Expression pattern of resynthesized allotetraploid Capsella is determined by hybridization, not whole-genome duplication

Polyploidization, the process leading to the increase in chromosome sets, is a major evolutionary transition in plants. Whole-genome duplication (WGD) within the same species gives rise to autopolyploids, whereas allopolyploids result from a compound process with two distinct components: WGD and interspecific hybridization. To dissect the instant effects of WGD and hybridization on gene expression and phenotype, we created a series of synthetic hybrid and polyploid Capsella plants, including diploid hybrids, autotetraploids of both parental species, and two kinds of resynthesized allotetraploids with different orders of WGD and hybridization. Hybridization played a major role in shaping the relative expression pattern of the neo-allopolyploids, whereas WGD had almost no immediate effect on relative gene expression pattern but, nonetheless, still affected phenotypes. No transposable element-mediated genomic shock scenario was observed in either neo-hybrids or neo-polyploids. Finally, WGD and hybridization interacted and the distorting effects of WGD were less strong in hybrids. Whole-genome duplication may even improve hybrid fertility. In summary, while the initial relative gene expression pattern in neo-allotetraploids was almost entirely determined by hybridization, WGD only had trivial effects on relative expression patterns, both processes interacted and had a strong impact on physical attributes and meiotic behaviors

How broad is the selfing syndrome? Insights from convergent evolution of gene expression across species and tissues in the Capsella genus

The shift from outcrossing to selfing is one of the main evolutionary transitions in plants. It is accompanied by profound effects on reproductive traits, the so-called selfing syndrome. Because the transition to selfing also implies deep genomic and ecological changes, one also expects to observe a genomic selfing syndrome. We took advantage of the three independent transitions from outcrossing to selfing in the Capsella genus to characterize the overall impact of mating system change on RNA expression, in flowers but also in leaves and roots. We quantified the extent of both selfing and genomic syndromes, and tested whether changes in expression corresponded to adaptation to selfing or to relaxed selection on traits that were constrained in outcrossers. Mating system change affected gene expression in all three tissues but more so in flowers than in roots and leaves. Gene expression in selfing species tended to converge in flowers but diverged in the two other tissues. Hence, convergent adaptation to selfing dominates in flowers, whereas genetic drift plays a more important role in leaves and roots. The effect of mating system transition is not limited to reproductive tissues and corresponds to both adaptation to selfing and relaxed selection on previously constrained traits

Polymorphism and Divergence in Two Willow Species, Salix viminalis L. and Salix schwerinii E. Wolf

We investigated species divergence, present and past gene flow, levels of nucleotide polymorphism, and linkage disequilibrium in two willows from the plant genus Salix. Salix belongs together with Populus to the Salicaceae family; however, most population genetic studies of Salicaceae have been performed in Populus, the model genus in forest biology. Here we present a study on two closely related willow species Salix viminalis and S. schwerinii, in which we have resequenced 33 and 32 nuclear gene segments representing parts of 18 nuclear loci in 24 individuals for each species. We used coalescent simulations and estimated the split time to around 600,000 years ago and found that there is currently limited gene flow between the species. Mean intronic nucleotide diversity across gene segments was slightly higher in S. schwerinii (πi = 0.00849) than in S. viminalis (πi = 0.00655). Compared with other angiosperm trees, the two willows harbor intermediate levels of silent polymorphisms. The decay of linkage disequilibrium was slower in S. viminalis compared with S. schwerinii, and we speculate that this is due to different demographic histories as S. viminalis has been partly domesticated in Europe

Genetic Differentiation and Delimitation between Ecologically Diverged Populus euphratica and P. pruinosa

The fixed genetic differences between ecologically divergent species were found to change greatly depending on the markers examined. With such species it is difficult to differentiate between shared ancestral polymorphisms and past introgressions between the diverging species. In order to disentangle these possibilities and provide a further case for DNA barcoding of plants, we examine genetic differentiation between two ecologically divergent poplar species, Populus euphratica Oliver and P. pruinosa Schrenk using three different types of genetic marker.We genotyped 290 individuals from 29 allopatric and sympatric populations, using chloroplast (cp) DNA, nuclear (nr) ITS sequences and eight simple sequence repeat (SSR) loci. Three major cpDNA haplotypes were widely shared between the two species and between-species cpDNA differentiation (F(CT)) was very low, even lower than among single species populations. The average SSR F(CT) values were higher. Bayesian clustering analysis of all loci allowed a clear delineation of the two species. Gene flow, determined by examining all SSR loci, was obvious but only slightly asymmetrical. However, the two species were almost fixed for two different nrITS genotypes that had the highest F(CT), although a few introgressed individuals were detected both in allopatric and sympatric populations.The two species shared numerous ancestral polymorphisms at cpDNA and a few SSR loci. Both ITS and a combination of nuclear SSR data could be used to differentiate between the two species. Introgressions and gene flow were obvious between the two species either during or after their divergence. Our findings underscore the complex genetic differentiations between ecologically diverged species and highlight the importance of nuclear DNA (especially ITS) differentiation for delimiting closely related plant species

Divergent selection predating the Last Glacial Maximum mainly acted on macro-phenotypes in Norway spruce

