ASSORTATIVE MATING AND NATURAL SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic variation among flowering plants and their progeny were examined to assess the levels of assortative mating and selection in a hybrid population of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of parental genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking intermediate genotypes. Early in the season only I. fulva genotypes produced flowers, but as flowering in these plants decreased, the hybrid genotypes and I. brevicaulis genotypes began flowering, resulting in a 24-d period of coincidental flowering. The genotypic distribution of seeds produced during the period of flowering overlap contained a high frequency of intermediate genotypes that were not present in the adult generation. The degree of effective assortative mating was examined by comparing the observed progeny genotypic distributions with expected distributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowers on the day the recipient flower was receptive. The observed genotypic distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribution. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate genotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were aborted, indicating that intermediate genotypes have lower viability. On the basis of the observed progeny genotypes and genetic disequilibria estimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry is most likely due to pollen-style interactions that reduce the fertilization ability of genetically dissimilar pollen, or preferential abortion of genetically intermediate zygotes by I. fulva-like genotypes. The lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genotypes.     

HYBRIDIZATION IN LEOPARD FROGS (RANA PIPIENS COMPLEX): LARVAL FITNESS COMPONENTS IN SINGLE-GENOTYPE POPULATIONS AND MIXTURES

Recognizing the predominant mode of selection in hybrid systems is important in predicting the evolutionary fate of recombinant genotypes. Natural selection is endogenous if hybrid genotypes are at a disadvantage relative to parental species independent of environment. Alternatively, relative fitness can vary in response to environmental variation (exogenous selection), and hybrid genotypes can possess fitness values equal to or greater than that of parental species. I investigated the nature of natural selection in a leopard frog hybrid system by rearing larvae of hybrid and parental genotypes between Rana blairi and R. sphenocephala in 1000-L outdoor experimental ponds. Three hybrid (F₁, backcrossj [B₁], backcross₂ [B₂]) and two parental (R. blairi [BB] and R. sphenocephala [SS]) larval genotypes were produced by artificial fertilzations using adult frogs from a natural population in central Missouri. Resultant larvae were reared in single-genotype populations and two-way mixtures at equal total numbers from hatching to metamorphosis. In single-genotype ponds, F₁ hybrid larvae had highest survival and BB were largest at metamorphosis. When F₁ and SS larvae were mixed together, F₁ hybrids had reduced survival and both F₁ and SS larvae metamorphosed at larger body masses than when reared separately. When mixed, both B₁ and SS larvae had shorter larval period lengths than when reared alone. Higher proportion of B₁ metamorphs were produced when larvae were mixed with either parental species than when reared alone. Larval fitness components as measured by survival, body mass at metamorphosis, proportion of survivors metamorphosing, and larval period length for B₂ hybrid and BB larvae were similar in single-genotype populations and mixtures. Comparison of composite fitness component estimates indicated hybrid genotypes possess equivalent or higher larval fitness relative to both parental species for the life-history fitness components measured. Despite reduced survival of F₁ hybrids in mixtures, backcross-generation hybrid genotypes demonstrated high levels of larval growth, survival, and metamorphosis in mixtures with parental species. Consequently, this study suggests natural hybridization and subsequent backcrossing between R. blairi and R. sphenocephala can produce novel and relatively fit hybrid genotypes capable of successful existence with parental species larvae. Thus, the evolutionary fate of hybrid and parental genotypes in this system may be influenced by exogenous selection mediated by genotypic composition of larval assemblages.     

Pollinator behaviour and the evolution of Louisiana iris hybrid zones

Pollinator preference may influence the origin and dynamics of plant hybrid zones. Natural hybrid populations between the red‐flowered Iris fulva and the blue‐flowered Iris brevicaulis are found in southern Louisiana. The genetic structure of these populations reflects a lack of intermediate genotypes. We observed pollinator behaviour in an experimental array with five plants each of I. fulva, I. brevicaulis, their F₁, and the first backcross generation in each direction, to obtain data on flower type preferences and transitions between flower types. The most abundant visitors were Ruby‐throated Hummingbirds (Archilochus colubris) and workers of the bumblebee Bombus pennsylvanicus. Hummingbirds visited I. fulva twice as often as I. brevicaulis and visited hybrids at intermediate frequencies. Bumblebee workers preferred the purple‐flowered F₁s and visited plants of I. fulva and the backcross to I. fulva more often than I. brevicaulis and its backcross. Overall, F₁ flowers were visited most frequently. Both hummingbirds and bumblebees visited nearest neighbours in almost 80% of the interplant movements. This meant that a majority of movements were between different flower types, rather than between plants of the same type. Findings from the present study suggest that pollinator preference is not a major causal factor for the lack of intermediate genotypes in natural iris hybrid populations. Instead, pollinator behaviour in our array promoted mixed mating between flower types belonging to different pollination syndromes. However, owing to predominant nearest‐neighbour visitation, the spatial distribution of parental and hybrid genotypes (in concert with pollinator behaviour) will have a strong influence on mating patterns and thus the genotypic structure and evolution of Louisiana iris hybrid zones.     

