Direct selection on allozymes is not required to explain heterogeneity among marker loci across a Mytilus hybrid zone

Unequal differentiation between two types of loci (allozyme and DNA markers) across a Mytilus hybrid zone has recently been claimed as evidence for direct selection on some allozyme loci. We provide here a counter-example: a noncoding DNA locus that exhibits as much differentiation as the incriminated allozymes do. The levels of genetic differentiation varied widely among both allozymes and noncoding DNA markers and no clear difference emerged between the two types of markers. This suggests that the strong interlocus variance in genetic differentiation has been confounded with a discrepancy between marker types as a result of an insufficient and unbalanced locus sampling. Heterogeneity in differentiation among neutral loci can be created by stochastic variance during the allopatric divergence preceding a secondary contact. In hybrid zones, a further source of variance is differential introgression among chromosomal regions after the secondary contact owing to the local influence of selected genes on more or less distant markers. However, the degree of differentiation alone gives no way to distinguish indirect pseudo-selection (a regular and ubiquitous feature of hybrid zones) from direct selection. More generally, we suggest that comparative neutrality tests based on discrepancies among marker types have to be applied with caution when the presence of semi-permeable genetic barriers to gene exchange is suspected.     

The 'rare allele phenomenon' in a ribosomal spacer

We describe the increased frequency of a particular length variant of the internal transcribed spacer 1 (ITS-1) of the ribosomal DNA in a hybrid zone of the land snail Albinaria hippolyti. The phenomenon that normally rare alleles or other markers can increase in frequency in the centre of hybrid zones is not new. Under the term 'hybrizyme' or 'rare allele' phenomenon it has been recorded in many organisms and different genetic markers. However, this is the first time that it has been found in a multicopy locus. On the one hand, the pattern fits well with the view that purifying selection in hybrid populations works on many loci across the genome and should thus have its effect on many independent molecular markers. On the other hand, the results are puzzling, given that the multiple copies of rDNA are not expected to respond in unison. We suggest two possible explanations for these conflicting observations.     

Genetic analysis of autosomal and X-linked markers across a mouse hybrid zone

In this paper, we present results of the first comprehensive study of the introgression of both autosomal and sex-chromosome markers across the central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and M. m. domesticus. More than 1800 individuals sampled from 105 sites were analyzed with a set of allozyme loci (hopefully representing neutral or nearly neutral markers) and X-linked loci (which are assumed to be under selection). The zone center is best modeled as a single straight line independent of fine-scale local geographic or climatic conditions, being maintained by a balance between dispersal and selection against hybrids. The width (w) of the multilocus autosomal cline was estimated as 9.6 km whereas the estimate for the compound X-chromosome cline was about 4.6 km only. As the former estimate is comparable to that of the Danish portion of the zone (assumed to be much younger than the central European one), zone width does not appear to be related to its age. The strength (B) of the central barrier was estimated as about 20 km; with dispersal (sigma) of about 1 km/gen(1/2), this means effective selection (s*) is approximately 0.06-0.09 for autosomal loci and about 0.25 for X-linked loci. The number of loci under selection was estimated as N= 56-99 for autosomes and about 380 for X-linked loci. Finally, we highlight some potential pitfalls in hybrid zone analyses and in comparisons of different transects. We suggest that conclusions about parts of the mouse genome involved in reproductive isolation and speciation should be drawn with caution and that analytical approaches always providing some estimates should not be used without due care regarding the support or confidence of such estimates, especially if conclusions are based on the difference between these estimates. Finally, we recommend that analysis in two-dimensional space, dense sampling, and rigorous treatment of data, including inspection of likelihood profiles, are essential for hybrid zone studies.     

