Apomixis and hybridization together contribute to taxonomic complexity in Amelanchier. Hybridization combines genetically divergent genomes and spawns new forms that apomixis perpetuates. Apomixis is aposporous, facultative, and pseudogamous in the genus, and apomicts are generally polyploid, pollen fertile, and pollinated by generalists. That gene flow actually occurs is empirically evident. As apomixis is genetically dominant over sexuality, hybrids involving at least one apomictic parent are apomictic. Clonal reproduction may thus perpetuate Fl individuals and generate agamospecies. Alternatively hybrids may interbreed and backcross to create hybrid swarms or cross with species other than the parents. In eastern North America, the abundance of published names and general taxonomic confusion in the genus doubtless result at least in part from this interplay of apomixis and hybridization. The roles of apomixis and hybridization in diversification within Amelanchier are examined in light of new data about breeding system of an apomictic, hybrid microspecies, informally named A. ''erecta'' and its formation of a hybrid swarm with another Amelanchier apomict, A. laevis.
Random amplified polymorphic DNA (RAPD) studies of a natural hybrid swarm between Eucalyptus amygdalina Labill. and E. risdonii Hook.f. and nearby allopatric stands revealed that, despite clear morphological differences, all bands were shared between species. However, frequency differences revealed genetic divergence between species, populations within species, and individuals within populations. Variation was greatest between individuals within populations and lowest between species. For both species, the direction of variation which distinguished the two populations was in a different direction to that which separated the two species, suggesting population differences were not due to introgression but were the result of genetic isolation and/or strong localised selection. Several morphologically typical individuals with intermediate RAPD profiles were detected in the hybrid swarm and nearby allopatric samples of both species, suggesting that some cryptic introgression may be occurring. Controlled F-1 crosses generally had closer genetic affinity to E. risdonii, raising the possibility that some parents used may have been advanced generation hybrids. While natural hybrids selected for their intermediate leaf phenotype were usually also intermediate between the two species using RAPD markers, some deviated markedly toward E. risdonii. The study suggests that morphological appearance does not necessarily reflect genetic (RAPD) status and in some cases detectable RAPD differences between spatially close populations of the same species may be as great or greater than the differences between species.
A comparison of allozyme and mtDNA frequencies was used for insight into a situation in the Pecos River, Texas where contact between the endemic pupfish (Cyprinodon pecosensis) and an introduced congener (C. variegatus) has resulted in rapid, geographically extensive genetic introgression. Temporal changes in mean frequencies of diagnostic allozyme markers indicate that the clinal pattern of introduced genetic material (Echelle and Connor 1989) is slowly decreasing in amplitude. Significant rank concordance in diagnostic allele frequencies among sites and across sampling years indicates directional influences upon temporal allele frequency change. These observations are consistent with the theory of gene flow in neutral dines. Levels of introgression indicated by each of four allozyme loci and mtDNA were roughly equivalent. The early history of the hybrid swarm is explained by genetic swamping, possibly mediated by selection for C. variegatus or C. variegatus X C. pecosensis, at a time when the normally abundant endemic species had been catastrophically depleted. High frequencies of an introduced GPI-A allele in all samples of intergrades suggests that the introduced genome originated with a single founding event.