ECOLOGICAL AND GENETIC ASSOCIATIONS IN AN IRIS HYBRID ZONE

Chloroplast DNA (cpDNA) markers and 12 nuclear (random amplified polymorphic DNA, or RAPD) markers were used to examine the distribution of genetic variation among individuals and the genetic and ecological associations in a hybrid iris population. Plants in the population occurred at various distances from the edge of a bayou in a relatively undisturbed mixed hardwood forest and in an adjacent pasture dominated by herbaceous perennials with interspersed oak and cypress trees. The majority of plants sampled possessed combinations of markers from the different Iris species. Genetic markers diagnostic for Iris fulva and I. brevicaulis occurred at high frequencies, whereas markers diagnostic for I. hexagona were infrequent. For the majority of the nuclear markers, significant levels of cytonuclear disequilibria existed because of intraspecific associations among the markers in both the pasture and the forest. The distribution of nuclear markers among individuals was bimodal; intermediate genotypes were absent and the majority of RAPD markers were associated with their intraspecific cpDNA haplotypes. Strong intraspecific associations existed among RAPD markers in the forest, but associations tended to be weaker in the pasture area. Ecological correlations were detected for all but one of the I. fulva and I. brevicaulis RAPD markers. The ecological associations of hybrids similar to I. brevicaulis resembled associations of I. brevicaulis parental genotypes, suggesting that these hybrid genotypes may be relatively fit in the same habitats. The hybrids similar to I. fulva, however, were distributed in habitats that were unique relative to the parental species. The patterns of genetic and environmental associations along with other available data suggest that (1) only advanced generation hybrids were present in the population; (2) formation of F₁ hybrids among Louisiana irises is rare, leading to sporadic formation of hybrid populations; and (3) selection and assortative mating have contributed to the formation of hybrid genotypes that tend to be similar to parental genotypes. The patterns of ecological and genetic associations detected in this population suggest that assortative mating and environmental and viability selection are important in the structuring and maintenance of this hybrid zone.     

Genomic collinearity and the genetic architecture of floral differences between the homoploid hybrid species Iris nelsonii and one of its progenitors,Iris hexagona

Hybrid speciation represents a relatively rapid form of diversification. Early models of homoploid hybrid speciation suggested that reproductive isolation between the hybrid species and progenitors primarily resulted from karyotypic differences between the species. However, genic incompatibilities and ecological divergence may also be responsible for isolation. Iris nelsonii is an example of a homoploid hybrid species that is likely isolated from its progenitors primarily by strong prezygotic isolation, including habitat divergence, floral isolation and post-pollination prezygotic barriers. Here, we used linkage mapping and quantitative trait locus (QTL) mapping approaches to investigate genomic collinearity and the genetic architecture of floral differences between I. nelsonii and one of its progenitor species I. hexagona. The linkage map produced from this cross is highly collinear with another linkage map produced between I. fulva and I. brevicaulis (the two other species shown to have contributed to the genomic makeup of I. nelsonii), suggesting that karyotypic differences do not contribute substantially to isolation in this homoploid hybrid species. Similar to other studies of the genetic architecture of floral characteristics, at least one QTL was found that explained >20% variance in each color trait, while minor QTLs were detected for each morphological trait. These QTLs will serve as hypotheses for regions under selection by pollinators.     

