Population genetic consequences of larval dispersal mode and hydrography: a case study with bryozoans

Genetic analysis of the marine bryozoans Celleporella hyalina and Electra pilosa using the RAPD technique revealed population structuring corresponding to the contrasting modes of larval dispersal. Samples of C. hyalina exhibited genetic differentiation over distances as small as 10 m, concordant with the limited dispersal predicted by a simulation model, based on the short pelagic phase of the lecithotrophic larvae and the local hydrography. In contrast, E. pilosa showed high levels of genetic heterogeneity only over much larger spatial scales, commensurate with its production of comparatively long-lived planktotrophic larvae. The population differentiation observed between samples of E. pilosa, collected from sites 70 km apart, is reconcilable with coastal water currents and frontal systems that restrict the exchange of water masses between the two sites. Hydrographic conditions and discontinuous distribution of suitable substrata therefore are seen to constrain gene flow, creating opportunities for local genetic differentiation despite the high dispersal potential of pelagic larvae. Received: 9 August 2000 / Accepted: 18 November 2000     

Bathymetric patterns of genetic variation in a deep-sea protobranch bivalve, Deminucula atacellana

The origin of the deep-sea benthic fauna is poorly understood and represents an enormous gap in our understanding of basic evolutionary phenomena. One obstacle to studying evolutionary patterns in the deep sea has been the technical difficulty of measuring genetic variation in species that are typically minute, rare, and must be recovered from extreme depths. We used molecular genetic techniques to quantify variation in the 16S rRNA mitochondrial gene within and among populations of the common protobranch bivalve Deminucula atacellana (Schenck, 1939). We analyzed 89 individuals from nine samples collected in the 1960s along a depth gradient from 1100 to 3800 m in the western North Atlantic. Genetic variability within populations is much lower than between populations, and peak haplotype numbers occur near the center of its depth distribution. Continental slope (<2500 m) and rise (>2500 m) populations were genetically distinct despite the lack of any obvious topographic or oceanographic features that would impede gene flow. These findings indicate that the deep-sea macrofauna can have strong population structure over small (134 km) spatial scales, similar to that observed in shallow-water and terrestrial organisms. This surprisingly high biodiversity at the genetic level affords the potential for adaptation and evolutionary diversification, the ultimate historical causes of high species diversity in the deep-sea benthos. Received: 24 July 1997 / Accepted: 26 January 1998     

Genetic structure of the subtidal red alga Delisea pulchra

We used random amplified polymorphic DNA (RAPDs) to examine small-scale spatial genetic structure in the red alga Delisea pulchra (Greville) Montagne at two locations near Sydney, Australia. We examined genetic structure among plants at four spatial scales ranging from 2 km apart down to <50 cm apart between locations, among sites within locations, among quadrats within sites, and among plants within quadrats. Haploid stages of D. pulchra were absent from the populations studied, suggesting that they are maintained through asexual reproduction of diploid plants. Consistent with this, we found that 19 RAPD phenotypes scored in this study had multiple individuals, indicating the presence of clones in these populations. However, there were no RAPD phenotypes common to two locations separated by only 2 km. Analysis of molecular variance revealed that strong genetic differences occurred between plants from these two locations, with 46.3% of the total genetic variation occurring at this scale, most probably reflecting limited gene flow. Within each location, <25% of the genetic variation was attributable to differences among sites or quadrats, indicating gene flow at those smaller scales. Most of the variation within each location occurred at the smallest spatial scale, among plants within 0.25 m² quadrats. Nonetheless, some pairwise genetic distances (φST) between sites or quadrats within locations were large, indicating some genetic divergence on smaller scales. Genetic distance was independent of spatial distance within both locations, suggesting that fine-scale differences within locations were most probably caused by variation in fine-scale patterns of water movement or fine-scale natural selection. We assessed the impact of one potential selective agent, grazing sea urchins, on the fine-scale genetic structure of D. pulchra. There was no evidence that grazing by sea urchins affected the genetic structure of D. pulchra. In combination with demographic data, our results indicated that local populations of D. pulchra within locations were relatively open and that fine-scale genetic structure was probably constrained by gene flow. At the larger scale however, strong genetic differentiation indicated little gene flow between locations and restricted dispersal of spores. Received: 22 April 1999 / Accepted: 29 November 1999     

