THE EFFECT OF A PARTIAL BARRIER ON THE MOVEMENT OF A HYBRID ZONE

Computer simulations of clines (Brues, 1972; Endler, 1977) as well as theoretical arguments (Nagylaki, 1975), have shown that steps in gene frequencies will be pulled to partial barriers (areas of reduced gene flow) if they form within approximately a cline width of the partial barrier. The behavior of a hybrid zone between two chromosomal taxa (“Moreton” and “Torresian”) of the acridine grasshopper Caledia captiva in southeast Queensland has been analyzed and found to comform qualitatively with a model of altered gene flow patterns. Clines in four enzyme systems were analyzed for 1983 and 1986 along a transect across the hybrid zone. The clines have shifted towards an area of regenerating forest, while homozygote frequencies have increased at this point. This forest barrier has broken the continuity of the spatial distribution of C. captiva, forming population islands in part of the hybrid zone, and thus reducing the amount of gene flow. The distance between the barrier and the original cline is approximately of the order of a cline width, so that they would be expected to interact. Historical information suggests that the secondary contact between the “Moreton” and “Torresian” taxa occurred very recently (1844–1940), due to the intensive land-clearing activities during the European settlement.     

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.     

Morphological clines across a karyotypic zone of house mice in Central Italy

Multivariate morphometric differentiation between chromosomal races of the mouse Mus domesticus in Central Italy was investigated using a population of 2n = 22 “CB” karyotype, three populations of standard 2n = 40 karyotype, five populations of 2n = 22 “CD” karyotype and three populations from the hybrid zone between the latter two karyotypes. Whilst populations of different karyotype generally have significantly different morphometry, canonical analysis does not reveal that the populations ordinate into distinct aggregations based on karyotype, largely because the 2n = 22 “CD” populations are so diverse. Nevertheless, canonical analysis does reveal a significant cline in morphology across the contact zone between the 2n = 40 and 2n = 22 “CD” mice. The nature of this transition, i. e. a cline 1. within the 2n = 40 range, 2. within the hybrid range (but unrelated to chromosome number) and 3. within the 2n = 22 “CD” range, tends to indicate that the morphometric divergence is due to adaptation to the different ecological regimes across which these mice are distributed rather than the phylogenetic divergence of the karyotypic races.     

Genetic, morphological, and ecological characterization of a hybrid zone that spans a migratory divide

This study characterizes a hybrid zone that spans a migratory divide between subspecies of the Swainson's thrush (Catharus ustulatus), a long distance migratory songbird, in the Coast Mountains of British Columbia. To assess the potential for a barrier to gene flow between the subspecies, I: (1) analyzed the shape and width of genetic and morphological clines relative to estimates of dispersal distance, (2) assessed the ratio of parental to hybrid genotypes across the hybrid zone, (3) estimated population density across the hybrid zone, and (4) compared the spatial relationship between the hybrid zone and an existing environmental gradient. The results indicate that the hybrid zone is characterized by mostly concordant character clines that are narrow relative to dispersal, the absence of a hybrid swarm, and low population density at the center of the zone. This hybrid zone and additional regions of contact between these subspecies are found on the border between coastal and interior climatic regions throughout the Pacific Northwest. An identified shift in the location, but not the width, of the mtDNA cline relative to the nuclear clines is consistent with asymmetrical hybridization. Neutral diffusion of populations following secondary contact and hybrid superiority within an ecotone are insufficient explanations for the observed patterns. The hypothesis that best fits the data is that the Swainson's thrush hybrid zone is a tension zone maintained by dispersal and ecologically mediated barriers to gene flow.     

Species distribution models contribute to determine the effect of climate and interspecific interactions in moving hybrid zones

Climate is a major factor delimiting species’ distributions. However, biotic interactions may also be prominent in shaping geographical ranges, especially for parapatric species forming hybrid zones. Determining the relative effect of each factor and their interaction of the contact zone location has been difficult due to the lack of broad scale environmental data. Recent developments in species distribution modelling (SDM) now allow disentangling the relative contributions of climate and species’ interactions in hybrid zones and their responses to future climate change. We investigated the moving hybrid zone between the breeding ranges of two parapatric passerines in Europe. We conducted SDMs representing the climatic conditions during the breeding season. Our results show a large mismatch between the realized and potential distributions of the two species, suggesting that interspecific interactions, not climate, account for the present location of the contact zone. The SDM scenarios show that the southerly distributed species, Hippolais polyglotta, might lose large parts of its southern distribution under climate change, but a similar gain of novel habitat along the hybrid zone seems unlikely, because interactions with the other species (H. icterina) constrain its range expansion. Thus, whenever biotic interactions limit range expansion, species may become ‘trapped’ if range loss due to climate change is faster than the movement of the contact zone. An increasing number of moving hybrid zones are being reported, but the proximate causes of movement often remain unclear. In a global context of climate change, we call for more interest in their interactions with climate change.     