Genetic structure in aquatic bladderworts: clonal propagation and hybrid perpetuation

• Background and Aims The free-floating aquatic bladderwort Utricularia australis f. australis is a sterile F₁ hybrid of U. australis f. tenuicaulis and U. macrorhiza. However, co-existence of the hybrids and parental species has not been observed. In the present study, the following questions are addressed. (a) Does the capacity of the two parental species to reproduce sexually contribute to higher genotypic diversity than that of sterile F₁ hybrid? (b) Are there any populations where two parental species and their hybrid co-exist? (c) If not, where and how do hybrids originate?• Methods The presence and absence of Utricularia was thoroughly investigated in two regions in Japan. An amplified fragment length polymorphism (AFLP) analysis was conducted for 397 individuals collected from all populations (33 in total) where Utricularia was observed.• Key Results The mean number of genotypes per population (G) and genotypic diversity (D) were extremely low irrespective of the capacity to reproduce sexually: G was 1·1–1·2 and D was 0·02–0·04. The hybrid rarely co-existed with either parental species, and the co-existence of two parental species was not observed. Several AFLP bands observed in the hybrid are absent in both parental genotypes, and parent and hybrid genotypes in the same region do not show greater genetic similarity than those in distant regions.• Conclusions The capacity to reproduce sexually in parental species plays no role in increasing genotypic diversity within populations. The observed genotypes of the hybrid could not have originated from hybridization between the extant parental genotypes within the study regions. Considering the distribution ranges of three investigated taxa, it is clear that the hybrid originated in the past, and hybrid populations have been maintained exclusively by clonal propagation, which may be ensured by both hybrid vigor and long-distance dispersal of clonal offspring.     

PATTERNS OF MATING,GENERATION OF DIVERSITY,AND FITNESS OF OFFSPRING IN A GEUM HYBRID SWARM

To understand the evolutionary consequences of hybridization between the outcrossing plant Geum rivale (Rosaceae) and the selfer Geum urbanum, we tested the predictions of two simple models that assume either (A) low or (B) high pollen fitness in hybrids. Model A predicts only four genotypic classes (G. rivale, G. rivale backcross [BC_R], F1, and Geum urbanum) and asymmetric introgression from inbreeding to outbreeding species. Model B predicts additional genotypic classes and potential generation of novel inbreeding lines in the hybrid swarm. Amplified fragment length polymorphism (AFLP) analysis of adults revealed only the four genotypes predicted by model A. However, microsatellite analysis of parent–progeny arrays demonstrated production of selfed offspring by F1 and BC_R maternal parents and contribution of these genotypes to outcross pollen pools, as predicted by model B. Moreover, AFLP and morphological analysis showed that the offspring generation comprised genotypes and phenotypes covering the entire spectrum of variation between the two parental species, in line with model B. A common garden experiment indicated no systematic reduction in fitness of offspring derived from hybrid parents. The genetic structure of the adults in the Geum hybrid swarm cannot be explained by restricted mating patterns but may result from ecological selection acting on a diverse offspring population.     

Recombinant hybrids retain heterozygosity at many loci: new insights into the genomics of reproductive isolation in Populus

The maintenance of species barriers in the face of gene flow is often thought to result from strong selection against intermediate genotypes, thereby preserving genetic differentiation. Most speciation genomic studies thus aim to identify exceptionally divergent loci between populations, but divergence will be affected by many processes other than reproductive isolation (RI) and speciation. Through genomic studies of recombinant hybrids sampled in the wild, genetic variation associated with RI can be observed in situ, because selection against incompatible genotypes will leave detectable patterns of variation in the hybrid genomes. To better understand the mechanisms directly involved in RI, we investigated three natural ‘replicate’ hybrid zones between two divergent Populus species via locus‐specific patterns of ancestry across recombinant hybrid genomes. As expected, genomic patterns in hybrids and their parental species were consistent with the presence of underdominant selection at several genomic regions. Surprisingly, many loci displayed greatly increased between‐species heterozygosity in recombinant hybrids despite striking genetic differentiation between the parental genomes, the opposite of what would be expected with selection against intermediate genotypes. Only a limited, reproducible set of genotypic combinations was present in hybrid genomes across localities. In the absence of clearly delimited ‘hybrid habitats’, our results suggest that complex epistatic interactions within genomes play an important role in advanced stages of RI between these ecologically divergent forest trees. This calls for more genomic studies that test for unusual patterns of genomic ancestry in hybridizing species.     