Gene flow at three spatial scales in a coral reef fish, the three-spot dascyllus, Dascyllus trimaculatus

Dispersal in coral reef fishes occurs predominantly during the larval planktonic stage of their life cycle. With relatively brief larval stages, damselfishes (Pomacentridae) are likely to exhibit limited dispersal. This study evaluates gene flow at three spatial scales in one species of coral reef damselfish, Dascyllus trimaculatus. Samples were collected at seven locations at Moorea, Society Islands, French Polynesia. Phylogenetic relationships and gene flow based on mitochondrial control region DNA sequences between these locations were evaluated (first spatial scale). Although spatial structure was not found, molecular markers showed clear temporal structure, which may be because pulses of settling larvae have distinct genetic composition. Moorea samples were then compared with individuals from a distant island (750 km), Rangiroa, Tuamotu Archipelago, French Polynesia (second spatial scale). Post-recruitment events (selection) and gene flow were probably responsible for the lack of structure observed between populations from Moorea and Rangiroa. Finally, samples from six Indo-West Pacific locations, Zanzibar, Indonesia, Japan, Christmas Island, Hawaii, and French Polynesia were compared (third spatial scale). Strong population structure was observed between Indo-West Pacific populations. Received: 26 May 2000 / Accepted: 10 October 2000     

Spatial autocorrelation analysis of small-scale genetic structure in a clonal soft coral with limited larval dispersal

The philopatric larval dispesal and small effective population sizes characteristic of many clonal species should promote the development of significant small-scale genetic structure within populations as a result of isolation-by-distance. We used spatial autocorrelation statistics to detect genetic structure, arising from both clonal reproduction and philopatric dispersal of sexual propagules, for five allozyme loci within populations of the soft coral Alcyonium sp. In a population on Tatoosh Island, Washington, USA, sampled in 1991/1992, we found significant positive spatial autocorrelation at all loci among individuals separated by <40 cm, reflecting the presence of significant smallscale genetic structure due to associations among clonemates. For 4 of 5 loci, however, we detected no significant spatial autocorrelation among the different clones within this population over distances of 1 to 40 m. Analysis of soft-coral populations from six additional, topographically diverse sites in the north-east Pacific also did not reveal significant spatial autocorrelation among clones at any loci. This general lack of spatial autocorrelation of genotypes among clones suggests that significant small-scale genetic structure has not arisen in populations of Alcyonium sp. as a consequence of isolation-by-distance.     

Genetic sub-structure and intermediate optimal outcrossing distance in the marine angiosperm <Emphasis Type="Italic">Zostera marina</Emphasis>

The spatial distribution of genetic variability depends on the spatial patterns of clonal and sexual reproduction, gene flow, genetic drift and natural selection. Species with restricted dispersal may exhibit genetic structuring within populations with immediate neighbours being close relatives, and may show differentiation among populations. Genetic structuring of a species may have important genetic, evolutionary and ecological consequences including distance-dependent mating success. In this study we used microsatellite markers to show that clones of Zostera marina in a population in the Ria Formosa, Portugal, were aggregated and covered distances of up to 3–4 m. Clones within 4 m of each other exhibited significant and positive coancestry values, reflecting the limited seed dispersal of this species. Hand-pollinations between near (0–10.9 m), intermediate (11–32 m) and far (15 km) individuals resulted in similar levels of seed set, although the near pollinations had higher, although not statistically significant, levels of seed abortion during maturation. Seeds from intermediate-distance pollinations had a significantly higher proportion of seeds germinate and shorter germination time than both the near and far seeds. Similarly, the average number of seedlings produced per pollination, used as an overall estimate of fitness, was significantly greater for the intermediate distance when compared to both near and far pollinations. These results suggest that the genetic structuring observed may result in both inbreeding and outbreeding depression, which gives rise to an intermediate optimal outcrossing distance.     

