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.     

MITOCHONDRIAL DNA AND PROTEIN DIFFERENTIATION BETWEEN HYBRIDIZING CYTOTYPES OF THE WHITE-FOOTED MOUSE,PEROMYSCUS LEUCOPUS

Restriction-enzyme analysis of mitochondrial DNA and protein electrophoresis were used to document patterns of gene flow across a hybrid zone between chromosomal races of Peromyscus leucopus. Chromosomal markers (three inversions) are such that individuals can be classified as potential F₁'s, backcrosses, or parental types. Allozymic characterization of the hybrid zone is congruent with the chromosomal data (Stangl, 1986) and indicates an assymetrical distribution of markers, with the northeastern markers being distributed at a higher frequency into southwestern populations. Restriction patterns of mtDNA suggest that the two cytotypes may have had different evolutionary histories, and the distribution of haplotypes is concordant with other genetic markers used to identify the hybrid zone. Concordant changes in chromosomes, allozymes, and mtDNA suggest that the most viable hypothesis for the origin of the zone is secondary contact. A unique aspect of this study is that the same individuals were used for protein electrophoresis, mtDNA analysis, and chromosomal analysis. Thus, patterns of genetic variation can be interpreted free of any historical bias associated with samples collected at different times.     

Concordance of genetic and phenotypic characters across a sapsucker hybrid zone

Hybridization has presented a challenge for taxonomists and conservation biologists, since hybridizing forms could be stable evolutionary entities or ephemeral forms that are blending together. However, hybrid zones also provide a unique opportunity for evolutionary biologists who study the interaction between gene flow and reproductive isolation in speciation. Three forms of woodpeckers (sapsuckers; genus Sphyrapicus) in North America that are mostly geographically separated but hybridize with each other where they come into contact present a remarkable system for the study of hybridization. We provide the first comprehensive analysis of phenotypic and genetic variation across a hybrid zone between two of these forms, the red‐breasted Sphyrapicus ruber and yellow‐bellied S. varius sapsuckers. The objective was to infer whether selection maintains the differences between forms. Our analysis of eight morphometric and 20 plumage traits, and two molecular markers showed clear differences between the forms and roughly concordant clinal variation across a narrow hybrid zone. Thirty percent of sampled birds in the hybrid zone had mixed west/east genotypes at the genetic markers examined. The center of the genetic cline was located 20 km west of the crest of the Rocky Mountains. The width of the zone was 122 km, narrower than would be expected under neutral blending given reasonable estimates of the age of the zone and individual dispersal distances. Heterozygote deficit and cytonuclear disequilibrium at the centre of the hybrid zone suggested nonrandom mating or limited hybridization. Given these patterns and lack of evidence for habitat segregation we conclude that this hybrid zone is maintained by selection, most likely in the form of hybrid inferiority. This study provides an illustrative example of extensive hybridization between stable entities, providing additional evidence against the historical practice of treating hybridizing forms as members of the same species.     

A hybrid zone between two subspecies of Chorthippus parallelus. X-chromosome variation through a contact zone

Two grasshopper subspecies, Chorthippus parallelus parallelus (Cpp) and Chorthippus parallelus erythropus (Cpe), meet along the Pyrenees where they hybridize and produce a hybrid zone. A contact zone located in the Col de Portalet has been analyzed for the distribution of chromosome markers on the sex (X) chromosome in pure and hybrid populations. C-banding allowed us to distinguish both pure subspecific forms and recombinant forms, and to demonstrate their different frequencies through the contact zone. Interestingly, the distal C-band (P) on the X chromosome that characterizes Cpp individuals occurs at very high frequency through the zone and then drops sharply, close to where pure Cpe populations are found. A novel interstitial C-band (H) has been found, probably derived from that characterizing Cpe individuals (E). This marker band is only present in hybrid populations. These data are discussed in the light of the dynamics of the hybrid zone.     

