Molecular bases of soft coral reproduction

Wessels, Wiebke (2016) Molecular bases of soft coral reproduction. PhD thesis, James Cook University.

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Abstract

The early development of the hard coral Acropora millepora has been described at both the morphological and molecular levels, and comparative studies are under way for other Scleractinia. Complementary data on Octocoral embryogenesis are, however, essential for a broader understanding of the different developmental strategies in Cnidaria and their evolution. Although the morphology of soft coral embryogenesis has been previously described, data are scarce and molecular data complementing the morphology are missing. This thesis involves the investigation of reproduction and early development in soft corals, using Lobophytum pauciflorum as model. As in the majority of soft corals, L. pauciflorum colonies are either female or male. The time of reproduction approximately coincides with the annual synchronised mass spawning of hard corals on the Great Barrier Reef (GBR), although in favourable years studied, it occurred a lunar month earlier. The process of gametogenesis is longer in the gonochoric soft coral than in the hermaphrodite hard coral A. millepora or the gonochoric hard coral Fungia concinna. However oogenesis is completed within a year, which is shorter than has been reported for a conspecific population in Taiwan. In addition, bidirectional sex change was observed in F. concinna, suggesting that this trait could also be a predominant reproductive feature in fungiid corals.

After studying the gametogenic cycle of L. pauciflorum on the GBR and identifying the time point of spawning as 4 − 5 nights after the austral spring full moon, investigation of early development was possible. Following mixing of eggs and sperm, the first obvious signs of successful fertilisation occurred at the 16 − 32 cell stages. Through a series of cell divisions leading to irregular shapes, embryos developed into a swimming planula within 48 hours of fertilisation. Planulae then stayed in the water column for up to a week before settling and developing into a primary polyp with 8 tentacular buds in the 24 hours after settlement. Within the first week of metamorphosis, sclerites were visible inside the polyp tissue. Early development in L. pauciflorum appears to be slower than in A. millepora, and it was not possible to identify developmental stages corresponding to those of hard corals on the basis of morphology. Some similarities with hard corals were observed in terms of timing of expression of genes involved in calcification.

In order to extend the knowledge on soft coral microbial communities, microbes associated with the entire colony were also investigated in Lobophytum pauciflorum. In a first approach the possibility of sex−specific differences in the core microbiome of L. pauciflorum was investigated. The profile of ribotypes present in at least 51 % of the samples was similar to the general profile when all ribotypes are considered. The microbial community in L. pauciflorum was dominated by Spirochaetes−related ribotypes, though a−Proteobacteria were the second most abundant class. Further, microbial profiles were very similar between male and female colonies in terms of highly abundant ribotypes, although sequences that cannot be assigned to any known bacteria were significantly more abundant among female than male colonies. Some sex−specific differences were detectable in ribotypes of low abundance. However, at class and family level, these differences do not persist, suggesting that the different ribotypes exhibit similar functions. The high abundance of unknown ribotypes provides the potential for discovery of novel bacterial strains with potentially different functions. In a second study, the effect of environmental stresses on the microbial community of L. pauciflorum was investigated. Microbial profiles of Lobophytum fragments exposed to 1) elevated temperatures, 2) reduced pH and 3) a combination of elevated temperatures and reduced pH for 12 days did not vary from control conditions. However, a clear pattern of colony specific microbial profiles at ribotype level emerged. A Spirochaete−dominated microbial community has only recently been identified in a gorgonian species in the Mediterranean Sea and deserves further characterisation. Combining the results of both studies showed that the microbial community of L. pauciflorum is conserved, with limited colony specific variability. Further, the microbial community appears robust to short term stresses, however the effect of longer−term stress deserves further investigation.

Item ID: 51476
Item Type: Thesis (PhD)
Keywords: Alcyonacea, alcyonacean, environmental stress, Lobophytum pausiforum, ocean acidification, Octocorallia, sex differentiation, soft coral microbiome, soft coral reproduction, Spirochaetes domoinated
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 4: Wessels, Wiebke, Sprungala, Susanne, Watson, Sue-Ann, Miller, David J., and Bourne, David G. (2017) The microbiome of the octocoral Lobophytum pauciflorum: minor differences between sexes and resilience to short-term stress. FEMS Microbiology Ecology, 93 (5). pp. 1-13.

Date Deposited: 07 Nov 2017 00:14
FoR Codes: 06 BIOLOGICAL SCIENCES > 0604 Genetics > 060403 Developmental Genetics (incl Sex Determination) @ 50%
06 BIOLOGICAL SCIENCES > 0605 Microbiology > 060504 Microbial Ecology @ 30%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 20%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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