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PubMed2001

The evolutionary history of brown trout (Salmo trutta L.) inferred from phylogeographic, nested clade, and mismatch analyses of mitochondrial DNA variation.

Phylogeographic, nested clade, and mismatch analyses of mitochondrial DNA (mtDNA) variation were used to infer the temporal dynamics of distributional and demographic history of brown trout (Salmo trutta). Both new and previously published data were analyzed for 1,794 trout from 174 populations. This combined analysis improved our knowledge of the complex evolutionary history of brown trout throughout its native Eurasian and North African range of distribution in many ways. It confirmed the existence of five major evolutionary lineages that evolved in geographic isolation during the Pleistocene and have remained largely allopatric since then. These should be recognized as the basic evolutionarily significant units within brown trout. Finer phylogeographic structuring was also resolved within major lineages. Contrasting temporal juxtaposition of different evolutionary factors and timing of major demographic expansions were observed among lineages. These unique evolutionary histories have been shaped both by the differential latitudinal impact of glaciations on habitat loss and potential for dispersal, as well as climatic impacts and landscape heterogeneity that translated in a longitudinal pattern of genetic diversity and population structuring at more southern latitudes. This study also provided evidence for the role of biological factors in addition to that of physical isolation in limiting introgressive hybridization among major trout lineages.

Animals↗

PubMed2002

Molecular evidence for the origin of workerless social parasites in the ant genus Pogonomyrmex.

Speciation of two social parasites from their respective hosts is tested using a molecular phylogeny. Alignment of 711 DNA base pairs of mitochondrial cytochrome b gene was used to assess phylogenetic relationships of inquiline species to their hosts and to other members of the genus. We show that the inquiline social parasites of the North American seed harvester ants are monophyletic, descending from one of the known hosts (Pogonomyrmex barbatus) in the recent past and shifting hosts in a pattern similar to that observed in other Hymenopteran social parasites. In addition, the host populations unexpectedly were found to be polyphyletic. Populations of Pogonomyrmex rugosus from an area east of the Chiricahua Mountains in Southern Arizona belong to a mitochondrial clade separate from the more western clade of P. rugosus from the Sonoran and Chihuahuan Deserts. Evidence of mitochondrial DNA introgression between P. rugosus and P. barbatus was also observed. We conclude that Emery's rule does not strictly hold for this system, but that the hosts and parasites are very closely related, supporting a loose definition of Emery's rule.

Animals↗

PubMed2002

Incipient speciation across a depth gradient in a scleractinian coral?

A few marine cases have demonstrated morphological and genetic divergence in the absence of spatial barriers to gene flow, suggesting that the initial phase of speciation is possible without geographic isolation. In the Bocas del Toro Archipelago of the Atlantic Coast of Panama, we found two morphotypes of the scleractinian coral Favia fragum with opposing depth distributions. One morphotype fit the classical description of F. fragum and was most abundant at 3 m depth. A second morphotype was distinguished by raised corallites and was restricted to < or = 1 m depth. The two morphotypes overlapped in distribution at 1 m depth. Multivariate analysis of polyp-level characters (shape and distribution of septa within corallites) divided samples into two groups corresponding to initial qualitative observations of colony shape and corallite relief. To determine whether reduced gene flow maintains morphological variation, we measured the frequencies of alleles at five allozyme loci in both morphotypes at three sites 1-2 km distant. While there were significant differences in allele frequencies between morphotypes within sites, there were also frequency differences among sites at most loci, with the exception of nearly fixed alleles at the PGM locus. Extremely low heterozygosity permitted us to use haplotypes to compare genetic distance between morphotypes and among sites. Comparisons between haplotype data and a null model assuming gene flow between morphotypes showed that the two morphotypes shared significantly fewer haplotypes than expected, and average genetic distance between morphotypes was significantly greater than expected. Partitioning haplotype variation with analysis of molecular variance demonstrated that 35% of the variation was explained by morphotype, whereas 28% of the variation was explained by site. Two PGM heterozygotes and several individuals homozygous for rare PGM alleles are consistent with hybridization, and perhaps introgression by selfing within morphotypes. We consider three hypotheses for this morphological and genetic divergence in F. fragum: (1) intraspecific polymorphism, (2) incipient species, (3) biological species; and discuss the role of reproductive characters in a divergence-with-gene flow mechanism of speciation.

