Characteristics of dispersal in the eastern mosquitofish Gambusia holbrooki

The demographic and genetic characteristics of dispersing Gambusia holbrooki were examined relative to changes in stream flow velocity, flow direction, and rapid saltwater inundation. Dispersal of all G. holbrooki sex classes decreased as flow velocity increased, dispersal for a given flow velocity being greater in female, than male and juvenile fish respectively. Juveniles and males dispersed at low (9 mm s⁻¹) but not high (109 mm s ¹) flow. Females dispersed at both flow rates. Dispersing adults were not larger than non-dispersing adults of the same sex. Direction of dispersal was sex and size dependent, but independent of flow velocity. Larger females (>22 mm) moved preferentially downstream, while smaller females (17–22 mm) remained stationary or moved in the upstream direction. Males dispersed equally in both directions. No relationship was found between dispersal characteristics and the presence of high salt concentrations. Movement patterns at high flow were independent of glucose-6-phosphate isomerase ( GPI-2* ) alleles carried by dispersing individuals. Significant non-random dispersal of sex or size cohorts, and the interaction of dispersal with flow velocity have significant implications for the maintenance of genetic diversity in stream ecosystems, particularly where localized selection or genetic drift may alter the genetic constitution of specific sex or size cohorts. The applicability of these results to field populations is discussed.     

Distribution of genetic variability in populations of two chromosomal races of Dichroplus pratensis (Melanoplinae, Acrididae) and their hybrid zone

We examined, through allozyme electrophoresis, the genetic structure of populations of the acridid grasshopper Dichroplus pratensis from two chromosomal races (Northern and Southern) and their hybrid zone in Argentina. No fixed alleles for any particular race were found, although genetic differentiation among parental races was significant (0 = 0.044, 95% CI: 0.004-0.068). Hybrid populations are genetically more similar to the Southern race (0 = 0.008, 95% CI: -0.005-0.018) than to Northern ones (0 = 0.018, 95% CI: 0.002-0.030). Differential viability or fertility of hybrids, or asymmetry in mating preferences in favour of one particular cross would cause a higher proportion of matings between hybrid individuals and those from the Southern race. This would explain the high genetic similarity between those groups, in spite of their geographical vicinity with northern race populations.     

Morphometric and genetic characterization of sympatric populations of Clarias gariepinus and C. anguillaris from Senegal

A sample of African Clarias catfishes from the Senegal River was studied using morphometry, allozyme variation, microsatellites and RFLPs of mitochondrial DNA. They all confirmed the presence of two species , C. gariepinus and C. anguillaris . The two species were closely related genetically and no diagnostic loci were found in allozymes and microsatellites studies. Two of the 11 haplotypes of mtDNA observed were shared by both species. Three of the four assays (morphometry, allozymes and microsatellites) allowed a precise characterization of both. One specimen occupied an intermediate position in the analysis of the data; it was considered an F_l hybrid whose possible origin is discussed.     

Small-scale patterns of genetic variation in the mayfly Bungona narilla (Ephemeroptera: Baetidae) in rainforest streams, south-east Queensland

SUMMARY 1. Genetic structure of the mayfly Bungona narilla was examined using allozymes and a section of the cytochrome oxidase I gene.
2. The study had two major aims. The first was to determine whether patterns of genetic variation in mitochondrial DNA were similar to those found previously for allozymes, i.e. that more variation was evident among pools within a single stream than among streams. The mitochondrial DNA results were similar to those reported previously for allozymes, supporting the idea that larvae within any particular pool were unrepresentative of the total population and may result from a few matings.
3. The second aim was to test the hypothesis that the variation among pools within a stream was greater after dry periods than after wet periods. This was because after wet periods, larvae would have greater opportunity for mixing because of movement among pools. This hypothesis was partly supported by the mitochondrial DNA data but not by the allozyme data, in which variation among pools was extremely low on both sampling occasions. The reasons for this difference are unclear.     

