Potential Gene Flow Research Articles

Genetic differentiation among populations of the Mediterranean hermaphroditic brooding coral Balanophyllia europaea (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.

Genetic differentiation in sympatric wood ants, Formica rufa and F. polyctena

Direct observations have suggested that the closely related wood ants Formica polyctena and F. rufa represent different social organizations, with high queen number in F. polyctena and a high frequency of monogynous nests in F. rufa. We examined social organization and genetic population structure in a setup where populations of the two species are sympatric and gene flow between the species is possible. Our aim was to compare social organization in the species, and study evolutionary relationships between them. The observed relatedness among colony workers suggested that the difference in the level of polygyny is quantitative rather than qualitative, with a higher queen number in F. polyctena. The observed difference in polygyny was not accompanied by a difference in spatial genetic differentiation which was weak in both species. The genetic distance between the species is consistent with limited interspecific gene flow. Identification of a few possible F. rufa migrants in F. polyctena populations suggests potential interspecific gene flow. Thus, reproductive isolation of the species may not be complete when they are sympatric.

Estuaries, lagoons and enclosed embayments: habitats that enhance population subdivision of inshore fishes

Several studies have suggested that estuaries, lagoons and enclosed embayments may offer special opportunities for local subdivision in marine species. We used data from published papers and unpublished theses to examine the effect of such water bodies on allozyme differentiation of seven species of inshore fishes in Western Australia. We included species that differ in their dispersal, and hence their intrinsic potential for gene flow. Over large distances, subdivision was generally greater among estuarine populations than among conspecific marine populations collected over similar distances. Over small distances, paired marine and estuarine samples were generally more divergent than pairs of marine samples separated by similar distances. For species with a low capability for dispersal, estuaries appear to add to the high levels of genetic subdivision that commonly result from other factors. Under special circumstances estuaries may also provide opportunities for genetic divergence in species with a high capability of dispersal that are rarely subdivided at a large geographical scale. These observations indicate that estuaries can increase the genetic subdivision of populations of inshore fishes, and that species that use both marine and estuarine habitats are likely to have greater genetic subdivision than those that are restricted to marine habitats.

RESISTANCE TO BIOTIC STRESS IN FRUIT TREES

Resistance to pests and diseases is a highly desirable objective in fruit breeding but is a long-term endeavor. It requires appropriate tests for early screening and an analysis of host-parasite interactions for durability of resistance. Host resistance is considered durable when it remains effective for a long period despite an intensive exposure to the parasite. Durability has to be the key-point of any fruit breeding programme for resistance. Non-durable resistance is usually associated with a hypersensitive reaction (HR) and major gene resistance which often occur in multiple allelic series and/or complex loci. HR resistance or race-specific resistance is often rendered ineffective because the pathogen population shifts genetically to avoid the resistance gene. Resistance of a more durable nature against fungi and bacteria is often polygenic and based upon the additive or interaction effects of a few to several genes not promoting a hypersensitive reaction. The more complex the genetics of the resistance, the more difficult it is for the pathogen to evolve to a form that can overcome the resistance. Polygenic resistance is usually associated with a quantitative expression that is characterized by a continuous variation between susceptibility and resistance. To approach the number of genes involved in polygenic resistance (QTL) and the relative effect of each of these genes, they have to be mapped on the genome. Marker-assisted selection (MAS) gives the possibility to recombine QTL for resistance; major genes could be associated with QTL. The combining of resistance genes is desired to: acquire a higher level of resistance against a particular pathotype, obtain resistance against a broader range of pathotypes, and combine different mechanisms of resistance. The current situation of fruit crops - monocultivar orchards established for several years - enhances the selection pressure on the pathogen population and results in population shifts which could be dramatic. Guidelines have to be proposed to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure; the most important parameters to consider are reproductive/mating systems and gene/genotype flow. Pathogens that have the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. Several examples of different strategies used by fruit breeders will be reported mainly on Coffea (coffee rust), Musa (black leaf streak disease), Prunus (root knot nematodes) and Malus (scab, mildew, fire blight). Fruit breeders can use the lessons learned from deploying resistance genes in annual crops and selecting resistance in forest trees and apply these to their strategies for fruit tree improvement

Using empirical data to model transgene dispersal.

