Fertile Hybrids Could Aid Coral Adaptation

Abstract

Fertile hybrids can enhance the adaptive capacity and resilience of species under stress by increasing genetic diversity within populations, masking the effects of deleterious recessive alleles, and facilitating the introgression of beneficial genetic variants into parental species. However, many hybrids are infertile. We compared the fertility of aquarium-reared F1 hybrid and purebred corals of the species Acropora loripes and Acropora kenti and examined the viability of early life stages of second-generation (F2) hybrid and back-crossed planula larvae and recruits. The F1 hybrids spawned viable gametes and the F2 hybrid and back-crossed embryos developed into planula larvae and settled to become sessile coral recruits. The F1 hybrids had greater reproductive fitness than the F1 A. loripes purebred stock in an aquarium environment based on their probability of spawning and their fertilization success in crosses using their gametes. Interspecific coral hybrids can therefore be fertile and have high reproductive fitness, which could benefit the persistence of threatened coral reefs.

Keywords: coral; coral reef; evolution; hybrid.

FIGURE 1

Box plots depicting the distribution of (A) the number of oocytes in the mesenteries of LL_F1 purebred (orange) and LK_F1 hybrid (blue) colonies, where each dot represents the oocyte count in a single mesentery, and (B) the mean diameter of oocytes in the LL_F1 purebred and LK_F1 hybrid colonies, where each dot represents the average diameter (mm) of a single oocyte. 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.

FIGURE 2

Bar chart of the percentage of the 39 LK_F1 hybrid and 31 LL_F1 purebred colonies spawning on each night of the December 2021 spawning period.

FIGURE 3

Photographs of a LK_F1 colony (A) setting and (B) spawning in December 2021, taken with an Olympus TG‐5 camera. Note that the image of the coral setting was taken under red light to minimize disruption of spawning behavior.

FIGURE 4

Box plots depicting the distribution of fertilization success (percentage of multicell embryos post fertilization) of the crosses conducted between LL_F1 and LK_F1 colonies in December 2021. The dam and sire F1 parental group are listed first and second, respectively, in the cross labels on the x‐axis. 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 fertilization reactions) are shown below each median line for each offspring group; note that duplicate counts were conducted for each unique pair of colonies crossed and that one duplicate was missing for the cross LL_F1 × LK_F1. Significant differences in fertilization success amongst the crosses have been inferred from Bayesian generalized linear mixed effects modeling and are annotated on the figure with *.

FIGURE 5

Images of F2 colonies taken using the Leica Stereo Microscope MZ16A that depict a LLLL_F2 and LKLL_F2 coral recruit, respectively, taken approximately 1 month after they settled in 2021; the Symbiodiniaceae photosymbionts can be seen as scattered, golden/brown dots in these images.