Multilocus Microsatellite Genotypes Research Articles

Natural hybridization and potential seed set of sympatric Populus nigra and Populus × canadensis along the river IJzer in Flanders (Belgium)

Background In Europe, Populus nigra generally grows sympatric with the exotic related taxon Populus x canadensis. Natural hybridization followed by introgression and genetic swamping is often mentioned as a potential threat for the conservation of P. nigra. However, as evidence for natural hybridization with P. x canadensis is scarce, the significance of this threat is unclear. Methods After first determining the genotypic diversity within 209 trees of P. nigra collected in western Flanders (northwest Belgium), we assessed natural hybridization events between P. nigra females and P. x canadensis males with diagnostic microsatellite markers. We also studied the possible paternity of the frequently planted P. nigra cv. Italica. Furthermore, we compared the potential seed set of P. x canadensis and P. nigra by counting the number of unfertilized ovules per ovary and the number of ovaries per pistillate inflorescences. Results and conclusions Only two different multilocus microsatellite genotypes were detected within the 209 black poplar trees sampled in western Flanders. In 57 % of the seedlings from the open pollinated black poplar trees, at least one species-specific allele of P. deltoides was detected. P. nigra cv. Italica could not be excluded as a father of one seedling. The potential for seed set and the number of ovules per ovary was significantly higher for P. x canadensis compared to P. nigra. Conclusions The extreme low number of P. nigra genotypes in western Flanders suggests a strong human influence on the species’ genetic diversity. The findings further indicate that natural hybridization events between female P. nigra and male P. x canadensis frequently occur in our study area. Furthermore, P. x canadensis has a higher seed set potential compared to P. nigra. Restoration projects of P. nigra in the valley of the IJzer river should initially aim reforestation measures with a mixture of different black poplar genotypes from neighbouring regions.

Genetically Derived Estimates of Contemporary Natural Straying Rates and Historical Gene Flow among Lake Michigan Lake Sturgeon Populations

AbstractNatural rates of straying are difficult to quantify over large spatial scales using direct observations, particularly for long‐lived fish species characterized by delayed sexual maturity and long interspawning intervals. Using multilocus microsatellite genotypes and likelihood‐based statistical methods, we quantified rates of immigration and emigration for six genetically differentiated (mean FST = 0.041) lake sturgeon Acipenser fulvescens populations in Lake Michigan based on adults (n = 437) captured in tributaries during the spawning season. Estimated rates of straying were high (mean = 0.105), asymmetrical, and highly variable across populations. We found no significant association between the total length (a surrogate measure of age) of individuals that strayed and those that did not. Linear distance between streams was more predictive of straying rates and FST than least‐cost distances estimated based on lakescape features (bathymetry and lake current patterns). Historical rates of gene flow estimated using coalescent analysis indicated a fully parameterized model with variable evolutionarily effective population sizes (θ range, 0.684–0.989), and variable and nonsymmetrical migration rates best explained the genetic data. Comparatively high estimates of relative historical gene flow from several numerically depressed populations suggest that these populations were once larger contributors to basinwide gene flow than indicated by estimates of contemporary straying rates. High rates of interpopulation straying contrast with high FST, suggesting that straying rates are poor indicators of successful reproduction following dispersal.

Breeding site fidelity and winter admixture in a long-distance migrant, the tufted duck (Aythya fuligula).