One element of the current public debate about genetically modified crops is that gene flow from transgenic cultivars into surrounding weed populations will lead to more problematic weeds, particularly for traits such as herbicide resistance. Evolutionary biologists can inform this debate by providing accurate estimates of gene flow potential and subsequent ecological performance of resulting hybrids. We develop a model for gene flow incorporating exponential distance and directional effects to be applied to windpollinated species. This model is applied to previously published data on gene flow in experimental plots of Agrostis stolonifera L. (creeping bentgrass), which assessed gene flow from transgenic plants resistant to the herbicide glufosinate to surrounding non-transgenic plants. Our results show that although pollen dispersal can be limited in some sites, it may be extensive in others, depending on local conditions such as exposure to wind. Thus, hybridization under field conditions is likely to occur. Given the nature of the herbicide resistance trait, we regard this trait as unlikely to persist in the absence of herbicide, and suggest that the ecological consequences of such gene flow are likely to be minimal.

Dispersal and gene flow of Populus nigra (Salicaceae) along a dynamic river system

Summary 1 We used genetic markers to study gene flow of the riparian pioneer tree species Populus nigra along the Drôme river (France). This dioecious species is supposed to have more efficient dispersal mechanisms for pollen (wind) and seeds (wind and water) than other trees. 2 Seedlings belonging to the same reproduction/migration event were sampled in 22 riparian forest fragments along the river and their genetic diversity assessed through six nuclear microsatellites. 3 We found a high level of diversity and significant differentiation among populations. The significant isolation by distance allowed us to reject the infinite island model of migration. 4 Gene flow parameters were higher in the upper, mountainous part than in the alluvial plain downstream. There was no accumulation of diversity downstream, indicating migration rates were symmetrical upstream and downstream. This was confirmed by computing individual migration parameters between adjacent populations. 5 The results are discussed with regard to the dispersal mechanisms of seeds and pollen. The discrepancy between potential gene flow and effective gene flow is interpreted as an effect of fragmentation, due to the alteration of the natural dynamics of the riparian ecosystem rather than to physical barriers.

Morphological and genetic variation in four Italian populations of Lebias fasciata (Teleostei, Cyprinodontidae)

Morphological and genetic analyses in four Italian populations of Lebias fasciata were carried out in order to assess the level of differentiation among populations. Fourteen meristic and 23 morphometric characters, relative to the skull, vertebral column, and the rays of the dorsal and anal fins were examined. In addition, variations at seven gene loci were investigated by allozyme electrophoresis. Both morphological and genetic results showed a noteworthy differentiation among the four populations which reflects their high degree of isolation. Allozyme results indicated that genetic variation is consistent with geographical distribution of these populations, while morphological differentiation cannot be interpreted biogeographically. The lack of dispersion phases in the life cycle of L. fasciata and its limited potential for gene flow account for the genetic divergence among populations. The inconsistency between morphological and genetic results for the two Sicilian populations may account for appreciable environmental (non‐genetic) influences on the morphological characters examined, which may obscure any enzyme‐morphological relation.

High mortality in introduced brown trout depressed potential gene flow to a wild population

High mortality in introduced brown trout depressed potential gene flow to a wild population

High mortality in introduced brown trout depressed potential gene flow to a wild population

Hatchery reared 0+ year brown trout Salmo trutta, with 51 mm mean LT, were released in a sea trout stream in June 1991 to compare the survival of wild and introduced trout during the freshwater stage from age 0+ to 2+ years. The introduced brown trout were homozygous for a genetic marker, enabling released individuals and their offspring to be distinguished phenotypically from the local sea trout. The mean size of 0+ and 1+ year introduced parr was larger than 0+ and 1+ year wild parr, while 2+ year parr of both groups were of the same size. Half year survival rates of both introduced and wild parr increased with size up to c. 80 mm (1+ years), but then decreased, and could be described by a polynomial regression function, with the same shape for both groups. The introduced parr had, however, a significant lower survival rate than the wild parr. The number of the introduced cohort decreased from 2200 at release in 1991 to c. 20 in March 1994 (c. 1% of the original number). Between 1994 and 2000 no introduced individuals or offspring have been observed in the study area. High mortality at the parr stage and additional mortality until the spawning, give a low probability for a genetic impact on the local population as long as releases are restricted, both in time and number of fish.