Animals↗

PubMed2002

A fine-scale spatial analysis of the mosaic hybrid zone between Gryllus firmus and Gryllus pennsylvanicus.

The pattern of character variation within a hybrid zone, the hybrid zone structure, has been used to infer the processes that maintain hybrid zones. Unfortunately it is difficult to infer process from structure alone because many different processes can produce the same pattern of character variation. Mosaic hybrid zones may be maintained by exogenous selection in a heterogeneous environment and/or endogenous selection against hybrid individuals; habitat preference, premating isolating barriers and/or fertility selection can also contribute. The spatial scale at which a hybrid zone is sampled affects its apparent structure; a hybrid zone may appear clinal at one scale and mosaic at another. Here, we sample the mosaic hybrid zone between two field crickets, Gryllus firmus and G. pennsylvanicus, at a scale that spans the boundaries between individual soil-habitat patches. From our analysis, we find that at fine scales, the mosaic hybrid zone resolves into a set of steep clines across patch boundaries. Both morphological and molecular traits exhibit sharp and generally concordant clines. However, clines for mitochondrial DNA and one anonymous nuclear marker are clearly displaced as a result of current hybridization or past introgression (the "ghost of hybridization past"). Thus, scale is important for the structure of this and probably other hybrid zones. The extremely sharp, concordant clines across patch boundaries indicate that the cricket hybrid zone is undoubtedly structured by selection. However, the detailed mechanisms responsible for the maintenance of the hybrid zone--whether endogenous selection against hybrids, exogenous selection by the environment, and/or behavioral preferences for mates or habitats--remain to be elucidated. Determining these mechanisms will depend on closer inspection of the organisms themselves and their interactions, as is the case for all hybrid zones.

Animals↗

PubMed2002

A test of ecologically dependent postmating isolation between sympatric sticklebacks.

Ecological speciation occurs when reproductive isolation evolves ultimately as a result of divergent natural selection between populations inhabiting different environments or exploiting alternative resources. I tested a prediction of the ecological model concerning the fitness of hybrids between two young, sympatric species of threespine sticklebacks (Benthics and Limnetics). The two species are ecologically and morphologically divergent: the Benthic is adapted to feeding on invertebrates in the littoral zone of the lake whereas the Limnetic is adapted to feeding on zooplankton in the open water. The growth rate of two types of hybrids, the Benthic backcross and the Limnetic backcross, as well as both parent species, was evaluated in enclosures in both parental habitats in the lake. The use of backcrosses is ideal because a comparison of their growth rates in the two habitats estimates an ecologically dependent component of their fitness while controlling for any intrinsic genetic incompatibilities that may exist between the Benthic and Limnetic genomes. The backcross results revealed a striking pattern of ecological dependence: in the littoral zone, Benthic backcrosses grew at approximately twice the rate of Limnetic backcrosses, while in the open water, Limnetic backcrosses grew at approximately twice the rate of Benthic backcrosses. Such a reversal of relative fitness of the two cross-types in the two environments provides strong evidence that divergent natural selection has played a central role in the evolution of postmating isolation between Benthics and Limnetics. Although the rank order of growth rates of all cross-types in the littoral zone was Benthic > Benthic backcross > Limnetic backcross > Limnetic, neither backcross differed significantly from the parent from which it was mainly derived. Implications of this result are discussed in terms of ecological speciation and possible introgressive hybridization between the species. Results in the open water were less clear and were not fully consistent with the ecological model of speciation, mainly as a result of the low growth rate of Limnetics. However, analysis of the diet of the fish in the open water suggests that these enclosures may not have been fully successful at replicating the food regimes characteristic of this habitat.

Animals↗

PubMed2002

Discovery and phylogenetic analysis of a riverine species flock of African electric fishes (Mormyridae: Teleostei).