HYBRIDIZATION AND GEOGRAPHIC VARIATION IN TWO MEADOW KATYDID CONTACT ZONES

In this study, previously unrecognized hybridization was documented between two meadow katydids in each of two disjunct contact zones, in the southeastern United States and along the Potomac River near Washington, DC. These two zones have very different histories and dynamics of interaction between the two taxa. Orchelimum nigripes and O. pulchellum (Tettigoniidae: Conocephalinae) are distributed west and east, respectively, of the Appalachian Mountains, from the Great Lakes to the Gulf Coast and along the Atlantic Coastal Plain from New York to the Florida Keys, but are not found in the Appalachians themselves. In addition, during this century O. nigripes has become established in a small area east of the Appalachians, in the Potomac River basin, where it has completely replaced O. pulchellum along the river corridor above Washington, DC. I sampled katydids from 40 sites across both hybrid zones and mapped geographic patterns of genetic variation (allele frequencies at two diagnostic loci) and variation in a morphometric index for males. Although the two taxa are quite distinct over most of their extensive distributions, there is clear evidence of introgression in both contact zones. In the Deep South, samples from a transect along the Gulf Coast define a broad hybrid zone of about 50–100 km, while samples from a transect 200 km to the north define a zone of about 150–250 km in width. Only one Deep South population shows a deviation from Hardy-Weinberg equilibrium at either locus, and there is no evidence of linkage disequilibrium in any Deep South population. In the Potomac region, there is a narrow upstream-downstream hybrid zone along the river. Within the Potomac River floodplains downriver from Washington, DC, as well as outside the floodplains throughout the region, O. pulchellum is present in abundance, but O. nigripes markers are virtually absent. Within the floodplains upriver from Washington, DC, O. nigripes is abundant, but O. pulch***ellum markers are virtually absent. All four mixed ancestry Potomac populations sampled show strong and highly significant linkage disequilibrium, although only one clearly deviates from single-locus Hardy-Weinberg equilibrium. The position of the Deep South hybrid zone is generally consistent with interspecific and intraspecific phylogeographic patterns previously reported for numerous taxa from the southeastern United States. The observed genetic and morphometric clines appear to be the result of neutral introgression over thousands of years. In the much younger Potomac hybrid zone, O. nigripes appears to be spreading downriver, interbreeding with O. pulchellum, and replacing it. The mechanism for this replacement remains uncertain, but may be clarified by ongoing behavioral, genetic, and breeding studies.     

RIBOSOMAL-DNA,MITOCHONDRIAL-DNA,CHROMOSOMAL, AND ALLOZYMIC STUDIES ON A CONTACT ZONE IN THE POCKET GOPHER,GEOMYS

We studied 75 individuals of the Plains pocket gopher, Geomys bursarius, from eastern New Mexico, where the subspecies major and knoxjonesi hybridize. Each individual was examined for chromosome number, ribosomal DNA, mitochondrial DNA, and three protein systems for which reference parental populations were fixed for alternative alleles. Twenty individuals were indistinguishable from parental major, 14 individuals were indistinguishable from parental knoxjonesi, and 41 individuals had genotypes composed of combinations of character states that distinguish the two parental types. The parental types appear to represent discrete genetic entities that have restricted introgression across a narrow hybrid zone (width approximately 3 km, using the 20/80 criterion). Parental types overlap in geographic distribution near the center of the zone, and changes in mitochondrial DNA and the five nuclear markers are concordant across the zone. It is probable that there is premating isolation between knoxjonesi males and major females. The frequencies of individuals with certain genotypic combinations within our sample imply differential reproductive success of certain genotypes. We propose that F₁'s and highly heterozygous males are sterile and that hybrid females are less fertile than parental females. These postmating factors, along with premating isolation for one of the reciprocal crosses, probably account for the restriction of gene flow across the contact zone. The structure of the zone can be explained by the “dynamic equilibrium” model.     

Hybridization between two gartersnake species (<Emphasis Type="Italic">Thamnophis</Emphasis>) of conservation concern: a threat or an important natural interaction?

