Ancient divisions, recent expansions: phylogeography and population genetics of the round goby Apollonia melanostoma

During the past two decades, the round goby Apollonia melanostoma (= Neogobius melanostomus ) has expanded its range via shipping transport and canals, extending north and west from the Ponto-Caspian region of Eurasia and to the North American Great Lakes. Exotic populations of the round goby have been very successful in the Baltic Sea and the Great Lakes regions, exerting significant ecological changes. Our study evaluates the population genetic and biogeographical structure of the round goby across its native and nonindigenous ranges, in light of geological history and its expansion pathways. We analyzed seven new nuclear microsatellite loci and mitochondrial DNA cytochrome b gene sequences from 432 individuals in 22 locations. Population structure was tested using F _ST-analogs, phylogenetic trees, clustering diagrams, Bayesian assignment tests and nested clade analyses. Results show that native populations in the Black vs. the Caspian Sea basins diverge by 1.4% and c. 350 000 years, corresponding to closure of their prior connections and supporting the taxonomic separation of the Black Sea A. m. melanostoma from the Caspian Sea A. m. affinis . Their within-basin populations diverge by ~0.4% and 100 000 years. Nonindigenous populations in the Baltic Sea and Danube and Dnieper Rivers trace to separate northern Black Sea origins, whereas the upper Volga River system houses mixed populations of A. m. melanostoma and A. m. affinis . Native populations average twice the genetic diversity of most exotic sites; however, sites in the Volga River system have high diversity due to mixing of the two taxa. Our results highlight how vicariance and anthropogenic disturbances have shaped a rapidly expanding species' genetic heritage.     

Genetic patterns across an invasion's history: a test of change versus stasis for the Eurasian round goby in North America

Biological invasions comprise accidental evolutionary experiments, whose genetic compositions underlie relative success, spread and persistence in new habitats. However, little is known about whether, or how, their population genetic patterns change temporally and/or spatially across the invasion's history. Theory predicts that most would undergo founder effect, exhibit low genetic divergence across the new range and gain variation over time via new arriving propagules. To test these predictions, we analyse population genetic diversity and divergence patterns of the Eurasian round goby Neogobius melanostomus across the two decades of its North American invasion in the Laurentian Great Lakes, comparing results from 13 nuclear DNA microsatellite loci and mitochondrial DNA cytochrome b sequences. We test whether ‘genetic stasis’, ‘genetic replacement’ and/or ‘genetic supplement’ scenarios have occurred at the invasion's core and expansion sites, in comparison with its primary native source population in the Dnieper River, Black Sea. Results reveal pronounced genetic divergence across the exotic range, with population areas remaining genetically distinct and statistically consistent across two decades, supporting ‘genetic stasis’ and ‘founder takes most’. The original genotypes continue to predominate, whose high population growth likely outpaced the relative success of later arrivals. The original invasion core has stayed the most similar to the native source. Secondary expansion sites indicate slight allelic composition convergence towards the core population over time, attributable to some early ‘genetic supplementation’. The geographic and temporal coverage of this investigation offers a rare opportunity to discern population dynamics over time and space in context of invasion genetic theory vs. reality.     

Ten polymorphic microsatellite markers in the invasive round goby (Neogobius melanostomus) and cross‐species amplification

Ten tetranucleotide, dinucleotide and compound microsatellite loci were isolated and characterized for the round goby, Neogobius melanostomus. Analysis of 64 gobies from one nonindigenous population in Lake Erie, Ontario, Canada, indicated that allele number varied from three to 12 per locus, while observed heterozygosity ranged between 0.33 and 0.86. Eight of these primers showed some amplification in other species in four genera. These newly developed microsatellite markers are a powerful tool that will provide insights into population structure and dispersal of the round goby in their novel environment.     