THE PONTIA DAPLIDICE-ED USA HYBRID ZONE IN NORTHWESTERN ITALY

The pierid butterflies Pontia daplidice and P. edusa, parapatrically distributed in southern Europe, have very similar morphologies and life histories, but show fixed differences at four allozyme markers. We sampled these allozymes in a 28-population transect north of Genoa in Italy, through the hybrid zone where these taxa meet. We used the numerical techniques developed for hybrid zone analysis to study the patterns of genetic differentiation and their underlying evolutionary causes. The hybrid zone is characterized by a very short and steep central region, flanked by broad tails of introgression extended up to 100 km in either direction. From mean two-locus disequilibium of D = 0.148 (maximum-likelihood two-unit support limits 0.139-0.153), and after accounting for minor differences in the center locations of the single-locus clines, which act to bias the dispersal estimate, we estimated a dispersal rate of σ = 4.4 (3.7-5.5) km/gen^1/2. The effective selection needed to maintain the steep central portion is strong, 0.47 < s? < 0.64, when combined over potential intrinsic (genetic background) and extrinsic (ecological) sources of selection. The clines in allozyme loci showed variation that was significantly different between the most divergent shapes, and the differences are attributable to different degrees of introgression on the edusa side of the zone. The average selection acting on individual allozyme loci was high at s_???e  1.5%, but because of the narrowness of the central region of the cline, we suspect that this estimate is somewhat biased by selection on loci closely linked to the allozyme markers. A common question for taxa that show fixed allozyme differences in parapatry is whether or not they are genetically isolated. A fairly general measure of genetic isolation across hybrid zones is the time, T, that it takes a neutral allele to cross the hybrid zone and recombine into the opposite genetic background, given by T = (β/σ)², where β is the barrier strength of the hybrid zone. Genetic isolation in the Pontia zone is weak, with T  25 generations for most allozyme markers. By this measure, populations of daplidice and edusa on opposite sides of the hybrid zone share more identical-by-descent alleles than do populations of phenotypically pure daplidice in, say, France and Morocco. Accordingly, we think it best for systematists to consider edusa as a well-marked subspecies of P. daplidice.     

Functional Effects of Pgi Allozymes in ESCHERICHIA COLI

Five alleles representing three electromorphs of phosphoglucose isomerase (PGI) have been transferred from natural isolates of E. coli into the genetic background of E. coli K12 and examined for their effect on growth rate in chemostats limited for glucose or fructose. With glucose limitation, all alleles are selectively neutral or nearly neutral within the limit of resolution of the technique, whether the genetic background is nonmutant or whether it contains a deletion of the locus of glucose-6-phosphate dehydrogenase, the enzyme that provides an alternative metabolic pathway for the substrate of PGI. With fructose limitation, one of the naturally occurring alleles has a small but reproducible detrimental effect on growth rate. A kinetic difference in this detrimental allozyme, apparently relating to an inhibition constant, has been observed in some, but not all, lots of substrate, and a similar difference has also been noted in one of the rare electromorphs that could not be transferred into E. coli K12. These results support a model of genetic variation in which the alleles are neutral or nearly neutral in the prevailing environment but have a potential for selection that can be expressed under the appropriate conditions of environment or genetic background. This hypothesis is discussed in the context of allozyme polymorphisms observed in other organisms.     

On the neutrality of molecular genetic markers: pedigree analysis of genetic variation in fragmented populations

Many studies employ molecular markers to infer ecological and evolutionary processes, assuming that variation found at genetic loci offers a reliable representation of stochastic events in natural populations. Increasingly, evidence emerges that molecular markers might not always be selectively neutral. However, only a few studies have analysed how deviations from neutrality could affect estimates of genetic variation, using populations with known genealogy. We monitored changes in allozyme variation over eight generations in captive metapopulations of the butterfly Bicyclus anynana. Population demography was recorded by individually marking 35 000 butterflies and constructing pedigrees. We designed a computer program that simulated the inheritance of founder allozyme alleles in butterfly pedigrees. We thus tested whether the observed transmission of allozyme alleles could be explained by random genetic drift alone, or whether there was evidence for positive or negative selection. This analysis showed that in the smallest metapopulations the loss of allozyme variation exceeded the neutral rate. Possibly, linkage disequilibria between deleterious mutations and marker alleles resulted in background selection and a faster erosion of allozyme variation. In larger metapopulations, one locus (MDH) showed a significant heterozygote excess and smaller than expected loss in heterozygosity, observations consistent with (associative) overdominance. This study demonstrates that the neutrality of molecular markers cannot always be assumed, particularly in small populations with a high mutation load.     

