinian corals and their algal endosymbionts (genus Symbiodinium) exhibit distinct bathymetric distributions on coral reefs. Few studies have assessed the evolutionary context of these ecological distributions by exploring the genetic diversity of closely related coral species and their associated Symbiodinium over large depth ranges. We know that Symbiodinium diversity in tropical scleractinian corals consists of several different phylogenetic clades [13,14,15], species [16,17] and hundreds of different subclades based on the ITS2 region of the ribosomal DNA [14,16] These different Symbiodinium genotypes often represent physiologically distinct entities that can be strongly partitioned across host species, geographic regions and particular reef environments [18,19,20,21,22,23]. Ecological niche partitioning across environmental gradients (e.g. light, temperature, turbidity) appears fundamental to maintaining the diversity of both scleractinian corals and their endosymbiotic dinoflagellates, the evolutionary context of this hostsymbiont niche partitioning remains poorly understood
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