Natural hybridization and reproductive isolation between two Primula species

Natural hybridization frequently occurs in plants and can facilitate gene flow between species,possibly resulting in species refusion. However,various reproductive barriers block the formation of hybrids and maintain species integrity. Here,we conducted a field survey to examine natural hybridization and reproductive isolation(RI) between sympatric populations of Primula secundiflora and P. poissonii using ten nuclear simple sequence repeat(SSR) loci. Although introgressive hybridization occurred,species boundaries between P. secundiflora and P. poissonii were maintained through nearly complete reproductive isolation. These interfertile species provide an excellent model for studying the RI mechanisms and evolutionary forces that maintain species boundaries.     

The Puzzle of Rice Domestication

The origin of cultivated rice has puzzled plant biologists for decades. This is due, at least in part, to the complex evolutionary dynamics in rice cultivars and wild progenitors, particularly rapid adaptive differentiation and continuous gene flow within and between cultivated and wild rice. The long-standing controversy over single versus multiple and annual versus perennial origins of cultivated rice has been brought into shaper focus with the rapid accumulation of genetic and phylogenetic data. Molecular phylogenetic analyses revealed ancient genomic differentiation between rice cultivars, suggesting that they were domesticated from divergent wild populations. However, the recently cloned domestication gene sh4, responsible for the reduction of grain shattering from wild to cultivated rice, seems to have originated only once. Herein, we propose two models to reconcile apparently conflicting evidence regarding rice domestication. The snowoballing model considers a single origin of cultivated rice. In this model, a core of critical domestication alleles was fixed in the founding cultivar and then acted to increase the genetic diversity of cultivars through hybridization with wild populations. The combination model considers multiple origins of cultivated rice. In this model, Initial cultivars were domesticated from divergent wild populations and fixed different sets of domestication alleles. Subsequent crosses among these semi-domesticated cultivars resulted in the fixation of a similar set of critical domestication alleles in the contemporary cultivars. In both models, introgression has played an important role in rice domestication. Recent and future introgression of beneficial genes from the wild gene pool through conventional and molecular breeding programs can be viewed as the continuation of domestication.     

Reproductive Isolation during Domestication

It has been hypothesized that reproductive isolation should facilitate evolution under domestication. However, a systematic comparison of reproductive barrier strength between crops and their progenitors has not been conducted to test this hypothesis. Here, we present a systematic survey of reproductive barriers between 32 economically important crop species and their progenitors to better understand the role of reproductive isolation during the domestication process. We took a conservative approach, avoiding those types of reproductive isolation that are poorly known for these taxa (e.g., differences in flowering time). We show that the majority of crops surveyed are isolated from their progenitors by one or more reproductive barriers, despite the fact that the most important reproductive barrier in natural systems, geographical isolation, was absent, at least in the initial stages of domestication for most species. Thus, barriers to reproduction between crops and wild relatives are closely associated with domestication and may facilitate it, thereby raising the question whether reproductive isolation could be viewed as a long-overlooked "domestication trait." Some of the reproductive barriers observed (e.g., polyploidy and uniparental reproduction), however, may have been favored for reasons other than, or in addition to, their effects on gene flow.     

Multiple reproductive barriers separate recently diverged sunflower ecotypes

Measuring reproductive barriers between groups of organisms is an effective way to determine the traits and mechanisms that impede gene flow. However, to understand the ecological and evolutionary factors that drive speciation, it is important to distinguish between the barriers that arise early in the speciation process and those that arise after speciation is largely complete. In this article, we comprehensively test for reproductive isolation between recently diverged (<10,000 years bp) dune and nondune ecotypes of the prairie sunflower, Helianthus petiolaris. We find reproductive barriers acting at multiple stages of hybridization, including premating, postmating–prezygotic, and postzygotic barriers, despite the recent divergence. Barriers include extrinsic selection against immigrants and hybrids, a shift in pollinator assemblage, and postpollination assortative mating. Together, these data suggest that multiple barriers can be important for reducing gene flow in the earliest stages of speciation.     