Ongoing movement of the hermit warbler X Townsend's warbler hybrid zone

Background

Movements of hybrid zones – areas of overlap and interbreeding between species – are difficult to document empirically. This is true because moving hybrid zones are expected to be rare, and because movement may proceed too slowly to be measured directly. Townsend''s warblers (Dendroica townsendi) hybridize with hermit warblers (D. occidentalis) where their ranges overlap in Washington and Oregon. Previous morphological, behavioral, and genetic studies of this hybrid zone suggest that it has been steadily moving into the geographical range of hermit warblers, with the more aggressive Townsend''s warblers replacing hermit warblers along ∼2000 km of the Pacific coast of Canada and Alaska. Ongoing movement of the zone, however, has yet to be empirically demonstrated.

Methodology/Principal Findings

We compared recently sampled hybrid zone specimens to those collected 10–20 years earlier, to test directly the long-standing hypothesis of hybrid zone movement between these species. Newly sampled specimens were more Townsend''s-like than historical specimens, consistent with ongoing movement of the zone into the geographical range of hermit warblers.

Conclusions/Significance

While movement of a hybrid zone may be explained by several possible mechanisms, in this case a wealth of existing evidence suggests that movement is being driven by the competitive displacement of hermit warblers by Townsend''s warblers. That no ecological differences have been found between these species, and that replacement of hermit warblers by Townsend''s warblers is proceeding downward in latitude and elevation – opposite the directions of range shifts predicted by recent climate change – further support that this movement is not being driven by alternative environmental factors. If the mechanism of competitive displacement is correct, whether this process will ultimately lead to the extinction of hermit warblers will depend on the continued maintenance of the dramatic competitive asymmetry observed between the species.     

PATTERNS OF VARIATION AND LINKAGE DISEQUILIBRIUM IN A FIELD CRICKET HYBRID ZONE

The distribution of multilocus genotypes found within a natural hybrid zone is determined by the sample of genotypes present when the hybrid zone first formed, by subsequent patterns of genetic exchange between the hybridizing taxa, and by drift and selection within each of the hybrid zone populations. We have used anonymous nuclear DNA restriction fragment polymorphisms (RFLPs) to characterize the array of multilocus genotypes present within a well-studied hybrid zone between two eastern North American field crickets, Gryllus pennsylvanicus and Gryllus firmus. These crickets hybridize along a zone of contact that extends from New England to Virginia. Previous studies have shown that both premating and postmating barriers exist between the two cricket species, but the absence of diagnostic morphological and allozyme markers has made it difficult to assess the consequences of these barriers for genetic exchange. Analyses based on four diagnostic anonymous nuclear markers indicate that hybrid zone populations in Connecticut contain few F₁ hybrids, and that nonrandom associations persist among nuclear gene markers, between nuclear and cytoplasmic markers, and between molecular markers and morphology. Field cricket populations within the hybrid zone are not “hybrid swarms” but consist primarily of crickets that are very much like one or the other of the parental species. Despite ample opportunity for genetic exchange and evidence for introgression at some loci, the two species remain quite distinct. Such a pattern appears to be characteristic of many natural hybrid zones.     

