Together stronger: Intracolonial genetic variability occurrence in Pocillopora corals suggests potential benefits

Abstract

We investigated the occurrence of intracolonial genetic variability (IGV) in Pocillopora corals in the southwestern Indian Ocean. Ninety-six colonies were threefold-sampled from three sites in Reunion Island. Nubbins were genotyped using 13 microsatellite loci, and their multilocus genotypes compared. Over 50% of the colonies presented at least two different genotypes among their three nubbins, and IGV was found abundant in all sites (from 36.7% to 58.1%). To define the threshold distinguishing mosaicism from chimerism, we developed a new method based on different evolution models by computing the number of different alleles for the infinite allele model (IAM) and the Bruvo's distance for the stepwise mutation model (SMM). Colonies were considered as chimeras if their nubbins differed from more than four alleles and if the pairwise Bruvo's distance was higher than 0.12. Thus 80% of the IGV colonies were mosaics and 20% chimeras (representing almost 10% of the total sampling). IGV seems widespread in scleractinians and beyond the disabilities of this phenomenon reported in several studies, it should also bring benefits. Next steps are to identify these benefits and to understand processes leading to IGV, as well as factors influencing them.

Keywords: Pocillopora; chimerism; intracolonial genetic variability; microsatellite; mosaicism; scleractinian.

Figure 1

Sampling sites of Pocillopora colonies in Reunion Island (represented by the black circles). For each site (N = 32 colonies), the species and cluster distribution are given. The hatched parts correspond to colonies removed as no comparison among nubbin genotypes was possible (no locus in common)

Figure 2

Proportions of the categories of genetic variability (a) per site, (b) per SSH05c cluster (SSH13a was not represented as only two colonies from REU4 were sampled), and (c) overall colonies (number of colonies in parentheses). The hatched parts correspond to colonies for which at least one intracolonial comparison was done with less than nine loci (N_L < 9). Distributions are not significantly different among sites (Fisher's exact test; p = .099) nor between clusters (Fisher's exact test; p = .626)

Figure 3

Thresholds between mosaicism and chimerism. (a) Distribution of the number of different alleles (N_A) between two multilocus genotypes (MLGs) and (b) distribution of the Bruvo's distance (D; Bruvo et al., 2004) between two MLGs. Only MLGs without missing data were compared by pair (N = 6,670 paired comparisons, including 4,987 intracluster, and 1,683 intercluster comparisons). The categories of genetic variability are indicated above each chart. N_{A  CHI/MOS} and D  _CHI/MOS are the genetic differentiation thresholds between mosaicism and chimerism

Figure 4

Pictures of the three bicolor Pocillopora SSH05c colonies. Colors are delimited with the dashed line and nubbin sampling spots (noted a, b, and c within each colony, referring to Table S2) are shown with the arrows. At the top of each photo, are indicated (1) the name of the colony (referring to Table S2), (2) the cluster and the site (in parentheses), and (3) the numbers of different alleles (N_A) between two multilocus genotypes (MLGs) for the intracolonial comparisons of nubbins a‐b, b‐c and a‐c, respectively