Long-distance migrants are, by definition, highly mobile but it is poorly understood if this leads to high rates of gene flow and an essentially panmictic global population structure. Genetic divergence in migratory species could be promoted, for example, by fidelity to distinct migratory pathways. In this study, we investigate the population genetic structure of tufted duck (Aythya fuligula), a long-distance migrant with a largely continuous breeding distribution across Eurasia. Distinct, longitudinally oriented flyways have been postulated based on geographically disjunct wintering areas and are supported by evidence from ringing data. We generated sequences of the mitochondrial control region and multi-locus microsatellite genotypes for several hundreds of samples from the European and Asian breeding and wintering grounds including some individuals infected with highly pathogenic avian influenza virus H5N1. Significant differentiation between breeding sites was observed for both marker types, but F(ST) values were approximately 10 times higher for maternally inherited mitochondrial DNA than for biparentally transmitted nuclear markers. The genetic differentiation between the postulated European and Asian flyways was similar to that observed within continents and, in general, genetic divergence was not associated with geographic distance. Neither marker type showed evidence of genetic substructure among aggregations on the European wintering grounds. Our results suggest some breeding site fidelity, especially in females, but extensive population admixture on the wintering grounds. Several scenarios may explain the observed lack of genetic divergence between Europe and Asia including non-equilibrium conditions following a recent range expansion or contemporary gene flow across the postulated migratory divides.

Discrimination of hybrid classes using cross‐species amplification of microsatellite loci: methodological challenges and solutions in Daphnia

Microsatellite markers are important tools in population, conservation and forensic studies and are frequently used for species delineation, the detection of hybridization and introgression. Therefore, marker sets that amplify variable DNA regions in two species are required; however, cross-species amplification is often difficult, as genotyping errors such as null alleles may occur. To estimate the level of potential misidentifications based on genotyping errors, we compared the occurrence of parental alleles in laboratory and natural Daphnia hybrids (Daphnia longispina group). We tested a set of 12 microsatellite loci with regard to their suitability for unambiguous species and hybrid class identification using F(1) hybrids bred in the laboratory. Further, a large set of 44 natural populations of D. cucullata, D. galeata and D. longispina (1715 individuals) as well as their interspecific hybrids were genotyped to validate the discriminatory power of different marker combinations. Species delineation using microsatellite multilocus genotypes produced reliable results for all three studied species using assignment tests. Daphnia galeata × cucullata hybrid detection was limited due to three loci exhibiting D. cucullata-specific null alleles, which most likely are caused by differences in primer-binding sites of parental species. Overall, discriminatory power in hybrid detection was improved when a subset of markers was identified that amplifies equally well in both species.

Discordant introgression in a rapidly expanding hybrid swarm.

The erosion of species boundaries can involve rapid evolutionary change. Consequently, many aspects of the process remain poorly understood, including the formation, expansion, and evolution of hybrid swarms. Biological invasions involving hybridization present exceptional opportunities to study the erosion of species boundaries because timelines of interactions and outcomes are frequently well known. Here, we examined clinal variation across codominant and maternally inherited genetic markers as well as phenotypic traits to characterize the expansion and evolution of a hybrid swarm between native Cyprinella venusta and invasive Cyprinella lutrensis minnows. Discordant introgression of phenotype, microsatellite multilocus genotype, and mtDNA haplotype indicates that the observable expansion of the C. venusta × C. lutrensis hybrid swarm is a false invasion front. Both parental and hybrid individuals closely resembling C. lutrensis are numerically dominant in the expansion wake, indicating that the non-native parental phenotype may be selectively favored. These findings show that cryptic introgression can extend beyond the phenotypic boundaries of hybrid swarms and that hybrid swarms likely expand more rapidly than can be documented from phenotypic variation alone. Similarly, dominance of a single parental phenotype following an introduction event may lead to instances of species erosion being mistaken for species displacement without hybridization.

Genetic identification of two putative world record Michigan salmonids resolves stakeholder and manager questions