Population genetic structure of Picea engelmannii, P. glauca and their previously unrecognized hybrids in the central Rocky Mountains

Areas of geographic overlap between potentially hybridizing species provide the opportunity to study interspecific gene flow and reproductive barriers. Here we identified hybrids between Picea engelmannii and P. glauca by their genetic composition at 17 microsatellite markers, and determined the broad-scale geographic distribution of hybrids in the central Rocky Mountains of North America, a geographic region where hybrids and isolation between species had not previously been studied. Parameter estimates from admixture models revealed considerable variation in ancestry within and among collection sites, suggesting that within this area of geographic overlap, the interaction of the two species varies extensively. The results document a previously unrecognized patchy distribution of hybrids between P. engelmannii and P. glauca, including locations where hybrids were not known or expected to exist. Further, the ancestry of many hybrids was consistent with multiple generations of hybridization, with probable directional backcrossing to P. engelmannii, suggesting a relatively porous species boundary. The identification and characterization of hybridization between these spruce in this region raises the question of what factors maintain barriers to gene flow in these long-lived forest trees. The current research lays the groundwork for future study of the ecological and evolutionary contexts of their hybridization, as well as of differential introgression and permeability of species boundaries.     

Species and “strange species” in zoology: Do we need a “unified concept of species”?

Eidonomy is the field of taxonomy, which addresses the “species problem”. The latter has several dimensions. The first is the confusions frequently made between species as an evolutionary unit, a taxon, a taxonomic category and a nomenclatural rank. The second is the reductionist temptation to recognize in nature only one kind of “basic entities” of biodiversity. From a practical viewpoint and for all non-systematist users of specific nomina, any animal organism should be referred to a taxon of nomenclatural rank species, designated by a Latin binomen. These nomina are indispensable for administrative and legal documents, for management and conservation of biodiversity, as well as for all biological research. But this does not imply that all these taxa should be referred to a single taxonomic category, a “unified concept of species”. In nature, several kinds of entities correspond to different “species concepts” or specions that are irreducible one to another. These categories can be defined according to the modalities of their reproduction, i.e., of their gametopoiesis (mode of formation of gametes), of their kinetogenesis (mode of initiation of the development of the ovum) and of the gene flow between individuals. The best known of these categories is that of mayron or “mixiological species concept”, which points to an independent bisexual panmictic entity, constituting a protected gene pool, but others do exist. In order to better understand their differences, it is useful to consider the various patterns of speciation. These can be referred to three main categories: monogeny (change within a single lineage), diplogeny (birth of two specions from a single one) and mixogeny (hybridization between two specions). The splitting in two of a single initial gene pool is a rather long process, which often shows different stages. The latter can be expressed taxonomically through the use of particular eidonomic categories. Speciation can be considered completed only when it has become irreversible in case of new contact between the two stocks after a period of separation. In allopatry, it is difficult or impossible to know if this stage has been reached. This is not a reason for abandoning the mixiological criterion for identifying sympatric or parapatric specions. In allopatry, the method of inference can be used and, in the absence of sufficient information, it is fully possible to use “by default” the “phylogenetic concept of species” or simpson. The situation is further complicated by the fact that some of the basic entities of biodiversity of hybrid origin, the kyons, are not mayrons, as they have particular reproductive modes. Contrary to isolated and “normal” hybrids, such entities may persist in the long term in nature. Their gametopoiesis implies either an ameiosis (mitoses) or a metameiosis (modified meiosis), and their kinetogenesis implies zygogenesis (fertilization), gynogenesis or parthenogenesis. Kyons are of two main categories. The klonons are unisexual female entities in which genetic transmission is clonal, reproduction being often through parthenogenesis, or following other mechanisms with similar results in genetic terms. The kleptons are unisexual or bisexual entities, which depend for their reproduction, at each generation, on a mayron or another klepton. Their metameiosis produces particular gametes, which start their development either by zygogenesis (zygokleptons), by gynogenesis (gynokleptons) or by a combination of both systems (tychokleptons). All these particular cases do not constitute, as it has long been believed, “evolutionary dead ends”. On the contrary, some of these forms are advantaged in some conditions, and may also, in some cases, be at the origin of allopolyploid specions. The latter include both “normal mayrons” or eumayrons (allotetraploid bisexual entities) and heteromayrons (allotriploid bisexual entities with metameiosis). Mayron, klepton, klonon and simpson, and their subcategories, are different taxonomic categories which correspond to a single nomenclatural rank, that of species. The existence of interspecific hybridization in nature is a very common phenomenon in animals, the importance of which has long been underestimated. Reticulous relationships between specions have played a major role in the scenario of evolution. This should be acknowledged by the rejection of the “universal tree of life” model for organic evolution and its replacement by a “network of life” model.     

