Genetic differentiation and connectivity of morphological types of the broadcast-spawning coral Galaxea fascicularis in the Nansei Islands, Japan

Abstract

Population connectivity resulting from larval dispersal is essential for the maintenance or recovery of populations in marine ecosystems, including coral reefs. Studies of species diversity and genetic connectivity within species are essential for the conservation of corals and coral reef ecosystems. We analyzed mitochondrial DNA sequence types and microsatellite genotypes of the broadcast-spawning coral, Galaxea fascicularis, from four regions in the subtropical Nansei Islands in the northwestern Pacific Ocean. Two types (soft and hard types) of nematocyst morphology are known in G. fascicularis and are significantly correlated with the length of a mitochondrial DNA noncoding sequence (soft type: mt-L; hard type: mt-S type). Using microsatellites, significant genetic differentiation was detected between the mitochondrial DNA sequence types in all regions. We also found a third genetic cluster (mt-L+), and this unexpected type may be a cryptic species of Galaxea. High clonal diversity was detected in both mt-L and mt-S types. Significant genetic differentiation, which was found among regions within a given type (F ST = 0.009-0.024, all Ps ≤ 0.005 in mt-L; 0.009-0.032, all Ps ≤ 0.01 in mt-S), may result from the shorter larval development than in other broadcast-spawning corals, such as the genus Acropora. Nevertheless, intraspecific genetic diversity and connectivity have been maintained, and with both sexual and asexual reproduction, this species appears to have a potential for the recovery of populations after disturbance.

Keywords: Conservation; cryptic species; genetic diversity; microsatellite; population genetics.

Figure 1

Collection sites for Galaxea fascicularis in the Nansei Islands in southwestern Japan.

Figure 2

Mitochondrial DNA sequences revealed a previously unknown type of Galaxea fascicularis. A haplotype network comparing the three types obtained from 10 specimens from O‐Kum in Okinawa with those defined in previous studies, based upon specimens collected in the Ryukyu Archipelago (Watanabe et al. 2005). The three types, readily identified by mitochondrial DNA, cannot be reliably distinguished based on appearance. Sequences of LA to LE and sequences of SA to SC were obtained from Watanabe et al. (2005). The sequence of LE was genetically closest to type mt‐L+ from this study. Colors correspond to the clusters in the Bayesian clustering analysis (K = 3, see Fig. 3A).

Figure 3

Mitochondrial DNA sequence types are genetically well differentiated. (A) Most multilocus lineages (MLLs) were correctly assigned by Bayesian clustering analysis when the number of clusters was set to 5. These clusters corresponded closely to the three G. fascicularis types. The X‐axis shows the probability of MLL membership in each cluster. Using STRUCTURE with the number of groups set to five (K = 5), three MLLs in mt‐L and two MLLs in mt‐S showed a reversed genetic cluster (see black triangles). (B) Finer genetic differentiation was detected in each type among regions using InStruct instead of STRUCTURE, which is sensitive to high inbreeding coefficients (see Table 2). Using microsatellite markers, STRUCTURE classified five MLLs differently than their mitochondrial DNA sequence types. These MLLs were excluded from analyses with InStruct.