Testing for selection on the androgen-binding protein in the Danish mouse hybrid zone

The three peripheral subspecies of the house mouse Mus musculus have fixed specific variants of the androgen-binding protein (ABP) that have been proposed to be part of a recognition mechanism that could participate in sexual isolation between the subspecies. We tested for selection on Abpa by characterizing the pattern of Abpa introgression across a transect of the hybrid zone between M. m. musculus and M. m. domesticus in Jutland. On the musculus side, the cline for Abpa resembled that of a nearly neutral allozyme more than that of strongly selected X and Y chromosome markers. However, the high central step which displaces the tail of introgression of Abpa to higher frequencies was best accounted for by linkage to a locus under strong selection against hybrids. Still, we cannot exclude that this pattern results from weak selection against Abpa in the tail of introgression, which would be compatible with an assortative choice mechanism. On the domesticus side there was little introgression close to the hybrid zone, presumably due to a geographical barrier to migration. However, substantial frequencies of musculus alleles occurred further away, suggesting mixed colonization patterns as well as flow across the hybrid zone.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 447–459.     

Conserving marine biodiversity: insights from life-history trait candidate genes in Atlantic cod (Gadus morhua)

Recent technological developments have facilitated an increased focus on identifying genomic regions underlying adaptive trait variation in natural populations, and it has been advocated that this information should be important for designating population units for conservation. In marine fishes, phenotypic studies have suggested adaptation through divergence of life-history traits among natural populations, but the distribution of adaptive genetic variation in these species is still relatively poorly known. In this study, we extract information about the geographical distribution of genetic variation for 33 single nucleotide polymorphisms (SNPs) associated with life-history trait candidate genes, and compare this to variation in 70 putatively neutral SNPs in Atlantic cod (Gadus morhua). We analyse samples covering the major population complexes in the eastern Atlantic and find strong evidence for non-neutral levels and patterns of population structuring for several of the candidate gene-associated markers, including two SNPs in the growth hormone 1 gene. Thus, this study aligns with findings from phenotypic studies, providing molecular data strongly suggesting that these or closely linked genes are under selection in natural populations of Atlantic cod. Furthermore, we find that patterns of variation in outlier markers do not align with those observed at selectively neutral markers, and that outlier markers identify conservation units on finer geographical scales than those revealed when analysing only neutral markers. Accordingly, results also suggest that information about adaptive genetic variation will be useful for targeted conservation and management in this and other marine species.     

Inferences from a rapidly moving hybrid zone

Anartia fatima and Anartia amathea (Lepidoptera: Nymphalidae) are sister taxa whose ranges abut in a narrow hybrid zone in eastern Panama. At the center of the zone, hybrids are abundant, although deviations from Hardy-Weinberg and linkage disequilibria are strong, due in part to assortative mating. We measured differences across the zone in four wing color-pattern characters, three allozyme loci, and mitochondrial haplotype. Wing pattern, allozyme, and mitochondrial clines were coincident (i.e., had the same positions) and concordant (i.e.. all markers had similar cline shapes, about 28 km wide). Repeated samples demonstrated that the hybrid zone has been moving eastwards at an average rate of 2.5 km/year over the past 20 years, accompanied by an equivalent movement of the mtDNA cline. No introgression of mtDNA haplotypes were found in the 'wake" of the moving cline, as might be expected for a neutral marker. The concordance of morphological and mtDNA clines between 1994 and 2000, in spite of hybrid zone movement, suggests strong epistasis between the mitochondrial genome and nuclear loci. Cline movement is achieved mainly by pure fatima immigrating into amathea populations; hybrids had little effect, and were presumably outcompeted by fitter pure fatima genotypes. This movement can be explained if random dispersal of 7-19 km x gen(1/2) is coupled with a competitive advantage to A. fatima genomes of 2-5%. Hybrid zone motion is equivalent to Phase III of Wright's shifting balance. Hybrid zone movement has rarely been considered likely in the past, but our results show that it may be more important in biogeography and evolution than generally realized.     