Genetic differentiation among populations of the brooding soft coral Clavularia koellikeri on the Great Barrier Reef

The contribution of sexual and asexual reproduction, the spatial patterns of genetic structure, and the potential gene flow among populations were determined for the soft coral Clavularia koellikeri (Octocorallia: Alcyonacea, Clavulariidae) at ten sites among six reefs from two well-separated regions of the Great Barrier Reef (GBR), Australia. Eight allozyme loci indicated that colonies of C. koellikeri separated ≥3 m were produced sexually. Genetic diversity was lower in the southern (18°S) compared with the northern (10°S) populations, suggesting that reefs closer to the southernmost limit of the distribution of C. koellikeri within the GBR (19°S) may represent a more marginal habitat for this species. High levels of genetic differentiation were significant at all spatial scales (sites within reefs, reefs, and regions) from <4 km up to 1,000 km, indicating that C. koellikeri has restricted dispersal, consistent with having brooded larvae.

Determination of the Paternity of Wheat (Triticum aestivum L) × Jointed Goatgrass (Aegilops cylindrica Host) BC1 Plants by Using Genomic In Situ Hybridization (GISH) Technique

The release of herbicide resistant wheat (Triticum aestivum L.) raises concerns with gene flow between wheat and jointed goatgrass (Aegilops cylindrica Host). Hybrids between the two species and backcrosses with either species have been observed in the field. Gene flow is dependent on jointed goatgrass being the paternal parent of the BC1 generation. Differences in the genomes of wheat (AABBDD) and jointed goatgrass (CCDD) could be used to determine the paternity of the BC1 generation. Twenty BC1 plants (10 of each paternal type) were used to determine if the number of C genome chromosomes based on genomic in situ hybridization (GISH) could be used to determine BC1 paternity. Differences between the two BC1 paternal types for number of C genome chromosomes indicates that C genome chromosome counts could be used to determine the paternity BC1 plants providing a more accurate estimate of the potential for gene flow between the two species.

The Frequency and Persistence of Volunteer Canola (Brassica napus) in Québec Cropping Systems*

The presence of volunteer canola is becoming a significant agro-ecological concern, given the large-scale use of herbicide-tolerant varieties in some areas. Our goal was to estimate the frequency and persistence of volunteer canola in Québec cropping systems by surveying fields that included a single canola crop since 1995. A survey was conducted in 131 fields in the main canola-growing areas of Québec: in the Saguenay-Lac Saint-Jean region and the Québec City–La Pocatière area. Volunteer canola plants were counted in 0.25-m2 quadrats every 10 m along a W pattern, and every 15 m along the margins of 88 fields. Volunteer canola plants were found in 90% of the fields surveyed and in a wide range of crops, including cereal, corn, and soybean. Average densities of 4.9 and 3.9 plants/m2 were found 1 yr after canola production in fields and field margins, respectively. Volunteer canola densities decreased significantly over time. However, volunteer plants were still present at low densities 4 and 5 yr after production. Dense stands of volunteer canola were found before postemergence herbicide application in no-till fields (9.8 ± 4.1 plants/m2), suggesting that, contrary to what was suggested in the literature, seeds could become dormant in no-till as well as in tilled systems. A small proportion of the volunteer canola plants observed in no-till fields near Québec City and Ottawa included plants that had overwintered, either originating from fall-germinated seedlings, harvested adult plants that had grown new leaves before the onset of winter, or spring regrowth from the base of unharvested adult plants from experimental plots. The presence and persistence of low densities of volunteer canola may not have been a cause of concern until now. However, producers should be made more aware of the potential short-and long-term problems associated with potential gene flow between different herbicide-tolerant canola (HT canola) varieties and also between HT canola and related weed species.Nomenclature: Canola, Brassica napus L.; corn, Zea mays L.; soybean, Glycine max (L.) Merr.Additional index words: Herbicide-tolerant canola, oilseed rape, weed survey.Abbreviations: HT, herbicide-tolerant (canola).

Pathogen population genetics, evolutionary potential, and durable resistance.

We hypothesize that the evolutionary potential of a pathogen population is reflected in its population genetic structure. Pathogen populations with a high evolutionary potential are more likely to overcome genetic resistance than pathogen populations with a low evolutionary potential. We propose a flexible framework to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. According to this framework, pathogens that pose the greatest risk of breaking down resistance genes have a mixed reproduction system, a high potential for genotype flow, large effective population sizes, and high mutation rates. The lowest risk pathogens are those with strict asexual reproduction, low potential for gene flow, small effective population sizes, and low mutation rates. We present examples of high-risk and low-risk pathogens. We propose general guidelines for a rational approach to breed durable resistance according to the evolutionary potential of the pathogen.

