Expanding the population genetic perspective of cnidarian‐Symbiodinium symbioses

The modern synthesis was a seminal period in the biological sciences, establishing many of the core principles of evolutionary biology that we know today. Significant catalysts were the contributions of R.A. Fisher, J.B.S. Haldane and Sewall Wright (and others) developing the theoretical underpinning of population genetics, thus demonstrating adaptive evolution resulted from the interplay of forces such as natural selection and mutation within groups of individuals occupying the same space and time (i.e. a population). Given its importance, it is surprising that detailed population genetic data remain lacking for numerous organisms vital to many ecosystems. For example, the coral reef ecosystem is well recognized for its high biodiversity and productivity, numerous ecological services and significant economic and societal values (Moberg & Folke 1999; Cinner 2014). Many coral reef invertebrates form symbiotic relationships with single‐celled dinoflagellates within the genus Symbiodinium Freudenthal (Taylor 1974), with hosts providing these (typically) intracellular symbionts with by‐products of metabolism and in turn receiving photosynthetically fixed carbon capable of meeting hosts’ respiratory demands (Falkowski et al. 1984; Muscatine et al. 1984). Unfortunately, the health and integrity of the coral reef ecosystem has been significantly and negatively impacted by onslaughts like anthropogenic eutrophication and disease in addition to global climate change, with increased incidences of ‘bleaching’ events (characterized as the loss of photosynthetic pigments from the algal cell or massive reduction of Symbiodinium density from hosts’ tissue) and host mortality leading to staggering declines in geographic coverage (Bruno & Selig 2007) that have raised questions on the viability of this ecosystem as we know it (Bellwood et al. 2004; Parmesan 2006). One avenue towards anticipating the future of the coral reef ecosystem is by developing a broader and deeper understanding of the current genotypic diversity encompassed within and between populations of their keystone species, the scleractinian corals and dinoflagellate symbionts, as they potentially possess functional variation (either singularly or in combination) that may come under selection due to the ongoing and rapid environmental changes they are experiencing. However, such studies, especially for members of the genus Symbiodinium, are sparse. In this issue, Baums et al. (2014) provide a significant contribution by documenting the range‐wide population genetics of Symbiodinium ‘fitti’ (Fig. 1 ) in the context of complementary data from its host, the endangered Caribbean elkhorn coral Acropora palmata (Fig. 1 ). Notable results of this study include a single S. ‘fitti’ genotype typically dominates an individual A. palmata colony both spatially and temporally, gene flow among coral host populations is a magnitude higher to that of its symbiont populations, and the partners possess disparate patterns of genetic differentiation across the Greater Caribbean. The implications of such findings are discussed herein.