GENETIC INTERACTIONS AND NATURAL SELECTION IN LOUISIANA IRIS HYBRIDS

Hybridization between divergent lineages has long been assumed to give rise to unfavorable interactions between the parental genomes. These deleterious genetic interactions are further assumed to result in the production of hybrid offspring with decreased levels of viability and/or fertility. To test this assumption, we investigated the role of both nuclear and cytonuclear epistatic interactions in determining the frequencies of F₂ genotypes produced in crosses between two species of Louisiana iris, Iris fulva and I. brevicaulis. Overall, these crosses revealed a significant deficit of intermediate hybrid genotypes accompanied by an excess of parental-like genotypes, suggesting that genetic interactions may promote postmating reproductive isolation between these species. However, analyses of single and multilocus segregation patterns revealed a variety of negative and positive interactions between the genomes of the parental taxa at the nuclear and cytonuclear levels. Taken together, these results indicate that the traditional view that interactions between divergent genomes are always deleterious is an oversimplification. Rather, it seems likely that crosses between divergent lineages can lead to the production of both fit and unfit hybrid genotypes.     

Determining population structure and hybridization for two iris species

Identifying processes that promote or limit gene flow can help define the ecological and evolutionary history of a species. Furthermore, defining those factors that make up “species boundaries” can provide a definition of the independent evolutionary trajectories of related taxa. For many species, the historic processes that account for their distribution of genetic variation remain unresolved. In this study, we examine the geographic distribution of genetic diversity for two species of Louisiana Irises, Iris brevicaulis and Iris fulva. Specifically, we asked how populations are structured and if population structure coincides with potential barriers to gene flow. We also asked whether there is evidence of hybridization between these two species outside Louisiana hybrid zones. We used a genotyping‐by‐sequencing approach and sampled a large number of single nucleotide polymorphisms across these species' genomes. Two different population assignment methods were used to resolve population structure in I. brevicaulis; however, there was considerably less population structure in I. fulva. We used a species tree approach to infer phylogenies both within and between populations and species. For I. brevicaulis, the geography of the collection locality was reflected in the phylogeny. The I. fulva phylogeny reflected much less structure than detected for I. brevicaulis. Lastly, combining both species into a phylogenetic analysis resolved two of six populations of I. brevicaulis that shared alleles with I. fulva. Taken together, our results suggest major differences in the level and pattern of connectivity among populations of these two Louisiana Iris species.     

Environment-dependent performance and fitness of Iris brevicaulis, I. fulva (Iridaceae), and hybrids

We tested the relative fitness of two Louisiana Iris species (Iris brevicaulis and I. fulva) and their first-generation backcross hybrids in three experimental watering treatments: dry, field capacity, and flooded. Leaf area expansion rate, gas exchange (A(max), g(s), c(i)), and biomass at final harvest were measured for each species and hybrid class in all three environmental treatments. Fitness (based on total biomass) of the four genotypic classes differed significantly with environment. All genotypic classes performed most poorly in the dry treatment. The fitness ranking of genotypic class also changed across environments (significant genotypic class by treatment interaction) with hybrid genotype fitness shifting relative to parental genotypes. Integrating over all treatments, backcrosses to I. fulva showed the lowest fitness, whereas backcrosses to I. brevicaulis outperformed I. fulva. The differences in fitness were apparently achieved by a combination of differences in photosynthesis and allocation. In this system, hybrids are not necessarily less fit than their parents, and the relationship between hybrid and parental fitness is influenced by environmental conditions, lending support to the Hybrid Novelty model of hybrid zone evolution.     

Intimately linked or hardly speaking? The relationship between genotype and environmental gradients in a Louisiana Iris hybrid population

Several models of hybrid zone evolution predict the same spatial patterns of genotypic distribution whether or not structuring is due to environment-dependent or -independent selection. In this study, we tested for evidence of environment-dependent selection in an Iris fulva x Iris brevicaulis hybrid population by examining the distribution of genotypes in relation to environmental gradients. We selected 201 Louisiana Iris plants from within a known hybrid population (80 m x 80 m) and placed them in four different genotypic classes (I. fulva, I. fulva-like hybrid, I. brevicaulis-like hybrid and I. brevicaulis) based on seven species-specific random amplified polymorphic DNA (RAPD) markers and two chloroplast DNA haplotypes. Environmental variables were then measured. These variables included percentage cover by tree canopy, elevation from the high water mark, soil pH and percentage soil organic matter. Each variable was sampled for all 201 plants. Canonical discriminant analysis (CDA) was used to infer the environmental factors most strongly associated with the different genotypic groups. Slight differences in elevation (-0.5 m to +0.4 m) were important for distinguishing habitat distributions described by CDA, even though there were no statistical differences between mean elevations alone. I. brevicaulis occurred in a broad range of habitats, while I. fulva had a narrower distribution. Of all the possible combinations, I. fulva-like hybrids and I. brevicaulis-like hybrids occurred in the most distinct habitat types relative to one another. Each hybrid class was not significantly different from its closest parent with regard to habitat occupied, but was statistically unique from its more distant parental species. Within the hybrid genotypes, most, but not all, RAPD loci were individually correlated with environmental variables. This study suggests that, at a very fine spatial scale, environment-dependent selection contributed to the genetic structuring of this hybrid zone.     

