Tension Zone Model Research Articles

Analyses of Hybrid Viability across a Hybrid Zone between Two Alnus Species Using Microsatellites and cpDNA Markers.

Diploid Alnus glutinosa s. str. and autotetraploid A. rohlenae form a narrow hybrid zone in a study area in southern Serbia, which results in triploid hybrid formation. The vast majority of previous studies have been focused on studies of maternal plants, but the offspring resulting from their crossing have not been much studied. Here, we use the variability of microsatellites and chloroplast DNA between these species and their putative hybrids to create an overall picture of the development of the hybrid zone and its predicted type. To elucidate the gene transfer within both species, the origins of individual ploidies and especially the role of triploid hybrids, a germination experiment was carried out linked with a flow cytometry study of the resulting seedlings. The tension zone model seems to offer the most adequate explanation of our observations, with selection against triploid hybrids and the spatial positioning of the hybrid zone. Despite selection against them, the triploid hybrids play an important role in the exchange of genes between the two species and therefore serve as a bridge for introgression. The presence of fertile triploids is essential for enriching the haplotype diversity between these species and for the development of new genetic lineages.

Dispersal Predicts Hybrid Zone Widths across Animal Diversity: Implications for Species Borders under Incomplete Reproductive Isolation.

Hybrid zones occur as range boundaries for many animal taxa. One model for how hybrid zones form and stabilize is the tension zone model, a version of which predicts that hybrid zone widths are determined by a balance between random dispersal into hybrid zones and selection against hybrids. Here, we examine whether random dispersal and proxies for selection against hybrids (genetic distances between hybridizing pairs) can explain variation in hybrid zone widths across 131 hybridizing pairs of animals. We show that these factors alone can explain ∼40% of the variation in zone width among animal hybrid zones, with dispersal explaining far more of the variation than genetic distances. Patterns within clades were idiosyncratic. Genetic distances predicted hybrid zone widths particularly well for reptiles, while this relationship was opposite tension zone predictions in birds. Last, the data suggest that dispersal and molecular divergence set lower bounds on hybrid zone widths in animals, indicating that there are geographic restrictions on hybrid zone formation. Overall, our analyses reinforce the fundamental importance of dispersal in hybrid zone formation and more generally in the ecology of range boundaries.

Replicated anthropogenic hybridisations reveal parallel patterns of admixture in marine mussels.

Human‐mediated transport creates secondary contacts between genetically differentiated lineages, bringing new opportunities for gene exchange. When similar introductions occur in different places, they provide informally replicated experiments for studying hybridisation. We here examined 4,279 Mytilus mussels, sampled in Europe and genotyped with 77 ancestry‐informative markers. We identified a type of introduced mussels, called “dock mussels,” associated with port habitats and displaying a particular genetic signal of admixture between M. edulis and the Mediterranean lineage of M. galloprovincialis. These mussels exhibit similarities in their ancestry compositions, regardless of the local native genetic backgrounds and the distance separating colonised ports. We observed fine‐scale genetic shifts at the port entrance, at scales below natural dispersal distance. Such sharp clines do not fit with migration‐selection tension zone models, and instead suggest habitat choice and early‐stage adaptation to the port environment, possibly coupled with connectivity barriers. Variations in the spread and admixture patterns of dock mussels seem to be influenced by the local native genetic backgrounds encountered. We next examined departures from the average admixture rate at different loci, and compared human‐mediated admixture events, to naturally admixed populations and experimental crosses. When the same M. galloprovincialis background was involved, positive correlations in the departures of loci across locations were found; but when different backgrounds were involved, no or negative correlations were observed. While some observed positive correlations might be best explained by a shared history and saltatory colonisation, others are likely produced by parallel selective events. Altogether, genome‐wide effect of admixture seems repeatable and more dependent on genetic background than environmental context. Our results pave the way towards further genomic analyses of admixture, and monitoring of the spread of dock mussels both at large and at fine spacial scales.

Ecological differentiation and habitat unsuitability maintaining a ground beetle hybrid zone.

