Genetic mapping in hybrid zones

Recent advances in genetic mapping methodologies make it feasible to localize quantitative trait loci (QTL) that contribute to adaptation and speciation. However, it has not been possible to employ these methods in many wild species because of difficulties associated with creating and propagating recombinant populations of sufficient size for QTL mapping. Natural hybrid zones contain recombinant individuals resulting from many generations of hybridization and thus offer a potential solution to these problems. For studies of speciation, hybrid zones offer the possibility of mapping QTL simultaneously with assessments of their effects on assortative mating, hybrid fitness, and interspecific gene flow. Here, we explore the problems and prospects associated with genetic map building and QTL analyses in natural hybrid zones by analyzing correlations among markers of known genomic location in four hybrid zones between the wild sunflower species Helianthus annuus and Helianthus petiolaris. Results indicate that mapping in hybrid zones presents many challenges. These include overlap in the strength of marker correlations between linked and unlinked markers, unevenness in marker frequencies along linkages, and heterogeneity in the relationship between marker distances and correlations. All make it difficult to accurately group and order markers or to estimate the distances between them. These problems can be ameliorated by sampling strategies that maximize the difference in linkage disequilibria between linked and unlinked markers and that minimize differences in frequencies among markers or QTL. In addition, studies that employ a previously determined molecular marker map for gene localization have a greater likelihood of success than those that rely on the hybrid zone data for both map construction and QTL analyses.     

Food web complexity and stability across habitat connectivity gradients

The effects of habitat connectivity on food webs have been studied both empirically and theoretically, yet the question of whether empirical results support theoretical predictions for any food web metric other than species richness has received little attention. Our synthesis brings together theory and empirical evidence for how habitat connectivity affects both food web stability and complexity. Food web stability is often predicted to be greatest at intermediate levels of connectivity, representing a compromise between the stabilizing effects of dispersal via rescue effects and prey switching, and the destabilizing effects of dispersal via regional synchronization of population dynamics. Empirical studies of food web stability generally support both this pattern and underlying mechanisms. Food chain length has been predicted to have both increasing and unimodal relationships with connectivity as a result of predators being constrained by the patch occupancy of their prey. Although both patterns have been documented empirically, the underlying mechanisms may differ from those predicted by models. In terms of other measures of food web complexity, habitat connectivity has been empirically found to generally increase link density but either reduce or have no effect on connectance, whereas a unimodal relationship is expected. In general, there is growing concordance between empirical patterns and theoretical predictions for some effects of habitat connectivity on food webs, but many predictions remain to be tested over a full connectivity gradient, and empirical metrics of complexity are rarely modeled. Closing these gaps will allow a deeper understanding of how natural and anthropogenic changes in connectivity can affect real food webs.     

The maintenance of hybrid zones across a disturbance gradient

The parapatric distribution of genetically divergent lineages in hybrid zones can be maintained by ecological differences (dispersal-independent 'ecotonal' hybrid zones), by frequency- and density-dependent interference when they intermingle and mate (dispersal-dependent 'tension' hybrid zones), or by both processes acting together. One potentially important ecological factor that has received little theoretical attention is gradients in habitat disturbance. Such gradients may be particularly important in contact zones in which the interacting lineages differ in their sexual system (e.g., self-fertile versus obligately outcrossing) because self-fertility promotes the colonization of open patches. Here we use a spatially explicit metapopulation model to examine the dynamics of a dispersal-dependent ecotonal hybrid zone across a gradient in the rate of habitat disturbance, where competing lineages differ in their sexual system. We found that self-fertility promoted the maintenance of one lineage over its outcrossing counterpart at high extinction rates, predominantly because self-fertility confers reproductive assurance. Additionally, greater seed and pollen production promoted a lineage's persistence by reducing the seed fertility of its counterpart through hybridization. Our results draw attention to the joint effects of ecological and endogenous selection in regulating the location of hybrid zones. Our study also casts new light on the maintenance of the parapatric distribution of incompatible lineages of Spanish populations of the plant Mercurialis annua. In particular, we expect the rate of movement of a contact zone in eastern Spain to increase as it moves further south, contrary to earlier predictions.     

Genetical and ecological differentiation across a hybrid zone

ABSTRACT.