Development of RAPD markers in two introduced fire ants, Solenopsis invicta and S. richteri, and their application to the study of a hybrid zone

We developed RAPD DNA markers useful in distinguishing the fire ants Solenopsis invicta and S. richteri. An initial survey of 200 primers revealed seven informative markers; family studies allowed us to determine expression patterns and to confirm Mendelian inheritance of these markers. The seven RAPD markers, one of which is inherited as a codominant marker, were employed along with three allozyme markers to describe the structure of a hybrid zone that has formed between the two species in the USA, where they have been introduced. The data suggest minor introgression of alleles from one parental species ( S. richteri ) into the other ( S. invicta ), which most likely reflects the documented recent movement of this hybrid zone. This pattern is interpreted as interspecific introgression rather than shared intraspecific polymorphism on the basis of comparisons with samples from native, non-hybridizing populations in South America that lack such polymorphism. The data further reveal that the structure of the hybrid zone in the USA varies geographically. One parental species ( S. invicta ) and the hybrids exhibit a mosaic distribution in the east; a gradual transition between the parental species occurs in the centre, with a large intervening area of hybrid genotypes only; and there is apparent contact between parental populations with a small or no intervening zone of hybridization in the west. These differing patterns in the structure of the hybrid zone presumably reflect the unique histories of colonization in different parts of the range of introduced fire ants.     

THE EXTENT OF INTROGRESSION OUTSIDE THE CONTACT ZONE BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (TELEOSTEI: CYPRINIDAE)

The cyprinid fishes, Notropis cornutus and N. chrysocephalus, hybridize in a long, narrow zone in the midwestern United States. To quantify the extent of introgression of genetic markers outside of this zone, samples were collected along transects starting near the region of contact (as defined by morphological characters), followed by samples progressively more distant. Diagnostic allozymic and mitochondrial DNA (mtDNA) restriction site markers were used to estimate the extent of introgression outside of the zone, while polymorphic allozyme and mtDNA markers were used to evaluate the potential for gene flow among populations within transects. Analysis of populations from the northern transect provided evidence for differentiation of populations for some of the markers; however, on average, enough gene flow has occurred to overcome substantial differentiation. Introgressed mtDNA and allozyme haplotypes were rare and found only in the population closest to the contact zone. The rarity of introgressed alleles in the more northern populations is consistent with the recent origin of these populations after the Wisconsin glaciation (less than 12,000 years bp) and/or selection maintaining the northern boundary of the contact zone. Analysis of populations from the southern transect revealed evidence for population subdivision but no evidence for introgression at the diagnostic allozyme loci; however, nearly all individuals from this transect possessed introgressed mtDNA haplotypes, with samples furthest from the contact zone exhibiting the highest frequencies of introgression. Patterns of variation for one of the polymorphic allozyme markers (Est-A) and introgressed mtDNAs were highly correlated, suggesting that allozymic heterogeneity at this locus is also the result of introgression. The most likely explanation for these data is that these introgressed haplotypes are indicators of a more southern position of the contact zone during the Pleistocene, with the contact zone shifting northward with the recession of the glacial front. Such movement implicates selection in the maintenance of distributional limits of these species, and hence, the width and position of the contact zone.     

SNP genotyping for detecting the ‘rare allele phenomenon’ in hybrid zones

Hybrid zones are regions where genetically distinct populations meet, mate and produce offspring. In such zones, genetically less compatible gene combinations are usually generated, resulting in reduced fitness, and hybrid zones are often maintained because of continuous removal of unfit genotypes, balanced by gene flow into the zone from the parental populations (and are then referred to as ‘tension zones’). Tension zones often display unexpectedly high frequencies of gene variants that are rare outside the zone. Previous work has shown that this ‘rare allele phenomenon’ is not the result of intragenic recombination or increased mutation rates. Further understanding of the population genetics of the phenomenon requires an approach in which both the numbers of individuals and the numbers of loci is increased. Here, we report an approach using a combination of Illumina next‐generation sequencing and mass spectrophotometer genotyping to identify markers that may be used for genome‐wide investigations of the rare allele phenomenon. We test this approach on a hybrid zone in the land snail Albinaria hippolyti from Greece.     