Genetic differentiation among populations of the Mediterranean hermaphroditic brooding coral<Emphasis Type="Italic"> Balanophyllia europaea</Emphasis> (Scleractinia: Dendrophylliidae)

Spatial models of genetic structure and potential gene flow were determined for five populations of Balanophyllia europaea, a simultaneous hermaphroditic and brooding coral, endemic to the Mediterranean. Six allozyme loci indicated a genetic structure that departed markedly from Hardy–Weinberg equilibrium, with a significant lack of heterozygotes. The genetic structure observed supports the hypothesis that self-fertilisation characterises the reproductive biology of B. europaea. Populations at small spatial scales (8–40 m) are genetically connected, while those at large scales (36–1,941 km) are genetically fragmented; the genetic differentiation of the populations is not correlated to geographic separation. This spatial model of genetic structure is compatible with an inbreeding mating system. Furthermore, it is also consistent with the expected dispersal potential of swimming larvae of brooding corals, i.e. larvae that are able to produce significant gene flows only within limited spatial scales.Communicated by R. Cattaneo-Vietti, Genova     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Population connectivity in the temperate damselfish <Emphasis Type="Italic">Parma microlepis</Emphasis>: analyses of genetic structure across multiple spatial scales

This study investigated the utility of microsatellite markers for providing information on levels of population connectivity for a low dispersing reef fish in New South Wales (NSW), Australia, at scales ≤400 km. It was hypothesized that the temperate damselfish Parma microlepis, which produces benthic eggs and has limited post-settlement dispersal, would exhibit spatial genetic structure and a significant pattern of isolation-by-distance (IBD). A fully nested hierarchical sampling design incorporating three spatial scales (sites, location and regions, separated by 1–2, 10–50 and 70–80 km respectively) was used to determine genetic variability at seven microsatellite loci. Broad-scale genetic homogeneity and lack of IBD was well supported by single and multi-locus analyses. The proportion of the total genetic variation attributable to differences among regions, locations or sites was effectively zero (Φ/R-statistics ≤0.007). The geographic distribution of genetic diversity and levels of polymorphism (H _E 0.21–0.95) indicate high mutation rates, large effective population sizes, and high rates of gene flow. Significant gene flow may be driven by factors influencing pre-settlement dispersal, including the East Australian Current (EAC) and habitat continuity. Genetic connectivity may not reflect demographically important connectivity, but does imply that P. microlepis populations are well connected from an evolutionary perspective. Total observed genetic diversity was accounted for within 1–2 km of reef and could be represented within small Marine Protected Areas. Reef fishes in NSW which have life histories similar to P. microlepis (e.g. pre-settlement durations ≥2 weeks) are also likely to exhibit genetic homogeneity. Genetic markers are, therefore, most likely to provide information on demographically relevant connectivity for species with lower dispersal capabilities, small population sizes, short life spans, and whose habitats are rare, or patchily distributed along-shore. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A Hierarchical View of Genetic Structure in the Rare Annual Plant Clarkia springvillensis

Genetic structure at several spatial scales was examined in the rare California annual, Clarkia springvillensis . Using seven isozyme-encoding loci as genetic markers, we assessed the amount and distribution of genetic variation among three populations and eight subpopulations. Total genetic variation was lower than in species with similar life history traits but equivalent to that of other endemic plants. Spatial autocorrelation showed some evidence for very limited differentiation within subpopulations at a scale of 1–2 m. The subpopulations, separated by tens of meters, were found to be more differentiated from each other ( F _sp = 0.084) on average than were populations ( F,pt = 0.017). This local genetic differentiation was not correlated with physical distance between subpopulations. The low F_pt estimates suggest that substantial gene flow is occurring among populations. However, the lack of correlation between genetic and geographic distances and the significant differentiation of subpopulations suggest that genetic drift is occurring within populations. Therefore, we believe the apparent homogeneity of populations is due to each population's gene frequencies' being an average of several divergent subpopulations. If drift is causing differentiation within populations, it may eventually cause differentiation between populations. The importance of using a hierarchical approach to evaluating genetic structure is clear. Patterns occurring at one spatial scale may not be evident at others. One should not necessarily conclude that gene flow is substantial and that the risk of genetic erosion via drift is negligible just because differentiation between populations is small; the system may not be at equilibrium. This lesson is particularly important when recent changes in climate or land use are apparent.     

