University of Tasmania

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The ecological and evolutionary significance of reproductive traits in corals

posted on 2023-05-27, 07:09 authored by Schmidt-Roach, S
In this thesis, I explore species boundaries in corals and I investigate how distinct reproductive strategies may in uence gene ow within and among species and ultimately how this aects processes of genetic isolation and speciation in corals. Reconstructing genealogical relationships among colonies of pocilloporid corals sampled along Eastern Australia in chapter 2 conrmed the hypothesis that the extensively studied species P. damicornis constitutes a cryptic species complex rather than a single reproductively and morphologically plastic species. Haplotype networks computed from two mitochondrial DNA regions (CR, ORF) recovered at least ve genetically distinct mitochondrial lineages within P. damicornis corresponding to distinct morphotypes previously considered ecomorphs. In addition, brooding and spawning observations on the Great Barrier Reef (GBR), suggested that the ancestral reproductive strategy in Pocillopora is broadcast spawning, while asexual brooding seems to be an apomorphic trait, characteristic to a single genetic clade that maintains a mixed mode of spawning and brooding. This conrmed the hypothesis for chapter 3 that P. damicornis (or hidden species within) brood larvae in Eastern Australia which are solely asexual. Further, the brooding clade contained three genetic lineages; two of them occur in sympatry in the Great Barrier Reef (GBR) and maintain brooding periods at opposite lunar phases. These GBR lineages showed clear nuclear divergences (HSP70, ITS2) conrming reproductive isolation likely due to dierences in reproductive timing. In contrast, other lineages within Pocillopora lacked sharp nuclear divergence despite belonging to dierent mitochondrial clades and exhibiting dierent reproductive traits, indicating that occasional hybridisation occurs even between distant, otherwise well-dened lineages. Further investigations, including additional Pocillopora species from Australia, conrmed mitochondrial molecular phylogenies are congruent with groups based on gross-morphology and symbiont association, therefore re ecting species-level dierentiation. Fine scale morphological variation, particularly the shape and type of columella, added additional support for the dierentiation of genetic lineages and provided an excellent signature of the evolutionary relationships amongst them. The apparent misinterpretation of taxonomical units within P. damicornis may explain the perceived variation in the ecology, biology and life history of this species across its range. In order to provide appropriate taxonomic units for future references, colonies genotyped from Eastern Australia were compared to type specimens and original descriptions, which allowed for the revision of the taxonomic status of eight closely related species to and within the P. damicornis species complex, including the description of two new taxa. This conrmed the hypothesis that genetic lineages identied within P. damicornis correspond to taxonomic morphospecies (Chapter 4 & 5). Much of what we know about reproductive relationships, including the ability of species to hybridise or self-fertilise, stems from ex situ fertilisation experiments and relies on the assumption that laboratory trials adequately re ect processes that are occurring in natural environments. However, this has never been tested empirically. Given that reproductive relationships within and among species play such an important role in the evolution of species, it is critical that we fully understand the links between laboratory trials and real world processes in order to make accurate predictions about species relationships and population dynamics. In chapter 6 I assessed the nature of fertilisation ex - versus in-situ in the hermaphroditic broadcast-spawning coral Goniastrea favulus, using seven newly developed microsatellite markers to compare estimates of selng and outcrossing under natural and experimental conditions. In particular, I tested for the hypotheses that self-fertilisation represents an important reproductive strategy in Goniastrea favulus and that G. favulus favours non-self over self-fertilisation in situations of sperm choice. In the presence of selfand non-self sperm, the ex situ experiments showed high levels of selng in G. favulus (30%), consistent with the known ability of this species to self-fertilise. However genotyping larvae fertilised under natural conditions revealed low levels of selng on the reef (<4%), suggesting that the occurrence of selng in this species may have been signicantly overestimated in laboratory trials and that laboratory experiments may fail to re ect real world conditions and lead to a distorted perception of reproductive relationships within and among coral species. In particular, the importance of inter-specic hybridisation between some coral species gained from similar laboratory fertilisation trials may have been similarly overestimated. Hybridisation has been thought to be an important aspect of coral reproduction and has been considered, in part, to explain blurred morphological species boundaries in many genera and consequently reticulate evolution has been considered central to coral speciation. However, conclusions from ex situ trials that hybridisation occurs commonly, and hence its signicance for coral evolution, may well be misleading. Nonetheless, my genetic data on Pocillopora suggest that occasional introgressive hybridisation at evolutionary time scales is likely to occur between some coral species. Within the genus Pocillopora semi-permeable species boundaries facilitated by occasional gene- ow among taxa may be an important evolutionary mechanism and may well contribute to the adaptive potential of this group as it allows for the exchange of benecial genetic information between species, e.g. genes supporting adaptation to new environmental conditions as a result of climate change. Genetic transfer between lineages may reduce the individual risk of extinction and accelerate evolutionary rates leading to a higher relative potential for adaptation. Consequently, evolutionary plasticity facilitated by variation in reproductive traits may raise hope for the persistence of corals facing future climate change.


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Copyright 2013 the Author Chapter 2 is the equivalent of a post-print article published as: Schmidt-Roach, S.; Lundgren, P.; Miller, K. J. ; Gerlach, G.; Noreen, A. M. E.; Andreakis N. (2013), Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia, Coral reefs, 32(1), 161-172. The final publication is available at Springer via Chapter 5 is the equivalent of a post-print article published as: Sebastian Schmidt-Roach, S.; Miller, K. J.; Lundgren, P.; Andreakis, N. (2014), With eyes wide open: a revision of species within and closely related to the pocillopora damicornis species complex (scleractinia; pocilloporidae) using morphology and genetics. Zoological Journal of the Linnean Society. 170, (1), 1-33

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