A hybrid swarm between Viola lutea subsp. sudetica (2n=50, native species) and Viola tricolor (2n=26, introduced species) originated in the 1970's in the Krkonose Mts. Analyses of chromosome numbers, reproductive biology, morphology, and ecology gave the following results: (1) Compared to the plants found in the 1970's, the number of colour morphs have decreased and the types now prevailing in the field are morphologically closer to V. lutea subsp, sudetica, forming a continuum. (2) In the field, hybrids having approximately the same chromosome number as the primary hybrids are most common. Some plants of the hybrid swarm have certain characters unknown to their parents. (3) In the field, autogamous types similar to V. tricolor were found. (4) A limited number of plants from the field and culture have higher somatic chromosome numbers than V. lutea subsp. sudetica; there was a tendency towards increasing chromosome numbers in their progeny (up to 2n=62). These plants have some new morphological characters (a small proportion of hexacolpate pollen) nor found in the parents (nor in the other hybrids with prevailing tetracolpate and pentacolpate pollen grains) and higher pollen fertility in comparison to the other hybrids. These plants also have the highest germination rate, (5) There is a tendency for chromosome numbers to decline in the progeny of open pollinated hybrid plants in the lowland experimental garden. (6) The phenology of the plants in the mountain grasslands and the lowland garden is different; the parents behave in a totally contrasting manner. (7) In the field, at least some hybrids extend outside the geographical and ecological ranges of the parental species, invading new communities. (8) Seeds of V. tricolor do not show any dormancy and germinate in the year of production; most of the V. lutea subsp. sudetica seeds germinate during the spring of the following year. Hybrids with intermediate chromosome numbers had both types of germination strategy. The germination rate of intermediates with high chromosome numbers was even higher than that of V. tricolor.
Native trout populations throughout western North America have declined because of habitat alteration, introgression with introduced trout, or competitive exclusion by nonnative species. Consequently, identification and preservation of native trout are now the goals of many management programs. We examined allozyme and mitochondrial DNA (mtDNA) variation in seven naturally occurring populations and one hatchery population of rainbow trout Oncorhynchus mykiss from southern Idaho and northern Nevada to determine their genetic origins. Allozyme and mtDNA results were concordant in identifying three populations as genetically pure interior rainbow trout and one population as a hybrid swarm. Results for the remaining four populations were discordant. However, these latter four populations were best classified as hybrid swarms due to the nature of either the allozyme or mtDNA data, which included genetic characteristics of both coastal and interior rainbow trout. Our study demonstrates the utility of mtDNA analysis in conjunction with independent criteria such as allozymes for detecting hybridization at the population level. Hybridized populations exhibited a greater number of mtDNA haplotypes than did genetically pure populations. Haplotypes within hybridized populations differed more from one another than did mtDNA haplotypes within nonhybridized populations.
Penstemon spectabilis is a putative stabilized diploid hybrid of P. centranthifolius and P. grinnellii. It is morphologically intermediate, and all three species have different pollinators. Penstemon centranthifolius and P. spectabilis have been proposed as parents of P. clevelandii, which is purportedly isolated by ecological factors. Although hybridization between the proposed parental species has been reported, hybrid swarm formation has not been observed and introgression is purported to be minimal. We tested hypotheses of diploid hybrid speciation and introgression among these species based on rDNA restriction-site and length variation for 56 populations within and outside of the hybrid complex. Unambiguous molecular markers clearly differentiated P. centranthifolius, P. grinnellii, and P. spectabilis, whereas P. spectabilis and P. clevelandii had the same rDNA type. The P. centranthifolius rDNA type was found in some populations of P. spectabilis and P. clevelandii, but there was no evidence of the additive profile documented for other recent diploid hybrid species. In contrast, the rDNA profile of P. x parishii had a completely additive profile of its proposed parental species P. centranthifolius and P. spectabilis. Ribosomal DNA markers for P. grinnellii were restricted to populations within the species and were not found in any population of P. spectabilis. Our data did not support hybrid-speciation hypotheses, but were in accord with allozyme data that provided evidence for introgression between P. centranthifolius and P. grinnellii, P. spectabilis and P. clevelandii. These results were used to propose criteria to differentiate ancient diploid hybrid speciation from patterns of introgression.
Hybridization and introgression have been suggested by previous workers as important factors in the evolution of the southeastern azaleas. However, because of high levels of intraspecific variation and lack of intensive sampling, documentation of this phenomenon has been difficult. We investigated a population of azaleas on Stone Mountain, Georgia, composed of Rhododendron flammeum, R. canescens, and their interspecific hybrids. After analysis of chloroplast DNA, we found evidence of extensive localized cytoplasmic introgression into R. flammeum. Many individuals in the sample population that are morphologically indistinguishable from R. flammeum possess the chloroplast genome of R. canescens. This suggests that some of the observed variation in some populations of R. flammeum may be due to past introgression from R. canescens.