Introgressive hybridization and morphological transgression in the contact zone between two Mediterranean Solea species

Hybrid zones provide natural experiments where new combinations of genotypes and phenotypes are produced. Studying the reshuffling of genotypes and remodeling of phenotypes in these zones is of particular interest to document the building of reproductive isolation and the possible emergence of transgressive phenotypes that can be a source of evolutionary novelties. Here, we specifically investigate the morphological variation patterns associated with introgressive hybridization between two species of sole, Solea senegalensis and Solea aegyptiaca. The relationship between genetic composition at nuclear loci and individual body shape variation was studied in four populations sampled across the hybrid zone located in northern Tunisia. A strong correlation between genetic and phenotypic variation was observed among all individuals but not within populations, including the two most admixed ones. Morphological convergence between parental species was observed close to the contact zone. Nevertheless, the samples taken closest to the hybrid zone also displayed deviant segregation of genotypes and phenotypes, as well as transgressive phenotypes. In these samples, deviant body shape variation could be partly attributed to a reduced condition index, and the distorted genetic composition was most likely due to missing allelic combinations. These results were interpreted as an indication of hybrid breakdown, which likely contributes to postmating reproductive isolation between the two species.     

Patterns of color and nectar variation across an Ipomopsis (Polemoniaceae) hybrid zone

Hybridization may uncouple adaptive trait combinations that are present in parental species. I studied variation in flower color and reward quality across a hybrid zone of Ipomopsis aggregata and I. tenuituba. Individuals from hybrid populations showed considerable variation in flower color using corolla reflectance measurements. Flower spectra of such individuals were either intermediate or else resembled those flowers from the parental species. Ipomopsis aggregata populations had consistently higher nectar production rates and higher nectar standing crops than either I. tenuituba or hybrids. Ipomopsis aggregata flowers also produced more dilute nectar than those of hybrids and I. tenuituba, but the actual concentration values were quite variable among populations of the same type. Overall, the nectar quality of hybrid flowers most resembled that of I. tenuituba flowers. Based on the observed interpopulation patterns of color-reward associations in this hybrid zone, pollinators should be able to discriminate against I. tenuituba and hybrid populations and against most individuals within hybrid populations. However, they may visit those hybrids that resemble the most rewarding flower type (I. aggregata). The results emphasize the need for studies that address how hybridization affects subsequent plant fitness and the evolutionary dynamics of the species involved.     

ECOLOGICAL SEGREGATION IN A SMALL MAMMAL HYBRID ZONE: HABITAT‐SPECIFIC MATING OPPORTUNITIES AND SELECTION AGAINST HYBRIDS RESTRICT GENE FLOW ON A FINE SPATIAL SCALE

The degree to which closely related species interbreed is determined by a complex interaction of ecological, behavioral, and genetic factors. We examine the degree of interbreeding between two woodrat species, Neotoma bryanti and N. lepida, at a sharp ecological transition. We identify the ecological association of each genotypic class, assess the opportunity for mating between these groups, and test whether they have similar patterns of year‐to‐year persistence on our study site. We find that 13% of individuals have a hybrid signature but that the two parental populations and backcrosses are highly segregated by habitat type and use. Also, we find that adult hybrids are comparable to parental types in terms of year‐to‐year persistence on our site but that, among juveniles, significantly fewer hybrids reach adulthood on site compared to their purebred counterparts. Our analyses show that this hybrid zone is maintained by occasional nonassortative mating coupled with hybrid fertility, but that these factors are balanced by lower apparent survival of juvenile hybrids and habitat‐based preference or selection that limits heterospecific mating while promoting backcrossing to habitat‐specific genotypes. This system presents a novel example of the role that sharp resource gradients play in reproductive isolation and the potential for genetic introgression.     