The mode of evolution of molecular markers in populations of house mice under artificial selection for locomotor behavior

A complete understanding of the mode of evolution of molecular markers is important for making inferences about different population genetic parameters, especially because a number of studies have reported patterns of allelic variation at molecular markers that are not in agreement with neutral evolutionary expectations. In the present study, house mice (Mus domesticus) from the fourteenth generation of a selection experiment for increased voluntary wheel-running activity were used to test how selection on a complex behavior affects the distribution of allelic variation by examining patterns of variation at six microsatellite and four allozyme loci. This population had a hierarchical structure that allowed for simultaneous testing of the effects of selection and genetic drift on the distribution of allelic variation by comparing observed patterns of allele frequencies and estimates of genetic divergence at multiple hierarchical levels to expectations under models of neutral evolution. The levels of genetic divergence among replicate lines and between selection groups, estimated from microsatellite data or pooled microsatellite and allozyme data, were not significantly different from expectations under neutral evolution. Furthermore, the pattern of change of allele frequencies between the base population and generation 14 was largely in agreement with expectations under neutral evolution (although the PGM locus exhibited a pattern of change within populations that was difficult to explain under neutral evolution). Overall the results generally provide support for the neutral evolution of molecular markers.     

Intron sequences of arginine kinase in an intertidal snail suggest an ecotype-specific selective sweep and a gene duplication

Many species with restricted gene flow repeatedly respond similarly to local selection pressures. To fully understand the genetic mechanisms behind this process, the phylogeographic history of the species (inferred from neutral markers) as well as the loci under selection need to be known. Here we sequenced an intron in the arginine kinase gene (Ark), which shows strong clinal variation between two locally adapted ecotypes of the flat periwinkle, Littorina fabalis. The ‘small-sheltered'' ecotype was almost fixed for one haplotype, H1, in populations on both sides of the North Sea, unlike the ‘large-moderately exposed ecotype'', which segregated for ten different haplotypes. This contrasts with neutral markers, where the two ecotypes are equally variable. H1 could have been driven to high frequency in an ancestral population and then repeatedly spread to sheltered habitats due to local selection pressures with the colonization of both sides of the North Sea, after the last glacial maximum (∼18 000 years ago). An alternative explanation is that a positively selected mutation, in or linked to Ark, arose after the range expansion and secondarily spread through sheltered populations throughout the distribution range, causing this ecotype to evolve in a concerted fashion. Also, we were able to sequence up to four haplotypes consistently from some individuals, suggesting a gene duplication in Ark.     

Structure and dynamics of hybrid zones at different stages of speciation in the common vole (Microtus arvalis)

The genetic structure and dynamics of hybrid zones provide crucial information for understanding the processes and mechanisms of evolutionary divergence and speciation. In general, higher levels of evolutionary divergence between taxa are more likely to be associated with reproductive isolation and may result in suppressed or strongly restricted hybridization. In this study, we examined two secondary contact zones between three deep evolutionary lineages in the common vole (Microtus arvalis). Differences in divergence times between the lineages can shed light on different stages of reproductive isolation and thus provide information on the ongoing speciation process in M. arvalis. We examined more than 800 individuals for mitochondrial (mtDNA), Y‐chromosome and autosomal markers and used assignment and cline analysis methods to characterize the extent and direction of gene flow in the contact zones. Introgression of both autosomal and mtDNA markers in a relatively broad area of admixture indicates selectively neutral hybridization between the least‐divergent lineages (Central and Eastern) without evidence for partial reproductive isolation. In contrast, a very narrow area of hybridization, shifts in marker clines and the quasi‐absence of Y‐chromosome introgression support a moving hybrid zone and unidirectional selection against male hybrids between the lineages with older divergence (Central and Western). Data from a replicate transect further support non‐neutral processes in this hybrid zone and also suggest a role for landscape history in the movement and shaping of geneflow profiles.     

