Bidirectional introgressive hybridization between Lepus capensis and Lepus yarkandensis

Lepus capensis and Lepus yarkandensis are two well-established hare species with different habitat preferences and obvious morphological distinctions. L. capensis is common and widespread while L. yarkandensis is an endemic species with a restricted distribution in the Tarim Basin of western China. During field sampling, individuals with a distinct phenotype were found in the contact zone between the two species. To understand the origin of these unclassified individuals, we analyzed morphological and genetic data from 700 individuals including L. yarkandensis, L. capensis and the unclassified individuals. Morphological analyses of the unclassified individuals revealed that they lie between L. yarkandensis and L. capensis. Genetic analyses based on species-specific molecular markers (mitochondrial control region and SRY) showed that the unclassified populations have mtDNA and SRY genes from both L. capensis and L. yarkandensis, suggesting that the unclassified populations result from hybridization between the two species. Interestingly, many identical and/or very similar mtDNA haplotypes were shared between L. capensis populations and L. yarkandensis populations close to their contact zones, which further suggests the existence of extensive bidirectional mtDNA introgression. Similarly, we found evidence for SRY introgression, though it existed at a lower level compared to mtDNA introgression.     

Cytonuclear genetic architecture in mosquitofish populations and the possible roles of introgressive hybridization

Spatial genetic structure in populations of mosquitofish ( Gambusia ) sampled throughout the south-eastern United States was characterized using mitochondrial (mt) DNA and allozyme markers. Both sets of data revealed a pronounced genetic discontinuity (along a broad path extending from south-eastern Mississippi to north-eastern Georgia) that corresponds to a recently recognized distinction between the nominal forms G. affinis to the west and G. holbrooki to the east. However, several populations from the general contact region exhibited unusual allelic associations in high frequency, suggestive of evolutionary processes within a zone of introgressive hybridization. These involve: (i) cytonuclear profiles representing combinations of nuclear and mitochondrial genotypes that tended to be more nearly species-specific and concordant elsewhere; and (ii) significant nuclear gametic disequilibria, perhaps attributable to positive assortative mating and/or differential fitnesses of homospecific vs. recombinant genotypes. However, outside this suspected hybrid region, 'heterospecific' genetic markers also appeared in low frequency, thus complicating interpretations. These discordant alleles on a broader geographic scale may reflect: (a) the retention of polymorphisms from an ancestral gene pool; (b) occasional evolutionary convergence (especially with respect to electrophoretic mobility of allozyme alleles); (c) the 'footprints' of a moving hybrid zone; or (d) differential introgressive penetrance across the current hybrid region.     

Interspecific hybridization and its evolutionary significance in the genus Aquilegia

Various hybrid combinations of 18 species and varieties ofAquilegia were produced, and several cross-pollinations were made which failed to produce hybrids. Pollen fertility of hybrids was generally high, with a total mean of approximately 50%, but highly variable among hybrid combinations and consistently lower than that of parental species. With the exception of frequent occurrence of giant pollen grains and rare synaptic failure of one or two homologous pairs, meiotic behavior in hybrid pollen mother cells appeared to be completely regular. Representatives of the Vulgaris Complex were observed to be intermediate genetically and morphologically leading to the interpretation of ancestral status among extant species. The greatest genetic diversity within the genus was shown to exist betweenA. ecalcarata and members of the Caerulea Complex.     

Morphological, cytogenetic, and molecular evidence for introgressive hybridization in birch.

Extensive morphological variation of tetraploid birch (Betula pubescens) in Iceland is believed to be due to gene flow from diploid dwarf birch (B. nana) by means of introgressive hybridization. A combined morphological and cytogenetic approach was used to investigate this phenomenon in two geographically separated populations of natural birch woodland in Iceland. The results not only confirmed introgressive hybridization in birch, but also revealed bidirectional gene flow between the two species via triploid interspecific hybrids. The populations showed continuous morphological variation connecting the species, but karyotypically they consisted of only three types of plants: diploids, triploids, and tetraploids. No aneuploids were found. Some of the tetraploid plants had B. pubescens morphology as expected, but most of them had intermediate characters. Most of the diploid plants were B. nana, but some were intermediates and a few had B. pubescens morphology. The triploid plants were either intermediates or they resembled one of the two species. Similar introgressive variation was observed among the diploid and triploid progeny of open-pollinated B. nana in a garden. Birch samples including field plants and artificial hybrids were further examined using a molecular method based on genomic Southern hybridization. The experiments verified introgression at the DNA level.     