Evolution of a hybrid zone of two willow species (Salix L.) in the European Alps analyzed by RAD-seq and morphometrics

Natural hybridization of plants can result in many outcomes with several evolutionary consequences, such as hybrid speciation and introgression. Natural hybrid zones can arise in mountain systems as a result of fluctuating climate during the exchange of glacial and interglacial periods, where species retract and expand their territories, resulting in secondary contacts. Willows are a large genus of woody plants with an immense capability of interspecific crossing. In this study, the sympatric area of two diploid sister species, S. foetida and S. waldsteiniana in the eastern European Alps, was investigated to study the genomic structure of populations within and outside their contact zone and to analyze congruence of morphological phenotypes with genetic data. Eleven populations of the two species were sampled across the Alps and examined using phylogenetic network and population genetic structure analyses of RAD Seq data and morphometric analyses of leaves. The results showed that a homoploid hybrid zone between the two species was established within their sympatric area. Patterns of genetic admixture in homoploid hybrids indicated introgression with asymmetric backcrossing to not only one of the parental species but also one hybrid population forming a separate lineage. The lack of F1 hybrids indicated a long-term persistence of the hybrid populations. Insignificant isolation by distance suggests that gene flow can act over large geographical scales. Morphometric characteristics of hybrids supported the molecular data and clearly separated populations of the parental species, but showed intermediacy in the hybrid zone populations with a bias toward S. waldsteiniana. The homoploid hybrid zone might have been established via secondary contact hybridization, and its establishment was fostered by the low genetic divergence of parental species and a lack of strong intrinsic crossing barriers. Incomplete ecological separation and the ability of long-distance dispersal of willows could have contributed to the spatial expansion of the hybrid zone.     

SPECIATION BY REINFORCEMENT OF PREMATING ISOLATION

The generation of premating isolation given partial or complete postzygotic isolation between populations is termed reinforcement or, in the case of complete isolation, reproductive character displacement. In this study we use computer simulations and a multilocus genetic model to reevaluate the theory of reinforcement. We consider the evolution of female preferences for a male secondary sexual trait. If the populations differ in mean female preference, there is direct selection on the preference for further divergence, which may be augmented by a correlated response to sexual selection on males. Two factors prevent divergence. First, if postzygotic isolation is not complete, gene flow can prevent divergence and lead to a hybrid swarm. This is the usual outcome whenever the average number of breeding adult offspring produced by a hybrid mating is sufficient to replace the parents. Second, one or the other population may become extinct because of the large number of hybrid matings it is involved in. The likelihood of extinction is lowered if population growth rates are high, if hybrids are inviable rather than infertile, or under some conditions when allopatric populations provide immigrants into the contact zone. Provided hybrid fitness is sufficiently low, there is a wide range of genetic and ecological conditions under which reinforcement rather easily occurs, and also a range under which it may occur because of stochastic effects on both the inheritance parameters and the population sizes.     

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.     

ECOLOGICAL GENETICS OF A MOSAIC HYBRID ZONE: MITOCHONDRIAL,NUCLEAR, AND REPRODUCTIVE DIFFERENTIATION OF CRICKETS BY SOIL TYPE

We investigated the effects that habitat variation has on the structure and dynamics of a hybrid zone between two closely related crickets in Connecticut. A collecting protocol was developed in which crickets were sampled from characteristic habitats on either side of the hybrid zone and from two distinct habitat types within the zone. Presumptive pure Gryllus pennsylvanicus were sampled from fields in northwestern Connecticut and represent “inland” populations. “Pure” Gryllus firmus were sampled from beaches along the coast and represent the “coastal” populations. Crickets from within the hybrid zone were sampled from two different soil types: the “loam” populations from loamy soils and the “sand” populations from sandy soils. Moreover, an attempt was made to identify closely adjacent sand and loam localities to determine the scale of habitat variation and its possible effects on hybrid-zone structure. In general, there was little variation in morphological traits or in allozyme and mtDNA genotype frequencies among localities from within each of the four habitat types. Between each of the closely situated sand and loam localities within the hybrid zone, however, there were very significant differences in each of these sets of markers. In addition, crickets from hybrid-zone populations were tested for reproductive isolation. The asymmetric outcome of hybrid crosses that exists across the zone (Harrison, 1983) also exists on a finer ecological scale within the zone. Thus, this hybrid zone is a mosaic of strikingly differentiated populations. The dynamics of hybrid zones with mosaic structures are discussed in contrast to the traditional clinal models. The data are also discussed in light of the semipermeable nature of species boundaries. The extent to which a species boundary is permeable varies not only from one genetic marker to the next, but also with the ecological and geographic context of species interaction.     

