Marine Biology (1999) 134: 753±760
Ó Springer-Verlag 1999
M. Adjeroud á M. Tsuchiya
Genetic variation and clonal structure in the scleractinian coral Pocillopora damicornis in the Ryukyu Archipelago, southern Japan
Received: 7 August 1998 / Accepted: 18 May 1999
Abstract Samples of the scleractinian coral Pocillopora damicornis were collected from six sites located around four islands in the Ryukyu Archipelago, southern Japan, and subjected to allozyme electrophoresis. Seven polymorphic loci were examined for their allelic patterns. The ratio of observed to expected genotypic diversity (0.30 < Go:Ge < 0.64), the ratio of the observed number of genotypes to the number of individuals (0.47 < Ng:Ni < 0.75), and deviations from Hardy± Weinberg equilibrium indicated that asexual reproduction plays a major role in the maintenance of established populations. However, populations were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples. The high frequency of typhoons in the region suggests that, in P. damicornis, fragmentation caused through occasional exposure to powerful waves is a major mode of asexual reproduction, but asexual production of planulae may also be contributing to the maintenance of populations. A signi®cant genetic dierentiation (FST) was found between the six populations examined (0.027 < FST < 0.092, average FST = 0.056). The
Communicated by S.A. Poulet, Rosco M. Adjeroud (&)1 á M. Tsuchiya University of the Ryukus, Department of Chemistry, Biology, and Marine Science, Nishihara, Okinawa 903±01 Japan M. Adjeroud á M. Tsuchiya Tropical Biosphere Research Center, Sesoko Station, University of the Ryukus, 2422 Sesoko, Motobu, Okinawa 905-02 Japan Present address: Centre de Biologie et d'Ecologie Tropicale et MeÂditerraneÂenne, Ecole Pratique des Hautes Etudes, URA CNRS 1453, Universite de Perpignan, F-66860 Perpignan Cedex, France 1
Fax: 0033 (0)4 6850 36-86 e-mail:
[email protected]
moderate gene ¯ow is discussed according to characteristics of the larval stage of the species, and to circulation patterns in the region.
Introduction Scleractinian corals have several modes of reproduction: sexual reproduction by broadcast of gametes or planulae, or asexual reproduction by fragmentation, budding, broadcast-spawning of asexual planulae, or polyp expulsion (Highsmith 1982; Fadlallah 1983; Stoddart 1983; Harrison and Wallace 1990; Kramarsky-Winter et al. 1997). Recently, examination of the genetic structure of populations by allozyme electrophoresis has allowed estimates of the relative importance of sexual and asexual reproduction in coral populations (Stoddart 1983; Willis and Ayre 1985; Stoddart et al. 1988; Ayre and Dufty 1994; Benzie et al. 1995). The results of these studies have revealed great variation in the relative contributions of sexual and asexual reproductions, not only among dierent species but also within single species. Stoddart (1984a) found that asexual reproduction by means of brooded larvae was particularly important in populations of Pocillopora damicornis in southwestern Australia and Hawaii, whereas Benzie et al. (1995) and Ayre et al. (1997) found that sexual reproduction was predominant in populations on the Great Barrier Reef (GBR). Sexual reproduction is followed by broadcast spawning of pelagic gametes in oviparous species and of planulae in viviparous, or brooding, species. The characteristics of the pelagic phase, such as its duration and its spatial dispersion, are poorly known because spawning is a temporally restricted phenomenon, and larvae are inaccessible by conventional methodology. Laboratory experiments, nevertheless, have shown that larval life is highly variable among species, ranging from a few minutes to nearly 300 d (Fadlallah 1983; Harrison and Wallace 1990; Carlon and Olson 1993). Therefore, dispersal of coral larvae has been mainly assessed by
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examination of its indirect eects on the genetic dierentiation and the gene ¯ow among populations (Ayre and Dufty 1994). The relative importance of sexual and asexual reproduction and the spatial scale of larval dispersal have profound eects on the genetic structure of populations (Jackson 1986), as well as on fundamental evolutionary phenomena such as speciation, selection, and extinction (Cook 1978). If asexual reproduction predominates, populations are generally dominated by a small number of genotypes that are supposedly particularly well adapted to local conditions (Shick et al. 1979; Sebens 1982; Ayre 1983; Neigel and Avise 1983; Stoddart 1984a; Ayre et al. 1991). A clonal life should also accelerate evolutionary change within, and divergence among, local populations (Wright 1978). In contrast, high rates of gene ¯ow and recruitment in strictly sexually-reproducing species reduce habitat selection and