Novel Genetic Diversity Through Somatic Mutations: Fuel for Adaptation of Reef Corals?

Abstract

Adaptation of reef corals to climate change is an issue of much debate, and often viewed as too slow a process to be of relevance over decadal time scales. This notion is based on the long sexual generation times typical for some coral species. However, the importance of somatic mutations during asexual reproduction and growth on evolution and adaptation (i.e., cell lineage selection) is rarely considered. Here we review the existing literature on cell lineage selection and show that the scope for somatic mutations to arise in the coral animal and associated Symbiodinium is large. For example, we estimate that ~100 million somatic mutations can arise within a branching Acropora coral colony of average size. Similarly, the large population sizes and rapid turn-over times of in hospite Symbiodinium likely result in considerable numbers of somatic mutations. While the fate of new mutations depends on many factors, including ploidy level and force and direction of selection, we argue that they likely play a key role in the evolution of reef corals

Rapid adaptive responses to climate change in corals

“…In long-lived corals, somatic mutations may accrue over the lifetime of modular colonies 18 , highlighting another mechanism that could potentially aid phenotypic responses to environmental changes within the lifespan of the colony. Evolution through somatic mutations, as in the case of transgenerational epigenetic inheritance, is more likely to have a role in organisms that lack distinct segregation of the somatic and germ lines, such as fungi, plants and corals (but see ref.…”

“…The dinoflagellate genus Symbiodinium contains enormous genetic and functional diversity 71 , and communities associated with corals vary among species, environments and host PERSPECTIVE NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3374 microhabitats 72 . The short generation time of Symbiodinium means that its rate of mutation is much faster than for the coral host 18 , and this, combined with its large within-host population sizes, potentially facilitates rapid responses to altered thermal environments, either through selection of existing genetic variants or through the evolution of novel adaptations 73,74 . Alternatively, the composition of host-associated Symbiodinium communities may vary temporally in response to environmental conditions or at different host lifehistory stages 75 , either through shuffling of existing symbionts 76 or through acquisition of new Symbiodinium types from the environment (that is, switching) 16 .…”

“…While the long lifespans and extensively overlapping generations typical of scleractinian corals might be expected to restrict the pace of genetic adaptation, this effect may be offset by other characteristics, particularly their close associations with a diverse range of microbes, high standing genetic variation (Box 1), colonial organization and high fecundity 18 .…”

Marginal coral populations: the densest known aggregation of Pocillopora in the Galápagos Archipelago is of asexual origin

“…While local density is quite high, the low genotypic diversity may limit the evolutionary potential to selfing and somatic mutations (Van Oppen et al, 2011). No evidence of selfing was found within the pools as that would have generated distinct albeit similar genotypes rather than identical ones.…”

Acclimatization of the Tropical Reef Coral Acropora millepora to Hyperthermal Stress

“…Field and laboratory evidence demonstrate that that ocean temperatures just 1°C warmer than the typical summer maxima can cause coral bleaching [10,12,14,15], a potentially lethal breakdown of the symbiotic relationship between the coral animal host and Symbiodinium, photosynthetic dinoflagellates which provide a nutritional symbiosis via their intracellular existence within the coral. The symptom of physiological distress, bleaching, is so termed because of the apparent loss of coral coloration, which is the result of the loss of Symbiodinium and/or their associated photopigments, and may ultimately end in the death of the coral colony [16].…”