Hybrid swarm between divergent lineages of mule deer (Odocoileus hemionus)

Studies of hybrid zones have revealed an array of evolutionary outcomes, yet the underlying structure is typically characterized as one of three types: a hybrid zone, a hybrid swarm or a hybrid taxon. Our primary objective was to determine which of these three structures best characterizes a zone of hybridization between two divergent lineages of mule deer (Odocoileus hemionus), mule deer and black-tailed deer. These lineages are morphologically, ecologically and genetically distinct, yet hybridize readily along a zone of secondary contact between the east and west slopes of the Cascade Mountains (Washington and Oregon, USA). Using microsatellite and mitochondrial DNA, we found clear evidence for extensive hybridization and introgression between lineages, with varying degrees of admixture across the zone of contact. The pattern of hybridization in this region closely resembles a hybrid swarm; based on data from 10 microsatellite loci, we detected hybrids that extend well beyond the F1 generation, did not detect linkage disequilibrium at the centre of the zone and found that genotypes were associated randomly within the zone of contact. Introgression was characterized as bidirectional and symmetric, which is surprising given that the zone of contact occurs along a sharp ecotone and that lineages are characterized by large differences in body size (a key component of mating success). Regardless of the underlying mechanisms promoting hybrid swarm maintenance, it is clear that the persistence of a hybrid swarm presents unique challenges for management in this region.     

REDUCED GENE FLOW AT PERICENTROMERIC LOCI IN A HYBRID ZONE INVOLVING CHROMOSOMAL RACES OF THE HOUSE MOUSE MUS MUSCULUS DOMESTICUS

The West European house mouse, Mus musculus domesticus, is a particularly suitable model to investigate the role of chromosomal rearrangements in reproductive isolation. In fact, it exhibits a broad range of chromosomal polymorphism due to Robertsonian (Rb) fusions leading to various types of contact zones between different chromosomal races. In the present study, we analyzed a parapatric contact in central Italy between the Cittaducale chromosomal race (CD: 2n= 22) and the surrounding populations with standard karyotype (2n= 40) to understand if Rb fusions play a causative role in speciation. One hundred forty‐seven mice from 17 localities were genotyped by means of 12 microsatellite loci. A telomeric and a pericentromeric locus situated on six chromosome arms (four Rbs and one telocentric) were selected to detect differences in the amount of gene flow for each locus in different chromosomal positions. The analyses performed on the two subsets of loci show differences in the level of gene flow, which is more restricted near the centromeres of Rb chromosomes. This effect is less pronounced in the homozygotes populations settled at the border of the hybrid zone. We discuss the possible cause of the differential porosity of gene flow in Rbs considering “hybrid dysfunctions” and “suppressed recombination” models.     

Dictionary of protein secondary structure: Pattern recognition of hydrogen-bonded and geometrical features

For a successful analysis of the relation between amino acid sequence and protein structure, an unambiguous and physically meaningful definition of secondary structure is essential. We have developed a set of simple and physically motivated criteria for secondary structure, programmed as a pattern-recognition process of hydrogen-bonded and geometrical features extracted from x-ray coordinates. Cooperative secondary structure is recognized as repeats of the elementary hydrogen-bonding patterns “turn” and “bridge.” Repeating turns are “helices,” repeating bridges are “ladders,” connected ladders are “sheets.” Geometric structure is defined in terms of the concepts torsion and curvature of differential geometry. Local chain “chirality” is the torsional handedness of four consecutive C^α positions and is positive for right-handed helices and negative for ideal twisted β-sheets. Curved pieces are defined as “bends.” Solvent “exposure” is given as the number of water molecules in possible contact with a residue. The end result is a compilation of the primary structure, including SS bonds, secondary structure, and solvent exposure of 62 different globular proteins. The presentation is in linear form: strip graphs for an overall view and strip tables for the details of each of 10.925 residues. The dictionary is also available in computer-readable form for protein structure prediction work.     