• 1 There is a narrow hybrid zone between two chromosomal races of the grasshopper Podisma pedestris (L.) in the French Alps.
• 2 There are no gross differences in habitat across the zone, and a multivariate analysis of the vegetation composition detected no significant transition either.
• 3 The hatch and development rates of Podisma in hybrid populations were lower than those found in matched habitat types outside the zone.
• 4 The simplest explanation for the persistence of the parapatric distribution of the two races is that hybrid disadvantage is maintaining a tension zone between them.

     

Finding habitat patches and directional connectivity

For animal species inhabiting patchy environments, the search behavior of individuals and the distance from which they can detect suitable habitat (perceptual range) are key determinants of the functional connectivity of landscapes. We examined the movement behavior and perceptual range of adult cactus bugs ( Chelinidea vittiger ), which are habitat specialists that feed and reproduce on Opuntia cactus. Movement pathways of walking individuals released into unsuitable matrix habitat (30–3000 m from Opuntia ) were highly directional. These results supported predictions of optimal search behavior from published simulation models. A release experiment within natural patch networks indicated that the perceptual range of C. vittiger depended on size of the target patch, matrix structure, and direction of the target patch relative to prevailing winds. A strong effect of wind direction on orientation behavior (and presumed perceptual range) was evident in a release experiment using 'artificial' patches of potted Opuntia . In these two experiments, individuals released 50–100 cm from Opuntia patches were more likely to orientate toward patches located upwind than to those located crosswind or downwind. A reexamination of the pathways of individuals walking in the matrix also revealed a strong bias for movement upwind. Such upwind movement by individuals, both within and outside of patch networks, suggests that C. vittiger uses olfaction to navigate and it complicates our ability to interpret search behavior and to estimate perceptual range. Current techniques for assessing perceptual range have limitations for olfactory-based species. Furthermore, we need to broaden our view of perceptual range and of patch and landscape connectivity. Perceptual range may be anisotropic and connectivity may be directional. An organism-based approach to spatial ecology requires that we consider the dominant senses used by species when navigating around patchy landscapes.     

Genetic connectivity in scleractinian corals across the Northern Gulf of Mexico: oil/gas platforms, and relationship to the Flower Garden Banks

The 3,000 oil/gas structures currently deployed in the northern Gulf of Mexico (GOM) provide hard substratum for marine organisms in a region where such has been rare since the Holocene. The major exception to this are the Flower Garden Banks (FGB). Corals are known to have colonized oil/gas platforms around the FGB, facilitating biogeographic expansion. We ask the question, what are the patterns of genetic affinity in these coral populations. We sampled coral tissue from populations of two species occurring on oil and gas platforms: Madracis decactis (hermatype) and Tubastraea coccinea (invasive ahermatype). We sampled 28 platforms along four transects from 20 km offshore to the continental shelf edge off 1) Matagorda Island, TX; 2) Lake Sabine, TX; 3) Terrebonne Bay, LA; and 4) Mobile, AL. The entire population of M. decactis was sampled between depths of 5 m and 37 m. T. coccinea populations were sub-sampled. Genetic variation was assessed using the PCR-based Amplified Fragment Length Polymorphisms (AFLPs). Data were analyzed via AFLPOP and STRUCTURE. Genetic connectivity among M. decactis platform populations was highest near the FGB and decreased to the east. Connectivity increased again in the eastern sector, indicating isolation between the populations from different sides of the Mississippi River (Transects 3 and 4). A point-drop in genetic affinity (relatedness) at the shelf edge south of Terrebonne Bay, LA indicated a population differing from all others in the northern GOM. Genetic affinities among T. coccinea were highest in the west and decreased to the east. Very low genetic affinities off Mobile, AL indicated a dramatic difference between those populations and those west of the Mississippi River, apparently a formidable barrier to larval dispersal.     

Genetic differentiation and habitat preference of flowering-time variants within Gymnadenia conopsea

Using fast-evolving microsatellites, more slowly evolving ITS markers and performing habitat analyses, we demonstrated a drastic genetic divergence and significant habitat differentiation between early- and late-flowering variants of plants morphologically belonging to Gymnadenia conopsea ssp conopsea. The two phenological variants can either be found in separate or in mixed populations. Information from microsatellite markers and ITS sequences indicated the occurrence of an early historical split between the two flowering-time variants, a split that has been maintained until the present time even within sympatric populations. Early-flowering variants were also far more genetically diverse, had more alleles per microsatellite locus and a wider habitat amplitude than late-flowering variants. As a comparison, we included G. odoratissima in the sequencing study. We found G. odoratissima to be most closely related to the early-flowering type. This indicates a more ancient divergence event between the two flowering-time variants within G. conopsea ssp conopsea than between the two different species G. odoratissima and the early-flowering variant of G. conopsea. Possible explanations to the results arrived at and possible mechanisms maintaining the genetic separation are discussed.     