Isolation by distance in the scleractinian coral <Emphasis Type="Italic">Seriatopora hystrix</Emphasis> from the Red Sea

Pelagic dispersal of larvae in sessile marine invertebrates could in principle lead to a homogeneous gene pool over vast distances, yet there is increasing evidence of surprisingly high levels of genetic differentiation on small spatial scale. To evaluate whether larval dispersal is spatially limited and correlated with distance, we conducted a study on the widely distributed, viviparous reef coral Seriatopora hystrix from the Red Sea where we investigated ten populations separated between ~0.150 km and ~610 km. We addressed these questions with newly developed, highly variable microsatellite markers. We detected moderate genetic differentiation among populations based on both F _ST and R _ST (0.089 vs. 0.136, respectively) as well as considerable heterozygote deficits. Mantel tests revealed isolation by distance effects on a small geographic scale (≤20 km), indicating limited dispersal of larvae. Our data did not reveal any evidence against strictly sexual reproduction among the studied populations.     

Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus franciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographically separated populations, seven from northern California and five from southern California (including Santa Rosa Island). Significant population heterogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring populations were often more genetically differentiated than distant populations. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variation and patterns of recruitment, large samples were collected from several northern California populations in 1996 and 1997, and were divided into three size classes, roughly representing large adults (>60 mm), medium-sized individuals (31 to 60 mm, “subadults”) and individuals <2 yr of age (≤30 mm test diam, referred to as “recruits”). Comparisons of allelic counts revealed significant spatial and temporal differentiation among size-stratified population samples. Recruit samples differed significantly from adult samples collected at the same locale, and showed extensive between-year variation. Genetic differentiation among recruit samples was much higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations for the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or “sweepstakes” recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, such differentiation across northern California populations indicates that the larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration. Received: 6 July 1999 / Accepted: 25 January 2000     

Microscale genetic differentiation in a sessile invertebrate with cloned larvae: investigating the role of polyembryony

Microscale genetic differentiation of sessile organisms can arise from restricted dispersal of sexual propagules, leading to isolation by distance, or from localised cloning. Cyclostome bryozoans offer a possible combination of both: the localised transfer of spermatozoa between mates with limited dispersal of the resulting larvae, in association with the splitting of each sexually produced embryo into many clonal copies (polyembryony). We spatially sampled 157 colonies of Crisia denticulata from subtidal rock overhangs from one shore in Devon, England at a geographic scale of ca. 0.05 to 130 m plus a further 21 colonies from Pembrokeshire, Wales as an outgroup. Analysis of molecular variance (AMOVA) revealed that the majority (67%) of genetic variation was distributed among individuals within single rock overhangs, with only 16% of variation among different overhangs within each shore and 17% of variation between the ingroup and outgroup shores. Despite local genetic variation, pairwise genetic similarity analysed by spatial autocorrelation was greatest at the smallest inter-individual distance we tested (5 cm) and remained significant and positive across generally within-overhang comparisons (<4 m). Spatial autocorrelation and AMOVA analyses both indicated that patches of C. denticulata located on different rock overhangs tended to be genetically distinct, with the switch from positive to negative autocorrelation, which is often considered to be the distance within which individuals reproduce with their close relatives or the radius of a patch, occurring at the 4–8 m distance class. Rerunning analyses with twenty data sets that only included one individual of each multilocus genotype (n = 97) or the single data set that contained just the unique genotypes (n = 67) revealed that the presence of repeat genotypes had an impact on genetic structuring (PhiPT values were reduced when shared genotypes were removed from the dataset) but that it was not great and only statistically evident at distances between individuals of 1–2 m. Comparisons to a further 20 randomisations of the data set that were performed irrespective of genotype (n = 97) suggested that this conclusion is not an artefact of reduced sample size. A resampling procedure using kinship coefficients, implemented by the software package GENCLONE gave broadly similar results but the greater statistical power allowed small but significant impacts of repeat genotypes on genetic structure to be also detected at 0.125–0.5 and 4–16 m. Although we predict that a proportion of the repeat multilocus genotypes are shared by chance, such generally within-overhang distances may represent a common distance of cloned larval dispersal. These results suggests that closely situated potential mates include a significant proportion of the available genetic diversity within a population, making it unlikely that, as previously hypothesised, the potential disadvantage of producing clonal broods through polyembryony is offset by genetic uniformity within the mating neighbourhood. We also report an error in the published primer note of Craig et al. (Mol Ecol Notes 1:281–282, 2001): loci Cd5 and Cd6 appear to be the same microsatellite. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999     