Observations on Living Diastoporidae (Bryozoa Cyclostomata), with Special Regard to Polymorphism

The Diastoporidan colony consists of three zones: The marginal common bud, a narrow middle zone with erect peristomes and active polypides nourishing the whole colony, and a large central zone where the erect peristomes are detached, the secondary orifices thus formed are closed by terminal diaphragms, and the feeding polypides have degenerated. In many Diastoporidae the terminal closure is complete, but in several species the diaphragm is raised into a narrow, open tubule. In the latter type a dwarfed polypide with one tentacle only is regenerated: The autozooid is transformed into a heterozooid (“secondary nanozooid”), a unique phenomenon. Its function is unknown. The single tentacle of the “true nanozooids” of Diplosolen performs sweeping movements indicating a cleaning function. In two Plagioecia species “occasional nanozooids” were found, induced by disturbances of the colony growth. The function, if any, is unknown.     

Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation‐in‐action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs Pseudacris feriarum and P. nigrita to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.     

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.     

A MODEL OF A HYBRID ZONE BETWEEN TWO CHROMOSOMAL RACES OF THE COMMON SHREW (SOREX ARANEUS)

The common shrew (Sorex araneus) is subdivided into several karyotypic races in Britain. Two of these races meet near Oxford o form the “Oxford-Hermitage” hybrid zone. We present a model which describes this system a; a “tension zone,” i.e., a set of clines maintained by a balance between dispersal and selection against chromosomal heterozygotes. The Oxford and Hermitage races differ by Robertsonian fusions with monobrachial homology (kq, no versus ko), and so Fl hybrids between them would have low fertility. However, the acrocentric karyotype is found at high frequency within the hybrid zone, so that complex Robertsonian heterozygotes (kg no/q ko n) are replaced by more fertile combinations, such as (kg no/k q n o). This suggests that the hybrid zone has been modified so as to increase hybrid fitness. Mathematical analysis and simulation show that, if selection against complex heterozygotes is sufficiently strong relative to selection against simple heterozygotes, acrocentrics increase, and displace the clines for kg and no from the cline for ko. Superimposed on this separation is a tendency for the hybrid zone to move in favor of the Oxford (kg no) race. We compare the model with estimates of linkage disequilibrium and cline shape made from field data.     

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.     

Behavioural discrimination among chromosomal races of the house mouse (Mus musculus domesticus)

This work is concerned with the extent of behavioural discrimination between three chromosomal races of the house mouse (the standard 40-chromosome race and a 32- and 36-chromosome races) found in the vicinity of a hybrid zone in northern Scotland. Mice were investigated for several elements of their social behaviour. Within-population dyadic encounters did not show consistent behavioural differences attributable to karyotype among five populations (two standard race, two 36-chromosome race, one 32-chromosome race). Between-population dyadic encounters revealed significant differences between three populations. The standard population examined appeared to be the most “open” to foreigners, the 32-chromosome population the most “closed” while the 36-chromosome mice displayed an intermediate response. Differences in behaviour displayed during between-population as compared to within-population dyadic encounters revealed the occurrence of behavioural discrimination between populations. The implication of these results on the dynamics of the hybrid zone are discussed.     

About nothing

In light of recent terminological controversy, this article reviews cladistic conceptions of character states coded as absences, symplesiomorphies, and secondary losses. The first section addresses absence as a question of ontology vs. epistemology. The second and third sections address the evidentiary status of symplesiomorphy in cladistics, the fourth contrasts primitive absence with secondary loss, and the fifth clarifies the meaning of “grouping”. While secondary losses (reversals) are often synapomorphies, symplesiomorphies (“absent” or otherwise) have no evidentiary import to cladistic hypotheses of relationship. Thus, we argue that identifying symplesiomorphic character states as “homologous” is conceptually vacuous, because they are either synapomorphies (homologues) of more inclusive taxa, or complementary absences that unite no group.     

Development of the “Third-Generation” Hybrid Rice in China

Rice is a major cereal crop for China. The development of the ‘‘three-line" hybrid rice system based on cytoplasmic male sterility in the 1970 s(first-generation) and the ‘‘two-line" hybrid rice system based on photoperiod-and thermo-sensitive genic male-sterile lines(second-generation)in the 1980 s has contributed significantly to rice yield increase and food security in China. Here we describe the development and implementation of the ‘‘third-generation" hybrid rice breeding system that is based on a transgenic approach to propagate and utilize stable recessive nuclear male sterile lines, and as such, the male sterile line and hybrid rice produced using such a system is nontransgenic. Such a system should overcome the intrinsic problems of the ‘‘first-generation" and‘‘second-generation" hybrid rice systems and hold great promise to further boost production of hybrid rice and other crops.