The evolution of species-specific mate recognition signals is of particular interest within speciose monophyletic groups with restricted distributions (known as "species flocks"). However, the explosive nature of speciation in these clades makes difficult the reconstruction of their phylogenetic history. Here we describe a species flock of riverine mormyrid fishes from west-central Africa in which electric signals may play a role in the reproductive isolation of sympatric species. In our recent field collections, totaling more than 1400 specimens from many localities, we recognize 38 forms that are distinct in their morphologies and electric organ discharge (EOD) characteristics. Of these 38, only four clearly correspond to described species. Here we treat these forms as operational taxonomic units (OTUs) in a phylogenetic analysis of cytochrome b sequence data from a sample of 86 specimens. We examined support in the molecular data for the monophyly of these 38 OTUs considered together, the monophyly of each phenotypically delimited OTU considered individually, and for relationships among OTUs congruent with those inferred from the distribution of morphological and EOD character states. Trees obtained by both maximum-parsimony and maximum-likelihood analyses, rooted with sequence data from outgroup taxa, provide evidence for the monophyly of these 38 OTUs with respect to other mormyrid fishes. The small genetic distances between many distinct forms suggest their recent divergence. However, in many instances the cytochrome b tree topology fails to support the monophyly of individual OTUs and close relationships between OTUs that are similar in morphology and EOD characteristics. In other cases, individuals from distinct OTUs share identical or nearly identical haplotypes. Close examination of these cases suggests that unnatural OTU definition is not the sole cause of this pattern, and we infer an incongruence between the mitochondrial gene tree and the organismal phylogeny caused by incomplete mitochondrial lineage sorting and/ or introgression across forms. The apparently rapid diversification in this clade of riverine electric fishes and the problems associated with recovering a meaningful species-level phylogeny from mitochondrial data parallel findings in other species flocks. Selection on EOD waveforms as mate recognition signals may be involved in the radiation of these fishes. This is the first description of a freshwater fish species flock from a riverine, as opposed to a lacustrine, environment.

Animals↗

PubMed2002

Inferences from a rapidly moving hybrid zone.

Anartia fatima and Anartia amathea (Lepidoptera: Nymphalidae) are sister taxa whose ranges abut in a narrow hybrid zone in eastern Panama. At the center of the zone, hybrids are abundant, although deviations from Hardy-Weinberg and linkage disequilibria are strong, due in part to assortative mating. We measured differences across the zone in four wing color-pattern characters, three allozyme loci, and mitochondrial haplotype. Wing pattern, allozyme, and mitochondrial clines were coincident (i.e., had the same positions) and concordant (i.e.. all markers had similar cline shapes, about 28 km wide). Repeated samples demonstrated that the hybrid zone has been moving eastwards at an average rate of 2.5 km/year over the past 20 years, accompanied by an equivalent movement of the mtDNA cline. No introgression of mtDNA haplotypes were found in the ''wake" of the moving cline, as might be expected for a neutral marker. The concordance of morphological and mtDNA clines between 1994 and 2000, in spite of hybrid zone movement, suggests strong epistasis between the mitochondrial genome and nuclear loci. Cline movement is achieved mainly by pure fatima immigrating into amathea populations; hybrids had little effect, and were presumably outcompeted by fitter pure fatima genotypes. This movement can be explained if random dispersal of 7-19 km x gen(1/2) is coupled with a competitive advantage to A. fatima genomes of 2-5%. Hybrid zone motion is equivalent to Phase III of Wright's shifting balance. Hybrid zone movement has rarely been considered likely in the past, but our results show that it may be more important in biogeography and evolution than generally realized.

Animals↗

PubMed2002

Ancient versus reticulate origin of a hemiclonal lineage.

