From population connectivity to the art of striping Russian dolls: the lessons from Pocillopora corals

Here, we examined the genetic variability in the coral genus Pocillopora , in particular within the Primary Species Hypothesis PSH 09, identified by Gélin, Postaire, Fauvelot and Magalon (2017) using species delimitation methods [also named Pocillopora eydouxi/meandrina complex sensu , Schmidt‐Roach, Miller, Lundgren, & Andreakis (2014)] and which was found to split into three secondary species hypotheses ( SSH 09a, SSH 09b, and SSH 09c) according to assignment tests using multi‐locus genotypes (13 microsatellites). From a large sampling (2,507 colonies) achieved in three marine provinces [Western Indian Ocean ( WIO ), Tropical Southwestern Pacific ( TSP ), and Southeast Polynesia ( SEP )], genetic structuring analysis conducted with two clustering analyses ( Structure and DAPC ) using 13 microsatellites revealed that SSH 09a was restricted to the WIO while SSH 09b and SSH 09c were almost exclusively in the TSP and SEP . More surprisingly, each SSH split into two to three genetically differentiated clusters, found in sympatry at the reef scale, leading to a pattern of nested hierarchical levels ( PSH  >  SSH  > cluster), each level hiding highly differentiated genetic groups. Thus, rather than structured populations within a single species, these three SSH s, and even the eight clusters, likely represent distinct genetic lineages engaged in a speciation process or real species. The issue is now to understand which hierarchical level ( SSH , cluster, or even below) corresponds to the species one. Several hypotheses are discussed on the processes leading to this pattern of mixed clusters in sympatry, evoking formation of reproductive barriers, either by allopatric speciation or habitat selection.