Establishment of Coral–Algal Symbiosis Requires Attraction and Selection

Citation: Yamashita H, Suzuki G, Kai S, Hayashibara T, Koike K ( Establishment of Coral-Algal Symbiosis Requires Attraction and Selection Hiroshi Yamashita 0 Go Suzuki 0 Sayaka Kai 0 Takeshi Hayashibara 0 Kazuhiko Koike 0 M onica Medina, Pennsylvania State University, United States of America 0 1 Research Center for Subtropical Fisheries, Seikai National Fisheries Research Institute, Fisheries Research Agency , Ishigaki, Okinawa , Japan , 2 Graduate School of Biosphere Science, Hiroshima University , Higashi-Hiroshima, Hiroshima , Japan Coral reef ecosystems are based on coral-zooxanthellae symbiosis. During the initiation of symbiosis, majority of corals acquire their own zooxanthellae (specifically from the dinoflagellate genus Symbiodinium) from surrounding environments. The mechanisms underlying the initial establishment of symbiosis have attracted much interest, and numerous field and laboratory experiments have been conducted to elucidate this establishment. However, it is still unclear whether the host corals selectively or randomly acquire their symbionts from surrounding environments. To address this issue, we initially compared genetic compositions of Symbiodinium within naturally settled about 2-week-old Acropora coral juveniles (recruits) and those in the adjacent seawater as the potential symbiont source. We then performed infection tests using several types of Symbiodinium culture strains and apo-symbiotic (does not have Symbiodinium cells yet) Acropora coral larvae. Our field observations indicated apparent preference toward specific Symbiodinium genotypes (A1 and D1-4) within the recruits, despite a rich abundance of other Symbiodinium in the environmental population pool. Laboratory experiments were in accordance with this field observation: Symbiodinium strains of type A1 and D1-4 showed higher infection rates for Acropora larvae than other genotype strains, even when supplied at lower cell densities. Subsequent attraction tests revealed that three Symbiodinium strains were attracted toward Acropora larvae, and within them, only A1 and D1-4 strains were acquired by the larvae. Another three strains did not intrinsically approach to the larvae. These findings suggest the initial establishment of corals-Symbiodinium symbiosis is not random, and the infection mechanism appeared to comprise two steps: initial attraction step and subsequent selective uptake by the coral. - Funding: This work was supported by a Grant-in-Aid for Scientific Research (B) (No. 21310011) and (C) (No. 24570028) from the Ministry of Education, Culture, Sports, Science and Technology of Japan to KK. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Reef-building corals engage zooxanthellae, as symbionts that supply them with photosynthetic products, which enable corals to effloresce in oligotrophic tropical seas. This coralalgal symbiosis is a fundamental pillar for biologically and economically important coral reef ecosystems. The symbiont algae, dinoflagellate genus Symbiodinium are divided into nine phylogenetically distinct genetic groups (clades AI) [1], and each clade consists of numerous genotypes (e.g. [2]). Physiological responses to environmental stresses may differ among different clades and genotypes [3,4]. Sexual progeny of corals can acquire Symbiodinium by either of the two modes: vertical transmission (maternal inheritance) or horizontal transmission (acquisition from environment), and corals that acquire Symbiodinium from the environment are predominant [5]. The horizontal transmission is considered to be advantageous by enabling corals to acquire Symbiodinium adapted to their newly settled environments, however, there is still scant evidence supporting this idea [6]. Furthermore, it is still unclear how corals that acquire Symbiodinium by horizontal transmission recognize and acquire their symbionts from the environmental population. To answer this question, several infection experiments have been carried out using Symbiodinium culture strains or freshly isolated cells from adult corals or other zooxanthellate animals [7 12]. Although such infections were usually successful, the results often differed between experiments. For example, Cumbo et al. [11] reported that Acropora larvae can acquire a wide variety of Symbiodinium clades, whereas Yuyama et al. [12] demonstrated that the infectivity of Symbiodinium cells in Acropora tenuis juveniles can differ among the Symbiodinium clades. Thus, it is still unclear whether the new generations of host corals acquire their own Symbiodinium randomly or selectively. Previous field observations revealed that in the common reef-building coral Acropora, Symbiodinium genotype compositions often differ between recruitment/juvenile stages and adult populations [13,14]. Yamashita et al. [14] only detected clade A and/or D Symbiodinium in 55 naturally settled 2-week-old Acropora recruits, whereas clade C Symbiodinium, which are the dominant symbionts in adult Acropora corals, were never detected. The Acropora recruits tested by Yamashita et al. [14] were identified to the species level by Suzuki et al. [15] and they comprised at least 10 Acropora species, including the dominant Acropora species in this area, e.g., Acropora hyacinthus, Acropora digitifera, Acropora nasuta, Acropora intermedia, and Acropora selago. Thus, it is plausible to suggest that clades A and D Symbiodinium play important roles during the initial symbiosis. In the present study, to clarify whether Acropora corals acquire their symbionts selectively or randomly, we compared the Symbiodinium genotype compositions (at a finer scale compared with Yamashita et al. [14]) in naturally settled Acropora recruits (approximately 2 weeks old) and in the adjacent seawater. Furthermore, we also conducted laboratory experiments in which the Acropora coral larvae were artificially infected with naturally occurring densities of various Symbiodinium genotypes. Our results demonstrated that in the initial stage of symbiosis, Acropora corals had an apparent preference for specific genotypes, even when these genotypes were present at much lower densities than the non-selected Symbiodinium. The mechanism of this preferential association was examined by an attraction experiment to determine whether the corals selected the symbionts or whether the Symbiodinium selected the corals. Our results from field observations and laboratory experiments suggest underlining two-step mechanisms of attraction and selection for the initial establishment of coralsSymbiodinium symbiosis. Symbiodinium composition in the environment and natural Acropora recruits Symbiodinium genotype composition occurring in the seawater and within naturally settled recruits of Acropora corals (approximately 2 weeks after spawning) are shown in Figure 1 (...truncated)