Distinguishing between hybrid zones formed by secondary contact versus parapatric divergence-with-gene-flow is an important challenge for understanding the interplay of geographic isolation and local adaptation in the origin of species. Similarly, distinguishing between natural hybrid zones and those that formed as a consequence of recent human activities has important conservation implications. Recent work has demonstrated the existence of a narrow hybrid zone between the plains gartersnake (Thamnophis radix) and Butler’s gartersnake (T. butleri) in the Great Lakes region of North America, raising questions about the history and conservation value of genetically admixed populations. Both taxa are of conservation concern, and it is not clear whether to regard hybridization as a threat or a natural interaction. Here we use phylogeographic and population genetic methods to assess the timescales of divergence and hybridization, and test for evidence that the hybrid zone is of recent origin. We assayed AFLP markers and ND2 mitochondrial DNA (mtDNA) sequences from T. radix, T. butleri, and the closely related short-headed gartersnake (T. brachystoma) throughout their North American ranges. We find shallow mtDNA divergence overall and high levels of variation within the contact zone. These patterns are inconsistent with recent contact of long-diverged taxa. It is not possible to distinguish true divergence-with-gene-flow from a long-term secondary contact zone, but we infer that the hybrid zone is a long-standing, natural interaction.     

PATTERNS OF HYBRIDIZATION IN THE PIRIQUETA CAROLINIANA COMPLEX IN CENTRAL FLORIDA: EVIDENCE FOR AN EXPANDING HYBRID ZONE

We have identified a broad zone of hybridization between two morphologically and ecologically distinct herbaceous perennial taxa (morphotypes) within the Piriqueta caroliniana complex, which extends more than 300 km across the central Florida peninsula. Phylogeographic analyses indicate that the caroliniana morphotype has been present in north and central Florida since the early Pleistocene and that the viridis morphotype has immigrated into southern Florida much more recently. We examine the distribution of diagnostic morphological characters and nuclear genetic markers to assess the extent and patterns of introgression in this system. The results are consistent with the hypothesis that this hybrid zone has expanded north in recent history as viridis alleles have introgressed into regions that were previously occupied by populations of caroliniana. Genetic markers diagnostic for caroliniana have consistently high frequencies across the hybrid zone, whereas markers for viridis are extremely variable among populations with frequency reversals in adjacent populations. The latter pattern is probably the result of the combined stochastic effects of dispersal and drift on viridis alleles as they introgressed northward. Additional evidence for the recent expansion of this hybrid zone comes from patterns of variation for morphological and genetic markers. As expected for an expanding hybrid zone, within-population morphological variation was greatest toward the advancing front of introgression and levels of genetic variation for neutral diagnostic markers were greatest in the region of initial contact and lower in areas of recent expansion. The observed patterns of variation suggest that at least some hybrid genotypes have high fitnesses, which has led to the expansion of the hybrid zone via the displacement of parental genotypes in central Florida.     

Hybridization between Townsend's Dendroica townsendi and black‐throated green warblers D. virens in an avian suture zone

Hybrid zones between species provide natural systems for the study of processes involved in divergence, reproductive isolation and speciation. Townsend's Dendroica townsendi and black‐throated green D. virens warblers are phenotypically and genetically divergent groups that occur in western and eastern North America respectively, with potential for range contact in the Rocky Mountains of British Columbia, where other west–east avian pairs come into contact. Although one potential hybrid (a phenotypic Townsend's warbler with the black‐throated green mitochondrial DNA) has been previously reported, there have been no studies of interactions between the taxa in potential areas of sympatry. To determine whether interbreeding between these species is a regular occurrence we examined variation in individuals across the area of putative range overlap. Analysis of plumage, morphology, and mitochondrial (COI) and nuclear molecular markers (CHD1Z and numt‐Dco1) shows surprisingly extensive hybridization between these species, with at least 38% of individuals in the hybrid zone being either hybrids or backcrosses. Each of the traits displays a sigmoidal cline centred along the eastern slope of the Rocky Mountains (molecular cline centres averaging 50 km east of the crest of the Rockies, ranging from 41 to 56 km). The clines are narrow (average molecular cline width is 60 km, ranging from 40 to 87 km) relative to the dispersal distance of related warbler species, suggesting that selection is maintaining the hybrid zone; we discuss possible sources of selection. Given the narrowness of the zone we recommend the two forms should continue to be treated as separate taxonomic species. Townsend's warblers also form an extensively studied hybrid zone with their more closely related southern relative, the hermit warbler D. occidentalis. The combined system of three discrete forms separated by narrow hybrid zones provides an excellent system for the study of hybridization, reproductive isolation and speciation.     

