Cladocopium community divergence in two Acropora coral hosts across multiple spatial scales

Abstract

17Many broadly-dispersing corals acquire their algal symbionts (Symbiodiniaceae) from their 18 environment upon recruitment. This horizontal transmission strategy has the potential to promote 19 coral fitness across diverse environments provided that corals can associate with diverse algae 20 across their range and that these symbionts exhibit reduced dispersal potential. Here we quantified 21 genetic structure of algal symbionts in symbiosis with two hosts (Acropora hyacinthus, Acropora 22 digitifera) across two spatial scales (across islands, within islands) in Micronesia using 23 microsatellites. We contrast these symbiont genetic structures to previously published coral host 24 structures across the same spatial gradient. We find that both hosts associated with two genetically 25distinct Cladocopium lineages (C3, C40), confirming that Acropora coral hosts associate with 26 diverse algae across this range. In addition, each Cladocopium lineage exhibited signatures of 27 moderate host specialization. Across locations, algal populations were significantly more 28genetically structured compared to their hosts: they demonstrated higher FST among islands and 29 often exhibited significant divergence among reefs on the same island, which was never observed 30 in coral hosts. While Cladocopium C3, like their hosts, broadly followed an isolation-by-distance 31 pattern across islands, there was one notable case of no genetic structure across more than 2,000 32 km (between Yap and Pohnpei), demonstrating that Cladocopium genetic structure can be more 33 complex than their hosts. Overall, our results support the view that horizontal transmission -34 establishing novel symbioses with local symbionts -has the potential to facilitate local fitness for 35 broadly dispersing coral species, and highlight the complexity of factors affecting the population 36 biology of Cladocopium. 37 38

Corals in the hottest reefs in the world exhibit symbiont fidelity not flexibility

“…Western Pacific (Cladocopium C3 and C40; Davies, Wham, Kanke, & Matz, 2019) that were previously viewed as "generalist" symbiont taxa (LaJeunesse et al, 2003). As in these examples, the sympatric divergence of closely related Symbiodiniaceae in the northeastern Arabian Peninsula indicates the specialization of symbionts to distinct host microenvironments.…”

“…This demonstrates that the fine‐scale symbiont specificity initially observed among these coral species (Smith, Ketchum, & Burt, ) extends across sites and regional divisions in Symbiodiniaceae genera. Similarly, several coral species host unique variants of common ITS2 types in the Caribbean ( Cladocopium C3, Breviolum B1: Santos et al, ; Finney et al, ; Thornhill et al, ) and Western Pacific ( Cladocopium C3 and C40; Davies, Wham, Kanke, & Matz, ) that were previously viewed as “generalist” symbiont taxa (LaJeunesse et al, ). As in these examples, the sympatric divergence of closely related Symbiodiniaceae in the northeastern Arabian Peninsula indicates the specialization of symbionts to distinct host microenvironments.…”

“…The fidelity of these partnerships, maintained by recognition and winnowing mechanisms (Fransolet, Roberty, & Plumier, ), results in reproductive isolation, and ultimately speciation (Finney et al, ; Thornhill et al, ). Investigations of fine‐scale genetic variation are lacking from the Indo‐Pacific beyond the Arabian Peninsula (but see Davies et al, ), and are required to re‐evaluate the generalist nature of ITS2 lineages that are common to many host species. However, it is likely that species‐level diversity within the Symbiodiniaceae is currently underestimated.…”

“…Indo-Pacific beyond the Arabian Peninsula (but see Davies et al, 2019), and are required to re-evaluate the generalist nature of ITS2 lineages that are common to many host species. However, it is likely that species-level diversity within the Symbiodiniaceae is currently underestimated.…”

Peer Review #3 of "Searching for phylogenetic patterns of Symbiodiniaceae community structure among Indo-Pacific Merulinidae corals (v0.2)"

“…For example, Siderastrea siderea in the Belize Mesoamerican Barrier Reef System contains a distinct Symbiodiniaceae community signal (Baumann et al, 2018). This host specificity reflects various interacting physiological needs of the coral species, such as their depth, light, temperature or nutrient preferences (Finney et al, 2010), and the endosymbiont populations can be further structured according to the environment (Thornhill et al, 2009;Davies et al, 2019). Interestingly, the host species signal breaks down at the genus level, as Symbiodiniaceae communities in Siderastrea radians are more similar to Pseudodiploria strigosa than its congeneric Siderastrea siderea (Baumann et al, 2018).…”