Genomic markers reveal introgressive hybridization in the Indo-West Pacific mangroves: a case study

Biodiversity of mangrove ecosystems is difficult to assess, at least partly due to lack of genetic verification of morphology-based documentation of species. Natural hybridization, on the one hand, plays an important role in evolution as a source of novel gene combinations and a mechanism of speciation. However, on the other hand, recurrent introgression allows gene flow between species and could reverse the process of genetic differentiation among populations required for speciation. To understand the dynamic evolutionary consequences of hybridization, this study examines genomic structure of hybrids and parental species at the population level. In the Indo-West Pacific, Bruguiera is one of the dominant mangrove genera and species ranges overlap extensively with one another. Morphological intermediates between sympatric Bruguiera gymnorrhiza and Bruguiera sexangula have been reported as a variety of B. sexangula or a new hybrid species, B. × rhynchopetala. However, the direction of hybridization and extent of introgression are unclear. A large number of species-specific inter-simple sequence repeat (ISSR) markers were found in B. gymnorrhiza and B. sexangula, and the additive ISSR profiling of B. × rhynchopetala ascertained its hybrid status and identified its parental origin. The varying degree of scatterness among hybrid individuals in Principal Coordinate Analysis and results from NewHybrids analysis indicate that B. × rhynchopetala comprises different generations of introgressants in addition to F(1)s. High genetic relatedness between B. × rhynchopetala and B. gymnorrhiza based on nuclear and chloroplast sequences suggests preferential hybrid backcrosses to B. gymnorrhiza. We conclude that B. × rhynchopetala has not evolved into an incipient hybrid species, and its persistence can be explained by recurrent hybridization and introgression. Genomic data provide insights into the hybridization dynamics of mangrove plants. Such information can assist in biodiversity assessment by helping detect novel taxa and/or define species boundaries.     

On the origin and evolution of the genetic code I. Wobbling and its potential significance

In this paper several properties of the genetic code are interpreted by assuming that wobbling or some remnant of wobbling has originally been a common phenomenon also in the first nucleotide of each codon, and not only in the third nucleotide. Some of the last steps in the evolution of the genetic code are described on the basis of this interpretation of genetic code features.An attempt to outline some of the earlier steps in the evolution of the genetic code is based on the assumption that at an earlier stage wobbling may also have been common in the central nucleotide of each codon.In the last part of the paper the possibility is considered that the pairing rules which characterize wobbling may have been much more common in the past not only in codon-anticodon pairing but also in polymer copying. The advantages of a freer purine-pyrimidine pairing like the one characteristic of wobbling in a primitive (or prebiologic) environment in which nucleotide production was not entirely (or not at all) under biologic control are stressed.This paper is based exclusively on the “Frozen accident” interpretation of the genetic code (Crick, 1968) with a few modifications introduced or implied in the text. No stereochemical codon interpretations and only a minimum of chemical considerations are involved.     

Auxin regulation of axial growth in bryophyte sporophytes: its potential significance for the evolution of early land plants

To identify developmental mechanisms that might have been involved in the evolution of axial sporophytes in early land plants, we examined the effects of auxin-regulatory compounds in the sporophytes of the hornwort Phaeoceros personii, the liverwort Pellia epiphylla, and the moss Polytrichum ohioense, members of the three divisions of extant bryophytes. The altered growth of isolated young sporophytes exposed to applied auxin (indole-3-acetic acid) or an auxin antagonist (p-chlorophenoxyisobutyric acid) suggests that endogenous auxin acts to regulate the rates of axial growth in all bryophyte divisions. Auxin in young hornwort sporophytes moved at very low fluxes, was insensitive to an auxin-transport inhibitor (N-[1-naphthyl]phthalamic acid), and exhibited a polarity ratio close to 1.0, implying that auxin moves by simple diffusion in these structures. Emerging liverwort sporophytes had somewhat higher auxin fluxes, which were sensitive to transport inhibitors but lacked any measurable polarity. Thus, auxin movement in liverwort sporophytes appears to result from a unique type of apolar facilitated diffusion. In young Polytrichum sporophytes, auxin movement was predominantly basipetal and occurred at high fluxes exceeding those measured in maize coleoptiles. In older Polytrichum sporophytes, acropetal auxin flux had increased beyond the level measured for basipetal flux. Insofar as acropetal and basipetal fluxes had different inhibitor sensitivities, these results suggested that moss sporophytes carry out bidirectional polar transport in different cellular pathways, which resembles the transport in certain angiosperm structures. Therefore, the three lineages of extant bryophytes appear to have evolved independent innovations for auxin regulation of axial growth, with similar mechanisms operating in moss sporophytes and vascular plants.     

