利用EST-SSR分子标记检测鹅掌楸种间渐渗杂交

引进物种常与本土近缘种间发生杂交而导致基因渐渗,进而影响本地种的遗传系统。鹅掌楸(Liriodendron chinense)为我国濒危树种之一,鹅掌楸种间杂交可配性高,近10年来随着北美鹅掌楸(L.tulipifera)的大量引种及杂交鹅掌楸的繁殖推广,鹅掌楸种间发生基因渐渗的可能性大大增加。本文以5个包含鹅掌楸、北美鹅掌楸、杂交鹅掌楸的人工混交林为实验群体,每个群体包含19–130个成年个体及60–117个自然更新小苗或半同胞子代,利用EST-SSR(simple sequence repeat based on expressed sequence tag)分子标记对各子代群体进行亲本分析,探究鹅掌楸种间自然交配方式以检测鹅掌楸种间渐渗杂交;同时,以亲子群体等位基因频率的回归系数作为渐渗系数,定量分析鹅掌楸属种间基因渐渗。结果表明:在研究的5个鹅掌楸混生群体中,均检测到种间渐渗杂交,且渐渗杂交的方向与效应大小在不同群体间有差异。研究还发现,各树种基因渐渗程度与其种群个体数有关,种群个体数越多,其渐渗能力越强。据此,我们还探讨了鹅掌楸种质资源保护策略。     

Genetic assessment of spatiotemporal evolutionary relationships and stocking effects in grayling (Thymallus thymallus, Salmonidae)

Domestically reared introduced (or escaped) individuals can have detrimental genetic effects on the indigenous populations into which they are released. Consequently, numerous studies have attempted to estimate whether non-native specimens have contributed to the gene pool of wild populations. So far, the key limiting factor of such studies has been their lack of appropriate baseline genetic material. Here, microsatellite DNA analyses of historical scale samples and contemporary wild and introduced populations were used to assess spatiotemporal population structure and stocking effects among endangered Lake Saimaa (eastern Finland) grayling ( Thymallus thymallus , Salmonidae). Significant decreases in genetic differentiation were detected between wild and introduced populations since the commencement of stocking in 1986. Accordingly, up to 15% of the contemporary wild grayling were confidently identified to be of pure hatchery origin, and recent hybridization between the hatchery and indigenous individuals appeared likely. Despite these clear genetic imprints of stocking, the contemporary populations exhibited evolutionary relationships congruent with the sampling locations, and up to 73% of contemporary individuals were identified to be of pure indigenous origin. The use of historical baseline material should prove efficient for monitoring gene flow between domesticated and wild populations in other species also, e.g. salmonids, game animals and plants.     

Recombination and introgression of nuclear and chloroplast genomes between the peat mosses, Sphagnum capillifolium and Sphagnum quinquefarium

Haploid hybrid gametophytes are often present at low frequencies in sympatric populations of Sphagnum capillifolium and Sphagnum quinquefarium. We used intersimple sequence repeat (ISSR) markers and polymerase chain reaction-restriction fragment length polymorphism of the trnL(UAA) intron of the chloroplast genome to reveal the nuclear and chloroplast composition of mature hybrid gametophytes from natural populations and of gametophytes derived from spores of hybrid sporophytes collected in nature. Asymmetrical nuclear inheritance was found in the progeny of the hybrid sporophytes, indicating that only spores with a low level of recombination of parental genomes were viable. A similarly skewed nuclear composition was found among the naturally occurring hybrid gametophytes. All hybrid genomes contained a larger proportion of S. capillifolium ISSR markers, combined with only two to five S. quinquefarium markers together with a chloroplast haplotype derived from S. quinquefarium. In this way, a pattern resembling introgression is created within a single generation. Some individuals possessed nuclear genomes typical for S. capillifolium in combination with the chloroplast haplotype of S. quinquefarium, possibly indicating backcrossing. Our results indicate that hybridization between S. capillifolium and S. quinquefarium is relatively common, but the resistance of large parts of the genome against heterospecific genes maintains the genetic distinctness of the species. Further evolutionary and phylogenetic consequences of restricted interspecific gene exchange are discussed.     

Testicular dysfunction in hybrids between parapatric subspecies of the grasshopper Chorthippus parallelus

Two subspecies of Chorthippus parallelus meet in a narrow hybrid zone in the Pyrenees. Laboratory Fl hybrids between the two races had atrophied testes and were virtually sterile. Backcross males had variable levels of dysfunction but were generally intermediate between Parental and Fl males; there were some reciprocal differences. No other effects on fertility and viability of males and females were apparent, and Fl males had normal accessory glands. These data provide another clear example of Haldane's rule and have implications for the structure of hybrid zones, the evolution of mating isolation and mating strategies.     

GENETIC DIFFERENTIATION AT NUCLEAR AND MITOCHONDRIAL LOCI AMONG LARGE WHITE‐HEADED GULLS: SEX‐BIASED INTERSPECIFIC GENE FLOW?

We measured genetic differentiation among species of large white-headed gulls using mitochondrial (cytochrome b haplotypes) and nuclear (microsatellites) markers. Additional information was added using a previously published study of allozymes on the same species. Levels of differentiation among species at nuclear markers are much lower than would be expected for avian species and are not concordant with the level of differentiation in mitochondrial markers. This discrepancy is best explained by a combination of recent species origin and interspecific gene flow after speciation. The data also suggest that female-mediated gene flow is reduced compared to male-mediated gene flow, either due to behavioral bias or due to stronger counterselection of female hybrids in accordance with Haldane's rule for ZW species. Whatever the reasons for the low differentiation of the species' nuclear gene pools, the extensive similarity of their nuclear genome demonstrates that selection on a limited number of characters is an important factor in establishing and maintaining clear-cut phenotypic differences between these species and suggests that the number of loci involved in this process is quite low. This situation may not be exceptional in birds, indeed a number of studies have found similarly low level of differentiation in nuclear markers among congeneric bird species, although usually based on a single set of markers. Because hybridization is a widespread phenomenon in birds, many of these cases might be due to interspecific gene flow.     