The population genetics of plant pathogens and breeding strategies for durable resistance

The durability of disease resistance is affected by the evolutionary potential of the pathogen population. Pathogens with a high evolutionary potential are more likely to overcome genetic resistance than pathogens with a low evolutionary potential. We will propose a set of guidelines to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. Under our model of pathogen evolution, the two most important parameters to consider are reproduction/mating system and gene/genotype flow. Pathogens that pose the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with at least one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. The lowest risk pathogens are those with strict asexual reproduction and low potential for gene flow. We will present examples of high- and low-risk pathogens. Knowledge of the population genetic structure of the pathogen may offer insight into the best breeding strategy for durable resistance. We will present broad guidelines suggesting a rational method for breeding durable resistance according to the population genetics of the pathogen.

POPULATION STRUCTURE OF PACIFIC COMMON EIDERS BREEDING IN ALASKA

We used satellite telemetry to study the migration routes and wintering areas of two allopatric breeding populations of Pacific Common Eiders (Somateria mollissimav-nigrum) in Alaska: the Yukon-Kuskokwim Delta, and the western Beaufort Sea coast. Only 6% (2 of 36) of females wintered within the wintering area of the other breeding population. Both breeding populations wintered in the closest available ice-free habitat, perhaps to minimize migratory distance. Two Beaufort Sea females wintered in areas used by Yukon-Kuskokwim Delta females, implying potential gene flow among breeding areas. Yet, we conclude that these two populations are largely geographically isolated throughout the annual cycle and the environmental factors influencing survival and reproduction likely differ between these groups of birds. Thus, regardless of the potential gene flow among breeding populations, we suggest that birds from these two breeding areas should be managed as separate populations. Estructura Poblacional Reproductiva de Somateria mollissimav-nigrum en Alaska Resumen. Usamos telemetría satelital para estudiar las rutas de migración y áreas de invernada de dos poblaciones alopáticas reproductivas de Somateria mollissimav-nigrum en Alaska: la del Delta Yukon-Kuskokwim, y la de la costa oeste del Mar de Beaufort. Solo el 6% (2 de 36) de las hembras invernaron dentro del área de invernada de la otra población reproductiva. Las dos poblaciones invernaron en el hábitat libre de hielo más cercano, tal vez para minimizar la distancia de migración. Dos hembras del Mar de Beaufort invernaron en áreas usadas por hembras del Delta de Yukon-Kuskokwim, implicando un potencial flujo génico entre las áreas de reproducción. Sin embargo, concluimos que estas dos poblaciones están aisladas geográficamente a lo largo del ciclo anual y que los factores ambientales que afectan la supervivencia y reproducción son probablemente diferentes entre estos grupos de aves. Por lo tanto, a pesar del flujo génico potencial entre las poblaciones reproductivas, sugerimos que las aves de estas dos áreas de reproducción deben ser manejadas como poblaciones separadas.

Determination of the Paternity of Wheat ( L) × Jointed Goatgrass ( Host) BC Plants by Using Genomic In Situ Hybridization (GISH) Technique

The release of herbicide resistant wheat (Triticum aestivum L.) raises concerns with gene flow between wheat and jointed goatgrass (Aegilops cylindrica Host). Hybrids between the two species and backcrosses with either species have been observed in the field. Gene flow is dependent on jointed goatgrass being the paternal parent of the BC1 generation. Differences in the genomes of wheat (AABBDD) and jointed goatgrass (CCDD) could be used to determine the paternity of the BC1 generation. Twenty BC1 plants (10 of each paternal type) were used to determine if the number of C genome chromosomes based on genomic in situ hybridization (GISH) could be used to determine BC1 paternity. Differences between the two BC1 paternal types for number of C genome chromosomes indicates that C genome chromosome counts could be used to determine the paternity BC1 plants providing a more accurate estimate of the potential for gene flow between the two species.