Cytonuclear genetic signatures of hybridization phenomena: Rationale,utility, and empirical examples from fishes and other aquatic animals

By definition, organisms of hybrid ancestry carry amalgamations of divergent genomes. Thus, exaggerated effects of genomic interactions might be anticipated in hybrid populations, thereby magnifying the impact of natural selection and making this and other evolutionary forces easier to document. Mating biases and other gender-based asymmetries also frequently characterize hybrid populations. Thus, maternally inherited cytoplasmic polymorphisms assayed jointly with those at biparentally inherited nuclear loci provide powerful genetic markers to dissect ethological, ecological, and evolutionary processes in hybrid settings. Population-level topics that can be addressed using cytonuclear markers include the frequency of hybridization and introgression in nature, behavioral and ecological factors (such as mating preferences and hybrid fitnesses) influencing the genetic architectures of hybrid zones, the degree of consistency in genetic outcomes across multiple hybrid contact regions, and environmental impacts (including the introduction of alien species) on hybridization processes. Several empirical studies on fish populations in hybrid settings illustrate the application of cytonuclear appraisals in such contexts.     

Patterns of population genetic variation in sympatric chiltoniid amphipods within a calcrete aquifer reveal a dynamic subterranean environment

Calcrete aquifers from the Yilgarn region of arid central Western Australia contain an assemblage of obligate groundwater invertebrate species that are each endemic to single aquifers. Fine-scale phylogeographic and population genetic analyses of three sympatric and independently derived species of amphipod (Chiltoniidae) were carried out to determine whether there were common patterns of population genetic structure or evidence for past geographic isolation of populations within a single calcrete aquifer. Genetic diversity in amphipod mitochondrial DNA (cytochrome c oxidase subunit I gene) and allozymes were examined across a 3.5 km² region of the Sturt Meadows calcrete, which contains a grid of 115 bore holes (=wells). Stygobiont amphipods were found to have high levels of mitochondrial haplotype diversity coupled with low nucleotide diversity. Mitochondrial phylogeographic structuring was found between haplogroups for one of the chiltoniid species, which also showed population structuring for nuclear markers. Signatures of population expansion in two of the three species, match previous findings for diving beetles at the same site, indicating that the system is dynamic. We propose isolation of populations in refugia within the calcrete, followed by expansion events, as the most likely source of intraspecific genetic diversity, due to changes in water level influencing gene flow across the calcrete.     

Genetic structure of natural populations of Castanea sativa in Turkey: evidence of a hybrid zone

This study points out the evidence of a hybrid zone between two groups of genetically differentiated populations of chestnut (Castanea sativa Mill.) in Turkey. Genetic structure, gene flow and introgression levels, based on 16 allozyme loci, were investigated on 34 population samples spanning the entire C. sativa distribution area in this country. The occurrence of the hybrid zone, located in the Bithynian region, was inferred in a group of populations showing the following genetic characteristics: (i) enhanced genetic variability and intermediate allelic frequencies between those of the Western and Eastern groups of populations; (ii) sharp and concordant changes in allele frequencies; (iii) decreased gene flow with the Western and Eastern populations. Starting from the cline width estimated to be 324 km, strength of selection was evaluated from the gene flow distance, as indicated from the degree of genetic structuring outside the hybrid zone. Evolutionary processes shaping the observed genetic differentiation and introgression are discussed on the basis of palynological data, palaeoclimatic events and evidence of hybridization found in other plant and animal species in the same region.     

Variation in mitochondrial DNA and maternal genetic ancestry of Ethiopian cattle populations

This study describes complete control region sequences of mitochondrial DNA (mtDNA) from 117 Ethiopian cattle from 10 representative populations, in conjunction with the available cattle sequences in GenBank. In total, 79 polymorphic sites were detected, and these defined 81 different haplotypes. The haplotype and nucleotide diversity of Ethiopian cattle did not vary among the populations studied. All mtDNA sequences from Ethiopian cattle converged into one main maternal lineage (T1) that corresponds to African Bos taurus cattle. According to the results of this study, no zebu mtDNA haplotypes have been found in Ethiopia, where the most extensive hybridization took place on the African continent.     