stochastic contributions to genetic dierentiation among populations (founder events and genetic drift: Williams 1975; Hedgecock 1986; Underwood and Fairweather 1989; Benzie 1994). Although early evidence suggested a negative correlation between larval dispersal and genetic dierentiation among geographically isolated populations, critical reviews (Burton and Feldman 1982; Hedgecock 1986) have shown the relationship to be less than clear-cut. Recent models have proposed that within species with mixed modes of reproduction, sexually-derived larvae provide the long-distance colonists and a constant gene ¯ow between distant populations, whereas asexual reproduction appears to be the major source of recruitment into local populations (Williams 1975; Bell 1982). The present paper aimed to examine the variation in the genetic composition of six populations of the viviparous scleractinian coral Pocillopora damicornis from four islands separated by up to 650 km in the Ryukyu Archipelago, southern Japan. The results were used to evaluate the relative contributions of sexual and asexual reproduction, and to assess the apparent scale of larval dispersal (gene ¯ow) between coral populations in this region. The Ryukyu Archipelago, located between 24° and 30°N, comprises more than 140 islands. The coral reefs bordering these islands are among the northernmost in the world. However, benthic communities, notably scleractinian corals, are highly diverse and abundant. Previous observations by Ayre et al. (1997) showed that life-history traits of Pocillopora damicornis vary with geographical location, which is expressed by the greater importance of asexual reproduction at the margins of its geographical range. Thus, the Ryukyu Islands, roughly aligned in a NE±SW direction, oer an ideal ®eld site for studying variation in the genetic composition of corals on a large spatial scale and to assess the relative contribution of sexual and asexual reproduction. The results of this study are expected to be not only fundamental in a purely biological sense, but also of great importance in devising eective methods for the management and
preservation of the biodiversity of coral reef ecosystems. Considering the fact that coral reefs in this region have been disturbed by various natural and man-induced perturbations (Nishihira 1987), the second issue should deserve particular attention.
Materials and methods Collection of specimens and electrophoresis Pocillopora damicornis is a branching coral, common throughout the tropical and subtropical Indo-Paci®c region. Three major modes of reproduction have been documented for this species. (1) Sexual reproduction, followed by broadcast-spawning of planulae; the periodicity of planulae release varies with geographic location (Harriott 1983; Richmond and Jokiel 1984; Ward 1992); P. damicornis is a simultaneous hermaphrodite, with gonads attached to the mesenteries by stalks (Harriott 1983). (2) Fragmentation, which is the major mode of asexual reproduction in the eastern Paci®c (Richmond 1987a). (3) Asexual production of planulae; this has been surmised by Stoddart (1983) on the basis of data from allozyme electrophoresis of populations from southwestern Australia and Hawaii. Pocillopora damicornis were collected from six sites around four islands in the Ryukyu Archipelago, two sites around Okinawa Island (Hentona and Abu), two sites around Aka Island (Kushibaru and Nishihama), one site around Sesoko Island, and one site around Ishigaki Island (Fig. 1). Coral reefs in the Ryukyus are relatively narrow, and consist of a shallow reef ¯at ( 0.05; *p < 0.05; **p < 0.01)
Mean no. alleles/locus
Heterozygosity (SD) Direct count
Expected
3.5 3.8 3.4 3.1 3.1 3.4
0.244 0.363 0.303 0.273 0.366 0.285
0.334 0.365 0.383 0.384 0.373 0.371
(0.132)* (0.251)NS (0.175)** (0.174)* (0.255)NS (0.162)*
Table 3 Pocillopora damicornis. Weir and Cockerham (1984) F-statistics by locus for six populations from Ryukyu Archipelago. FST standardised genetic variance between populations; FIS standardised genetic variance within populations. Statistical signi®cances of values were calculated with permutation tests followed by Bonferroni adjustments. NS p > 0.05; *p < 0.05; **p < 0.01 Locus
FST
FIS
GPI PGM HK LGG LT-1 LT-2 LP
0.111** 0.048** 0.069** 0.040** 0.010NS 0.018NS 0.004NS
0.116NS 0.099NS 0.174** 0.288** 0.299** 0.161NS 0.343**
Average
0.056**
0.180**
(Table 2), and in each population the number of observed similar genotypes was signi®cantly higher than the number of similar genotypes that can be obtained by chance (permutation tests with Bonferroni adjustments, p < 0.05). Both of these measures (Go:Ge and Ng:Ni) suggested a high degree of asexual reproduction within the six populations examined. Populations were not, however, dominated by a single or a small number of clones.