Personality-dependent inter- and intraspecific foraging competition in the invasive round goby,Neogobius melanostomus

This study examines the impact of boldness on foraging competition of the highly invasive round goby Neogobius melanostomus Pallas 1815. Individual risk tolerance, or boldness, was measured as the time to resume movement after a simulated predation strike. Fish that resumed movement faster were categorized as “bold,” fish that took more time to resume movement were categorized as “shy” and those that fell in between these two categories were determined to have “intermediate” boldness. Competitive impacts of boldness in N. melanostomus were determined in a laboratory foraging experiment in which interspecific (juvenile Atlantic cod Gadus morhua Linnaeus 1758) and intraspecific (intermediate N. melanostomus) individuals were exposed to either bold or shy N. melanostomus competitors. G. morhua consumed fewer prey when competing with bold N. melanostomus than when competing with shy N. melanostomus, whereas intermediately bold N. melanostomus foraging was not affected by competitor boldness. Bold and shy N. melanostomus consumed similar amounts of prey, and the number of interactions between paired fish did not vary depending on the personality of N. melanostomus individuals. Therefore, intraspecific foraging competition was not found to be personality dependent. This study provides evidence that individual differences in boldness can mediate competitive interactions in N. melanostomus; nonetheless, results also show that competition is also governed by other mechanisms that require further study.     

Invasion genetics of the Eurasian spiny waterflea: evidence for bottlenecks and gene flow using microsatellites

The Eurasian spiny waterflea (Bythotrephes longimanus) is a predacious zooplankter that has increased its range in Europe and is rapidly invading inland water-bodies throughout North America's Great Lakes region. To examine the genetics of these invasions, we isolated five microsatellite DNA loci with between 5 and 19 alleles per locus. We sampled three populations where B. longimanus has been historically present (Switzerland, Italy, and Finland) as well as an introduced European population (the Netherlands) and three North American populations (Lakes Erie, Superior, Shebandowan). Consistent with a bottleneck during colonization (i.e. founder effect), average heterozygosities of the four European populations ranged from 0.310 to 0.599, and were higher than that of three North American populations (0.151-0.220). Pairwise F(ST) estimates among North American populations (0.002-0.063) were not significantly different from zero and were much lower than among European populations (0.208-0.474). This is consistent with a scenario of high gene flow among North American populations relative to that of European ones. Contrary to an invasion bottleneck, however, Erie and Superior populations contained similar numbers of rare alleles as European populations. Assignment tests identified several migrant genotypes in all introduced populations (the Netherlands, Erie, Superior, Shebandowan), but rarely in native ones (Switzerland, Italy and Finland). A large number of genotypes from North America were assigned to our Italian population suggesting a second, previously unidentified, invasion source somewhere in the region of northern Italy. Together, our results support an invasion bottleneck for North American populations that has been largely offset by gene flow from multiple native sources, as well as gene flow among introduced populations.     

Temporal patterns in growth and survival of the round goby Neogobius melanostomus

Monthly, overwinter and annual instantaneous growth rates for round goby Neogobius melanostomus were calculated with maximal growth occurring in July and August and almost no growth observed between ice appearance (October) and melt (March). Annual absolute growth rates averaged 27·3 ± 1·9 mm for males and 19·8 ± 2·4 mm for females. The most parsimonious Cormack–Jolly–Seber model indicated that both the survival and recapture probabilities were dependent on sampling date, but not sex. Survival estimates remained high throughout the 13 month study with a median weekly survival probability of 0·920 (25 and 75% quartiles: 0·767 and 0·991), an overwinter survival probability of 99% and an annual survival rate of 67%. Survival probabilities were lowest for both sexes near the completion of the N. melanostomus reproductive season in July and August which supports existing evidence of higher mortality after reproduction, while challenging the paradigm that male N. melanostomus suffer comparatively higher mortality as a result of reproduction than females. Evidence indicating that growth and mortality rates are highest at the end of the reproductive season not only highlights seasonal variability in N. melanostomus natural history, but may also guide the control of this invasive species to periods when they are most vulnerable.     