On the ecology and evolution of annual plants in disturbed environments

This paper is concerned with the effect of disturbance on some crucial characteristics of annual plants. The theoretically optimal life-history traits that maximize individual fitness in disturbed environments are described and critically evaluated. It seems that none of them holds for all annual species.Self-pollination and especially seed polymorphism are considered important adaptations to life in unpredictable environments. The thesis is put forward that amphicarpic annuals, which exhibit both self-pollination and extreme seed polymorphism, are best adapted to life in hazardous habitats. The hypothetical course of the evolution of amphicarpy is demonstrated on the grounds of the comparison of contemporary annual species producing chasmogamous and cleistogamous flowers on a single individual.     

Chemical and morphological analysis of introgressive hybridization between iris tenax and I. chrysophylla

Iris tenax andI. chrysophylla were collected along a linear transect extending through narrow areas of sympatry from two sites in the Willamette River drainage of western Oregon. Statistical analysis of correlations between morphological characteristics was used to construct a hybrid index for each species. Morphological evaluation and 2-dimensional paper chromatography of the major flavonoid constituents were prepared on a plant by plant basis along the transects. The combined morphological and chemical data indicated that hybridization was occurring and that, in contrast to previous reports, introgression was taking place in each of the parental species.     

On the ecological and evolutionary significance of storage in clonal plants

Environmental heterogeneity has received wide attention in clonal plant research over the last decade. Most studies have focussed on the effects of spatial variation in environmental conditions on the performance of ramets and genets, while the effects of temporal heterogeneity have only occasionally been studied. As a consequence, our current knowledge about functional responses of clonal plants to habitat patchiness is biased towards spatial aspects of environmental heterogeneity. Nevertheless, temporal changes in biotic and abiotic conditions do occur in most natural habitats, and they are very likely to affect plant growth and performance, and to create positive selection pressures on traits that can buffer plants against unfavorable consequences of this variability. Storage of resources is a widespread phenomenon in clonal plant species. Typical clonal structures such as stolons, rhizomes and hibernacles serve as storage organs. However, the ecologic significance of storage in clonal plant structures remains partly unclear. We suggest that the lack of understanding with respect to resource storage in clonal plants be related to our poor knowledge of ecologic implications of temporal habitat heterogeneity in natural environments. Resource storage can be understood as a safety measure against temporal changes in the growing conditions of plants. This paper summarizes existing information about the ecologic relevance of storage in clonal plants and it tries to develop a framework for further investigation of resource storage as a strategy to enhance the performance of clonal plants in temporally variable environments.     

On the possible significance of the transamidination reaction in evolution

The origin of the various muscle phosphagens during evolution is considered in the context of the need to conserve ornithine for the synthesis of proline for connective tissue necessary for structural strength and flexibility and/or a complicated musculature. In each phosphagen, arginine is known to have contributed its amidine moiety thus maintaining the function of the phosphagen and setting free the proline precursor ornithine. Tissues from an earthworm, a starfish and a sea-squirt have been found to contain the enzymes arginase, ornithine aminotransferase and pyrroline-5-carboxylate reductase which are necessary to convert arginine to proline. For each of the animals studied analysis for the relevant free amino acids and for the characteristic amino acids (Pro, Oh-Pro, Oh-Lys, Gly) of collagen are presented. The amino acid composition of the diet of the sea-squirt Pyura stolonifera and of the starfish Coscinasterias calamaria is presented along with the level of the phosphagen kinases of the animals studied. The significance of the experimental results is discussed in connection with the importance of the transamidination reaction.     