Crossing experiments revealed that a diploid hybridogenetic fish (genus Poeciliopsis) from the Río Mocorito (Sinaloa, Mexico) is trihybrid. Its haploid maternal genome is inherited clonally (i.e., hemiclonally), and it expresses a mixture of morphological traits found in the closely related species P. monacha and P. viriosa. Its haploid paternal genome is replaced in each generation by mating with males of a more distantly related sexual species, P. lucida. However, expression of mixed (monacha X viriosa) traits by this hemiclone is also consistent with retention of shared ancestral polymorphisms. If true, this hemiclonal lineage would be one of the few examples of an ancient asexual taxon. We used mitochondrial DNA and allozymes to test whether the maternal progenitor of the Mocorito hybridogen was a recent P. monacha X P. viriosa hybrid or a remnant of their most recent common ancestor. Our results clearly link the hemiclonal genome to contemporary P. monacha and therefore support the hypothesis of a recent origin. Additionally, our findings suggest that this unisexual fish may serve as a vehicle for introgression between two allopatric sexual species.

Animals↗

PubMed2002

The Africanization of honeybees (Apis mellifera L.) of the Yucatan: a study of a massive hybridization event across time.

Until recently, African and European subspecies of the honeybee (Apis mellifera L.) had been geographically separated for around 10,000 years. However, human-assisted introductions have caused the mixing of large populations of African and European subspecies in South and Central America, permitting an unprecedented opportunity to study a large-scale hybridization event using molecular analyses. We obtained reference populations from Europe, Africa, and South America and used these to provide baseline information for a microsatellite and mitochondrial analysis of the process of Africanization of the bees of the Yucatan Peninsula, Mexico. The genetic structure of the Yucatecan population has changed dramatically over time. The pre-Africanized Yucatecan population (1985) comprised bees that were most similar to samples from southeastern Europe and northern and western Europe. Three years after the arrival of Africanized bees (1989), substantial paternal gene flow had occurred from feral Africanized drones into the resident European population, but maternal gene flow from the invading Africanized population into the local population was negligible. However by 1998, there was a radical shift with both African nuclear alleles (65%) and African-derived mitochondria (61%) dominating the genomes of domestic colonies. We suggest that although European mitochondria may eventually be driven to extinction in the feral population, stable introgression of European nuclear alleles has occurred.

Africa↗

PubMed2002

Host-associated genetic differentiation in the goldenrod elliptical-gall moth, Gnorimoschema gallaesolidaginis (Lepidoptera: Gelechiidae).

Careful study of apparently generalist phytophagous insects often reveals that they instead represent complexes of genetically differentiated host races or cryptic species. The goldenrod elliptical-gall moth, Gnorimoschema gallaesolidaginis, attacks two goldenrods in the Solidago canadensis complex: S. altissima and S. gigantea (Asteraceae). We tested for host-associated genetic differentiation in G. gallaesolidaginis via analysis of variation at 12 allozyme loci among larvae collected at six sites in Iowa, Minnesota, and Nebraska. Gnorimoschema gallaesolidaginis from each host are highly polymorphic (3.6-4.7 alleles/locus and expected heterozygosity 0.28-0.38 within site-host combinations). Although there were no fixed differences between larvae from S. altissima and S. gigantea at any site, these represent well differentiated host forms, with 11 of 12 loci showing significantly different allele frequencies between host-associated collections at one or more sites. Host plant has a larger effect on genetic structure among populations than does location (Wright's FST = 0.16 between host forms vs. F(ST) = 0.061 and 0.026 among altissima and gigantea populations, respectively). The estimated F(ST) between host forms suggests that the historical effective rate of gene flow has been low (N(e)m approximately 1.3). Consistent with this historical estimate is the absence of detectable recombinant (hybrid and introgressant between host form) individuals in contemporary populations (none of 431 genotyped individuals). Upper 95% confidence limits for the frequency of recombinant individuals range from 5% to 9%. Host association is tight, but imperfect, with only one likely example of a host mismatch (a larva galling the wrong host species). Our inferences about hybridization and host association are based on new maximum-likelihood methods for estimating frequencies of genealogical classes (in this case, two parental classes, F1 and F2 hybrids, and backcrosses) in a population and for assigning individuals to genealogical classes. We describe these new methods in the context of their application to genetic structure in G. gallaesolidaginis. Population phenograms are consistent with the origin of the host forms (at least in the midwestern United States) via a single host shift: altissima and gigantea moth populations form distinct lineages with 100% bootstrap support. Genetic structure in Gnorimoschema is of particular interest because another gallmaking insect attacking the same pair of hosts, the tephritid fly Eurosta solidaginis, includes a pair of host races with partial reproductive isolation. Gnorimoschema gallaesolidaginis and E. solidaginis therefore represent the first reported case of parallel host-associated differentiation, that is, differentiation by evolutionarily independent insect lineages across the same pair of host plants.