Sleeping with the ‘enemy’: hybridization of an endangered tree weta

While hybridization is an important part of the evolutionary process, for rare species mating with more common species hybridization can increase the risks of extinction. By mating with heterospecifics rare species waste valuable reproductive resources and as a result population sizes may decline. If introgression occurs, the rare species can become genetically swamped by alleles from the more common species, rendering it effectively extinct. As a consequence of these risks, hybridization with the more common species Hemideina femorata (Canterbury tree weta) may lead to the extinction of the range restricted species H. ricta (Banks Peninsula tree weta) on Banks Peninsula. The current study uses spatial interpolation to model the distribution of each species and the potential sympatric zone to guide sampling efforts. Polymorphic microsatellite markers and mitochondrial sequence data were used to determine the extent of hybridization between H. ricta and H. femorata. The results confirm that hybridization is possible between these species. However, hybrids and introgression appear to be very rare, suggesting that reproductive isolating barriers are present but incomplete. The threat of extinction to H. ricta via hybridization with H. femorata is low but extreme loss of habitat may cause changes to population densities that could increase the risks of hybridization. Therefore, landowners should be encouraged to conserve native bush.     

Mammalian hybrid zones: a review

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Genetic diversity, structure, gene flow and evolutionary relationships within the Sorghum bicolor wild�Cweedy�Ccrop complex in a western African region

Gene flow between domesticated plants and their wild relatives is one of the major evolutionary processes acting to shape their structure of genetic diversity. Earlier literature, in the 1970s, reported on the interfertility and the sympatry of wild, weedy and cultivated sorghum belonging to the species Sorghum bicolor in most regions of sub-Saharan Africa. However, only a few recent surveys have addressed the geographical and ecological distribution of sorghum wild relatives and their genetic structure. These features are poorly documented, especially in western Africa, a centre of diversity for this crop. We report here on an exhaustive in situ collection of wild, weedy and cultivated sorghum assembled in Mali and in Guinea. The extent and pattern of genetic diversity were assessed with 15 SSRs within the cultivated pool (455 accessions), the wild pool (91 wild and weedy forms) and between them. F (ST) and R (ST) statistics, distance-based trees, Bayesian clustering methods, as well as isolation by distance models, were used to infer evolutionary relationships within the wild-weedy-crop complex. Firstly, our analyses highlighted a strong racial structure of genetic diversity within cultivated sorghum (F (ST) = 0.40). Secondly, clustering analyses highlighted the introgressed nature of most of the wild and weedy sorghum and grouped them into two eco-geographical groups. Such closeness between wild and crop sorghum could be the result of both sorghum's domestication history and preferential post-domestication crop-to-wild gene flow enhanced by farmers' practices. Finally, isolation by distance analyses showed strong spatial genetic structure within each pool, due to spatially limited dispersal, and suggested consequent gene flow between the wild and the crop pools, also supported by R (ST) analyses. Our findings thus revealed important features for the collection, conservation and biosafety of domesticated and wild sorghum in their centre of diversity.     