Table 4 Pocillopora damicornis. Weir and Cockerham (1984) FST (above diagonal), average number of migrants per generation (Nem, in parentheses above diagonal ), and Nei's unbiaised genetic distance (below diagonal) between six populations collected from four Sesoko Sesoko
(0.169) (0.237) (0.230) (0.212) (0.221) (0.197)
Go:Ge
Ng:Ni
0.30 0.64 0.41 0.37 0.41 0.43
0.47 0.75 0.58 0.62 0.64 0.62
FST values were signi®cantly dierent from 0 at all loci except LT-1 and LP, and FST calculated for the seven loci and the six populations indicated a signi®cant genetic dierentiation (permutation tests with Bonferroni adjustments, p < 0.05; Table 3). FST calculated between pairs of populations indicated a signi®cant genetic differentiation (Table 4). The highest FST values were found between the Kushibaru sample and samples collected around Sesoko Island and Okinawa (Hentona and Abu), whereas the lowest value was found between Sesoko and Hentona samples. The number of migrants per generation (Nem) calculated from FST values is valid only at equilibrium but, making this assumption, our estimations ranged between 2.4 and 8.9 migrants per generation (Table 4). The genetic distances between populations ranged between 0.016 and 0.066 (average 0.037; Table 4). As shown in the neighbor-joining tree in Fig. 2, the highest similarities were found between populations from Okinawa Island (Hentona and Abu), and between populations from Aka Island (Kushibaru and Nishihama). The population from Sesoko Island was close to the populations from Okinawa Island, whereas the population from Ishigaki Island was clearly distinct from the other populations. However, there was no signi®cant correlation between genetic and geographical distances between populations (r2 = 0.006, p > 0.05).
islands in Ryukyu Archipelago. Statistical signi®cances of FST values were calculated with permutation tests followed by Bonferroni adjustments (NS p > 0.05; *p < 0.05; **p < 0.01)
Hentona
Abu
Kushibaru
Nishihama
Ishigaki
0.027** (8.9)
0.035* (6.7) 0.033** (7.3)
0.070** (3.2) 0.092** (2.4) 0.078** (2.9)
0.056** (4.1) 0.076** (3.0) 0.052** (4.5) 0.044** (5.3)
0.041** (5.7) 0.066** (3.5) 0.056* (4.1) 0.043** (5.4) 0.059** (3.9)
Hentona
0.016
Abu
0.022
0.022
Kushibaru
0.046
0.066
0.057
Nishihama
0.034
0.050
0.036
0.030
Ishigaki
0.026
0.044
0.039
0.031
0.040
758
Fig. 2 Pocillopora damicornis. Neighbor-joining tree showing pattern of similarity expressed by Nei's unbiased genetic distance among six populations collected around four islands. Bootstrap values are from a consensus tree
Discussion Genotypic diversity, the number of observed genotypes, and deviations from Hardy±Weinberg equilibrium, which include both excesses and de®cits of heterozygotes, clearly demonstrated that asexual reproduction plays a major role in the maintenance of established populations of Pocillopora damicornis in the Ryukyu Islands. These results are concordant with those obtained for 25 populations of P. damicornis from southwestern Australia (average Go:Ge = 0.40; Stoddart 1984a, b). In contrast, Benzie et al. (1995) and Ayre et al. (1997) found a predominance of sexual reproduction (average Go:Ge = 0.91 and 0.88, respectively) in populations of P. damicornis from One Tree Island (Central GBR), and along the GBR, respectively. Therefore, our results con®rm that the relative importance of asexual reproduction in P. damicornis varies geographically, and support the assumption that asexual reproduction has a higher importance at the margin of the geographical range of the species. The relative importance of sexual and asexual reproduction is probably related to the physical and ecological characteristics of the reef considered. Despite the high contribution of asexual reproduction, the populations surveyed in the present study were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples, as also found for the zoanthid Zoanthus coppingeri by Burnett et al. (1995). In contrast, populations of P. damicornis from southwestern Australia (Stoddart 1984a) were dominated by few clones, as found for the sea anemone Actinia tenebrosa by Black and Johnson (1979). Fragmentation (Highsmith 1982) and asexual production of planulae (Stoddart 1983) are the two major modes of asexual reproduction reported for Pocillopora damicornis. A colony produced by fragmentation is genetically indistinguisable from a colony derived from asexually-produced larvae, and therefore it is uncertain which mode is predominant. However, two factors suggest that fragmentation is likely to be a major mode of asexual reproduction in populations of the Ryukyu Islands, as was found in the eastern Paci®c (Richmond
1987a), and for other scleractinians such as Pavona cactus (Willis and Ayre 1985; Ayre and Willis 1988), Montipora digitata (Jokiel et al. 1983), Acropora cervicornis (Neigel and Avise 1983), and A. aspera, A. pulchra, A. robusta (Bothwell 1981): (1) All populations were collected on the shallow reef ¯at (