Animals↗

PubMed2003

Paternal, maternal, and biparental molecular markers provide unique windows onto the evolutionary history of macaque monkeys.

We report the results of one of the first intrageneric analyses to simultaneously survey mitochondrial, Y-chromosomal, and autosomal loci from the same individuals representing the same taxa. Phylogenetic trees were constructed for each of these genetic systems from a pool of 63 macaques, representing all 19 recognized species in this genus, and eight outgroup taxa. The mitochondrial locus analyzed here (1.5 Kb) spans the 3' end of 12S rDNA, tRNA-VAL, and the 5' end of 16S rDNA; the Y chromosome dataset (3.1 Kb) consists of the genes SRY and TSPY; the two autosomal datasets include IRBP intron 3 (1.6 Kb) and the 5' half of C4 "long" intron 9 (3.3 Kb). A total of 1.35 million bases were read, revealing 682 variable sites within the genus Macaca. With regard to earlier unresolved issues of macaque evolution, a comparison of topologies reconstructed from each of the three genetic systems suggests: (1) four monophyletic species groups; (2) an initial bifurcation among Asian macaques between the silenus group progenitor and a M. fascicularis-like taxon, with the latter representing the probable common ancestor to all non-silenus group Asian macaques; (3) a possible hybrid origin of M. arctoides from proto-M. assamensis/thibetana and proto-M. fascicularis; and (4) contemporary introgression between M. mulatta and M. fascicularis in Indochina. Inferences 3 and 4 are of particular interest, because episodes of reticulate evolution often go undetected in analyses employing a single genetic system. Finally, divergence calculations suggest that, in female-philopatric taxa, mitochondrial bifurcations may typically predate Y-chromosomal divergences at the same node.

Animals↗

PubMed2003

A model for chloroplast capture.

Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

Biological Evolution↗

PubMed2003

The maintenance of reproductive isolation in a mosaic hybrid zone between the fire-bellied toads Bombina bombina and B. variegata.

Mosaic hybrid zones arise when ecologically differentiated taxa hybridize across a network of habitat patches. Frequent interbreeding across a small-scale patchwork can erode species differences that might have been preserved in a clinal hybrid zone. In particular, the rapid breakdown of neutral divergence sets an upper limit to the time for which differences at marker loci can persist. We present here a case study of a mosaic hybrid zone between the fire-bellied toads Bombina bombina and B. variegata (Anura: Discoglossidae) near Apahida in Romania. In our 20 x 20 km study area, we detected no evidence of a clinal transition but found a strong association between aquatic habitat and mean allele frequencies at four molecular markers. In particular, pure populations of B. bombina in ponds appear to cause massive introgression into the surrounding B. variegata gene pool found in temporary aquatic sites. Nevertheless, the genetic structure of these hybrid populations was remarkably similar to those of a previously studied transect near Pescenica (Croatia), which had both clinal and mosaic features: estimates of heterozygote deficit and linkage disequilibrium in each country are similar. In Apahida, the observed strong linkage disequilibria should stem from an imperfect habitat preference that guides most (but not all) adults into the habitats to which they are adapted. In the absence of a clinal structure, the inferred migration rate between habitats implies that associations between selected loci and neutral markers should break down rapidly. Although plausible selection strengths can maintain differentiation at those loci adapting the toads to either permanent or temporary breeding sites, the divergence at neutral markers must be transient. The hybrid zone may be approaching a state in which the gene pools are homogenized at all but the selected loci, not dissimilar from an early stage of sympatric divergence.