Species' borders and dispersal barriers

Range limits of species are determined by combined effects of physical, historical, ecological, and evolutionary forces. We consider a subset of these factors by using spatial models of competition, hybridization, and local adaptation to examine the effects of partial dispersal barriers on the locations of borders between similar species. Prompted by results from population genetic models and biogeographic observations, we investigate the conditions under which species' borders are attracted to regions of reduced dispersal. For borders maintained by competition or hybridization, we find that dispersal barriers can attract borders whose positions would otherwise be either neutrally stable or moving across space. Borders affected strongly by local adaptation and gene flow, however, are repelled from dispersal barriers. These models illustrate how particular biotic and abiotic factors may combine to limit species' ranges, and they help to elucidate mechanisms by which range limits of many species may coincide.     

Overcoming hybridization barriers by the secretion of the maize pollen tube attractant ZmEA1 from Arabidopsis ovules

A major goal of plant reproduction research is to understand and overcome hybridization barriers so that the gene pool of crop plants can be increased and improved upon. After successful pollen germination on a receptive stigma, the nonmotile sperm cells of flowering plants are transported via the pollen tube (PT) to the egg apparatus for the achievement of double fertilization. The PT path is controlled by various hybridization mechanisms probably involving a larger number of species-specific molecular interactions. The egg-apparatus-secreted polymorphic peptides ZmEA1 in maize and LURE1 and LURE2 in Torenia fournieri as well as TcCRP1 in T. concolor were shown to be required for micropylar PT guidance, the last step of the PT journey. We report here that ZmEA1 attracts maize PTs in vitro and arrests their growth at higher concentrations. Furthermore, it binds to the subapical region of maize PT tips in a species-preferential manner. To overcome hybridization barriers at the level of gametophytic PT guidance, we expressed ZmEA1 in Arabidopsis synergid cells. Secreted ZmEA1 enabled Arabidopsis ovules to guide maize PT in vitro in a species-preferential manner to the micropylar opening of the ovule. These results demonstrate that the egg-apparatus-controlled reproductive-isolation barrier of PT guidance can be overcome even between unrelated plant families.     

Can the problem of hybridization in threatened species be evaluated using a fieldwork research? A case study in snapdragons

Hybridization, natural or artificial, is considered disadvantageous for species biodiversity when it threatens the population integrity of endangered species. Frequently, studies investigating whether hybridization poses a legitimate risk to rare species are based on genetic data obtained in molecular biology laboratories. In this study, we used field research to approach the problem that hybridization could cause for the viability of a population of a rare species and to be able to propose the most appropriate initial conservation strategy. Specifically, using the model genus Antirrhinum, the reproductive barriers between the rare A. pulverulentum and its common congener A. litigiosum have been analysed under the reproductive isolation index (RI). A. pulverulentum had a high value of total RI, indicating that there are barriers to gene flow from A. litigiosum towards this species, and also had a high value for the intrinsic RI, reflecting a low inherent capacity for production of hybrid plants; in addition, the possibility of successful backcrosses between this species with hybrids produced from A. litigiosum ovules were low, given the high intrinsic RI of A. litigiosum. These data indicate the current existence of strong and permanent barriers to hybridization between the two species, suggesting that hybridization does not seem to be a serious problem for the conservation of A. pulverulentum in the studied population, nor for the near future. This study shows how the RI index can provide useful information for conservation purposes and proposes different management recommendations.     

Incomplete reproductive isolation between Rhododendron taxa enables hybrid formation and persistence

The evolutionary consequences of hybridization ultimately depend on the magnitude of reproductive isolation between hybrids and their parents. We evaluated the relative contributions of pre-and post-zygotic barriers to reproduction for hybrid formation, hybrid persistence and potential for reproductive isolation of hybrids formed between two Rhododendron species,R. spiciferum and R. spinuliferum. Our study established that incomplete reproductive isolation promotes hybrid formation and persistence and delays hybrid speciation.All pre-zygotic barriers to reproduction leading to hybrid formation are incomplete: parental species have overlapping flowering; they share the same pollinators;reciprocal assessments of pollen tube germination and growth do not differ among parents. The absence of post-zygotic barriers between parental taxa indicates that the persistence of hybrids is likely. Reproductive isolation was incomplete between hybrids and parents in all cases studied, although asymmetric differences in reproductive fitness were prevalent and possibly explain the genetic structure of natural hybrid swarms where hybridization is known to be bidirectional but asymmetric. Introgression, rather than speciation, is a probable evolutionary outcome of hybridization between the two Rhododendron taxa. Our study provides insights into understanding the evolutionary implications of natural hybridization in woody plants.     