The ability of fishery managers to quickly and effectively answer stakeholder questions using the best available science is crucial for successful management. The 2009 capture of a potential world record brown trout (Salmo trutta) and the 2010 capture of a potential world record land-locked Atlantic salmon (S. salar) in Michigan required managers to acquire genetic verification of the species identity. Given the variety of hatchery strains used to maintain Great Lakes fisheries for brown trout and in the absence of physical markings, managers also were interested to determine the strain of origin for information on assessing performance. DNA barcoding techniques using sequences from the mitochondrial (mt) DNA cytochrome c oxidase I (COI) gene and frequency-based analysis of species-specific microsatellite genotypes provided data to establish the species of both fish. The putative brown trout was confirmed to be a new world record specimen. Using individual assignment tests based on maximum likelihood estimators informed by multi-locus microsatellite genotypes, we determined the fish to be from the Seeforellen hatchery strain (p<0.01). Analysis of the COI gene in the putative Atlantic salmon resulted in assignment as a brown trout. The presence of only brown trout alleles at all six microsatellite loci examined revealed that the individual was not an inter-specific hybrid. Given sufficient genetic divergence exists among species, populations, or hatchery strains, the combination of mtDNA barcoding and microsatellite genetic analysis can provide accurate and rapid identification to address stakeholder and management questions.

Genetic relationships and gene flow between resident and migratory brook trout in the Salmon Trout River

Genetic differentiation among brook trout (Salvelinus fontinalis) of different life history forms and populations can result from reproductive isolation imposed by natural or anthropogenically derived barriers to gene flow, behavioral incompatibilities, or differential exposure to environmental cues. We used multi-locus microsatellite genotypes and likelihood and Bayesian-based analyses to characterize the degree of genetic differentiation and evidence of introgression among stream resident brook trout above a natural barrier, and putative stream residents and adfluvial (coaster) brook trout from below the barrier in the Salmon Trout River (STR); the sole tributary along the southern shore of Lake Superior known to be inhabited by a viable remnant population of coaster brook trout. Two genetically differentiated populations were identified, generally associated with individuals inhabiting sections of the STR above and below the falls. No evidence of differentiation was found between a priori classified resident and coaster brook trout from below the falls. Gene flow from individuals above the falls was detected based on evidence of interbreeding between upper river individuals and coasters below the falls. We collected only a relatively small number of individuals that we a priori classified as being stream residents below the falls, and these individuals had a high probability of having ancestry originating from the population above the barrier, which suggests that the stream-resident life history may be exceptionally rare or absent in the lower Salmon Trout River.

Range‐wide genetic population structure of common pochard (Aythya ferina): a potentially important vector of highly pathogenic avian influenza viruses

An understanding of the distribution and spatial structure of the natural vectors of zoonothic pathogens is of interest for effective disease control and prevention. Here, we investigate the range-wide population genetic structure of common pochard (Aythya ferina), a long-distance migratory duck and potential vector of highly pathogenic avian influenza. We collected several hundred samples from breeding and wintering grounds across Eurasia including some H5N1-positive individuals and generated partial sequences of the mitochondrial control region and multilocus microsatellite genotypes. Genetic differentiation among breeding populations was significant for both marker types but higher for maternally inherited mtDNA than for biparentally inherited nuclear markers. There was only weak genetic divergence between ducks sampled in Europe and East Asia, and genetic differentiation between populations was not generally associated with geographical distance. No evidence of genetic substructure was detected for ducks sampled on the European wintering grounds. Our results suggest limited breeding-site fidelity, especially in females, but extensive population admixture on the wintering grounds. The specific role of pochards as natural vectors of zoonotic pathogens and in particular H5N1 remains to be clarified but our results point to wintering grounds as potential hotspots for disease transmission.

Genetic analysis of the endemic island loggerhead shrike, Lanius ludovicianus anthonyi