Patterns of hybridization and population genetic structure in the terrestrial orchids Liparis kumokiri and Liparis makinoana (Orchidaceae) in sympatric populations

We investigated the potential for gene flow and genetic assimilation via hybridization between common and rare species of the terrestrial orchid genus Liparis, focusing specifically on sympatric and allopatric populations of the common Liparis kumokiri and the rare Liparis makinoana. We utilized analyses of genetic diversity, morphology, and the spatial distributions of individuals and genotypes to quantify the dynamics of interspecific gene flow at within- and among-population scales. High levels of allozyme genetic diversity (HE) were found in populations of the rare L. makinoana (0.317), whereas the common L. kumokiri (N = 1744 from 14 populations) revealed a complete lack of variation. This contrast may reflect different breeding systems and associated rates of genetic drift (L. makinoana is self-incompatible, whereas L. kumokiri is self-compatible). At the two known sympatric sites, individuals were found that recombined parental phenotypes, possessing floral characteristics of L. kumokiri and vegetative characteristics of L. makinoana. These putative hybrids were the only individuals found segregating alleles diagnostic of both parental species. Analysis of these individuals indicated that hybrid genotypes were skewed towards L. kumokiri and later generation recombinants of L. kumokiri at both sympatric sites. Furthermore, Ripley's bivariate L(r) statistics revealed that at one site these hybrids are strongly spatially clustered with L. kumokiri. Nonetheless, the relatively low frequency of hybrids, absence of ongoing hybridization (no F1s or first generation backcrossess), and strong genetic differentiation between morphologically 'pure' parental populations at sympatric sites (FST = 0.708-0.816) indicates that hybridization was not an important bridge for gene flow. The results from these two species suggest that natural hybridization has not played an important role in the diversification of Liparis, but instead support the view that genetic drift and limited gene flow are primarily responsible for speciation in Liparis. Based on genetic data and current status of the species, implications of the research for conservation are considered to provide guidelines for appropriate conservation and management strategies.     

Fitness and genetic architecture of parent and hybrid willows in common gardens

Models of hybrid zone dynamics incorporate different patterns of hybrid fitness relative to parental species fitness. An important but understudied source of variation underlying these fitness differences is the environment. We investigated the performance of two willow species and their F1, F2, and backcross hybrids using a common-garden experiment with six replicated gardens that differed in soil moisture. Aboveground biomass, catkin production, seed production per catkin, and seed germination rate were significantly different among genetic classes. For aboveground biomass and catkin production, hybrids generally had intermediate or inferior performance compared to parent species. Salix eriocephala had the highest performance for all performance measures, but in two gardens F, plants had superior or equal performance for aboveground biomass and female catkin production. Salix eriocephala and backcrosses to S. eriocephala had the highest numbers of filled seeds per catkin and the highest estimates of total fitness in all gardens. Measures of filled seeds per catkin and germination rate tend to support the model of endogenous hybrid unfitness, and these two measures had major effects on estimates of total seed production per catkin. We also estimated how the two willow species differ genetically in these fitness measures using line cross analysis. We found a complex genetic architecture underlying the fitness differences between species that involved additive, dominance, and epistatic genetic effects for all fitness measures. The environment was important in the expression of these genetic differences, because the type of epistasis differed among the gardens for above-ground biomass and for female catkin production. These findings suggest that fine-scale environmental variation can have a significant impact on hybrid fitness in hybrid zones where parents and hybrids are widely interspersed.     