Genetic divergence between cave and surface populations of Astyanax in Mexico (Characidae,Teleostei)

A study of genetic diversity at microsatellite loci and the mitochondrial DNA (mtDNA) cytochrome b gene was carried out to assess genetic relationships among four Mexican cave (Pachon, Sabinos, Tinaja, Chica) and four surface populations of Astyanax fasciatus (Characidae) from northeast Mexico and the Yucatan. With the exception of Chica, the cave populations were all characterized by extremely low microsatellite variability, which most likely resulted from bottleneck events. Population analyses of the microsatellite data indicated no measurable levels of gene flow between all cave and surface populations (F(ST) > 0.0707). Phylogenetic analyses of mtDNA data showed that only two cave populations - Sabinos and Tinaja - group together to the exclusion of surface populations. From the microsatellite data these cave populations cluster with the Pachon cave fish population. The mtDNA thus appears to have been replaced in Pachon because of introgressive hybridization. It is likely that these three cave populations have descended from a surface ancestor in common with current surface populations, rather than evolving recently from one of the extant surface populations. Like Pachon, the Chica population clustered with the surface populations according to mtDNA data, but was not clearly associated with either the surface or the other cave populations according to the microsatellite data. Our data indicate that the Chica population evolved recently from a surface population, and subsequently hybridized with a phylogenetically older cave population. In conclusion, both the microsatellite and mtDNA data suggest multiple origins of cave populations and the Chica and Sabinos/Tinaja/Pachon were founded after at least two independent invasions from surface populations.     

Molecular evidence for multiple introductions of garlic mustard (Alliaria petiolata, Brassicaceae) to North America

Invasive species offer excellent model systems for studying rapid evolutionary change. In this context, molecular markers play an important role because they provide information about pathways of introduction, the amount of genetic variation introduced, and the extent to which founder effects and inbreeding after population bottlenecks may have contributed to evolutionary change. Here, we studied microsatellite variation in eight polymorphic loci among and within 27 native and 26 introduced populations of garlic mustard (Alliaria petiolata), a European herb which is a current serious invader in North American deciduous forests. Overall, introduced populations were genetically less diverse. However, considerable variability was present and when compared to the probable source regions, no bottleneck was evident. Observed heterozygosity was very low and resulted in high inbreeding coefficients, which did not differ significantly between native and introduced populations. Thus, selfing seems to be equally dominant in both ranges. Consequently, there was strong population differentiation in the native (F(ST) = 0.704) and the introduced (F(ST) = 0.789) ranges. The high allelic diversity in the introduced range strongly suggests multiple introductions of Alliaria petiolata to North America. Out of six European regions, the British Isles, northern Europe, and central Europe had significantly higher proportions of alleles, which are common to the introduced range, and are therefore the most probable source regions. The genetic diversity established by multiple introductions, and the lack of inbreeding depression in this highly selfing species, may have contributed to the invasion success of Alliaria petiolata.     

Genetic and biogeographic relationships of the racer goby Neogobius gymnotrachelus (Gobiidae: Teleostei) from introduced and native Eurasian locations

The racer goby Neogobius gymnotrachelus , along with several other neogobiin fishes, has been spreading north and west from its native Ponto-Caspian range for the past two decades via shipping and canals. It has been predicted as a likely future invader of the North American Great Lakes, where it would join its neogobiin relatives – the round and freshwater tubenose gobies. The present study is the first to analyse the population genetic and biogeographic relationships of the racer goby, establishing a baseline to aid interpretation of its future spread patterns and likely donor-recipient population relationships. The mitochondrial DNA cytochrome b gene was sequenced from representative areas of the racer goby's range, including rivers of the northern Black Sea and areas of spread upstream in the Danube River and outside the Ponto-Caspian region to the Vistula River in Poland. Results discerned nine haplotypes, with few shared among drainages of native rivers and most sites housing unique alleles. Racer goby populations significantly diverged among watersheds, supporting historic low migration and little non-anthropogenic gene flow. The Dnieper River was identified as a likely donor source for the Vistula River colonization, where appreciable variability suggests a relatively large number of founding genotypes.     

Isolation and characterization of microsatellites in Neogobius kessleri (Perciformes,Gobiidae) and cross‐species amplification within the family Gobiidae

Fourteen polymorphic microsatellites were isolated from Neogobius kessleri, a benthic fish of Ponto‐Caspian origin which has been recently introduced into the Middle and Upper Danube River. Number of alleles and heterozygosity per locus in a sample of 32 fish individuals ranged from two to four and from 0.13 to 0.75, respectively. These primers will be useful in determining the population structure of N. kessleri. In addition, successful cross‐amplification was obtained for four related species, N. melanostomus, N. fluviatilis, N. gymnotrachelus and Proterorhinus marmoratus. These microsatellite loci may be useful for the evaluation of the origin of non‐native goby populations.     