小果油茶与普通油茶居群遗传结构及种间杂交渐渗

利用11对SSR引物对5个同域分布的小果油茶和普通油茶居群进行了遗传多样性、居群遗传结构,以及种间杂交渐渗的研究.结果表明:两物种的共有等位基因数为57个,占总数的96.6％;在普通油茶中仅检测到2个特异等位基因,而在小果油茶中没有检测到特异等位基因,但两物种的等位基因频率存在一定的差别.两物种的遗传多样性均较高.在物种和居群层次上,两物种的多态信息含量的变化幅度均为0.25 ～0.50,属于中度多态性.同域分布的两物种间的平均遗传分化系数为0.057,明显低于物种内异域居群间的遗传分化系数的平均值(小果油茶0.332,普通油茶0.201),也低于木本被子植物种内居群间平均水平(0.102).5个同域分布物种间基因流的平均值为10.072,明显高于物种内异域间基因流的平均值(小果油茶1.006,普通油茶1.990).UPGMA聚类结果显示,小果油茶和普通油茶之间的亲缘关系非常接近,明显存在着因杂交而产生的基因渐渗.小果油茶或许是普通油茶的一个变种.     

Phylogeography and introgressive hybridization of the ground beetle Carabus yamato in Japan based on mitochondrial gene sequences

To study the phylogeography of the ground beetle Carabus yamato in Japan, we compared 1,020-bp sequences of the mitochondrial NADH dehydrogenase subunit 5 (ND5) gene from 373 specimens from 37 localities with those of three parapatric species (C. albrechti, C. kimurai, and C. japonicus) that might share mitochondrial lineages with C. yamato through introgressive hybridization. We found 81 haplotypes from C. yamato. Of these, 17 haplotypes were considered to be of an introgressed lineage from C. albrechti, based on the phylogeny and geographic distribution. In addition, one haplotype of C. kimurai was likely an introgressant from C. yamato. Putative introgression events among the four species were restricted to these two directional cases. We analyzed the phylogeography of C. yamato using nested clade phylogeographical analysis and population genetic parameters. The mitochondrial lineages of C. yamato were estimated to have diverged no more than approximately 1.12 million years ago, implying that the estimated historical events occurred after the Early Pleistocene. Carabus yamato was inferred to have experienced a contraction of its distribution range, followed by recent range expansion. Populations in the western and eastern regions, segregated by Ise Bay and the Nobi and Okazaki Plains, diverged in the mitochondrial clades. The northern and most western populations possessed one clade only (except an introgressed lineage), whereas eastern and some southwestern populations possessed several diverged clades, which were considered to be ancestral; these populations may have been associated with refugia during glacial periods.     

Patterns of hybridization in plants

Hybridization plays an important role in the evolution of many taxonomic groups, but large-scale phylogenetic patterns of hybridization are poorly known. Here, we investigate patterns of hybridization in vascular plants. Our dataset included 282 families, 3212 genera and ≈37,000 species accounts from eight regional floras covering continental Europe, two island regions, and parts of North America and Australia. Interspecific hybrids were common in the wild, occurring in 40% of families and 16% of genera, with an overall frequency of 0.09 hybrids per nonhybrid species. Taxon species richness explained a large amount of variation in the number of hybrids, but taxon bias (study effort) did not. We accounted for species richness in calculating hybridization propensities, and found that both families and genera differed in hybridization propensity. Hybridization propensity of a given group was generally consistent across regions (with the exception of Hawaii), suggesting that hybridization behavior may be determined more by intrinsic properties of a group than by environmental conditions. We found evidence of a strong phylogenetic signal (λ=0.93) in hybridization propensity as hybrids were not uniformly distributed across orders of vascular plants. Characterization of the hybridization behavior of groups should lead to increased predictive power regarding their traits and evolutionary trajectories, and will allow comparative tests of the traits driving differences in hybridization propensity.     