Long-term genetic monitoring reveals contrasting changes in the genetic composition of newly established populations of the intertidal snail Bembicium vittatum

Newly established populations are susceptible to founder events that reduce genetic variation. This may be counterbalanced by gene flow after populations become established or founders coming from genetically different populations. However, initial gains in genetic diversity may be short-lived if there is limited mixing between lineages and subsequent inbreeding, or if one lineage sweeps to fixation through selection or genetic drift. Here, we report on the genetic changes taking place within two newly established populations of intertidal snail over a 15-year period (～ 10 generations). Each translocation was set up using multiple, genetically distinct source populations. Our data show that higher levels of variation in the translocated populations compared to the source populations were maintained over time for both nuclear (microsatellite) and mitochondrial genes. Small changes in allele and haplotype frequencies were observed in the source populations and in one of the translocated populations, but marked changes were evident in the other, where there was a dramatic shift towards the genetic make-up of one of the source populations. These genetic changes occurred despite relatively large numbers of founders (200-374 adults) and no evidence of the population experiencing a severe reduction in effective population size. Our study shows that the genetic composition of newly established populations can vary greatly over time and that genetic outcomes can be highly variable, and significantly different from initial expectations, even when they are established using high numbers of individuals and involve source populations from the same geographic regions.     

Hybrid zones between two genetically differentiated forms of the pond frog Rana lessonae in southern Italy

Two distinct population groups of the pond frog Rana lessonae were detected in peninsular Italy and Sicily by multilocus electrophoresis: one group inhabits the peninsula down to northern Calabria, the second occurs in southern Calabria and on Sicily. Fixed alternative alleles distinguish the two groups at 5 of the 25 loci examined; marked allele frequency differences were observed at two additional loci. On average, the two groups differ by a Nei's standard genetic distance of 0.4. A wide hybrid zone (about 120 km) occurs between the two groups, with high genotypic diversity and absence of pure parental genotypes in central Calabria. Patterns of allozyme variation suggest that at least two distinct contact and hybridization events occurred, one in the Catanzaro, the other in the Crati-Sibari plains, about 70 km to the north. Geological evidence indicates that these areas correspond to two main marine-flooded grabens that would have repeatedly interrupted or reduced genetic exchange during Plio-Pleistocene times. The finding of a fixed difference at the Mdhp-1 locus between Sicilian and Calabrian R. lessonae witnesses their continuing differentiation following their last separation by definitive opening of the Strait of Messina, about 50 000 years ago. The wide hybrid zone, the diversity of genotypes and the agreement with Hardy-Weinberg expectations suggest complete hybrid fertility. Different patterns of introgression were observed at the various loci. The pattern of allelic variation at loci in R. lessonae is paralleled by the pattern of variation in lessonae genomes of the sympatric hemiclonal hybrid Rana esculenta, into which lessonae genomes are introduced each generation as a result of hybridogenesis.     

Three genetically divergent lineages of the Oryx in eastern Africa: Evidence for an ancient introgressive hybridization

Phylogeographic and population genetic studies using sequence information are frequently used to infer species boundaries and history; and to assess hybridization and population level processes. In this study, partial mitochondrial DNA (mtDNA) control region (423 bp) and cytochrome b sequences (666 bp) of Oryx beisa sampled from five isolated localities in its entire current range in Africa were analyzed to investigate the extent of genetic variation and differentiation between populations. We observed high nucleotide diversity at the control region in the total sample (6.3%) but within populations, it varied considerably ranging from 1.6% to 8.1%. Population pairwise genetic differentiation was generally significantly high (ranging from F _ST = 0.15, P<0.01 to F _ST = 0.54, P<0.001). In the total sample, 29 and 12 haplotypes were observed in the control region and the cytochrome b data sets respectively. For both data sets, the haplotypes cluster into three distinct clades (sequence divergence ranged from 6.0%–12.9% to 0.8%–1.0% for the control region and cytochrome b sequences, respectively) that do not correspond to sampling locations. Two of these clades are found in the same localities (Samburu and Marsabit), which represent the O.beisa beisa subspecies, whereas the last clade represents the fringe-eared oryx (O. beisa callotis). We interpret these findings in terms of an ancient hybridization and introgression between two formerly isolated taxa of Oryx beisa.     

