Cryptic diversity hides host and habitat specialization in a gorgonian‐algal symbiosis

Abstract

Shallow water anthozoans, the major builders of modern coral reefs, enhance their metabolic and calcification rates with algal symbionts. Controversy exists over whether these anthozoan-algae associations are flexible over the lifetimes of individual hosts, promoting acclimative plasticity, or are closely linked, such that hosts and symbionts co-evolve across generations. Given the diversity of algal symbionts and the morphological plasticity of many host species, cryptic variation within either partner could potentially confound studies of anthozoan-algal associations. Here, we used ribosomal, organelle and nuclear sequences, along with microsatellite variation, to study the relationship between lineages of a common Caribbean gorgonian and its algal symbionts. The gorgonian Eunicea flexuosa is a broadcast spawner, composed of two recently diverged, genetically distinct lineages largely segregated by depth. We sampled colonies of the two lineages across depth gradients at three Caribbean locations. We find that each host lineage is associated with a unique Symbiodinium B1/184 phylotype. This relationship between host and symbiont is maintained when host colonies are reciprocally transplanted, although cases of within phylotype switching were also observed. Even when the phylotypes of both partners are present at intermediate depths, the specificity between host and symbiont lineages remained absolute. Unrecognized cryptic diversity may mask host-symbiont specificity and change the inference of evolutionary processes in mutualistic associations. Symbiotic specificity thus likely contributes to the ecological divergence of the two partners, generating species diversity within coral reefs.

Species are hypotheses: avoid connectivity assessments based on pillars of sand

“…Clearly, these technical difficulties should be overcome thanks to single-copy nuclear markers (e.g. Prada et al 2014) and to new methods based on next-generation sequencing, such as RAD sequencing (Baird et al 2008). In a single run, many independent loci covering a large part of the genome are sequenced.…”

The extended phenotypes of marine symbioses: ecological and evolutionary consequences of intraspecific genetic diversity in coral–algal associations

“…An example would be the Acropora palmata–Symbiodinium “fitti” association, where pairings of single host and symbiont genotypes produce holobionts that may each exhibit unique extended phenotypes ( Figure 1 ; Baums et al, 2014, Parkinson et al, submitted). In fact, in studies where microsatellite markers have been used to characterize both partners, the host:symbiont genotype ratio is one:one in >70% of colonies (Goulet and Coffroth, 2003a,b; Santos et al, 2003b; Kirk et al, 2005; Pettay and LaJeunesse, 2007, 2009, 2013; Thornhill et al, 2009, 2013; Andras et al, 2011; Pettay et al, 2011; Pinzon et al, 2011; Baums et al, 2014; Prada et al, 2014b). This outcome falls in line with the predictions of basic population theory, as closely related organisms generally compete for similar resources, leading to competitive exclusion among similar species (Gause, 1934; Hardin, 1960).…”

“…Armed with such markers, it is now possible to sample a single coral colony and determine not only its host and symbiont species compositions, but also to resolve unique multilocus genotypes (i.e., individuals) within each species. However, only rarely have both host and symbiont genotype composition been analyzed in concert (Andras et al, 2011, 2013; Pettay et al, 2011; Pettay and LaJeunesse, 2013; Thornhill et al, 2013; Baums et al, 2014; Prada et al, 2014b). So far this has only been done in a general population survey context, with most evidence suggesting that the genetic structuring of the host and the symbiont are not the same (e.g., Baums et al, 2014).…”

Identifying cryptic diversity with predictive phylogeography

“…Third, the discovery of cryptic diversity is critical to understanding anthropogenic biotic changes, invasions and ecosystem health [7]. Thus, cryptic diversity is a vital component of biological diversity [8,9], and its discovery is central to documenting fundamental units in ecology [10], evolution and conservation [2].…”