Based on limited research, the island loggerhead shrike, Lanius ludovicianus anthonyi has been considered a distinct subspecies endemic to the northern California Channel Islands. We used mtDNA control region sequences and microsatellite genotyping to compare loggerhead shrikes from the southern California mainland (L. l. gambeli), San Clemente Island (L. l. mearnsi), and the northern islands (L. l. anthonyi). Habitats on the islands are recovering due to the removal of non-native ungulates on the islands, but may be transitioning to habitats less supportive of loggerhead shrikes, so this evaluation comes at a critical time. We utilized 96 museum specimens that were collected over a century to evaluate both spatial and temporal genetic patterns. Analysis of multilocus microsatellite genotypes indicated that historical specimens of loggerhead shrikes (collected between 1897 and 1986) from the two northern islands of Santa Rosa and Santa Cruz are genetically distinct from adjacent mainland and island shrikes. Birds from Santa Catalina Island showed mixed ancestry and did not cluster with the northern island birds. Historical specimens of L. l. mearnsi from San Clemente Island also showed mixed ancestry. Our study provides evidence that a genetically distinct form of loggerhead shrikes, L. l. anthonyi, occurred on the islands of Santa Rosa and Santa Cruz.

Colonization of Ireland: revisiting ‘the pygmy shrew syndrome’ using mitochondrial, Y chromosomal and microsatellite markers

There is great uncertainty about how Ireland attained its current fauna and flora. Long-distance human-mediated colonization from southwestern Europe has been seen as a possible way that Ireland obtained many of its species; however, Britain has (surprisingly) been neglected as a source area for Ireland. The pygmy shrew has long been considered an illustrative model species, such that the uncertainty of the Irish colonization process has been dubbed 'the pygmy shrew syndrome'. Here, we used new genetic data consisting of 218 cytochrome (cyt) b sequences, 153 control region sequences, 17 Y-intron sequences and 335 microsatellite multilocus genotypes to distinguish between four possible hypotheses for the colonization of the British Isles, formulated in the context of previously published data. Cyt b sequences from western Europe were basal to those found in Ireland, but also to those found in the periphery of Britain and several offshore islands. Although the central cyt b haplotype in Ireland was found in northern Spain, we argue that it most likely occurred in Britain also, from where the pygmy shrew colonized Ireland as a human introduction during the Holocene. Y-intron and microsatellite data are consistent with this hypothesis, and the biological traits and distributional data of pygmy shrews argue against long-distance colonization from Spain. The compact starburst of the Irish cyt b expansion and the low genetic diversity across all markers strongly suggests a recent colonization. This detailed molecular study of the pygmy shrew provides a new perspective on an old colonization question.

Conservation genetic management of a critically endangered New Zealand endemic bird: minimizing inbreeding in the Black Stilt Himantopus novaezelandiae

For threatened species with small captive populations, it is advisable to incorporate conservation management strategies that minimize inbreeding in an effort to avoid inbreeding depression. Using multilocus microsatellite genotype data, we found a significant negative relationship between genetic relatedness (inbreeding) and reproductive success (fitness) in a captive population of the critically endangered Black Stilt or KakīHimantopus novaezelandiae. In an effort to avoid inbreeding depression in this iconic New Zealand endemic, we recommend re-pairing closely related captive birds with less related individuals and pairing new captive birds with distantly related individuals.

Spatial genetic structure of a vector-borne generalist pathogen.

Vector-borne generalist pathogens colonize several reservoir species and are usually dependent on polyphagous arthropods for dispersal; however, their spatial genetic structure is generally poorly understood. Using fast-evolving genetic markers (20 simple sequence repeat loci, resulting in a total of 119 alleles), we studied the genetic structure of the vector-borne plant-pathogenic bacterium Xylella fastidiosa in Napa Valley, CA, where it causes Pierce's disease when it is transmitted to grapevines from reservoir plants in adjacent riparian vegetation. Eighty-three different X. fastidiosa multilocus microsatellite genotypes were found in 93 isolates obtained from five vineyards, resulting in an index of clonal fraction closer to 0 and a Simpson's genotypic diversity index (D) closer to a maximum value of 1. Moderate values of Nei's gene diversity (H(Nei); average H(Nei) = 0.41) were observed for most of the X. fastidiosa populations. The low Wright's index of genetic diversity among populations calculated by the FSTAT software (Wright's F(ST) index) among population pairs (0.0096 to 0.1080) indicated a weak or absent genetic structure among the five populations; a panmictic population was inferred by Bayesian analyses (with the STRUCTURE and BAPS programs). Furthermore, a Mantel test showed no significant genetic isolation by distance when both Nei (r = -0.3459, P = 0.268) and linearized (r = -0.3106, P = 0.269) indices were used. These results suggest that the riparian vegetation from which vectors acquire the pathogen prior to inoculation of grapevines supports a diverse population of X. fastidiosa.