Potential gene flow from cultivated sunflower to volunteer, wild Helianthus species in Europe

The possibility of gene flow between cultivated and wild species of different crops was assessed, focussing on sunflower and its wild relatives or cultivated forms (annual Helianthus annuus, H. argophyllus, H. bolanderi, H. petiolaris, and perennial rigidus, H. tuberosus). Data were compiled from the literature and an attempt was made at determining the efficiency of possible crosses between annual and perennial species. The risk of a cultivated allele escaping from sunflower to uncontrolled forms (volunteers, H. annuus and H. tuberosus feral forms) was addressed, the fate of such alleles in the wild also being examined. An attempt was made to define some of the pressures likely to limit the spread of wild forms in nature, or that could prevent or impede the spread and increase the frequency of alleles from cultivated plants. Risks of spread of transgenes from sunflower varieties in Europe where feral forms are present but no natural Helianthus species were assessed. The emergence of transgenic or biologically engineered varieties of sunflower demands that attention be focussed on this field.

Fine-scale genetic structure, estuarine colonization and incipient speciation in the marine silverside fish Odontesthes argentinensis.

The identification of incipient ecological species represents an opportunity to investigate current evolutionary process where adaptive divergence and reproductive isolation are associated. In this study we analysed the genetic structure of marine and estuarine populations of the silverside fish Odontesthes argentinensis using nine microsatellite loci and 396 bp of the mitochondrial DNA (mtDNA) control region. Our main objective was to investigate the relationship among estuarine colonization, divergent selection and speciation in silversides. Significant genetic structure was detected among all marine and estuarine populations. Despite the low phylogeographic structure in mtDNA haplotypes, there was clear signal of local radiations of haplotypes in more ancient populations. Divergence among marine populations was interpreted as a combined result of homing behaviour, isolation by distance and drift. On the other hand, ecological shifts due to the colonization of estuarine habitats seem to have promoted rapid adaptive divergence and reproductive isolation in estuarine populations, which were considered as incipient ecological species. This conclusion is supported by the existence of a set of environmental factors required for successful reproduction of estuarine ecotypes. The pattern of genetic structure indicates that phenotypic and reproductive divergence evolved in the face of potential gene flow between populations. We suggest that the 'divergence-with-gene-flow' model of speciation may account for the diversification of estuarine populations. The approach used can potentially identify 'incipient estuarine species', being relevant to the investigation of the evolutionary relationships of silversides in several coastal regions of the world.

Gene flow from wheat (Triticum aestivum L.) to jointed goatgrass (Aegilops cylindrica Host.), as revealed by RAPD and microsatellite markers

In order to estimate the potential of gene flow between wheat (Triticum aestivum L.) and jointed goatgrass (Aegilops cylindrica Host.), we carried out mixed pollinations in experimental and natural conditions. A set of species-specific RAPD (random amplified polymorphic DNA) and microsatellite markers were used to detect the presence of parental markers in the progeny of the plants used in these experiments. No hybrids were found within the offsprings of the plants used for the greenhouse experiments, while 85 Ae. cylindrica×T. aestivum hybrids were found within 2400 analyzed F1 plants resulting from the field pollinations. The hybridization rates for individuals of different populations of the wild species differed considerably: 1% for two populations known for more than 90 years versus 7% for a newly discovered population. Most of the hybrids were completely sterile, but five of them produced 13 seeds (BC1) by backcross with Ae. cylindrica. Twelve seeds germinated and generated viable and partly fertile plants. About 25% of the wheat specific RAPD markers were found in the BC1 plants, indicating that introgression of wheat DNA into Ae. cylindrica is possible. In addition, one microsatellite marker, known to be situated on the D genome (a genome shared by both species), was also found in the BC1 plants.

Abundance and diversity of Schizophyllum commune spore clouds in the Caribbean detected by selective sampling.

Selective spore trapping and molecular genotyping methods were employed to examine potential long-distance gene flow among Caribbean populations of the common mushroom Schizophyllum commune. Spore-trap samples from five locations were analysed using restriction fragment polymorphisms of five enzymatically amplified gene regions. Successful trappings suggested S. commune spores to be abundant in the air, with an estimated sedimentation rate of approximately 18 spores/m2/h. High levels of genetic diversity characterized the spore-trap samples, with as many as 12 alleles observed at a single locus (chitin synthase) over all samples. In addition, spore-trap samples showed significant among sample heterogeneity including geographical population substructure. The ribosomal DNA (rDNA) intergenic spacer displayed the greatest allele frequency differences among samples, clearly separating the samples into those possessing only a South American-type allele and those segregating for both North and South American-type alleles. The molecular variation provided no clear evidence for dispersal over large, aquatic barriers within the Caribbean region, and instead suggested that spore-trapping experiments are primarily reflective of the local, established population.