Implications of Macroalgal Isolation by Distance for Networks of Marine Protected Areas

The global extent of macroalgal forests is declining, greatly affecting marine biodiversity at broad scales through the effects macroalgae have on ecosystem processes, habitat provision, and food web support. Networks of marine protected areas comprise one potential tool that may safeguard gene flow among macroalgal populations in the face of increasing population fragmentation caused by pollution, habitat modification, climate change, algal harvesting, trophic cascades, and other anthropogenic stressors. Optimal design of protected area networks requires knowledge of effective dispersal distances for a range of macroalgae. We conducted a global meta‐analysis based on data in the published literature to determine the generality of relation between genetic differentiation and geographic distance among macroalgal populations. We also examined whether spatial genetic variation differed significantly with respect to higher taxon, life history, and habitat characteristics. We found clear evidence of population isolation by distance across a multitude of macroalgal species. Genetic and geographic distance were positively correlated across 49 studies; a modal distance of 50–100 km maintained F_ST < 0.2. This relation was consistent for all algal divisions, life cycles, habitats, and molecular marker classes investigated. Incorporating knowledge of the spatial scales of gene flow into the design of marine protected area networks will help moderate anthropogenic increases in population isolation and inbreeding and contribute to the resilience of macroalgal forests. Implicaciones del Aislamiento por Distancia de Macroalgas para Redes de Áreas Marinas Protegidas     

Sex-biased dispersal in a long-lived polygynous reptile (Crocodylus johnstoni )

We compared natal dispersals of freshwater crocodiles (Crocodylus johnstoni) against the prediction of male dispersal bias for a polygynous mating system. The crocodiles inhabited a linear series of pools and we calculated the net distances from natal pools to recapture locations some 12–18 years later, at maturity. Philopatry was assessed in terms of adult social distances. A female social distance was 0.46 pools and a male social distance was 1.0 pool. By these criteria, both sexes showed low levels of philopatry (7–12%). However, individuals of both sexes dispersed from the natal site long before they were sexually mature. Divergence in dispersal patterns by sex occurred after the maturity threshold, as males dispersed two to three times farther than females. Intrasexual competition by males is resolved by a size-based hierarchy. The displacement of small males from local mating access is a probable cause of the longer dispersals undertaken by males. Competition, rather than inbreeding avoidance, is driving dispersal in this population of freshwater crocodiles. Received: 8 May 1998 / Accepted after revision: 26 June 1998     

Stepping-stone gene flow in the solitary coral Balanophyllia elegans: equilibrium and nonequilibrium at different spatial scales

Limited dispersal should result in genetic differences between populations proportional to geographic distances of separation. This association between gene flow and distance can be disrupted by (1) continuing genetic exchange among distant populations, (2) historical changes in gene flow, and (3) physical barriers or corridors to dispersal. The movements of larvae are thought to determine dispersal capability in benthic marine invertebrates. The solitary scleractinian Balanophyllia elegans Verrill possesses crawling larvae capable of only limited dispersal. Paradoxically, however, inferred levels of gene flow between pairs of localities spread over much of the 4000 km range of B. elegans exhibited a weaker relationship with geographical separation than that expected for a linear array of populations in which all genetic exchange takes place between adjacent populations. In this paper, I examined the pattern of gene flow (inferred from the frequencies of eight polymorphic allozyme loci) in B. elegans at a smaller (1 to 50 km) spatial scale to determine (1) whether gene flow at this spatial scale conformed to the expectations of the stepping-stone model, and (2) whether continuing long-distance gene flow or historical changes in gene flow were responsible for the weak relationship between gene flow and distance observed previously at the rangewide spatial scale. Between May and August 1992, I collected 75 adults from each of 18 localities along the coast of Sonoma County, California, USA. These populations of B. elegans were significantly subdivided both among localities separated by 1 to 50 km (F _LT =0.053, Se=0.0075) and among patches separated by 4 to 8 m (F PL=0.026, SE=0.0023). The observed slope and correlation (r ²=0.54) between inferred levels of gene flow and the geographic distance at the 1 to 50 km spatial scale conformed to equilibrium expectations (obtained by simulation) for a linear stepping-stone model, although those from the rangewide spatial scale did not. This implies that the mechanisms conferring patterns of inferred genetic differentiation between localities in B. elegans differ fundamentally with spatial scale. At a scale of 1 to 50 km, continuing gene flow and drift have equilibrated and the process of isolation-bydistance may facilitate local adaptive change. At a broader spatial scale, historical changes in gene flow, perhaps affected by late Pleistocene climatic fluctuations, disrupt the equilibration of gene flow and genetic drift, so that genetic differentiation may not increase continuously with separation between populations.     

