Trends in Ecology & Evolution
OpinionGenome Evolution of Coral Reef Symbionts as Intracellular Residents
Section snippets
Symbiodiniaceae: Critical Symbionts of Coral Reefs
Dinoflagellates of the family Symbiodiniaceae are the most prevalent photosynthetic symbionts in tropical and subtropical coral reef ecosystems. Although dinoflagellates are primarily free-living, Symbiodiniaceae have diversified mainly as symbiotic lineages. Previously classified within the genus Symbiodinium and colloquially known as zooxanthellae, Symbiodiniaceae are associated with diverse hosts including corals, jellyfish, clams, and foraminiferans. These symbiotic interactions can occur
Free-Living Species
At one end of the spectrum, some Symbiodiniaceae species have not been found to be associated with a host. These free-living taxa include some species in the Symbiodinium genus (former Clade A; e.g., the type species S. natans and S. pilosum), the exclusively free-living Effrenium genus (former Clade E), and Fugacium (former Clade F) 2., 4..
A free-living lifestyle presents opportunities for the exchange of genetic material (e.g., recombination via sexual reproduction) with conspecifics,
Facultative Symbionts
Most Symbiodiniaceae are symbiotic, representing a broad spectrum of symbiotic associations and a range of host specificity. Their genomes would have experienced the phase of genome instability during the early transition stages to an intracellular lifestyle (and symbiosis). The genomes during this phase may be larger than those of well-established residents and have accumulated extensive structural rearrangements, mobile elements, and pseudogenes (Figure 1B), as observed in other facultative
Obligate Symbionts
At the other end of the spectrum, some Symbiodiniaceae may be obligate symbionts. These taxa are rarely, if at all, found in the environment or reported in culture. However, one cannot dismiss that brief ex hospite stages may still occur due to regular adjustments of symbiont density by the hosts. In the scenario of strict obligate symbionts (Figure 1C), genomes are expected to follow the evolutionary trajectory postulated in the resident genome syndrome (Box 1).
The effective population size (Ne
Concluding Remarks and Future Perspectives
The recent availability of draft genomes from Symbiodiniaceae and systematic revision of these taxa are allowing investigators to venture deeper into the evolutionary history of these ecologically important organisms. In this Opinion article, we discuss the genome evolution of Symbiodiniaceae across the broad spectrum of symbiotic associations. We acknowledge that Symbiodiniaceae ecology is highly intricate. For instance, host specificity does not always correlate with transmission mode or with
Outstanding Questions
How often do Symbiodiniaceae reproduce sexually and under what conditions?
How viable are ex hospite cells in distinct symbiotic Symbiodiniaceae taxa?
Does the burst of mobile element activity in symbiodiniacean genomes relate to the transition to symbiosis and/or the diversification of these lineages?
To what extent do genomes of symbiotic Symbiodiniaceae and other eukaryote residents exhibit symptoms of the resident genome syndrome?
Can the accelerated evolutionary rates typical of intracellular
Acknowledgments
R.A.G-P. is supported by an International Postgraduate Research Scholarship and a University of Queensland Centenary Scholarship. This work is supported by Australian Research Council grant DP150101875 awarded to M.A.R., C.X.C., and D.B. and DP190102474 awarded to C.X.C. and D.B.
Glossary
- Effective population size (Ne)
- in population genetics, the number of effectively reproducing individuals under the assumption of an ideal population.
- Genetic drift
- evolutionary mechanism in which the changes in allele frequencies of a population are driven by chance.
- Genome phasing
- statistical estimation of alleles (or haplotypes) from potentially heterozygous genome data.
- Horizontal transmission
- mode of symbiont transmission in corals in which the dinoflagellate symbionts can be acquired from the
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Facultative lifestyle drives diversity of coral algal symbionts
2024, Trends in Ecology and EvolutionIn the shadow of a giant reef: Palaeoecology of mesophotic coral communities from the Givetian of Anti-Atlas (Morocco)
2022, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :Unfortunately then, the taxonomic affinity of the photosymbionts available for Palaeozoic reef builders remains unknown. Nevertheless, symbionts of the Palaeozoic corals could have possessed a similar suite of adaptation for living on high latitudes like modern dinoflagellates (see González-Pech et al., 2019). This study illustrates the paleoecology of the mesophotic coral ecosystem that developed at the base of one of the most prominent reef structures in the north-western Gondwana – the Givetian Aferdou el Mrakib reef.
Genome-powered classification of microbial eukaryotes: focus on coral algal symbionts
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