Evidence for a quiet revolution: seasonal variation in colonies of the specialist tansy aphid,Macrosiphoniella tanacetaria(Kaltenbach) (Hemiptera: Aphididae) studied using microsatellite markers

In cyclical parthenogens, clonal diversity is expected to decrease due to selection and drift during the asexual phase per number of asexual generations. The decrease in diversity may be counteracted by immigration of new genotypes. We analysed temporal variation in clonal diversity in colonies of the monophagous tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach), sampled four times over the course of a growing season. In a related field study, we recorded aphid colony sizes and the occurrence of winged dispersers throughout the season. The number of colonies increased from April, when asexual stem mothers hatched from the sexually produced eggs, to the end of June. The proportion of colonies with winged individuals also increased over this period. After a severe reduction in colony sizes in late summer, a second expansion phase occurred in October when sexuals were produced. At the season's end, the only winged forms were males. A linked genetic study showed that the number of microsatellite multilocus genotypes and genetic variability assessed at three polymorphic loci per colony decreased from June to October. Overall, the relatedness of wingless to winged individuals within colonies was lower than average relatedness among wingless individuals, suggesting that winged forms mainly originated in different colonies. The results demonstrate that patterns of genetic diversity within colonies can be explained by the antagonistic forces of clonal selection, migration and genetic drift (largely due to midsummer population bottlenecks). We further suggest that the males emigrate over comparatively longer distances than winged asexual females.

Genetic Structure ofPlasmodium falciparumand Elimination of Malaria, Comoros Archipelago

The efficacy of malaria control and elimination on islands may depend on the intensity of new parasite inflow. On the Comoros archipelago, where falciparum malaria remains a major public health problem because of spread of drug resistance and insufficient malaria control, recent interventions for malaria elimination were planned on Moheli, 1 of 4 islands in the Comoros archipelago. To assess the relevance of such a local strategy, we performed a population genetics analysis by using multilocus microsatellite and resistance genotyping of Plasmodium falciparum sampled from each island of the archipelago. We found a contrasted population genetic structure explained by geographic isolation, human migration, malaria transmission, and drug selective pressure. Our findings suggest that malaria elimination interventions should be implemented simultaneously on the entire archipelago rather than restricted to 1 island and demonstrate the necessity for specific chemoresistance surveillance on each of the 4 Comorian islands.

Validation of swabs as a non‐destructive and relatively non‐invasive DNA sampling method in fish

Non-destructive methods of collecting DNA from small fish species can be problematic, as fin clips can potentially affect behaviour or survivorship in the wild. Swabbing body mucus may provide a less invasive method of DNA collection. However, risk of contamination from other individuals in high density groups could give erroneous genotyping results. We compared multilocus microsatellite genotypes from the same individuals when collected at low and high density and compared this with fin clips. We found no differences between these categories, with a genotyping error rate of 0.42%, validating the use of body mucus swabbing for DNA collection in fish.