Exogenous selection via interactions between organisms and environments may influence the dynamics of hybrid zones between species in multiple ways. Two major models of a hybrid zone allowed us to hypothesize that environmental conditions influence hybrid zone dynamics in two ways. In the first model, an environmental gradient determines the mosaic distribution at the boundary between ecologically differentiated species (mosaic hybrid zone model). In the second model, a patch of unsuitable habitat traps a hybrid zone between species whose hybrids are unfit (tension zone model). To test these, we examined the environmental factors influencing the spatial structure of a hybrid zone between the ground beetles Carabus maiyasanus and C. iwawakianus using GIS-based quantification of environmental factors and a statistical comparison of species distribution models (SDMs). We determined that both of the hypothetical processes can be important in the hybrid zone. We detected interspecific differences in the environmental factors in presence localities and their relative contribution in SDMs. SDMs were not identical between species even within contact areas, but tended to be similar within the range of each species. These results suggest an association between environments and species, and provide evidence that ecological differentiation between species plays a role in the maintenance of the hybrid zone. Contact areas were characterized by a relatively high temperature, low precipitation, and high topological wetness. Thus, the contact areas were regarded as being located in an unsuitable habitat with a drier climate, where those populations are likely to occur in patches with limited precipitation concentrated. A comparison of spatial scales suggests that exogenous selection via environmental factors may be weaker than endogenous selection via genitalic incompatibility.

Clinal variation in a brown lemur (Eulemur spp.) hybrid zone: Combining morphological, genetic and climatic data to examine stability

Studies of hybrid zones can inform our understanding of reproductive isolation and speciation. Two species of brown lemur (Eulemur rufifrons and E. cinereiceps) form an apparently stable hybrid zone in the Andringitra region of southeastern Madagascar. The aim of this study was to identify factors that contribute to this stability. We sampled animals at 11 sites along a 90-km transect through the hybrid zone and examined variation in 26 microsatellites, the D-loop region of mitochondrial DNA, six pelage and nine morphological traits; we also included samples collected in more distant allopatric sites. Clines in these traits were noncoincident, and there was no increase in either inbreeding coefficients or linkage disequilibrium at the centre of the zone. These results could suggest that the hybrid zone is maintained by weak selection against hybrids, conforming to either the tension zone or geographical selection-gradient model. However, a closer examination of clines in pelage and microsatellites indicates that these clines are not sigmoid or stepped in shape but instead plateau at their centre. Sites within the hybrid zone also occur in a distinct habitat, characterized by greater seasonality in precipitation and lower seasonality in temperature. Together, these findings suggest that the hybrid zone may follow the bounded superiority model, with exogenous selection favouring hybrids within the transitional zone. These findings are noteworthy, as examples supporting the bounded superiority model are rare and may indicate a process of ecologically driven speciation without geographical isolation.

Genomic and phenotypic architecture of a spruce hybrid zone (Picea sitchensis × P. glauca)

Interspecific hybridization may enhance the capacity of populations to adapt to changing environments, and has practical implications for reforestation. We use genome-wide estimates of admixture and phenotypic traits for trees in a common garden to examine the extent and direction of gene flow across a Picea hybrid zone, testing assumptions of the bounded hybrid superiority and tension zone models of hybrid zone maintenance. Seeds were collected from the ecological transition zone spanning from maritime to continental climates across the Picea sitchensis-P.glauca contact zone, and 721 trees were planted in a common garden experiment within the hybrid zone. Individuals were genotyped using a panel of 384 candidate-gene single nucleotide polymorphisms (SNPs) putatively associated with adaptive traits in Picea, and phenotyped at age ten for height and autumn cold hardiness. Low interspecific heterozygosity in hybrids indicated that intrinsic reproductive barriers were too weak to prevent widespread recombination, although introgression appeared asymmetric with P.sitchensis dominating the zone. Whereas marker-based hybrid index was strongly correlated with climate and geography, phenotypic traits exhibited weak or no significant clines. Our results indicated that exogenous selection appeared to play a strong role in the distribution and structure of this hybrid zone, indicative of an environmentally determined bounded hybrid superiority model of hybrid zone maintenance, although endogenous mechanisms could not be ruled out. This study provides insight into the mechanisms underlying adaptation across ecologically transitional hybrid zones that will ultimately provide an additional tool in managing these economically important tree species.

Analysis of photoelastic stress fields in beam

We have investigated the stress state of models of beams with damage in the form of cuts, simulating the vertical and inclined cracks. The experiments were performed in pure and direct lateral bending. The crack was located in the tension zone model of a beam of piezooptical plexiglass. To study was used method of the photo elasticity. Was ended collating of the stress fields in models of beams with reinforcing layer of carbon fiber and in its absence.