植物物种形成的模式与机制

物种形成是指由已有的物种通过各种进化机制进化出新物种的过程。持续不断的物种形成产生了地球上灿烂的生物物种多样性。然而,研究人员对物种形成的模式与机制的了解却非常有限。一直以来,谱系分裂被认为是最重要的物种形成模式,但在植物中,谱系融合,即通过杂交形成新物种的过程,也是一个非常重要的物种形成模式。经过几十年的研究才逐渐认识到,生殖隔离是差异适应和遗传漂变的副产品,而不是物种形成的前提。相比合子形成后隔离,合子形成前的隔离在物种形成过程中更早地发挥作用。合子形成前的隔离,尤其是生态地理的隔离是植物中最重要的隔离机制。一些基于QTLs分析的研究发现,基因组中的少数主效位点在物种形成中起了关键作用,并且这些位点往往受到自然选择的作用。适应性辐射往往发生在隆起的山脉和新形成的岛屿上,很可能与这些地方能够提供很多可利用的生态位有关。最新的物种形成理论认为,基因是物种形成的基本单位,不同的物种可以在非控制物种差异适应性状的位点上存在基因流。这一观点为植物物种形成的研究提供了新的思路。随着植物物种形成研究的深入,越来越多植物物种形成基因被分离,包括花色素苷合成通路和S-基因座上的一些关键基因,揭示了植物物种形成的分子机制。前期的研究主要集中在模式植物和农作物上,许多生态上非常有趣的非模式植物还未得到广泛的研究。在未来的研究中,还需要更多来自非模式植物的例子以全面理解植物物种形成的多样化机制。     

Strong postmating reproductive isolation in Mimulus section Eunanus

Postmating reproductive isolation can help maintain species boundaries when premating barriers to reproduction are incomplete. The strength and identity of postmating reproductive barriers are highly variable among diverging species, leading to questions about their genetic basis and evolutionary drivers. These questions have been tackled in model systems but are less often addressed with broader phylogenetic resolution. In this study we analyse patterns of genetic divergence alongside direct measures of postmating reproductive barriers in an overlooked group of sympatric species within the model monkeyflower genus, Mimulus. Within this Mimulus brevipes species group, we find substantial divergence among species, including a cryptic genetic lineage. However, rampant gene discordance and ancient signals of introgression suggest a complex history of divergence. In addition, we find multiple strong postmating barriers, including postmating prezygotic isolation, hybrid seed inviability and hybrid male sterility. M. brevipes and M. fremontii have substantial but incomplete postmating isolation. For all other tested species pairs, we find essentially complete postmating isolation. Hybrid seed inviability appears linked to differences in seed size, providing a window into possible developmental mechanisms underlying this reproductive barrier. While geographic proximity and incomplete mating isolation may have allowed gene flow within this group in the distant past, strong postmating reproductive barriers today have likely played a key role in preventing ongoing introgression. By producing foundational information about reproductive isolation and genomic divergence in this understudied group, we add new diversity and phylogenetic resolution to our understanding of the mechanisms of plant speciation.     

Multiple origins of the determinate growth habit in domesticated common bean (Phaseolus vulgaris)

Background and Aims

The actual number of domestications of a crop is one of the key questions in domestication studies. Answers to this question have generally been based on relationships between wild progenitors and domesticated descendants determined with anonymous molecular markers. In this study, this question was investigated by determining the number of instances a domestication phenotype had been selected in a crop species. One of the traits that appeared during domestication of common bean (Phaseolus vulgaris) is determinacy, in which stems end with a terminal inflorescence. It has been shown earlier that a homologue of the arabidopsis TFL1 gene – PvTFL1y – controls determinacy in a naturally occurring variation of common bean.