The two West African black-and-white colobus taxa, Colobus polykomos and C. vellerosus, are distinct species. A supposedly intermediate subspecies, C. polykomos dollmani, is actually a hybrid swarm, in which C. vellerosus genes greatly predominate. We propose a hypothesis to explain this situation.
Male sterility is studied in hybrid zones by different measures of cytonuclear disequilibria, D, D1, D2, and D3. Of particular interest are the dynamics of disequilibria as the system evolves to equilibrium. Our first model, the hybrid swarm model, yields equilibrium results identical to those observed in a model with random mating. In our second model of a hybrid zone, predictions of the sign pattern of disequilibrium values can be made based on migration values. A characteristic sign pattern may help to distinguish cytoplasmic male sterility (CMS) from other mechanisms of selection. Our simple CMS model with migration is successfully fit to cytonuclear data on a hybrid population of cottonwoods.
Isozyme diversity is described among a collection of 365 Manihot esculenta cultivars plus 109 accessions from wild relatives (M. glaziovii and spontaneous hybrids) from Africa. The study is based on 17 polymorphic loci. A natural hybrid swarm is detected between the two species. Although they were recently introduced, M. esculenta and M. glaziovii show high levels of polymorphism: heterozygosity estimates are 0.225 and 0.252 respectively. For the wild species, diversity is structured at the unilocus level, and the multilocus approach reveals a geographical pattern. The organization of the diversity is not so clear for the cultivated cassava, but a multilocus approach, based on both common and rare alleles, led us to identify different groups of clones with many intermediate genotypes between them. Elements of the secondary diversification process of Manihot in Ivory Coast are discussed.
We sampled 100 adult milkweeds along a 3 km transect through the Dolly Sods Wilderness Area in northeastern West Virginia. These plants were measured for 13 flower and 17 leaf characters. We also used horizontal starch-gel electrophoresis to score variation in 11 enzyme loci. A composite character index was constructed that yielded negative scores for species-specific alleles of A. syriaca and positive scores for species-specific alleles of A. exaltata. The 30 plants from the middle portion of the transect were highly variable, with many plants achieving scores near zero. These plants presumably represent a complex hybrid swarm, consisting of F1 interspecific hybrids and backcrosses. Plants with intermediate character-index scores were significantly more likely not to flower than were plants with higher (A. exaltata-like) or, especially, lower (A. syriaca-like) scores. On the basis of a study of reproductive characters alone, one might conclude that there is no evidence of hybridization, as there appear to be no intermediates among the 54 flowering plants. Vegetative characters, however, reveal a more complex pattern of variation that agrees better with the isozyme analysis of all 100 plants. It does appear that hybridization and localized introgression are occurring between A. exaltata and A. syriaca in the Dolly Sods region. Several unique features of milkweed reproduction appear to promote interspecific hybridization and to magnify its effects.
In this study we examine the cytoplasmic inheritance patterns of an interspecific hybridizing population of Fremont and narrowleaf cottonwoods, using mitochondrial DNA. Three mitochondrial probes showing polymorphisms were used to distinguish between trees of known nuclear inheritance. Every tree screened had only one cytoplasmic genotype, either Fremont or narrowleaf. Thus, these results demonstrate that mitochondria are uniparentally inherited in these trees. Previous studies of the nuclear inheritance of this interspecific hybridizing population of cottonwood trees indicated an asymmetry in the frequency of parental genes. Using mitochondrial markers we tested one hypothesis potentially responsible for this asymmetric distribution (i.e., trees of mixed genotypes will be sterile or will not survive if their cytoplasm is derived from one or the other parent). Our results, however, show that both Fremont and narrowleaf mitochondrial markers are found in trees with mixed nuclear genotypes. Thus, nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to explain the asymmetric distribution of nuclear alleles within the hybrid zone, nonrandom associations between nuclear and cytoplasmic genotypes do exist. For example, all F1 hybrids had Fremont mitochondrial genotypes. Furthermore, backcrosses between F1 hybrid and narrowleaf trees have a higher than expected proportion of heterozygous loci and a higher than expected proportion of Fremont mitochondria. We propose that seeds, seedlings, or trees with high proportions of heterozygous loci are at a disadvantage unless they also have the Fremont mitochondrial genotype. While it is generally difficult to infer dynamic processes from static patterns, studies such as ours enable one to gain new insights to the dynamics of plant hybrid zones. A hybridization pattern of decreasingly complex backcrosses as one proceeds from higher to lower elevation within the hybrid swarm, a residue of Fremont cytoplasmic DNA within the pure narrowleaf population, and the unidirectional nature of these crosses suggest that the narrowleaf population may be spreading down the canyon and the Fremont population receding. The eventual fate of the hybrid zone, in relation to these processes, is discussed.