Natural hybridisation between two coastal endemic species of Armeria (Plumbaginaceae) from Portugal. 2. Ecological investigations on a hybrid zone

A clinal range of morphological variation between two coastal endemic species of the Genus Armeria in Portugal (A. pseudoarmeria and A. welwitschii) was investigated to test the hypothesis of hybrid advantage in contact zones between species. The putative F1 hybrids morphotypes could be submitted to endogenous selection as most of them show deficiencies in pollen fertility and seed setting in comparison with the parental species. Reciprocal transplant experiments in three sectors along the cline revealed that the intermediate F1 morphotypes are less productive in biomass than the parents even in their sector of origin. These data do not show evidence for the Hybrid Bound Superiority Model. Rather they are in favour of the Tension Zone Model where a dynamic balance is obtained between the continuous production of hybrids and the selection against them. Nevertheless the Tension Zone Model postulates similar fitness for the parents along the cline, which is not the case as the parents are clearly differentiated for nutrient uptake. Soil and plant leaf elemental analysis revealed that, in transplantation, some of the hybrids correspond to a geochemical specialisation diverging from that of the parents suggesting a local adaptation. Owing the high frequency and the healthy developmental aspect of the intermediate morphs in central and northern sector selection must be weak against hybrids. It is concluded that in natural situations the two models could not be strictly exclusive.

Divergent character clines across a recent secondary contact zone in a Hispaniolan lizard

AbstractStudies of genetic contact zones provide valuable information regarding the processes of population divergence, adaptation and speciation. In this paper, I examine transitions in morphology, mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) haplotypes across a recent secondary contact zone in a Hispaniolan lizard Ameiva chrysolaema. Maximum likelihood cline fitting analyses suggest non‐coincidence of cline centers and that the mtDNA cline is significantly displaced to the west of the remaining clines. nDNA and morphological clines are coincident and tend to be associated with the prevailing environmental gradient. The lack of cytonuclear disequilibrium near the center of the contact zone and the non‐coincidence of character clines suggest that this zone does not conform to a tension zone model of hybridization; thus, gene flow across the zone does not seem to be impeded. The extremely narrow width of the dorsal scale size cline and the close association of this cline with the steepness of the environmental (precipitation) gradient suggest that this character may be under environmental selection. Taken together, this contact zone appears to be structured by a combination of mtDNA introgression, possibly associated with eastward movement of the zone, and environmental selection on some characters.

Estuarine habitats protect hybrid mussels from selection

The marine mussels, Mytilus edulis and Mytilus galloprovincialis, form an extensive hybrid zone in Europe where F 2 hybrids and mussels of mixed genetic ancestry are often locally abundant. Hybrid zones are maintained by the interplay of dispersal and selection on hybrid genotypes but there has been vigorous debate on the form of selection that may occur in these systems. Tension zone models argue that selection is against hybrids because of developmental misregulation and is independent of the external environment. Exogenous selection models argue that selection is habitat-dependent and the structure of the hybrid zone is due to the distribution of habitat patches that vary in selection intensity. We test this prediction by comparing the genetic structure of mussel populations in open-coast habitats, where selection on hybrids is strong, to those within two independent estuaries. We show that mussels within these estuaries are protected from selection and thus selection is strongly dependent on habitat, which supports the exogenous selection hypothesis. Hybrid mussel populations on the open-coast experience strong selection against M. edulis-like genotypes, which has been postulated to be the result of differential dislodgment by waves. This hypothesis is supported by our results since mussels within sheltered habitats are protected from selection. There was, despite previous suggestions, no evidence of selection in favor of M. edulis-like mussels within either estuary.

Theoretical model for cracked tension zone of reinforced concrete under creep and cyclic load

The cracked tension zone model of Tue and Konig is extended to take account of changes in slip between steel and concrete as a result of maintained and/or cyclic loading. The bond stress–slip curve given in CEB Model Code 90 is adapted for the extended analysis, which enables load history to be considered. The predicted results are compared with test results, and found to be in good agreement.

Theoretical model for cracked tension zone of reinforced concrete under creep and cyclic load

The cracked tension zone model of Tue and Konig is extended to take account of changes in slip between steel and concrete as a result of maintained and/or cyclic loading. The bond stress–slip curve given in CEB Model Code 90 is adapted for the extended analysis, which enables load history to be considered. The predicted results are compared with test results, and found to be in good agreement.