Integrating phylogeographic patterns of microsatellite and mtDNA divergence to infer the evolutionary history of chamois (genus Rupicapra)

BackgroundThe chamois, distributed over most of the medium to high altitude mountain ranges of southern Eurasia, provides an excellent model for exploring the effects of historical and evolutionary events on diversification. Populations have been grouped into two species, Rupicapra pyrenaica from southwestern Europe and R. rupicapra from eastern Europe. However, a previous study of cytochrome b revealed that the two proposed species were non-monophyletic. The reconstruction of phylogenetic relationships between animal species often depends on the markers studied. To further elucidate the evolutionary history of chamois, we extended earlier studies by analysing DNA sequences of four mitochondrial regions (ND1, 12S, tRNApro and Control Region) and microsatellites (20 loci) to include all subspecies and cover its entire distribution range.ResultsWe found discordant microsatellite (μsat) and mitochondrial (mt) DNA phylogenies. Mitochondrial phylogenies form three clades, West, Central and East (mtW, mtC and mtE), at variance with taxonomic classification. Our divergence age estimates indicate an initial separation into branches mtW-mtC and mtE 1.7 million years ago (mya), in the late Pliocene-early Pleistocene, quickly followed by the split of clades mtW and mtC. Clade mtW contains haplotypes from the Iberian peninsula and the western Alps, Clade mtC includes haplotypes from the Apennines and the Massif of Chartreuse and Clade mtE comprises populations to the east of the Alps. Divergence among populations within these three major clades is recent (< 0.5 mya). New microsatellite multilocus genotypes added to previously published data revealed differences between every pair of subspecies, forming three well defined groups (μsatW, μsatC and μsatE) also with a strong geographic signature. Grouping does not correspond with the mitochondrial lineages but is closer to morphology and taxonomic classification. Recent drastic reductions in population size can be noted for the subspecies ornata as an extremely low diversity.ConclusionsThe phylogeographic patterns for mtDNA and microsatellites suggest an evolutionary history with limited range contractions and expansions during the Quaternary period and reflect a major effect of the Alpine barrier on west-east differentiation. The contrasting phylogenies for mtDNA and microsatellites indicate events of hybridization among highly divergent lineages in the central area of distribution. Our study points to the importance of reticulate evolution, with periods of isolation and reduction of population size followed by expansions and hybridizations, in the diversification at the level of close species or subspecies.

Realized gene flow within mixed stands of Quercus robur L. and Q. petraea (Matt.) L. revealed at the stage of naturally established seedling

The estimates of contemporary gene flow assessed based on naturally established seedlings provide information much needed for understanding the abilities of forest tree populations to persist under global changes through migration and/or adaptation facilitated by gene exchange among populations. Here, we investigated pollen- and seed-mediated gene flow in two mixed-oak forest stands (consisting of Quercus robur L. and Q. petraea [Matt.] Liebl.). The gene flow parameters were estimated based on microsatellite multilocus genotypes of seedlings and adults and their spatial locations within the sample plots using models that attempt to reconstruct the genealogy of the seedling cohorts. Pollen and seed dispersal were modelled using the standard seedling neighbourhood model and a modification--the 2-component seedling neighbourhood model, with the later allowing separation of the dispersal process into local and long-distance components. The 2-component model fitted the data substantially better than the standard model and provided estimates of mean seed and pollen dispersal distances accounting for long-distance propagule dispersal. The mean distance of effective pollen dispersal was found to be 298 and 463 m, depending on the stand, while the mean distance of effective seed dispersal was only 8.8 and 15.6 m, which is consistent with wind pollination and primarily seed dispersal by gravity in Quercus. Some differences observed between the two stands could be attributed to the differences in the stand structure of the adult populations and the existing understory vegetation. Such a mixture of relatively limited seed dispersal with occasional long distance gene flow seems to be an efficient strategy for colonizing new habitats with subsequent local adaptation, while maintaining genetic diversity within populations.