Amount of introgression in flycatcher hybrid zones reflects regional differences in pre and post-zygotic barriers to gene exchange

Introgression is the incorporation of alleles from one species or semispecies into the gene pool of another through hybridization and backcrossing. The rate at which this occurs depends on the frequency of hybridization and the fitness of hybrids and backcrosses compared to 'pure' individuals. The collared flycatcher (Ficedula albicollis) and the pied flycatcher (F. hypoleuca) co-exist and hybridize at low to moderate frequencies in a clinal hybrid zone in Central Europe and on the islands of Gotland and Oland off the Swedish east coast. Data on hatching success suggest that hybrids are less fertile in Central Europe compared to on the islands. Direct fitness estimates using molecular markers to infer paternity are consistent with the demographic data. Applying a tag-array-based minisequencing assay to genotype interspecific substitutions and single nucleotide polymorphisms we demonstrate that the amount of introgression from the pied to the collared flycatcher is higher in the two island populations (Gotland and Oland) than in two geographically distinct areas from the Central European hybrid zone (Czech Republic and Hungary). In all areas the amount of introgression from collared to pied flycatchers is very low or seemingly absent. The different patterns of introgression are consistent with regional differences in rates of hybridization and fitness of hybrids. We suggest that barriers to gene exchange may have been partly broken down on the islands due to asymmetric gene flow from allopatry. Alternatively, or in addition, more pronounced reinforcement of prezygotic isolation in Central Europe might have increased post-zygotic isolation through hitchhiking, since genes affecting pre and post-zygotic isolation are both sex-linked in these birds. One of our genetic markers appears to introgress from pied to collared flycatchers at a much higher rate than the other markers. We discuss the possibility that the introgressed marker may be linked to a gene which is under positive selection in the novel genetic background.     

Genome scan of hybridizing sunflowers from Texas (Helianthus annuus and H. debilis) reveals asymmetric patterns of introgression and small islands of genomic differentiation

Although the sexual transfer of genetic material between species (i.e. introgression) has been documented in many groups of plants and animals, genome‐wide patterns of introgression are poorly understood. Is most of the genome permeable to interspecific gene flow, or is introgression typically restricted to a handful of genomic regions? Here, we assess the genomic extent and direction of introgression between three sunflowers from the south‐central USA: the common sunflower, Helianthus annuus ssp. annuus; a near‐endemic to Texas, Helianthus debilis ssp. cucumerifolius; and their putative hybrid derivative, thought to have recently colonized Texas, H. annuus ssp. texanus. Analyses of variation at 88 genetically mapped microsatellite loci revealed that long‐term migration rates were high, genome‐wide and asymmetric, with higher migration rates from H. annuus texanus into the two parental taxa than vice versa. These results imply a longer history of intermittent contact between H. debilis and H. annuus than previously believed, and that H. annuus texanus may serve as a bridge for the transfer of alleles between its parental taxa. They also contradict recent theory suggesting that introgression should predominantly be in the direction of the colonizing species. As in previous studies of hybridizing sunflower species, regions of genetic differentiation appear small, whether estimated in terms of F_ST or unidirectional migration rates. Estimates of recent immigration and admixture were inconsistent, depending on the type of analysis. At the individual locus level, one marker showed striking asymmetry in migration rates, a pattern consistent with tight linkage to a Bateson–Dobzhansky–Muller incompatibility.     

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.     

Review. Genetic exchange and the origin of adaptations: prokaryotes to primates

Data supporting the occurrence of adaptive trait transfer (i.e. the transfer of genes and thus the phenotype of an adaptive trait through viral recombination, lateral gene transfer or introgressive hybridization) are provided in this review. Specifically, we discuss examples of lateral gene transfer and introgressive hybridization that have resulted in the transfer or de novo origin of adaptations. The evolutionary clades in which this process has been identified include all types of organisms. However, we restrict our discussion to bacteria, fungi, plants and animals. Each of these examples reflects the same consequence, namely that the transfer of genetic material, through whatever mechanism, may result in adaptive evolution. In particular, each of the events discussed has been inferred to impact adaptations to novel environmental settings in the recipient lineage.     

Bayesian analysis of hybridization and introgression between the endangered european mink (Mustela lutreola) and the polecat (Mustela putorius)

Human-mediated global change will probably increase the rates of natural hybridization and genetic introgression between closely related species, and this will have major implications for conservation of the taxa involved. In this study, we analyse both mitochondrial and nuclear data to characterize ongoing hybridization and genetic introgression between two sympatric sister species of mustelids, the endangered European mink (Mustela lutreola) and the more abundant polecat (M. putorius). A total of 317 European mink, 114 polecats and 15 putative hybrid individuals were collected from different localities in Europe and genotyped with 13 microsatellite nuclear markers. Recently developed Bayesian methods for assigning individuals to populations and identifying admixture proportions were applied to the genetic data. To identify the direction of hybridization, we additionally sequenced mtDNA and Y chromosomes from 78 individuals and 29 males respectively. We found that both hybridization and genetic introgression occurred at low levels (3% and 0.9% respectively) and indicated that hybridization is asymmetric, as only pure polecat males mate with pure European mink females. Furthermore, backcrossing and genetic introgression was detected only from female first-generation (F1) hybrids of European mink to polecats. This latter result implies that Haldane's rule may apply. Our results suggest that hybridization and genetic introgression between the two species should be considered a rather uncommon event. However, the current low densities of European mink might be changing this trend.