Distribution and habitat associations of billfish and swordfish larvae across mesoscale features in the Gulf of Mexico

Ichthyoplankton surveys were conducted in surface waters of the northern Gulf of Mexico (NGoM) over a three-year period (2006-2008) to determine the relative value of this region as early life habitat of sailfish (Istiophorus platypterus), blue marlin (Makaira nigricans), white marlin (Kajikia albida), and swordfish (Xiphias gladius). Sailfish were the dominant billfish collected in summer surveys, and larvae were present at 37.5% of the stations sampled. Blue marlin and white marlin larvae were present at 25.0% and 4.6% of the stations sampled, respectively, while swordfish occurred at 17.2% of the stations. Areas of peak production were detected and maximum density estimates for sailfish (22.09 larvae 1000 m(-2)) were significantly higher than the three other species: blue marlin (9.62 larvae 1000 m(-2)), white marlin (5.44 larvae 1000 m(-2)), and swordfish (4.67 larvae 1000 m(-2)). The distribution and abundance of billfish and swordfish larvae varied spatially and temporally, and several environmental variables (sea surface temperature, salinity, sea surface height, distance to the Loop Current, current velocity, water depth, and Sargassum biomass) were deemed to be influential variables in generalized additive models (GAMs). Mesoscale features in the NGoM affected the distribution and abundance of billfish and swordfish larvae, with densities typically higher in frontal zones or areas proximal to the Loop Current. Habitat suitability of all four species was strongly linked to physicochemical attributes of the water masses they inhabited, and observed abundance was higher in slope waters with lower sea surface temperature and higher salinity. Our results highlight the value of the NGoM as early life habitat of billfishes and swordfish, and represent valuable baseline data for evaluating anthropogenic effects (i.e., Deepwater Horizon oil spill) on the Atlantic billfish and swordfish populations.     

Genetic differentiation of the Chlorospingus ophthalmicus complex in Mexico and Central America

Genetic variation in the common bush-tanager Chlorospingus ophthalmicus complex in Mesoamerica was studied. An 800-bp mitochondrial DNA fragment, including a portion of COII, full tRNA-Lys, ATP8, and partial ATP6, was sequenced and analyzed for 96 individuals of the species and related taxa, resulting in a detailed framework of genetic differentiation in the northern half of the distribution of the complex. Phylogenetic analyses based on maximum parsimony, maximum likelihood, and Bayesian optimality criteria found deep divergence between South American and Mesoamerican isolates, and seven differentiated populations corresponding to clear geographic breaks across the highlands of Mesoamerica. These distinct populations coincide with geographic structure found in previous analyses of morphological and molecular data for Chlorospingus ophthalmicus , as well as in previous phylogeographic studies of other taxa in the region. Species status for these populations is discussed under the evolutionary and biological species concepts.     

Genotypic proportions in hybrid zones

We study a hybrid zone between two populations of a diploid organism. The populations differ at one locus. Homozygotes have equal fitnesses and the heterozygote fitness is reduced by + ( is the birth rate deviation and is the death rate deviation). The populations extend along a one dimensional continuous habitat, and migration occurs by diffusion of individuals. The model is formulated as a set of simple continuous time demographic models without age structure for the three genotypes, and the system is transformed into three new variables, the total population size N, the gene frequency p, and the deviation from Hardy-Weinberg proportions F. The gene frequency in a steady state cline always follows a hyperbolic tangent closely. Analysis of the asymptotic behavior of the cline far from the hybrid zone suggests a qualitative prediction of the shape of N, p and F over the zone. For weak selection the shape is determined by a central steepness of , as observed by Bazykin in 1969, where D is the diffusion coefficient. For strong selection the cline is less steep than the Bazykin cline, and the form is dominated by the migration process. The steepness at the center of the cline is close to where b is the birth rate of homozygotes.Supported in part by grant 11-9639-1 from the Danish Natural Science Research Council and grant 5.18.2023 from the Danish Biotechnological Research and Development Programme     

Morphological differentiation in introduced pumpkinseed Lepomis gibbosus (L.) occupying different habitat zones in Portuguese reservoirs