Animal Migration↗

PubMed2003

Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. III. Heterogeneous accumulation of hybrid incompatibilities, degree of dominance, and implications for Haldane's rule.

The genetic basis of Haldane's rule was investigated through estimating the accumulation of hybrid incompatibilities between Drosophila simulans and D. mauritiana by means of introgression. The accumulation of hybrid male sterility (HMS) is at least 10 times greater than that of hybrid female sterility (HFS) or hybrid lethality (HL). The degree of dominance for HMS and HL in a pure D. simulans background is estimated as 0.23-0.29 and 0.33-0.39, respectively; that for HL in an F1 background is unlikely to be very small. Evidence obtained here was used to test the Turelli-Orr model of Haldane's rule. Composite causes, especially, faster-male evolution and recessive hybrid incompatibilities, underlie Haldane's rule in heterogametic male taxa such as Drosophila (XY male and XX female). However, if faster-male evolution is driven by sexual selection, it contradicts Haldane's rule for sterility in heterogametic-female taxa such as Lepidoptera (ZW female and ZZ male). The hypothesis of a faster-heterogametic-sex evolution seems to fit the current data best. This hypothesis states that gametogenesis in the heterogametic sex, instead of in males per se, evolves much faster than in the homogametic sex, in part because of sex-ratio selection. This hypothesis not only explains Haldane's rule in a simple way, but also suggests that genomic conflicts play a major role in evolution and speciation.

Animals↗

PubMed2004

Gene flow across a climatic barrier between hybridizing avian species, California and Gambel's quail (Callipepla californica and C. gambelii).

Allopatric species commonly interbreed in a restricted margin between their ranges. The particular factors that permit interbreeding between species determine the extent of hybridization and its significance for evolution and conservation. Using California quail and Gambel's quail (Callipepla californica and C. gambelii) that naturally hybridize in a narrow region between relatively mesic and xeric environments, I assessed the exchange of genetic and phenotypic traits in relation to vegetative and climatic features (temperature and precipitation) that characterize the area of range overlap, and I examined genetic and phenotypic traits within the hybrid zone over a five-year period in relation to variation in precipitation. Using microsatellite markers, this study reveals that genetic, plumage, and morphometric traits are tightly associated with vegetation, rainfall, and temperature profiles through the abrupt transition from one parental species to the other across the hybrid zone. Results show that the hybrid zone has remained clinal, stationary, and bounded over the five-year study period. There was no evidence of introgression outside the narrow hybrid zone. Interannual climatic fluctuations are associated with internal hybrid zone dynamics but did not alter the shape and position of the zone. A transect through the hybrid zone revealed rapid and episodic genetic mixing within the zone. Possible long-term consequences of this restricted hybridization for the evolution of the two parental species are discussed in the light of changing environments.

Animals↗

PubMed2004

Barriers to sympatry between avian sibling species (Paridae: Baeolophus) in local secondary contact.

Range limits and secondary contact zones often occur at ecotones between major associations of habitat and climate. Therefore, understanding processes that limit sympatry between species in such areas provides an important framework for testing biogeographic and evolutionary hypotheses. Theoretical and empirical work has shown that the evolution of species borders is influenced by a complexity of factors, including gene flow from central to peripheral populations and the ability of species to adapt locally to environmental conditions. However, few studies have used bioclimatic models, combined with molecular and morphological data, to predict geographic range limits in the context of gene flow across a secondary contact zone. In this study, I applied these methods to test specific hypotheses about barriers to sympatry between closely related species where they approach and contact each other. Specifically, I examined the importance of historical isolation, local adaptation, and symmetry of gene flow in limiting sympatry and range expansion of ecologically distinct species across environmental gradients. Molecular (mitochondrial DNA, allozymes), morphological, and bioclimatic data were obtained for two avian sibling species (Baeolophus inornatus and B. ridgwayi) that exist in recent, narrow secondary contact in northern California. These species are broadly allopatric and occupy rangewide associations of oak and pinyon-juniper woodlands, respectively, although B. inornatus also inhabits mixed or juniper woodlands locally. Patterns of molecular variation generally were congruent with morphological and bioclimatic data, and support prior evidence for a history of isolation, adaptation, and divergence in distinctive, species-specific vegetation-climate associations. However, molecular and morphological clines fall east of the limit of oaks, and individuals of B. inornatus in this juniper-associated contact zone experience bioclimates that are more similar to B. ridgwayi than to B. inornatus in oak habitat. Thus, B. inornatus is able to adapt and expand locally into the range of its close relative, but not vice versa. These data support the hypothesis that gene flow is asymmetrical where peripheral populations meet at range boundaries. Physiological differences between species may play an important role in influencing these patterns. Empirical studies that highlight the importance of local adaptation and patterns of gene flow in which closely related species contact across ecotones are central to understanding limits on geographic ranges, sympatry, and introgression-a cornerstone of biogeographic and speciation theory.