Genetic structure of Plasmodium falciparum populations between lowland and highland sites and antimalarial drug resistance in Western Kenya

Human travel to malaria endemic lowlands from epidemic highlands has been shown to increase the risk of malaria infections in the highlands. In order to gain insight on the impact of human travel, we examined prevalence, genetic variability and population genetic structure of Plasmodium falciparum in asymptomatic children from one highland site and three surrounding malaria endemic lowland sites in Western Kenya, using multilocus microsatellite genotyping. We further analyzed the frequencies of mutations at the genes conferring resistance to chloroquine and sulfadoxine–pyrimethamine. We found a significant decrease in malaria prevalence in the highland site from 2006 to 2007, 1 year after the introduction of the artemisinin-based combination therapy as first-line treatment for uncomplicated malaria and the scale-up of insecticide-treated bed nets. Population genetic diversity, measured by the number of observed and effective microsatellite alleles and Nei's unbiased genetic diversity, was high and comparable for both highland and lowland populations. Analysis of molecular variance did not detect a significant genetic structure across highland and lowland regions. Similarly, mutations at key antimalarial-resistance codons of the pfcrt, pfmdr1, pfdhfr and pfdhps genes were found at comparable high frequencies in all four sites. High level of gene flow and lack of significant genetic structure in malaria parasites between highland and lowland areas suggest the importance of human travel in shaping parasite population structure.

Flow cytometric analysis and microsatellite genotyping reveal extensive DNA content variation in Trypanosoma cruzi populations and expose contrasts between natural and experimental hybrids

Trypanosoma cruzi exhibits remarkable genetic heterogeneity. This is evident at the nucleotide level but also structurally, in the form of karyotypic variation and DNA content differences between strains. Although natural populations of T. cruzi are predominantly clonal, hybrid lineages (TcIId and TcIIe) have been identified and hybridisation has been demonstrated in vitro, raising the possibility that genetic exchange may continue to shape the evolution of this pathogen. The mechanism of genetic exchange identified in the laboratory is unusual, apparently involving fusion of diploid parents followed by genome erosion. We investigated DNA content diversity in natural populations of T. cruzi in the context of its genetic subdivisions by using flow cytometric analysis and multilocus microsatellite genotyping to determine the relative DNA content and estimate the ploidy of 54 cloned isolates. The maximum difference observed was 47.5% between strain Tu18 cl2 (TcIIb) and strain C8 cl1 (TcI), which we estimated to be equivalent to ∼73Mb of DNA. Large DNA content differences were identified within and between discrete typing units (DTUs). In particular, the mean DNA content of TcI strains was significantly less than that for TcII strains (P<0.001). Comparisons of hybrid DTUs TcIId/IIe with corresponding parental DTUs TcIIb/IIc indicated that natural hybrids are predominantly diploid. We also measured the relative DNA content of six in vitro-generated TcI hybrid clones and their parents. In contrast to TcIId/IIe hybrid strains these experimental hybrids comprised populations of sub-tetraploid organisms with mean DNA contents 1.65–1.72 times higher than the parental organisms. The DNA contents of both parents and hybrids were shown to be relatively stable after passage through a mammalian host, heat shock or nutritional stress. The results are discussed in the context of hybridisation mechanisms in both natural and in vitro settings.

Genetic population structure of the Swordfish ( Xiphias gladius) in the southwest Indian Ocean: Sex-biased differentiation, congruency between markers and its incidence in a way of stock assessment

Genetic variation was surveyed at 11 microsatellite loci and at 517 bp of the mitochondrial control region to investigate the presence of genetic stock structure in swordfish ( Xiphias gladius) in four proximal localities of the southwest Indian Ocean. One aim of this study was to serve as a preliminary examination for congruency of structure detected by these two genetic markers, prior to conducting a more comprehensive basin-wide survey of the Indian Ocean and nearby surrounding areas. Analyses of multilocus microsatellite genotypes and mitochondrial control region sequences both revealed a great homogeneity between samples. Genetic diversity detected at the regional scale was not significantly higher than detected at the local scale. Results suggest that the southwest Indian Ocean globally functions as a unique panmictic population. However, some discrete genetic differences appeared that could possibly indicate influence from a second genetic pool in the northern part of the Indian Ocean. This structure appeared to be sex-dependent with genetic differences higher among female than among male samples. This result may indicate a higher level of spawning area fidelity for females with a subsequent sampling bias tending to homogenise male genotypic distributions.