ASSORTATIVE FERTILIZATION AND SELECTION AT LARVAL STAGE IN THE MUSSELS MYTILUS EDULIS AND M. GALLOPROVINCIALIS

Assortative mating (prezygotic isolation) and reduced hybrid fitness (postzygotic isolation) are typically invoked to explain the stability of hybrid zones. In the tension zone model, these factors work in opposition to migration, which promotes genetic homogeneity. Many marine animals migrate over long distances through a planktonic larval stage. Therefore, strong reproductive isolation is needed to maintain stable marine hybrid zones. However, surprisingly little is known about mating preferences and hybrid fitness in marine organisms. Smooth-shelled mussels (Mytilus spp.) form a well-known species complex, with hybridization over extensive areas such as the contact zone of M. edulis and M. galloprovincialis around European Atlantic coasts. This paper reports direct experimental evidence of assortative fertilization, hybrid larval inviability, and early heterosis for growth rate in M. edulis and M. galloprovincialis. Four crosses between pure M. edulis and M. galloprovincialis were analyzed with a new polymerase-chain-reaction-based diagnostic marker. Gamete competition between taxa was allowed in two out of the four crosses. Genotype frequencies observed at an early stage (36 h after fertilization) unambiguously revealed assortative fertilization when gamete competition was allowed. A significant reduction in hybrid viability was subsequently observed during the larval stage. At the same stage an antagonistic effect, heterosis, was observed on growth rate. However, even if heterosis is observed in the F1, it is expected to vanish in subsequent hybrid generations. Although specialization for different habitats and asynchronous spawning have been mentioned as factors contributing to the maintenance of the blue mussel hybrid zone in Europe, we argue that assortative fertilization and reduced hybrid fitness are important factors that also contribute to the stabilization of this zone. These results emphasize that multiple factors may act concomitantly in a barrier to gene flow, especially in complex life cycles. Furthermore, they show that assortative mating through gamete preference, as already demonstrated for sea urchins, may play a role in speciation processes taking place in the sea.Corresponding Editor: R. Burton

Spatial structure and habitat variation in a grasshopper hybrid zone.

A hybrid zone between the grasshoppers Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) in northern Spain has been analyzed for variation in morphology and ecology. These species are readily distinguished by the number of stridulatory pegs on the hind femur. Both sexes are fully winged and inhabit disturbed habitats throughout the study area. We develop a maximum-likelihood approach to fitting a two-dimensional cline to geographical variation in quantitative traits and for estimating associations of population mean with local habitat. This method reveals a cline in peg number approximately 30 km south of the Picos de Europa Mountains that shows substantial deviations in population mean compared with the expectations of simple tension zone models. The inclusion of variation in local vegetation in the model explains a significant proportion of the residual variation in peg number, indicating that habitat-genotype associations contribute to the observed spatial pattern. However, this association is weak, and a number of populations continue to show strong deviations in mean even after habitat is included in the final model. These outliers may be the result of long-distance colonization of sites distant from the cline center or may be due to a patchy pattern of initial contact during postglacial expansion. As well as contrasting with the smooth hybrid zones described for Chorthippus parallelus, this situation also contrasts with the mosaic hybrid zones observed in Gryllus crickets and in parts of the hybrid zone between Bombina toad species, where habitat-genotype associations account for substantial amounts of among-site variation.

Distribution and habitat use of water frog hybrid complexes in France

AbstractHybrid zones are either distributed along clines or in a mosaic of patches. This distribution may depend upon variation in taxon habitat use. Habitat use and distribution of diverse taxa of water frogs (Rana ridibunda,R. lessonae,R. perezi,R.kl.grafiandR.kl.esculenta) in France are analysed to determine whether water frog complexes conform to the mosaic or clinal model. Biogeographical scenarios may be invoked in order to explain the distribution of water frogs. However, the distribution ofR. pereziandR.kl.grafi, being restricted to regions characterized by Mediterranean or Oceanic climatic conditions, suggests that these frogs do not endure cold winters.R. ridibundais widespread in Southern France and its distribution suggests multiple introductions. It is concluded that water frogs conform to the mosaic zone model rather than to the tension zone model because: (i) taxa exhibited differences in habitat use, (ii) pure parental species were documented and (iii) hybrids are not unfit relative to parental species.