Analysis of Variance↗

PubMed2004

Resistance of Phaseolus species to ice crystallization at subzero temperature.

Dry bean (Phaseolus vulgaris L.) cultivars possess little or no freezing tolerance and are killed at the temperature of ice formation in their tissues. An increase in frost tolerance by 2-3 degrees C would expand dry bean production in the short growing seasons of the Canadian prairies and possibly to higher altitudes in the tropics where episodic frosts occur during the growing season. The objective of this study was to determine the differences in frost resistance of Phaseolus species in both controlled and field environments. Leaflets of dry bean cv. CDC Nighthawk, and wild relatives from the primary gene pool (P. vulgaris var. mexicanus Freytag and P. vulgaris var. aborigineus (Burkart) Baudet) and the tertiary gene pool (P. acutifolius var. tenuifolius A. Gray, P. filiformis Bentham, P. angustissimus A. Gray and P. ritensis M.E. Jones) were subjected to subzero temperatures with and without ice nucleation to determine the levels of tolerance and avoidance, respectively. The lethal temperature at which 50% of the leaflets were killed (LT(50)) was 0.5-1 degrees C lower for species of the tertiary gene pool compared to those from the primary gene pool. Leaflets of species from the tertiary gene pool were also characterized by extensive supercooling compared to leaflets of species from the primary gene pool. Resistance of Phaseolus species to spring and autumn frosts were determined on seedlings transplanted to the field. Phaseolus angustissimus, a species of the tertiary gene pool had the highest seedling survival in response to both autumn and spring frosts, when the minimum air temperatures were -5 and -7 degrees C, respectively. Frost resistance of Phaseolus angustissimus, if successfully introgressed into bean germplasm, may enable the development of frost resistant dry bean cultivars.

Journal Article↗

PubMed2004

Consomic rat model systems for physiological genomics.

A consomic rat strain is one in which an entire chromosome is introgressed into the isogenic background of another inbred strain using marker-assisted selection. The development and physiological screening of two inbred consomic rat panels on two genetic backgrounds (44 strains) is well underway. Consomic strains enable one to assign traits and quantitative trait loci (QTL) to chromosomes by surveying the panel of strains with substituted chromosomes. They enable the rapid development of congenic strains over a narrow region and enable one to perform F2 linkage studies to positionally locate QTL on a single chromosome with a fixed genetic background. These rodent model systems overcome many of the problems encountered with segregating crosses where even if linkage is found, each individual in the cross is genetically unique and the combination of genes cannot be reproduced or studied in detail. For physiologists, consomics enable studies to be performed in a replicative or longitudinal manner to elucidate in greater detail the sequential expression of genes responsible for the observed phenotypes of these animals. They often provide the best available inbred control strains for physiological comparisons with the parental strains and they enable one to assess the impact of a causal gene region in a genome by allowing comparisons of the effect of replacement of a specific chromosome on a disease susceptible or a resistant genomic background. Consomic rat strains are proving to be a unique scientific resource that can greatly extend our understanding of genes and their role in the regulation of complex function and disease.

Animals↗