Large discrepancies in differentiation of allozymes, nuclear and mitochondrial DNA loci in recently founded Pacific populations of the pearl oyster Pinctada margaritifera

This study presents a comparative analysis of population structure applied to the pearl oyster (Pinctada margaritifera) from the Central Pacific islands using three classes of molecular markers: two mitochondrial genes (mtDNA), five anonymous nuclear loci (anDNA), and eight polymorphic allozymes. Very low levels of haplotype diversity and nucleotidic divergence detected for mtDNA validate the hypothesis of a recent (re)colonization of Polynesian lagoons after their exondation during the last glaciations. Some nuclear loci, however, showed highly significant FST values, indicating a reduced amount of larval exchange between archipelagos at present. A large interlocus variance of FST was nevertheless observed. We discuss whether this pattern is inherent to the stochasticity of the drift process since recolonization, or if it could result from balancing selection acting on certain loci. This study illustrates once more the need to combine the analysis of several kinds of loci when unrelated phenomena are likely to leave their footprints on genetic structure.     

POPULATION CAGE EXPERIMENTS WITH A VERTEBRATE: THE TEMPORAL DEMOGRAPHY AND CYTONUCLEAR GENETICS OF HYBRIDIZATION IN GAMBUSIA FISHES

The dynamics of mitochondrial and multilocus nuclear genotypic frequencies were monitored for 2 yr in experimental populations established with equal numbers of two poeciliid fishes (Gambusia affinis and Gambusia holbrooki) that hybridize naturally in the southeastern United States. In replicated “small-pool” populations (experiment I), 1018 sampled individuals at six time periods revealed an initial flush of hybridization, followed by a rapid decline in frequencies of G. affinis nuclear and mitochondrial alleles over 64 wk. Decay of gametic and cytonuclear disequilibria differed from expectations under random mating as well as under a model of assortative mating involving empirically estimated mating propensities. In two replicate “large-pond” populations (experiment II), 841 sampled individuals across four reproductive cohorts revealed lower initial frequencies of F₁ hybrids than in experiment I, but again G. holbrooki alleles achieved high frequencies over four generations (72 wk). Thus, evolution within experimental Gambusia hybrid populations can be extremely rapid, resulting in consistent loss of G. affinis nuclear and cytoplasmic alleles. Concordance in results between experiments and across genetic markers suggests strong directional selection favoring G. holbrooki genotypes. Results are interpreted in light of previous reports of genotype-specific differences in life-history traits, reproductive ecology, patterns of recruitment, and size-specific mortality, and in the context of patterns of introgression previously studied indirectly from spatial observations on cytonuclear genotypes in natural Gambusia populations.     

Genetic evidence for historical continuity between populations of the Australian freshwater fish Craterocephalus stercusmuscarum (Atherinidae) east and west of the Great Dividing Range

The freshwater Craterocephalus stercusmuscarum (Atherinidae) has a broad northern and south-eastern Australian distribution, and has been divided into two sub-species. Craterocephalus stercusmuscarum fulvus occurs in eastern and western flowing drainages of the southeast, while C. s. stercusmuscarum occurs in north eastern and northern flowing drainages. Four populations of each sub-species were sampled from different river systems to examine if genetic diversity was consistent with this nomenclature or a previously proposed vicariance hypothesis. Allozyme data did not support the notion that the subspecies were genetically distinct, but the mtDNA data showed that haplotypes from one sub-species, regardless of geography, were reciprocally monophyletic to haplotypes of the other subspecies. Thus, mtDNA genetic diversity was partitioned by prevailing taxonomy and the data suggest that C. s. fulvus populations in eastern and western flowing drainages may have had a relatively recent connection subsequently interrupted by geological events.     