GENETIC STRUCTURE AND EVOLUTION OF A FIRE ANT HYBRID ZONE

Two introduced fire ants, Solenopsis invicta and S. richteri, hybridize over an extensive area in the United States spanning central Mississippi, Alabama, and western Georgia. We studied a portion of this hybrid zone in northwestern Mississippi in detail by sampling ants at many sites along two transects extending across the zone and examining gene frequency and size distributions at a large number of genetic and morphological markers. The distributional patterns at these markers are most consistent with the mosaic hybrid zone model, whereby the distribution of various fire ant genotypes is determined initially by the historical patterns of colonization of newly available habitats. However, these distributional patterns probably do not reflect the equilibrium state of interactions because of the very recent secondary contact of the species (< 60 yr) and the dynamic nature of available nesting habitats in this area. Our data suggest that, with prolonged contact and interaction, differential fitness of various hybrid genotypes due to intrinsic and extrinsic selective factors is important in structuring the hybrid zone. For instance, consistent differential introgression of morphological and genetic markers, combined with previous evidence of differences in developmental stability among genotypes, suggest reduced fitness of hybrids relative to parentals due to intrinsic selection (as may be caused by breakup of parental gene complexes). Furthermore, marked reductions in the occurrence of parental-like hybrids in areas where the similar parental species is common suggest reduced fitness of these parental-like hybrids in competition with the parentals (i.e., extrinsic selection). Because the relative roles of such deterministic as well as stochastic forces apparently vary both spatially and temporally, the eventual distribution of the various fire ant genotypes and the fate of the hybrid zone in the United States is difficult to predict.     

POPULATION VARIATION AND HYBRIDIZATION IN SEA-ROCKETS (CAKILE,CRUCIFERAE): SEED GLUCOSINOLATE CHARACTERS

In a study of intra- and interpopulation variation in seed glucosinolates, remarkable uniformity was found among individuals within the two chemically distinct subspecies of East Coast Cakile edentula. Glucosinolates were used as taxonomic markers in a study of hybridization between ssp. edentula and ssp. harperi where they form mixed populations on the Outer Banks of North Carolina. Thirteen plants with hybrid chemical profiles were detected in a sample of 89 individuals from four populations; the zone of hybridization is narrow, and has persisted for at least eight years. Artificial hybrids of the two subspecies are additive for kinds and intermediate for proportions of seed glucosinolates, and families of F₂'s show a range of recombinant chemical profiles. Natural hybridization was confirmed by recovering hybrid and parental types in 47 progeny of three presumptive wild hybrids. Glucosinolate profile characteristics segregated independently of the segregation patterns for fruit length and seed weight. Selection against particular glucosinolate phenotypes is one possible mechanism maintaining this narrow hybrid zone on the Outer Banks.     

Natural hybridization between extremely divergent chromosomal races of the common shrew (Sorex araneus, Soricidae, Soricomorpha): hybrid zone in European Russia