SPATIAL STRUCTURE AND HABITAT VARIATION IN A GRASSHOPPER HYBRID ZONE

A hybrid zone between the grasshoppers Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) in northern Spain has been analyzed for variation in morphology and ecology. These species are readily distinguished by the number of stridulatory pegs on the hind femur. Both sexes are fully winged and inhabit disturbed habitats throughout the study area. We develop a maximum-likelihood approach to fitting a two-dimensional cline to geographical variation in quantitative traits and for estimating associations of population mean with local habitat. This method reveals a cline in peg number approximately 30 km south of the Picos de Europa Mountains that shows substantial deviations in population mean compared with the expectations of simple tension zone models. The inclusion of variation in local vegetation in the model explains a significant proportion of the residual variation in peg number, indicating that habitat-genotype associations contribute to the observed spatial pattern. However, this association is weak, and a number of populations continue to show strong deviations in mean even after habitat is included in the final model. These outliers may be the result of long-distance colonization of sites distant from the cline center or may be due to a patchy pattern of initial contact during postglacial expansion. As well as contrasting with the smooth hybrid zones described for Chorthippus parallelus, this situation also contrasts with the mosaic hybrid zones observed in Gryllus crickets and in parts of the hybrid zone between Bombina toad species, where habitat-genotype associations account for substantial amounts of among-site variation.

Is thePrunella(Lamiaceae) hybrid zone structured by an environmental gradient? Evidence from a reciprocaltransplant experiment

Hybrid zones may be structured by environmentally independent selection against intrinsically unfit hybrids (tension zone models), or by environmentally dependent fitness differences among parental species and hybrids (ecological selection-gradient models). A 30-m slope in a mountain grassland harbors a hybrid zone of the clonal perennials, Prunella grandiflora and P. vulgaris (Lamiaceae), with P. grandiflora in the upper, P. vulgaris in the lower, and both parental species and P. grandiflora × P. vulgaris Hybrids in a narrow middle part. We found gradients for soil depth and water content, and vegetation height and biomass along the slope. A reciprocal transplant experiment yielded crossing reaction norms for vegetative reproduction. Parental species were locally adapted to their home sites, while the three taxa did not differ in vegetative reproduction in the Hybrid transplant site. Local adaptation for vegetative reproduction of P. grandiflora was mediated through higher survival and that of P. vulgaris through higher ramet number, indicating adaptation of their clonal growth strategies (phalanx vs. guerrilla) to the different habitats. Hybrid performance was intermediate between that of the parental species in all three sites, although Hybrids flowered more often than the parental species in the Hybrid site. Our results support ecological selection-gradient rather than tension zone models.

Hybrids and hybrid zones

Arnold's new book on hybridization[1xSee all References][1], recently reviewed by Ritchie in TREE[2xRitchie, M. Trends Ecol. Evol. 1998; 13: 123–124Abstract | Full Text PDFSee all References][2], has brought attention to an old, controversial but revitalized topic in evolutionary biology. The two reviews[2xRitchie, M. Trends Ecol. Evol. 1998; 13: 123–124Abstract | Full Text PDFSee all References, 3xBarton, N.H. Genet. Res. 1997; 70: 178–180CrossRefSee all References]we have read share a sceptical attitude towards studies of hybridization lying outside the hybrid zone theory. They consider Arnold's book too `opinionated'[2xRitchie, M. Trends Ecol. Evol. 1998; 13: 123–124Abstract | Full Text PDFSee all References][2]and `an argument for a greater emphasis on the positive role of hybridization in evolution' rather than a `comprehensive review'[3xBarton, N.H. Genet. Res. 1997; 70: 178–180CrossRefSee all References][3]. However, one of the merits of the book is that it devotes a great deal of effort towards reconciling divergent approaches to the topic. Arnold dedicates equal importance to animal and plant studies, as acknowledged by Ritchie[2xRitchie, M. Trends Ecol. Evol. 1998; 13: 123–124Abstract | Full Text PDFSee all References][2], and also puts much emphasis on the analysis of hybrid zones. In fact, when proposing a model for the birth of new evolutionary hybrid lineages he places his `new conceptual framework (the evolutionary novelty model)' within the hybrid zone framework. Our criticism of this proposal is that it may be too rigid to fit scenarios departing from the specific model of hybrid speciation proposed by Grant[4xSee all References][4], that is, recombinational speciation.In our opinion, relying exclusively on the hybrid zone framework to assess the role of hybrids in evolution is misleading. Hybrid zones usually imply relatively recent events and species with strong reproductive barriers. The tension zone model assumes that hybrid zones are maintained by a balance between selection against hybrid individuals and dispersal of parental individuals into the hybrid zone[5xBarton, N.H. and Hewitt, G.M. Annu. Rev. Ecol. Syst. 1985; 16: 113–148CrossRefSee all References][5]. As we allow other factors (basically, interactions between the hybrids and a heterogeneous environment) to play a role in the model, we move into the other interpretation of hybrid zones. The latter, often called the bounded-hybrid-superiority model, seems more accurate—at least when applied to plants[6xWang, H. et al. Evolution. 1997; 51: 95–102CrossRefSee all References][6]. Yet, dedicating much effort to studying the genetic and ecological factors involved in the maintenance of hybrid zones does not justify neglecting the ample molecular and morphological evidence available.From a pattern-oriented approach, it is true that reticulation may preclude a ready interpretation of results and that alternative explanations, such as `lineage sorting following speciation in a polymorphic ancestor', are sometimes possible. Adopting a phylogenetic approach has risks and difficulties, some of which are inherent to the subject of hybridization: (1) phylogenetic inference methods are designed to detect hierarchical structure resulting from divergence; (2) no single predictable pattern can undeniably reveal the occurrence of reticulation. However, a combination of different sources of data may be conclusive to document reticulate evolution as the references gathered in Arnold's book convincingly show. Certainly, the difficulties involved in a phylogenetic approach do not justify Ritchie's untenable statement that `most of the evidence of the significance of hybridization relies on well studied hybrid zones'[2xRitchie, M. Trends Ecol. Evol. 1998; 13: 123–124Abstract | Full Text PDFSee all References][2]nor his conclusion that he `need[s] more persuading that hybridization is a creative force in speciation before [he]can recommend this [book] as essential primer reading'.We do not claim that the pattern-versus-process dichotomy is all that matters in the way the impact of hybridization is perceived. Process-oriented studies have produced insights both at the genomic and populational levels. Our point is that this kind of study, as exemplified by the excellent research on sunflowers[7xRieseberg, L.H. et al. Science. 1996; 272: 741–745CrossRef | PubMedSee all References][7]and Galapagos finches[8xGrant, P.R. and Grant, B.R. Science. 1992; 256: 193–197CrossRef | PubMedSee all References][8], need not be immersed in the hybrid zone paradigm.The hybrid zone approach is a critical avenue to study the initial stages of the formation of hybrid lineages under certain circumstances. However, relying exclusively on studies framed by the hybrid zone theory is insufficient for a comprehensive perception of a complex phenomenon like hybridization. Furthermore, it is certainly inadequate to test or document the occurrence of past events of reticulation between divergent lineages.