Methods

Sequence variation was analysed for PvTFL1y in a sample of 46 wild and domesticated accessions that included determinate and indeterminate accessions.

Key Results

Indeterminate types – wild and domesticated – showed only synonymous nucleotide substitutions. Determinate types – observed only among domesticated accessions – showed, in addition to synonymous substitutions, non-synonymous substitutions, indels, a putative intron-splicing failure, a retrotransposon insertion and a deletion of the entire locus. The retrotransposon insertion was observed in 70 % of determinate cultivars, in the Americas and elsewhere. Other determinate mutants had a more restricted distribution in the Americas only, either in the Andean or in the Mesoamerican gene pool of common bean.

Conclusions

Although each of the determinacy haplotypes probably does not represent distinct domestication events, they are consistent with the multiple (seven) domestication pattern in the genus Phaseolus. The predominance of determinacy in the Andean gene pool may reflect domestication of common bean prior to maize introduction in the Andes.     

Plant Domestication and Crop Evolution in the Near East: On Events and Processes

Reconstructing the evolutionary history of crop plants is fundamental for understanding their adaptation profile and the genetic basis of yield-limiting factors, which in turn are critical for future crop improvement. A major topic in this field is the recent claim for a millennia-long ‘protracted’ domestication process. Here we evaluate the evidence for the protracted domestication model in light of published archaeobotanical data, experimental evidence and the biology of the Near Eastern crops and their wild progenitors. The crux of our discussion is the differentiation between events or ‘domestication episodes’ and the later following crop evolutionary processes under domestication (frequently termed ‘crop improvement stage’), which are by definition, still ongoing. We argue that by assuming a protracted millennia-long domestication process, one needlessly opts to operate within an intellectual framework that does not allow differentiating between the decisive (critical) domestication traits and their respective loci, and those that have evolved later during the crop dissemination and improvement following the episodic domestication event. Therefore, in our view, apart from the lack of experimental evidence to support it, the protracted domestication assumption undermines the resolution power of the study of both plant domestication and crop evolution, from the cultural as well as from the biological perspectives.     

Evolutionary domestication in Drosophila subobscura

The domestication of plants and animals is historically one of the most important topics in evolutionary biology. The evolutionary genetic changes arising from human cultivation are complex because of the effects of such varied processes as continuing natural selection, artificial selection, deliberate inbreeding, genetic drift and hybridization of different lineages. Despite the interest of domestication as an evolutionary process, few studies of multicellular sexual species have approached this topic using well-replicated experiments. Here we present a comprehensive study in which replicated evolutionary trajectories from several Drosophila subobscura populations provide a detailed view of the evolutionary dynamics of domestication in an outbreeding animal species. Our results show a clear evolutionary response in fecundity traits, but no clear pattern for adult starvation resistance and juvenile traits such as development time and viability. These results supply new perspectives on the confounding of adaptation with other evolutionary mechanisms in the process of domestication.     

The molecular and evolutionary basis of reproductive isolation in plants

Reproductive isolation is defined as processes that prevent individuals of different populations from mating, survival or producing fertile offspring. Reproductive isolation is critical for driving speciation and maintaining species identity, which has been a fundamental concern in evolutionary biology. In plants, reproductive isolation can be divided into prezygotic and postzygotic reproductive barriers, according to its occurrence at different developmental stages. Postzygotic reproductive isolation caused by reduced fitness in hybrids is frequently observed in plants, which hinders gene flow between divergent populations and has substantial effects on genetic differentiation and speciation, and thus is a major obstacle for utilization of heterosis in hybrid crops. During the past decade, China has made tremendous progress in molecular and evolutionary basis of prezygotic and postzygotic reproductive barriers in plants. Present understandings in reproductive isolation especially with new data in the last several years well support three evolutionary genetic models, which represent a general mechanism underlying genomic differentiation and speciation. The updated understanding will offer new approaches for the development of wide-compatibility or neutral varieties, which facilitate breeding of hybrid rice as well as other hybrid crops.