The Moscow and Seliger chromosomal races of the common shrew differ by Robertsonian fusions and possibly whole‐arm reciprocal translocations (WARTs) such that their F₁ hybrids produce a chain‐of‐eleven configuration at meiosis I and are expected to suffer substantial infertility. Of numerous hybrid zones that have been described in the common shrew, those between the Moscow and Seliger races involve the greatest chromosomal difference. We collected 211 individuals from this zone to generate a total dataset of 298 individuals from 187 unique global positioning system (GPS) locations within the vicinity of interracial contact. We used a geographic information system (GIS) to map the location of the hybrid zone, which follows a direct route between two lakes, as would be anticipated from tension zone theory. Even within the central area of the hybrid zone, there is a much higher frequency of pure race individuals than hybrid, making this a clear example of a bimodal zone in the sense of Jiggins & Mallet (2000) . The zone runs through good habitat for common shrews, but nevertheless it is very narrow (standard cline widths: 3–4 km), as would be anticipated from low hybrid fitness. There is clear potential for an interruption to gene flow and build‐up of reproductive isolation. As found in some other hybrid zones, there is a high frequency of novel genetic variants, in this case, new chromosomal rearrangements. Here, we report a de novo Robertsonian fission and a de novo reciprocal translocation, both for the first time in the common shrew. There is an extraordinarily high frequency of de novo mutations recorded in F₁ hybrids in the zone and we discuss how chromosomal instability may be associated with such hybrids. The occurrence of a de novo Robertsonian fission is of considerable significance because it provides missing evidence that fissions are the basis of the novel acrocentric forms found and apparently selected for in certain common shrew hybrid zones.     

From refugia to contact: Pine processionary moth hybrid zone in a complex biogeographic setting

Contact zones occur at the crossroad between specific dispersal routes and are facilitated by biogeographic discontinuities. Here, we focused on two Lepidoptera sister species that come in contact near the Turkish Straits System (TSS). We aimed to infer their phylogeographic histories in the Eastern Mediterranean and finely analyze their co‐occurrence and hybridization patterns in this biogeographic context. We used molecular mitochondrial and nuclear markers to study 224 individuals from 42 localities. We used discordances between markers and complementary assignment methods to identify and map hybrids and parental individuals. We confirmed the parapatric distribution of Thaumetopoea pityocampa (Lepidoptera: Notodontidae) in the west and Thaumetopoea wilkinsoni in the east and identified a narrow contact zone. We identified several glacial refugia of T. wilkinsoni in southern Turkey with a strong east–west differentiation in this species. Unexpectedly, T. pityocampa crossed the TSS and occur in northern Aegean Turkey and some eastern Greek islands. We found robust evidence of introgression between the two species in a restricted zone in northwestern Turkey, but we did not identify any F₁ individuals. The identified hybrid zone was mostly bimodal. The distributions and genetic patterns of the studied species were strongly influenced both by the Quaternary climatic oscillations and the complex geological history of the Aegean region. T. pityocampa and T. wilkinsoni survived the last glacial maximum in disjoint refugia and met in western Turkey at the edge of the recolonization routes. Expanding population of T. wilkinsoni constrained T. pityocampa to the western Turkish shore. Additionally, we found evidence of recurrent introgression by T. wilkinsoni males in several T. pityocampa populations. Our results suggest that some prezygotic isolation mechanisms, such as differences in timing of the adult emergences, might be a driver of the isolation between the sister species.     

LINKAGE DISEQUILIBRIUM AND A CONTACT ZONE IN PLETHODON CINEREUS ON THE DEL-MAR-VA PENINSULA

A contact zone between two electrophoretically differentiated forms of Plethodon cinereus extends across the Del-Mar-Va Peninsula. Based on the presumed nature of regional climates during the late Pleistocene, it is suggested that the climate was inhospitable for P. cinereus on the Del-Mar-Va Peninsula at the Wisconsin maximum. It is postulated that, after the Laurentide ice sheet receded about 14,000 years BP, the peninsula was invaded by populations of P. cinereus from the north and south. When the two forms of P. cinereus met, a secondary zone of contact was produced. This contact zone is now apparent in an area in which six electrophoretic loci have concordant clinal changes in allele frequency. The lack of evidence for heterozygote deficiencies, the broad width of the contact zone, and the overall symmetry of introgression suggest that fusion of the populations is occurring freely. Two esterase loci are in linkage disequilibrium in four of eight populations to the north of the contact zone. As all populations of P. cinereus studied to date have disequilibrium coefficients of similar sign, selection is likely important in the maintenance of this linkage disequilibrium. Within the contact zone, the incidence of the linkage disequilibrium increases, but exchange of genes between the two introgressing populations may account for the higher incidence of linkage disequilibrium.     