Introduced pumpkinseed Lepomis gibbosus sampled from four habitat zones (fluvial pelagic, fluvial littoral, lacustrine pelagic and lacustrine littoral) in three Portuguese reservoirs were used to test the hypotheses that habitats with the least similar characteristics will show the most differentiation, and that morphological differences will relate to functional adaptations to flow and trophic habitats. Results from discriminant function analysis and ANCOVA showed that there were significant differences in external morphology in pumpkinseed captured from the four habitat zones in all three reservoirs. Littoral and pelagic differentiation was stronger than fluvial and lacustrine differentiation in all of the reservoirs, and the most significant variable that differentiated pumpkinseed from the littoral and pelagic habitats was body depth. The illustration of external morphological differentiation in pumpkinseed along both habitat dimensions demonstrates the high degree of morphological plasticity of this introduced species.     

Three-trophic-level interactions in cattail hybrid zones

Plant-herbivore and herbivore-parasitoid wasp interactions were examined in three hybrid zones of the cattails Typha latifolia and T. angustifolia in south-eastern Michigan over a 2-year period. Patterns of resource use by two lepidopteran species of seed-eating herbivores were studied and herbivore densities sustained by the hybrid cattail T. x glauca and its parental species were determined. Densities of the commoner seed-eating lepidopteran, Lymnaecia phragmitella, were found to be highest in seed heads of both parental species and lowest in hybrid seed heads in zones of hybridization, thus supporting the hybrid resistance hypothesis of Fritz et al. (1994). Densities of the second herbivore, Dicymolomia julianalis, on the hybrid were lower than on the parental T. latifolia, but did not differ from the mean of the combined herbivore densities of the two parental species. D. julianalis in cattails appears to fit the additive hypothesis (Fritz et al. 1994). Parasitism of L. phragmitella by four species of parasitoid wasp, Itoplectis conquisitor (Ichneumonidae), Scambus hispae (Ichneumonidae), Macrocentrus delicatus (Braconidae), and Temelucha gracilipes (Ichneumonidae), was studied to determine if rates of parasitization by these natural enemics on the different cattails were similar to the patterns of plant exploitation exhibited by their hosts. Parasitism rates of L. phragmitella were significantly higher in larvae reared on hybrid cattails. This study reveals two different patterns of hybrid utilization by two very similar herbivores. Opposite patterns of plant utilization were found for the parasitoids compared with their host (L. phragmitella) in cattail hybrid zones.     

Transgressive character expression in hybrid zones between the native invasives Tithonia tubaeformis and Tithonia rotundifolia (Asteraceae) in Mexico

Natural hybridization frequently promotes gene introgression among closely related species in sympatric populations, producing complex patterns of morphological variation. Therefore, a detailed understanding of the dynamics of interspecific gene flow and its morphological patterns is of widespread interest. We tested if introgressive hybridization promotes an increase in transgressive characters in comparison with the parental species. A sunflower species complex occurring in Mexico formed by two native invasive species, Tithonia tubaeformis and Tithonia rotundifolia, was analyzed using 46 morphological characters (leaf, flower and fruit) in five hybrid zones (N = 150 individuals) and two pure sites for each parental species (N = 80 individuals). In general, T. tubaeformis differed significantly from T. rotundifolia in all the examined characters, except six foliar and one inflorescence character. Morphological characters support the hypothesis of hybridization in this complex, even though both species remain morphological distinct in mixed stands. Individual hybrids appear to be a mosaic of parent-like (24.8 % of traits), intermediate (26.1 %) and transgressive (37.8 %) phenotypes (the remaining 11.3 % of the traits did not differ significantly from both parental species). Our results suggest that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence indicates a unidirectional pattern of gene flow toward T. rotundifolia.     

Genetic mapping in maize with hybrid progeny across testers and generations: plant height and flowering