The current distribution and population structure of many species were, to a large extent, shaped by cycles of isolation in glacial refugia and subsequent population expansions. Isolation in and postglacial expansion through heterogeneous environments led to either neutral or adaptive divergence. Norway spruce is no exception, and its current distribution is the consequence of a constant interplay between evolutionary and demographic processes. We investigated population differentiation and adaptation of Norway spruce for juvenile growth, diameter of the stem, wood density, and tracheid traits at breast height. Data from 4461 phenotyped and genotyped Norway spruce from 396 half-sib families in two progeny tests were used to test for divergent selection in the framework of Q(ST) vs. F-ST. We show that the macroscopic resultant trait (stem diameter), unlike its microscopic components (tracheid dimensions) and juvenile growth, was under divergent selection that predated the Last Glacial Maximum. Altogether, the current variation in these phenotypic traits in Norway spruce is better explained by local adaptation to ancestral environments than to current ones, where populations were partly preadapted, mainly through growth-related traits

Genomic data provide new insights on the demographic history and the extent of recent material transfers in Norway spruce

Primeval forests are today exceedingly rare in Europe, and transfer of forest reproductive material for afforestation and improvement has been very common, especially over the last two centuries. This can be a serious impediment when inferring past population movements in response to past climate changes such as the last glacial maximum (LGM), some 18,000 years ago. In the present study, we genotyped 1,672 individuals from three Picea species (P. abies, P. obovata, and P. omorika) at 400K SNPs using exome capture to infer the past demographic history of Norway spruce (P. abies) and estimate the amount of recent introduction used to establish the Norway spruce breeding program in southern Sweden. Most of these trees belong to P. abies and originate from the base populations of the Swedish breeding program. Others originate from populations across the natural ranges of the three species. Of the 1,499 individuals stemming from the breeding program, a large proportion corresponds to recent introductions from mainland Europe. The split of P. omorika occurred 23 million years ago (mya), while the divergence between P. obovata and P. abies began 17.6 mya. Demographic inferences retrieved the same main clusters within P. abies than previous studies, that is, a vast northern domain ranging from Norway to central Russia, where the species is progressively replaced by Siberian spruce (P. obovata) and two smaller domains, an Alpine domain and a Carpathian one, but also revealed further subdivision and gene flow among clusters. The three main domains divergence was ancient (15 mya), and all three went through a bottleneck corresponding to the LGM. Approximately 17% of P. abies Nordic domain migrated from P. obovata ~103K years ago, when both species had much larger effective population sizes. Our analysis of genomewide polymorphism data thus revealed the complex demographic history of Picea genus in Western Europe and highlighted the importance of material transfer in Swedish breeding program

Ancient polymorphisms and divergence hitchhiking contribute to genomic islands of divergence within a poplar species complex

How genome divergence eventually leads to speciation is a topic of prime evolutionary interest. Genomic islands of elevated divergence are frequently reported between diverging lineages, and their size is expected to increase with time and gene flow under the speciation-with-gene-flow model. However, such islands can also result from divergent sorting of ancient polymorphisms, recent ecological selection regardless of gene flow, and/or recurrent background selection and selective sweeps in low-recombination regions. It is challenging to disentangle these nonexclusive alternatives, but here we attempt to do this in an analysis of what drove genomic divergence between four lineages comprising a species complex of desert poplar trees. Within this complex we found that two morphologically delimited species, Populus euphratica and Populus pruinosa, were paraphyletic while the four lineages exhibited contrasting levels of gene flow and divergence times, providing a good system for testing hypotheses on the origin of divergence islands. We show that the size and number of genomic islands that distinguish lineages are not associated with either rate of recent gene flow or time of divergence. Instead, they are most likely derived from divergent sorting of ancient polymorphisms and divergence hitchhiking. We found that highly diverged genes under lineage-specific selection and putatively involved in ecological and morphological divergence occur both within and outside these islands. Our results highlight the need to incorporate demography, absolute divergence measurement, and gene flow rate to explain the formation of genomic islands and to identify potential genomic regions involved in speciation.Publisher PDFPeer reviewe

A calibration method for non-overlapping cameras based on mirrored phase target

A novel calibration method for non-overlapping cameras is proposed in this paper. A LCD screen is used as a phase target to display two groups of orthogonal phase-shifted sinusoidal patterns during the calibration process. Through a mirror reflection, the phase target is captured by the cameras respectively. The relations between each camera and the phase target can be obtained according the proposed algorithm. Then the relation between the cameras can be calculated by treating the phase target as an intermediate value. The proposed method is more flexible than conventional mirror-based approach, because it do not require the common identification points and is robust to out-of-focus images. Both simulation work and experimental results show the proposed calibration method has a good result in calibrating a non-overlapping cameras system

Teasing apart the joint effect of demography and natural selection in the birth of a contact zone

Vast population movements induced by recurrent climatic cycles have shaped the genetic structure of plant species. During glacial periods species were confined to low-latitude refugia from which they recolonized higher latitudes as the climate improved. This multipronged recolonization led to many lineages that later met and formed large contact zones. We utilize genomic data from 5000 Picea abies trees to test for the presence of natural selection during recolonization and establishment of a contact zone in Scandinavia. Scandinavian P. abies is today made up of a southern genetic cluster originating from the Baltics, and a northern one originating from Northern Russia. The contact zone delineating them closely matches the limit between two major climatic regions. We show that natural selection contributed to its establishment and maintenance. First, an isolation-with-migration model with genome-wide linked selection fits the data better than a purely neutral one. Second, many loci show signatures of selection or are associated with environmental variables. These loci, regrouped in clusters on chromosomes, are often related to phenology. Altogether, our results illustrate how climatic cycles, recolonization and selection can establish strong local adaptation along contact zones and affect the genetic architecture of adaptive traits