Dispersal strategies, secondary range expansion and invasion genetics of the nonindigenous round goby, Neogobius melanostomus, in Great Lakes tributaries

Dispersal strategies are important mechanisms underlying the spatial distribution and colonizing ability of all mobile species. In the current study, we use highly polymorphic microsatellite markers to evaluate local dispersal and colonization dynamics of the round goby (Neogobius melanostomus), an aquatic invader expanding its range from lake to river environments in its introduced North American range. Genetic structure, genotype assignment and genetic diversity were compared among 1262 round gobies from 20 river and four lake sites in three Great Lakes tributaries. Our results indicate that a combination of short-distance diffusion and long-distance dispersal, collectively referred to as 'stratified dispersal', is facilitating river colonization. Colonization proceeded upstream yearly (approximately 500 m/year; 2005-2009) in one of two temporal replicates while genetic structure was temporally stable. Contiguous dispersal from the lake was observed in all three rivers with a substantial portion of river fish (7.3%) identified as migrants. Genotype assignment indicated a separate introduction occurred upstream of the invasion front in one river. Genetic diversity was similar and relatively high among lake and recently colonized river populations, indicating that founder effects are mitigated through a dual-dispersal strategy. The remarkable success of round goby as an aquatic invader stresses the need for better diffusion models of secondary range expansion for presumably sessile invasive species.     

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.     

PERMANENT GENETIC RESOURCES: Isolation and characterization of polymorphic trinucleotide and dinucleotide microsatellite loci for the ice goby, Leucopsarison petersii

Eleven microsatellite markers were isolated and characterized for the ice goby, Leucopsarion petersii, from genomic libraries enriched for (ATG)(9) and (CA)(16) . Twenty individuals from a single population were used to screen polymorphism in these loci. The number of alleles per locus and observed heterozygosity ranged from four to 22 and from 0.35 to 1.00, respectively. All loci did not significantly deviated from Hardy-Weinberg equilibrium, and there was no evidence of linkage disequilibrium between all loci-pairs. These loci showed Mendelian inheritance in a full-sib family. The high level of polymorphism of these loci will be useful for studies of population genetics.     

Ship Mediated Fish Invasions in Australia: Two New Introductions and A Consideration of Two Previous Invasions

Two species of fishes, the northwest Pacific Acentrogobius pflaumi (Bleeker 1853) (Perciformes: Gobiidae) and the New Zealand Forsterygion lapillum Hardy (1989) (Perciformes: Tripterygiidae), are newly reported as recent introductions in Port Phillip Bay, southeastern Australia. Forsterygion lapillum is restricted to a commercial port in the western area of the bay, while A. pflaumi occurs in all areas of the bay except the entrance. Both species are among the most abundant fishes collected in occupied habitats. It seems probable that they were introduced as eggs, larvae or juveniles in ships ballast water. While both A. pflaumi and F. lapillum appear to be well suited to conditions in Australia, populations of two previously introduced marine fishes (the gobiidsAcanthogobius flavimanus and Tridentiger trigonocephalus) have failed to rapidly expand their distributions and seem to be controlled by factors yet to be determined.     

Glacial history of Saxifraga paniculata (Saxifragaceae): molecular biogeography of a disjunct arctic-alpine species from Europe and North America

The molecular biogeography of the disjunctly distributed and morphologically highly variable species Saxifraga paniculata Mill. was analysed using amplified fragment length polymorphism (AFLP) and chloroplast microsatellites. The study comprised 77 samples from mountain regions in Europe and North America throughout the complete range of distribution. AFLP data revealed clear genetic differentiation between samples from the Arctic, the Caucasus, and the eastern European mountains. Samples from the Alps were divided into two groups. One group clustered with the samples from central Europe and the Pyrenees, whereas another group with individuals from southern Norway. AFLP diversity was lowest in the Arctic and highest in the Alps. Chloroplast microsatellite analysis revealed eight haplotypes but no unequivocal phylogeographical pattern. However, haplotype diversity was highest in the Alps and central Europe whereas, in the Arctic, only few widespread haplotypes could be found. The results indicate in situ survival of S. paniculata in the Caucasus, the eastern European mountains, and the Alps. The Arctic has presumably been colonized postglacially from North American refugia south of the ice shield. Southern Norway and the Pyrenees have most likely been colonized from two phylogeographically different groups in the Alps. The origin of the central European samples remains ambiguous. In situ survival seems to be as possible as several postglacial recolonization events from the Alps. The obtained molecular data clearly support the subdivision of S. paniculata into three subspecies: ssp. cartilaginea from the Caucasus, ssp. laestadii from northern Norway, Iceland, and North America, and ssp. paniculata from the other geographical regions.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 385–398.     