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.

THE EFFECT OF HABITAT ON PARENTAL AND HYBRID FITNESS: TRANSPLANT EXPERIMENTS WITH LOUISIANA IRISES.

We performed transplant experiments with Louisiana irises to test the assumptions of three models of hybrid zone structure: the bounded hybrid superiority model, the mosaic model, and the tension zone model. Rhizomes of Iris fulva, I. hexagona, and F1 and F2 hybrids were planted at four sites in southeastern Louisiana in 1994. Wild irises grew at all four sites, but differed in genotypic composition among sites. The sites were characterized by (1) pure I. fulva plants; (2) I. fulva-like hybrids; (3) I. hexagona-like hybrids; and (4) pure I. hexagona plants. The sites differed significantly in light availability, soil moisture and chemical composition, and vegetation. Survival of transplants was high in all sites and did not differ significantly among plant classes. Iris hexagona produced significantly more leaf material than I. fulva at the I. hexagona and I. hexagona hybrid sites. The two species did not differ in leaf production at the I. fulva and I. fulva hybrid sites. Leaf production by both classes of hybrid was as great as, or significantly greater than, both parental classes in all sites. Iris hexagona rhizomes gained mass in the I. hexagona and I. hexagona hybrid sites, but lost mass in the I. fulva and I. fulva hybrid sites. Iris fulva rhizomes lost mass in all sites. There were no significant differences in rhizome growth among classes at the I. fulva site. At all other sites, F1 rhizomes grew significantly more than all other classes except for I. hexagona at the I. hexagona hybrid site. There were no significant differences among classes in the production of new ramets. Overall blooming frequencies were 30% for I. fulva, 10% for F1 s, 3% for F2 s, and 0.7% for I. hexagona. Blooming frequency did not differ among sites for I. fulva, but significantly more F1 s bloomed at the I. hexagona site than at the I. fulva site. These results are inconsistent with all three models of hybrid zone structure. They suggest that once rhizomes become established, hybrids can reproduce by clonal growth as successfully as parents in all habitats, and can outperform them in some habitats. Clonal reproduction may ensure the long-term survival of early generation hybrids and allow the establishment of introgressed populations, despite the fact that F1 hybrids are rarely produced in nature.