HYBRID DYSFUNCTION IN FIRE-BELLIED TOADS (BOMBINA)

Reproductive isolation between two taxa may be due to endogenous selection, which is generated by incompatibilities between the respective genomes, to exogenous selection, which is generated by differential adaptations to alternative environments, or to both. The continuing debate over the relative importance of either mode of selection has highlighted the need for unambiguous data on the fitness of hybrid genotypes. The hybrid zone between the fire-bellied toad (Bombina bombina) and the yellow-bellied toad (B. variegata) in central Europe involves adaptation to different environments, but evidence of hybrid dysfunction is equivocal. In this study, we followed the development under laboratory conditions of naturally laid eggs collected from a transect across the Bombina hybrid zone in Croatia. Fitness was significantly reduced in hybrid populations: Egg batches from the center of the hybrid zone showed significantly higher embryonic and larval mortality and higher frequencies of morphological abnormalities relative to either parental type. Overall mortality from day of egg collection to three weeks after hatching reached 20% in central hybrid populations, compared to 2% in pure populations. There was no significant difference in fitness between two parental types. Within hybrid populations, there was considerable variation in fitness, with some genotypes showing no evidence of reduced viability. We discuss the implications of these findings for our understanding of barriers to gene flow between species.     

Population genetic analysis of Elliottiaracemosa (Ericaceae), a rare Georgia shrub

Genetic and genotypic diversity found within populations of threatened plant species can have important implications for their conservation and management. In this study we describe genetic and genotypic diversity found within 10 populations of the endemic shrub Elliottiaracemosa (Ericaceae), the Georgia plume. E. racemosa is a threatened species known from fewer than 50 locations, all within the state of Georgia, USA. Seedset is limited to nonexistent in some E. racemosa populations and sexual recruitment has not been documented. However, the species is known to spread vegetatively via root-sprouts. Twenty-one allozyme loci were resolved for E. racemosa, nine of which were polymorphic. Compared with other woody taxa, E. racemosa has low genetic (i.e. allelic) diversity within populations (H_ep = 0.063) and at the species level (H_es = 0.091). Most of the genetic variation (82%) was found within populations, and genetic identities between populations were high (mean I = 0.96). However, genotypic diversity (i.e. the number of multilocus genotypes) differed markedly among populations. Two of the 10 populations consisted almost entirely of single multilocus genotypes, whereas more than 20 multilocus genotypes (in samples of 48 stems) were detected at three sites. Sites in which few multilocus genotypes were detected have low seedset, suggesting that the lack of clonal diversity limits reproduction in some populations of this reportedly self-incompatible species.     

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

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

ELECTROPHORETIC CONFIRMATION OF INTERSPECIFIC HYBRIDIZATION IN AESCULUS (HIPPOCASTANACEAE) AND THE GENETIC STRUCTURE OF A BROAD HYBRID ZONE

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the southeastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F₁; and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F₁ and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.     

SHADE TOLERANCE AND ITS EFFECT ON THE SEGREGATION OF TWO SPECIES OF LOUISIANA IRIS AND THEIR HYBRIDS

Iris fulva Ker. Gawler and Iris hexagona Walter have overlapping geographic ranges in Louisiana. In areas of overlap hybrids are fairly common. Iris hexagona occupies the borders of freshwater marshes of southern Louisiana while I. fulva can be found farther north along edges of natural levees, canals and swamps. Where the natural levee penetrates the marsh, natural hybridization can occur between I. hexagona and I. fulva. It has been suggested that one principal explanation for the segregation of the two species is that I. fulva grows best in semishade and I. hexagona grows best in full sun. A greenhouse study was conducted using rhizomes collected from the field to test this hypothesis and determine the relative shade tolerance of two natural hybrid types. Iris fulva, I. hexagona, and the two hybrid taxa were grown under 0% (control), 50% (medium shade), and 80% (high shade) reduction of sunlight for 6 months and then harvested. Iris fulva was found to be more tolerant of shading than I. hexagona and the two hybrids. Further, I. fulva was found to grow as well in control as in medium shade. Both hybrid taxa were more shade tolerant than I. hexagona. Iris hexagona was greatly affected by all levels of shade. In general, the results suggest that these hybrids are intermediate to the parental taxa in terms of shade tolerance.