DNA markers were used to identify quantitative trait loci (QTLs) for plant height, ear height, and three flowering traits in hybrid progeny of two generations (F_2:3, F_6:8) of lines from a Mo17×H99 maize population. For both generations, testcross (TC) progeny were developed by crossing the lines to three inbred testers (B91, A632, B73). The hybrid progeny from the two generations were evaluated at the same locations but in different years as per an early generation testing program. QTLs were identified within each TC population and for mean testcross (MTC) performance. Overall, more QTLs were detected in the F_6:8 than the F_2:3 generation. Totalled over all five traits, 41 (B91) to 69% (B73) of the QTLs for tester effects and 67% of the QTLs for MTC detected in the F_2:3 generation were verified in the F_6:8 generation. Although differences in relative rank of the QTL effects across generations were observed, especially for the flowering traits, parental contributions were nearly always consistent. Several (8–11) QTLs were identified with effects for all three tester populations and for all traits except the anthesis-silk interval, which had only two such regions. Over all five traits, previous evaluations in this population identified 26 QTLs with consistent effects for two (F_2:3, F_6:8) inbred-progeny evaluations, and 20 (77%) were also associated with MTC in at least one of the generations evaluated herein. In all instances of common inbred and TC QTLs, parental contributions were the same. Received: 26 November 1999 / Accepted: 18 April 2000     

Global patterns of fragmentation and connectivity of mammalian carnivore habitat

Although mammalian carnivores are vulnerable to habitat fragmentation and require landscape connectivity, their global patterns of fragmentation and connectivity have not been examined. We use recently developed high-resolution habitat suitability models to conduct comparative analyses and to identify global hotspots of fragmentation and connectivity for the world's terrestrial carnivores. Species with less fragmentation (i.e. more interior high-quality habitat) had larger geographical ranges, a greater proportion of habitat within their range, greater habitat connectivity and a lower risk of extinction. Species with higher connectivity (i.e. less habitat isolation) also had a greater proportion of high-quality habitat, but had smaller, not larger, ranges, probably reflecting shorter distances between habitat patches for species with restricted distributions; such species were also more threatened, as would be expected given the negative relationship between range size and extinction risk. Fragmentation and connectivity did not differ among Carnivora families, and body mass was associated with connectivity but not fragmentation. On average, only 54.3 per cent of a species' geographical range comprised high-quality habitat, and more troubling, only 5.2 per cent of the range comprised such habitat within protected areas. Identification of global hotspots of fragmentation and connectivity will help guide strategic priorities for carnivore conservation.     

Clustering of contact zones, hybrid zones, and phylogeographic breaks in North America

A recent test for the existence of suture zones in North America, based on hybrid zones studied since 1970, found support for only two of the 13 suture zones identified by Remington in 1968 (Swenson and Howard 2004). One limitation of that recent study was the relatively small number of hybrid zones available for mapping. In this study, we search for evidence of clustering of contact zones between closely related taxa using data not only from hybrid zones but from species range maps of trees, birds, and mammals and from the position of phylogeographic breaks within species. Digital geographic range maps and a geographic information system approach allowed for accurate and rapid mapping of distributional data. Areas of contact between closely related species and phylogeographic breaks within species clustered into areas characterized by common physiographic features or predicted by previously hypothesized glacial refugia. The results underscore the general importance of geographic barriers to dispersal (mountain chains) and climate change (periods of cooling alternating with periods of warming, which lead to the contraction and expansion of species ranges) in species evolution.     

Levels of herbivory and parasitism in host hybrid zones

Hybrid zones often are intermediate in their habitat, and have individuals that are intermediate genetically to those of either parental species. Recently, a number of studies has addressed the distribution of herbivores and parasites in hybrid zones of their hosts. Mechanisms that underlie these distributions include behavioral, environmental and genotypic differences between hybrids and their parental species.     

Metacommunity diversity depends on connectivity and patch arrangement in heterogeneous habitat networks

Connectivity is critical to the maintenance of biodiversity in fragmented landscapes, but its effects differ depending on the arrangement of linkages within a habitat network. Additionally, heterogeneity in habitat quality within the habitat network can alter patterns of diversity at local and regional scales in the metacommunity. Using a controlled experiment we examined the interactive effects of habitat connectivity, network form (linear vs square), and habitat patch quality on a moss‐inhabiting microarthropod community. We fragmented moss habitat while controlling for habitat loss, and altered habitat patch quality by regulating moisture conditions in landscapes differing in patch arrangement. Habitat patch quality had a significant effect on patterns of species richness, extinction, abundance and biomass. The effects of network form on diversity were strongest in heterogeneous landscapes. Gamma and beta diversity were greatest in continuous and linear landscapes. However, linear habitat networks showed marked patch specific edge effects that were detrimental to diversity under heterogeneous conditions. We provide direct evidence that habitat network structure impacts species community properties through mass effects, that are most evident when heterogeneity in habitat patch quality is present within the network. We conclude that habitat quality at the individual patch level and the distribution of high‐quality habitat within the network are important factors affecting biodiversity in metacommunities.