The application of molecular markers to the study and conservation of fish populations, with special reference to Salmo

The main molecular techniques which can be used to generate genetic markers, and the applications of these markers to studies of fish populations are outlined. Published and ongoing studies, in the authors' laboratories, on brown trout and Atlantic salmon are used to compare the resolution and applicability of allozyme, mitochondrial DNA and minisatellite (variable number of tandem repeats) markers for studies on population structuring, genetic variation within populations, and the impact of the accidental and deliberate introduction of non-native salmonids on the genetic make-up of natural populations.     

Variation in life history and genetic traits of Hawaiian mosquitofish populations

Mosquitofish (Gambusia affinis) were collected from 17 reservoirs on three islands in Hawaii, USA. Genetic and life history traits for adult females from these populations were used to evaluate hypotheses concerning short-term evolutionary divergence of populations recently established from a common ancestral source. The effects of founder events and drift on genetic variability and population differentiation were also examined. Significant differences in life history characteristics, allele frequencies, and multi-locus heterozygosities (H) were found among fish populations collected from different reservoirs and between reservoirs classified as stable or fluctuating on the basis of temporal fluctuation in water level. Females from stable reservoirs exhibited greater standard length (35.1 vs 32.8 mm), lower fecundity (11.9 vs 15.2 embryos), lower reproductive allocation (18.2% vs 22.8%), but larger mean embryo size (1.95 vs 1.67 mg) than females from fluctuating reservoirs. Consistency in means among replicates of each reservoir class and concordance in direction and magnitude of differences reported here and results of sampling conducted from these same locations 10 years previously (Stearns, 1983a) suggest that ecological factors intrinsic to these two environments are important in determing population life history traits. Females from stable reservoirs exhibited lower heterozygosity than females from fluctuating reservoirs (0.134 vs 0.158, respectively). Levels and direction od differences in heterozygosity, the high proportion of polymorphic loci and lack of fixation of alternative alleles argue against a purely stochastic explanation for genetic and life history variation among reservoir populations. Levels of genetic variability and interpopulation differentiation were similar to those observed in mainland populations of this species. A high proportion of the genetic diversity was apportioned between populations and within populations due to differences between juveniles and adults. Significant genotypic differences between adult and juvenile age classes suggest that the genetic divergence of local populations may occur over short periods of time.     

Untangling the evolutionary history of a highly polymorphic species: introgressive hybridization and high genetic structure in the desert cichlid fish Herichtys minckleyi

Understanding the origin of biodiversity requires knowledge on the evolutionary processes that drive divergence and speciation, as well as on the processes constraining it. Intraspecific polymorphisms can provide insight into the mechanisms that generate and maintain phenotypic, behavioural and life history diversification, and can help us understand not only the processes that lead to speciation but also the processes that prevent local fixation of morphs. The ‘desert cichlid’ Herichtys minckleyi is a highly polymorphic species endemic to a biodiversity hotspot in northern Mexico, the Cuatro Ciénegas valley. This species is polymorphic in body shape and trophic apparatus, and eco‐morphotypes coexist in small spring‐fed lagoons across the valley. We investigated the genetic structure of these polymorphisms and their phylogeographic history by analysing the entire control region of the mitochondrial DNA and 10 nuclear microsatellite markers in several populations from different sites and morphs. We found two very divergent mitochondrial lineages that most likely predate the closing of the valley and are not associated with morphotypes or sites. One of these lineages is also found in the sister species Herichthys cyanoguttatus. Data from neutral microsatellite markers suggest that most lagoons or drainages constitute their own genetic cluster with sympatric eco‐morphotypes forming panmictic populations. Alternative mechanisms such as phenotypic plasticity and a few loci controlled traits provide possible explanations for the sympatric coexistence of discrete nonoverlapping eco‐morphotypes with apparent lack of barriers to gene flow within multiple lagoons and drainages.