Three genetically divergent lineages of the Oryx in eastern Africa: Evidence for an ancient introgressive hybridization

Phylogeographic and population genetic studies using sequence information are frequently used to infer species boundaries and history; and to assess hybridization and population level processes. In this study, partial mitochondrial DNA (mtDNA) control region (423 bp) and cytochrome b sequences (666 bp) of Oryx beisa sampled from five isolated localities in its entire current range in Africa were analyzed to investigate the extent of genetic variation and differentiation between populations. We observed high nucleotide diversity at the control region in the total sample (6.3%) but within populations, it varied considerably ranging from 1.6% to 8.1%. Population pairwise genetic differentiation was generally significantly high (ranging from F _ST = 0.15, P<0.01 to F _ST = 0.54, P<0.001). In the total sample, 29 and 12 haplotypes were observed in the control region and the cytochrome b data sets respectively. For both data sets, the haplotypes cluster into three distinct clades (sequence divergence ranged from 6.0%–12.9% to 0.8%–1.0% for the control region and cytochrome b sequences, respectively) that do not correspond to sampling locations. Two of these clades are found in the same localities (Samburu and Marsabit), which represent the O.beisa beisa subspecies, whereas the last clade represents the fringe-eared oryx (O. beisa callotis). We interpret these findings in terms of an ancient hybridization and introgression between two formerly isolated taxa of Oryx beisa.     

Widespread mito‐nuclear discordance with evidence for introgressive hybridization and selective sweeps in Lycaeides

We investigated the extent and potential cause(s) of mitochondrial introgression within the polytypic North American Lycaeides species complex (Lepidoptera). By comparing population genetic structure based on mitochondrial DNA (COI and COII) and nuclear DNA (251 polymorphic amplified fragment length polymorphism markers), we detected substantial mito‐nuclear discordance, primarily involving a single mitochondrial haplotype (h01), which is likely due to mitochondrial introgression between differentiated Lycaeides populations and/or species. We detected reduced mitochondrial genetic diversity relative to nuclear genetic diversity in populations where mitochondrial haplotype h01 occurs, suggesting that the spread of this haplotype was facilitated by selection. We found no evidence that haplotype h01 is associated with increased fitness (in terms of survival to eclosion, fresh adult weight, and adult longevity) in a polymorphic Lycaeides melissa population. However, we did find a positive association between mitochondrial haplotype h01 and infection by the endoparasitic bacterium Wolbachia in one out of three lineages tested. Linkage disequilibrium between mitochondrial haplotype h01 and Wolbachia infection status may have resulted in indirect selection favouring the spread of haplotype h01 in at least one lineage of North American Lycaeides. These results illustrate the potential for introgressive hybridization to produce substantial mito‐nuclear discordance and demonstrate that an individual's mitochondrial and nuclear genome may have strikingly different evolutionary histories resulting from non‐neutral processes and intrinsic differences in the inheritance and biology of these genomes.     

Somatic hybridization in higher plants

Summary Somatic hybridization in higher plants has come into focus since methods have been established for protoplast fusion and uptake of foreign DNA and organelles by protoplasts. Polyethylene glycol (PEG) was an effective agent for inducing fusion. Treatment of protoplasts with PEG resulted in 5 to 30% heterospecific fusion products. Protoplasts of different species, genera and even families were compatible when fused. A number of protoplast combinations (soybean + corn, soybean + pea, soybean + tobacco, carrot + barley, etc.) provided fusion products which underwent cell division and callus formation. Fusion products initially were heterokaryocytes. In dividing heterokaryocytes, random distribution of mitotic nuclei was observed to be accompanied by multiple wall formation and to result in chimeral callus. Juxtaposition of mitotic nuclei suggested nuclear fusion and hybrid formation. Fusion of heterospecific interphase nuclei was demonstrated in soybean + pea and carrot + barley heterokaryons. Provided parental protoplasts carry suitable markers, the fusion products can be recognized. For the isolation and cloning of hybrid cells, fusion experiments must be supplemented with a selective system. Complementation of two nonallelic genes that prevent or inhibit growth under special culture conditions appears as the principle on which to base the selection of somatic hybrids. As protoplasts of some species have been induced to regenerate entire plants, the development of hybrid plants from protoplast fusion products is feasible and has already been demonstrated for tobacco. Presented in the formal symposium on Somatic Cell Genetics at the 27th Annual Meeting of the Tissue Culture Association, Philadelphia, Pennsylvania, June 7–10, 1976.