Interspecific hybridisation provides a low-risk option for increasing genetic diversity of reef-building corals

Abstract

Interspecific hybridisation increases genetic diversity and has played a significant role in the evolution of corals in the genus Acropora. In vitro fertilisation can be used to increase the frequency of hybridisation among corals, potentially enhancing their ability to adapt to climate change. Here, we assessed the field performance of hybrids derived from the highly cross-fertile coral species Acropora sarmentosa and Acropora florida from the Great Barrier Reef. Following outplanting to an inshore reef environment, the 10-month survivorship of the hybrid offspring groups was intermediate between that of the purebred groups, although not all pairwise comparisons were statistically significant. The A. florida purebreds, which had the lowest survivorship, were significantly larger at 10 months post-deployment compared to the other three groups. The four offspring groups harboured the same intracellular photosymbiont communities (Symbiodiniaceae), indicating that observed performance differences were due to the coral host and not photosymbiont communities. The limited differences in the performance of the groups and the lack of outbreeding depression of the F1 hybrids in the field suggest that interspecific hybridisation may be a useful method to boost the genetic diversity, and as such increase the adaptive capacity, of coral stock for restoration of degraded and potentially genetically eroded populations.

Keywords: Acropora; Coral reef; Great Barrier Reef; Hybrid; Symbiodiniaceae.

Fig. 1.

Box plots depicting the distribution of fertilisation success (percentage of multicell embryos at 2.5 h post fertilisation) of each of the offspring groups. The horizontal lines of the boxes represent the lower quartile, median, and upper quartile values, the whiskers represent the extreme values and dots represent single outlier datapoints. Sample sizes (number of fertilisation counts) are shown below each box for each offspring group.

Fig. 2.

The probability of recruits surviving over the 10-month reef deployment shown for each of the offspring groups. (A. florida purebred – purple, FS hybrid – green, SF hybrid – red, and A. sarmentosa purebred – blue). The data points on the graph represent the mean probability and the upper and lower vertical limits of the shaded areas represent the 95% confidence intervals around the mean. Sample sizes (number of recruits that were counted as dead or alive) are shown in the table for each offspring group and timepoint.

Fig. 3.

Recruit surface area (mm²) over the 10 months post-deployment shown for each of the offspring groups. The data points represent the mean area (mm²) and the upper and lower limits of each ribbon represent the standard error around the mean. Sample sizes (number of recruits) are included in boxes next to the data points. The 10-month post-deployment size estimates (annotated with an arrow) were obtained through 3D modelling whilst sizes from earlier time points were estimated using 2D imaging.

Fig. 4.

Observed temperatures at the deployment site. (A) Mean (solid line), maximum (upper dashed line), and minimum (lower dashed line) daily temperatures (°C) at a 5 m depth in Geoffrey Bay. (B) Cumulative degree heating weeks (°C) relative to the parental colony collection location, Davies Reef, throughout the course of the deployment. (C) Recruit calibrated relative colour scores, derived from grey values (proxy for bleaching) shown for each of the offspring groups over the 10-month deployment from September 2020 to July 2021. The data points represent the colour score relative to the colour score of the recruit at the time of deployment, and the upper and lower limits of each ribbon represent the standard error around the mean. Sample sizes (number of recruits) are included in boxes next to the data points. Colour score is used as a proxy here for the density of algal symbionts in the coral tissue where a lower number/lighter colour can represent a lower algal symbiont density that is indicative of coral bleaching.

Fig. 5.

Symbiodiniaceae community information where each aligned row/tree branch represents data from one sample. The bar plot shows the relative abundance of Symbiodiniaceae profiles that are putatively characteristic of unique taxa, where each colour represents a profile. A tree visualises the hierarchical clustering of samples according to their unifrac distances, where the tips (representing samples) are coloured by offspring group (A. florida purebred – purple, FS hybrid – green, SF hybrid – red, and A. sarmentosa purebred – blue).