Molecular genetic variation of boll weevil populations in North America estimated with microsatellites: Implications for patterns of dispersal

The boll weevil (Anthonomus grandis Boheman) is an insect pest of cotton that underwent a well-documented range expansion across the southeastern U.S. from Mexico beginning about 110 years ago. Eleven microsatellite loci were surveyed to infer the magnitude and pattern of genetic differentiation among boll weevil populations from 18 locations across eight U.S. states and northeast Mexico. Estimates of genetic diversity (allelic diversity and heterozygosity) were greater in Southern than Northern populations, and were greater in the west than the east among Northern populations. Boll weevil populations were genetically structured as a whole across the geographic range sampled, with a global F _ST of 0.241. South-central populations exhibit classic isolation by distance, but evidence suggests that populations within the Eastern and Western regions have not yet reached genetic equilibrium. Gene flow appears to be relatively high among populations within the Eastern region. Population assignment data and estimates of gene flow indicate that migration between locations separated by < 300 km is frequent. The database of microsatellite genotypes generated in this study now makes it possible, through population assignment techniques, to identify the most likely geographic source of a boll weevil reintroduced to an eradication zone, which will help action agencies decide the most appropriate mitigation response. Kyung Seok Kim - Formerly: USDA-ARS, Areawide Pest Management Research Unit, 2771 F & B Rd., College Station,TX 77845, USA.     

Symbiodinium population genetics: testing for species boundaries and analysing samples with mixed genotypes

Population genetic markers are increasingly being used to study the diversity, ecology and evolution of Symbiodinium, a group of eukaryotic microbes that are often mutualistic with reef‐building corals. Population genetic markers can resolve individual clones, or strains, from samples of host tissue; however, samples may comprise different species that may confound interpretations of gene flow and genetic structure. Here, we propose a method for resolving species from population genetic data using tests for genetic recombination. Assigning individuals to genetically recombining populations prior to further analyses avoids critical errors in the interpretation of gene flow and dispersal. To demonstrate the effectiveness of the approach, we first apply this method to a simulated data set. We then use the method to resolve two species of host generalist Symbiodinium that commonly co‐occur in reef‐building corals collected from Indo‐West Pacific reefs. We demonstrate that the method is robust even when some hosts contain genotypes from two distinct species. Finally, we examine population genetic data sets from two recently published papers in Molecular Ecology. We show that each strongly supports a two species interpretation, which significantly changes the original conclusions presented in these studies. When combined with available phylogenetic and ecological evidence, the use of population genetic data offers a robust method for unambiguously delimiting morphologically cryptic species.     

Invasion genetics of the Mediterranean fruit fly: variation in multiple nuclear introns

Biological invasions generally start from low initial population sizes, leading to reduced genetic variation in nuclear and especially mitochondrial DNA. Consequently, genetic approaches for the study of invasion history and population structure are difficult. An extreme example is the Mediterranean fruit fly, Ceratitis capitata (Medfly), for which successive invasions during this century have resulted in a loss of 60% of ancestral genetic variation in isozymes and 75% of variation in mitochondrial DNA. Using Medflies as an example, we present a new approach to invasion genetics that measures DNA sequence variation within introns from multiple nuclear loci. These loci are so variable that even relatively recently founded Medfly populations within California and Hawaii retain ample genetic diversity. Invading populations have only lost 35% of the ancestral genetic variation. Intron variation will allow high-resolution genetic characterization of invading populations in both natural and managed systems, although non-equilibrium methods of analysis may be necessary if the genetic diversity represents sorting ancestral polymorphism.