Foliar terpenes and unidentified volatiles differed significantly between western larch (Larix occidentalis Nutt.) and alpine larch (Larix lyallii Parl.) populations and were useful in identifying putative hybrids at a site where the species are sympatric. Camphene, sabinene, beta-pinene, limonene, beta-caryophyllene, beta-cubebene, four unidentified volatiles, total volatile concentration, and terpene evenness were significantly different (p less-than-or-equal-to 0.05) between the parental populations. Linear discriminant functions for these terpenes and volatiles were developed for species classes (alpine larch, western larch, and hybrids) previously identified by morphological features. Of 15 trees judged to be hybrids based on morphology, 12 were classified as hybrids by the discriminant functions. All of the alpine larch (n = 10) and western larch (n = 10) in the parental populations were correctly classified by the terpenes. Foliar terpenes appear to be under tight genetic control in the parent species but sort out in the hybrids. Basal area growth of two of the putative hybrids was much greater than growth of either alpine or western larch on the sympatric site, suggesting heterosis. Concentration of ocimene was threefold higher in one of the fast-growing hybrids than in the parental types, suggesting heterosis at the biochemical level.
Analysis of bud exudate by gas chromatography-mass spectrometry (GC-MS) of 14 clones belonging to a natural hybrid swarm involving Populus angustifolia, P. balsamifera, and P. deltoides produced results consistent with those obtained by previous analysis of leaf characteristics. Specimens that had leaves most characteristic of a pure species also produced bud exudate GC-MS profiles which were characteristic of those species. GC-MS profiles of interspecific hybrid clones were intermediate between the parental species. This demonstrates the usefulness of GC-MS analysis of bud exudate as a chemotaxonomic method for the study of intersecific poplar hybrids and also supports the accuracy of analysis of foliar morphology for taxonomic assessment.
Flavonoids were isolated from leaf extracts of three putative hybrid milkweeds and the four presumed parental species of Asclepias. A total of 16 flavonoids, all of which are glycosides of the flavonols quercetin and kaempferol, was identified. The hybrid A. exaltata x A. quadrifolia contained both of the marker compounds characteristic of A. quadrifolia and three of the four compounds usually produced by A. exaltata. In addition, two novel flavonoids were detected. Naturally occurring A. exaltata x A. syriaca produced three of four compounds diagnostic for A. exaltata and all but one of the common constituents of A. exaltata, yielding a profile similar to that of known hybrids produced by experimental crosses. Four novel compounds occurred in field-collected hybrids but were present only in trace amounts. Detailed analyses of a larger population of milkweeds in West Virginia suggested that F1 interspecific hybrids of A. exaltata and A. syriaca had backcrossed to the parents, yielding a complex hybrid swarm. The putative hybrid A. purpurascens x A. syriaca contained two of the four compounds characteristic of A. syriaca and four of the six variably present in A. purpurascens. Two novel flavonoids were observed. Analyses of flower extracts indicated that the novel compounds detected in leaves of hybrid milkweeds usually result from a breakdown in tissue-specific regulation. Chemical data therefore lend strong support to morphological evidence of hybridization between these species of Asclepias.