Populations composed entirely of hybrid colonies: bidirectional hybridization and polyandry in harvester ants

In eusocial Hymenoptera, haplodiploid life cycles, obligate sterile castes, and polyandry may facilitate selection for hybridization. We analyzed a broad hybrid zone between the ecologically distinct seed‐harvester ants Pogonomyrmex occidentalis (Cresson) and Pogonomyrmex maricopa (Wheeler) using mitochondrial (mt)DNA sequence data, eight morphological markers, and 14 random amplified polymorphic DNA (RAPD) markers. Average mtDNA sequence divergence among parental species was 11.34%, indicating secondary contact. RAPD markers were significantly correlated with morphological variation, confirming the interspecific hybrid origin of all morphologically putative hybrid colonies. A morphological hybrid index indicates an abundance of both F₁ hybrids and parental morphotypes within colonies. Individual character frequencies plotted against distance show coincident and concordant clines, suggesting little to no introgression. The structure of the hybrid zone is two‐fold. Within the western region, stark reversals in character frequencies coincide with overt soil differences, indicating a mosaic hybrid zone structure. The eastern region is a riparian habitat where four adjacent populations were composed entirely of hybrid colonies. These habitat associations suggest that hybrid worker genomes permit dispersal into intermediate environments that select against one or both parental species. The present study suggests that, in addition to retaining reproductive compatibility, ecologically distinct species of ants may generate hybrid colonies maintained by environmental selection. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 320–336.     

HYBRID FITNESS IN THE LOUISIANA IRISES: ANALYSIS OF PARENTAL AND F1 PERFORMANCE

The assumption of hybrid inferiority is central to the two models most widely applied to the prediction of hybrid zone evolution. Both the tension zone and mosaic models assume that natural selection acts against hybrids regardless of the environment in which they occur. To test this assumption, we investigated components of fitness in Iris fulva, I. hexagona and their reciprocal F₁ hybrids under greenhouse conditions. The four cross types were compared on the basis of seed germination, vegetative and clonal growth, and sexual reproduction. In all cases, the hybrids performed as well as, or significantly better than, both of their parents. These results suggest that F₁ hybrids between I. fulva and I. hexagona are at least as fit as their parents. The results of this study are therefore inconsistent with the assumptions of both the tension zone and mosaic models of hybrid zone evolution.     

Hybridization of two forms of ninespine stickleback,Pungitius pungitius and P. platygaster,under experimental conditions and an attempt to predict the consequences of their contact in nature

Synopsis In aquaria and in ponds crosses were made between two closely related allopatric forms of ninespine stickleback of the genus Pungitius (P. pungitius and P. platygaster). There were no ethological isolation mechanisms; F₁, F₂ and F₃ hybrids are fertile; experimental hybrid stocks as well as backcrosses maintain themselves for at least three generations. F₁ hybrids are readily distinguishable from parental forms by a combination of morphological characters; F₂ hybrids grown in the north near the Arctic circle, are morphologically similar to P. pungitius. The southern form P. platygaster does not survive in northern conditions; eggs of the northern form P. pungitius die at a temperature of 28°C, normal for the spawning of the southern form. By 9 out of 19 studied morphological characters there are distinctions between the two forms. Therefore, ecologically and morphologically the two forms have diverged rather significantly. On the other hand, ethologically and genetically (as judged by the absence of postmating isolating mechanisms) the forms have diverged only slightly. As a result, the differences between the two forms are considered not to exceed the subspecies or at least semispecies rank.In recent years, due to a disturbance of the ranges of both these forms in Eurasia, a tendency for a secondary contact between P. pungitius and P. platygaster appeared in the basins of the rivers Volga and Irtysh. In the immediate future the two forms are expected to come into contact. From our data we predict possible introgressive hybridization between the northern and sourthern forms in natural conditions.     