Latitudinal variability in spatial genetic structure in the invasive ascidian, Styela plicata

Increases in temperature can shorten planktonic larval durations, so that higher temperatures may reduce dispersal distances for many marine animals. To test this prediction, we first quantified how minimum time to settlement is shortened at higher temperatures for the ascidian Styela plicata. Second, using latitude as a correlate for ocean temperature and spatial genetic structure as a proxy for dispersal, we tested for a negative correlation between latitude and spatial genetic structure within populations, as measured by anonymous DNA markers. Spatial genetic structure was variable among latitudes, with significant structure at low and intermediate latitudes (high and medium temperatures) and there was no genetic structure within high-latitude (low temperature) populations. In addition, we found consistently high genetic diversity across all Australian populations, showing no evidence for recent local bottlenecks associated S. plicata’s history as an invasive species. There was, however, significant genetic differentiation between all populations indicating limited ongoing gene flow.     

Interplay of host morphology and symbiont microhabitat in coral aggregations

The Belizean reef coral Agaricia tenuifolia Dana forms aggregations in which rows of thin, upright blades line up behind each other. On average, the spacing between blades increases with depth and hence with decreasing ambient irradiance. We designed and built a small, inexpensive light meter and used it to quantify the effect of branch spacing on light levels within colonies at varying distances from branch tips. Concurrently, we measured photosynthetic pigment concentrations and population densities of symbiotic dinoflagellates (zooxanthellae) extracted from coral branches of colonies with tight (≤3 cm) vs wide (≥6 cm) branch spacing, collected at 15 to 17 m and from colonies with tight branch spacing collected at 1 to 2 m. Light levels decreased significantly with tighter branch spacing and with distance from the branch tips. Total cellular pigment concentrations (chlorophylls a, c ₂ and peridinin) as well as chlorophyll a:c ₂ and chlorophyll a: peridinin ratios all increased significantly with distance from the branch tip, indicating very localized differences in photoacclimation within individual branches. Zooxanthellae from colonies with widely-spaced branches displayed significantly lower chlorophyll a:c ₂ and chlorophyll a:peridinin ratios, and were present at significantly higher population densities than those from colonies with tightly-spaced branches collected at the same depth (15 m). Tightly-spaced colonies collected from shallow environments (1 to 2 m) displayed pigment ratios similar to those from widely-spaced colonies from deeper water (15 m), but maintained zooxanthellae populations at levels similar to those in tightly-branched colonies from deeper water. Thus, variation in colony morphology (branch spacing and distance from branch tip) can affect symbiont physiology in a manner comparable to an increase of over 15 m of water depth. These results show that a host's morphology can strongly determine the microhabitat of its symbionts over very small spatial scales, and that zooxanthellae can in turn display steep gradients in concordance with these altered physical conditions. Received: 12 June 1997 / Accepted: 24 June 1997     

Low to moderate levels of genetic differentiation detected across the distribution of the New Zealand abalone, Haliotis iris

Two regions of the mitochondrial genome (cytochrome oxidase I and ATPase 8–ATPase 6) were used to examine the population genetic structure of New Zealand’s endemic abalone (Haliotis iris). Samples were collected from 28 locations around New Zealand between January 2005 and February 2008. At least four phylogeographic breaks were present and occurred across the Chatham rise, in the western Cook Strait region, along the southeast coast of the South Island, and at East Cape in the North Island. Gene flow across the Chatham rise is probably limited due to infrequent dispersal across large geographic distances (~850 km), while factors limiting gene flow around the North and South Islands are less clear, and understanding these may require intense temporal and spatial sampling in complex hydrographic regions. High genetic diversity and weak genetic structure may be a general feature of abalone potentially reflecting large and/or ancient populations.