Reproductive division of labor between hybrid and nonhybrid offspring in a fire ant hybrid zone

Abstract.— Interspecific hybridization can often impose a substantial fitness cost due to reduced hybrid viability or fecundity. In social insects, however, such costs disproportionately impact reproductive offspring, whereas hybrids who become sterile workers can be functional, and even beneficial, colony members. Genomic imprinting of the paternal genome in reproductive, but not worker female offspring has been proposed as a mechanism to avoid genomic incompatibilities in hybrid queens in a hybrid zone between two fire ant species, Solenopsis geminata and S. xyloni. A study of allozyme variation demonstrated differences between the worker caste displaying a hybrid phenotype, and the winged queen caste displaying only the mother's phenotype. In this study, we investigate whether these differences are caused by genomic imprinting or genetic differences between castes by comparing variability of proteins to that of microsatellite markers. Workers and winged queens differed genetically at both classes of marker, indicating that allozyme differences were caused by underlying genetic differences between castes rather than differences in gene expression due to imprinting. Workers were F1 S. geminata X S. xyloni hybrids, whereas nearly all winged queens were of pure S. xyloni ancestry. Thus, S. xyloni within the hybrid zone appears to have evolved social hybridogenesis, in which the loss of worker potential in pure-species offspring necessitates hybridization for worker production, but prevents hybrids from being represented in the reproductive caste.     

Influence of Environment and Mitochondrial Heritage on the Ecological Characteristics of Fish in a Hybrid Zone

Background

Ecological characteristics (growth, morphology, reproduction) arise from the interaction between environmental factors and genetics. Genetic analysis of individuals'' life history traits might be used to improve our understanding of mechanisms that form and maintain a hybrid zone.

Methodology/Principal Findings

A fish hybrid zone was used to characterize the process of natural selection. Data were collected during two reproductive periods (2001 and 2002) and 1117 individuals (nase, Chondrostama nasus nasus, sofie C. toxostoma toxostoma and hybrids) were sampled. Reproductive dates of the two parental species overlapped at sympatric sites. The nase had an earlier reproductive period than the sofie; males had longer reproductive periods for both species. Hybridisation between female nase and male sofie was the most likely. Hybrids had a reproductive period similar to the inherited parental mitochondrial type. Growth and reproductive information from different environments has been synthesised following a bayesian approach of the von Bertalanffy model. Hybrid life history traits appear to link with maternal heritage. Hybrid size from the age of two and size at first maturity appeared to be closer to the size of the maternal origin species (nase or sofie).Median growth rates for hybrids were similar and intermediate between those of the parental species. We observed variable life history traits for hybrids and pure forms in the different parts of the hybrid zone. Geometrical analysis of the hybrid fish shape gave evidence of two main morphologies with a link to maternal heritage.

Conclusions/Significance

Selective mating seemed to be the underlying process which, with mitochondrial heritage, could explain the evolution of the studied hybrid zone. More generally, we showed the importance of studies on hybrid zones and specifically the study of individuals'' ecological characteristics, to improve our understanding of speciation.     

DEVELOPMENT OF THE HYBRID SWARM BETWEEN PECOS PUPFISH (CYPRINODONTIDAE: CYPRINODON PECOSENSIS) AND SHEEPSHEAD MINNOW (CYPRINODON VARIEGATUS): A PERSPECTIVE FROM ALLOZYMES AND mtDNA

A comparison of allozyme and mtDNA frequencies was used for insight into a situation in the Pecos River, Texas where contact between the endemic pupfish (Cyprinodon pecosensis) and an introduced congener (C. variegatus) has resulted in rapid, geographically extensive genetic introgression. Temporal changes in mean frequencies of diagnostic allozyme markers indicate that the clinal pattern of introduced genetic material (Echelle and Connor 1989) is slowly decreasing in amplitude. Significant rank concordance in diagnostic allele frequencies among sites and across sampling years indicates directional influences upon temporal allele frequency change. These observations are consistent with the theory of gene flow in neutral clines. Levels of introgression indicated by each of four allozyme loci and mtDNA were roughly equivalent. The early history of the hybrid swarm is explained by genetic swamping, possibly mediated by selection for C. variegatus or C. variegatus × C. pecosensis, at a time when the normally abundant endemic species had been catastrophically depleted. High frequencies of an introduced GPI-A allele in all samples of intergrades suggests that the introduced genome originated with a single founding event.