Integrating phylogeography and paleoecology to investigate the origin and dynamics of hybrid zones: insights from two widespread North American firs

Secondary contact between closely related taxa routinely occurs during postglacial migrations. After initial contact, the location of hybrid zones may shift geographically or remain spatially stable over time in response to various selective pressures or neutral processes. Studying the extent and direction of introgression using markers having contrasted levels of gene flow can help unravel the historical dynamics of hybrid zones. Thanks to their contrasted maternal and paternal inheritance, resulting in different levels of gene flow for mitochondrial and chloroplast DNA (mtDNA and cpDNA), the Pinaceae stand out as a relevant biological model for this purpose. The objective of the study was to assess whether the hybrid zone between Abies balsamea and Abies lasiocarpa (two largely distributed Pinaceae) has moved or remained stable over time by analysing the distribution of cytoplasmic DNA variation as well as published palaeobotanical data. Interspecific gene flow was higher for cpDNA than mtDNA markers; hence, the geographic distribution of mitotypes was more congruent with species distributions than chlorotypes. This genetic signature was contrary to expectations under a moving hybrid zone scenario, as well as empirical observations in other conifers. Genetic evidence for this rare instance of stable hybrid zone was corroborated by the colonization chronology derived from published fossil data, indicating that the two fir species initially came into contact in the area corresponding to the current sympatric zone 11 kyr ago. While an explanatory analysis suggested the putative influence of various environmental factors on the relative abundance of cytoplasmic genome combinations, further research appears necessary to assess the role of both demographic history and selective factors in driving the dynamics of hybrid zones.     

Concordant morphological and molecular clines in a contact zone of the Common and Spined toad (<Emphasis Type="Italic">Bufo bufo</Emphasis> and <Emphasis Type="Italic">B. spinosus</Emphasis>) in the northwest of France

Background

Hybrid zones are regions where individuals of two species meet and produce hybrid progeny, and are often regarded as natural laboratories to understand the process of species formation. Two microevolutionary processes can take place in hybrid zones, with opposing effects on population differentiation. Hybridization tends to produce genetic homogenization, reducing species differences, whereas the presence of mechanisms of reproductive isolation result in barriers to gene flow, maintaining or increasing differences between taxa.

Results

Here we study a contact zone between two hybridizing toad species, Bufo bufo and B. spinosus, through a combination of molecular (12 polymorphic microsatellites, four nuclear and two mitochondrial SNP markers) and morphological data in a transect in the northwest of France. The results show largely concordant clines across markers, defining a narrow hybrid zone of ca. 30 km wide. Most hybrids in the centre of the contact zone are classified as F₂ or backcrossed individuals, with no individuals assigned to the F₁ hybrid class.

Conclusions

We discuss the implications of these results for our understanding of the evolutionary history of these species. We anticipate that the toad contact zone here described will become an important asset in the study of hybrid zone dynamics and evolutionary biology because of its easy access and the abundance of the species involved.

     

An anomalous hybrid zone in Drosophila

Abstract Despite the genetic tractability of many of Drosophila species, the genus has few examples of the “classic” type of hybrid zone, in which the ranges of two species overlap with a gradual transition from one species to another through an area where hybrids are produced. Here we describe a classic hybrid zone in Drosophila that involves two sister species, Drosophila yakuba and D. santomea, on the island of SaTo Tomé. Our transect of this zone has yielded several surprising and anomalous findings. First, we detected the presence of an additional hybrid zone largely outside the range of both parental species. This phenomenon is, to our knowledge, unique among animals. Second, the genetic analysis using diagnostic molecular markers of the flies collected in this anomalous hybrid zone indicates that nearly all hybrid males are F₁s that carry the D. santomea X chromosome. This F₁ genotype is much more difficult to produce in the laboratory compared to the genotype from the reciprocal cross, showing that sexual isolation as seen in the laboratory is insufficient to explain the genotypes of hybrids found in the wild. Third, there is a puzzling absence of hybrid females. We suggest several tentative explanations for the anomalies associated with this hybrid zone, but for the present they remain a mystery.