Integrating phylogeography and paleoecology to investigate the origin and dynamics of hybrid zones: insights from two widespread North American firs

The Syngameon Enigma

Despite their evolutionary relevance, multispecies networks or syngameons are rarely reported in the literature. Discovering how syngameons form and how they are maintained can give insight into processes such as adaptive radiations, island colonizations, and the creation of new hybrid lineages. Understanding these complex hybridization networks is even more pressing with anthropogenic climate change, as syngameons may have unique synergistic properties that will allow participating species to persist. The formation of a syngameon is not insurmountable, as several ways for a syngameon to form have been proposed, depending mostly on the magnitude and frequency of gene flow events, as well as the relatedness of its participants. Episodic hybridization with small amounts of introgression may keep syngameons stable and protect their participants from any detrimental effects of gene flow. As genomic sequencing becomes cheaper and more species are included in studies, the number of known syngameons is expected to increase. Syngameons must be considered in conservation efforts as the extinction of one participating species may have detrimental effects on the survival of all other species in the network.

Glacial Expansion or Interglacial Expansion? Contrasting Demographic Models of Four Cold-Adapted Fir Species in North America and East Asia

Understanding and forecasting species’ response to climate change is a critical need for future conservation and management. Two expansion hypotheses, the glacial expansion versus the interglacial expansion, have been proposed to interpret how cold-adapted organisms in the northern hemisphere respond to Quaternary climatic fluctuations. To test these two hypotheses, we originally used two pairs of high-low elevation firs from North America (Abies lasiocarpa and Abies balsamea) and East Asia (Abies chensiensis and Abies nephrolepis). Abies lasiocarpa and Abies chensiensis are widely distributed in high-elevation regions of western North America and central China. Abies balsamea and Abies nephrolepis occur in central North America and northeast China, with much lower elevations. These fir species are typical cold-adapted species and sensitive to climate fluctuations. Here, we integrated the mtDNA and cpDNA polymorphisms involving 44 populations and 585 individuals. Based on phylogeographic analyses, recent historical range expansions were indicated in two high-elevation firs (Abies lasiocarpa and Abies chensiensis) during the last glaciation (43.8–28.4 or 21.9–14.2 kya, 53.1–34.5 or 26.6–17.2 kya). Such glacial expansions in high-elevation firs were further confirmed by the evidence of species distribution modelling, geographic-driven genetic patterns, palynological records, and current distribution patterns. Unlike the north American firs, the SDM models indicated unremarkable expansion or contraction in East Asia firs for its much more stable conditions during different historical periods. Taken together, our findings highly supported that high-elevation firs experienced glacial expansion during the Quaternary climate change in East Asia and North America, as interglacial expansion within low-elevation firs. Under this situation, the critically endangered fir species distributed in high elevation would have no enough higher elevational space to migrate. Facing the increasing global warming, thus we proposed ex-situ conservation of defining conservation units as the most meaningful strategy.

Finding common ground: Toward comparable indicators of adaptive capacity of tree species to a changing climate

Adaptive capacity, one of the three determinants of vulnerability to climate change, is defined as the capacity of species to persist in their current location by coping with novel environmental conditions through acclimation and/or evolution. Although studies have identified indicators of adaptive capacity, few have assessed this capacity in a quantitative way that is comparable across tree species. Yet, such multispecies assessments are needed by forest management and conservation programs to refine vulnerability assessments and to guide the choice of adaptation measures. In this paper, we propose a framework to quantitatively evaluate five key components of tree adaptive capacity to climate change: individual adaptation through phenotypic plasticity, population phenotypic diversity as influenced by genetic diversity, genetic exchange within populations, genetic exchange between populations, and genetic exchange between species. For each component, we define the main mechanisms that underlie adaptive capacity and present associated metrics that can be used as indices. To illustrate the use of this framework, we evaluate the relative adaptive capacity of 26 northeastern North American tree species using values reported in the literature. Our results show adaptive capacity to be highly variable among species and between components of adaptive capacity, such that no one species ranks consistently across all components. On average, the conifer Picea glauca and the broadleaves Acer rubrum and A. saccharinum show the greatest adaptive capacity among the 26 species we documented, whereas the conifers Picea rubens and Thuja occidentalis, and the broadleaf Ostrya virginiana possess the lowest. We discuss limitations that arise when comparing adaptive capacity among species, including poor data availability and comparability issues in metrics derived from different methods or studies. The breadth of data required for such an assessment exemplifies the multidisciplinary nature of adaptive capacity and the necessity of continued cross-collaboration to better anticipate the impacts of a changing climate.

Backbone phylogeny and evolution of Apioideae (Apiaceae): New insights from phylogenomic analyses of plastome data

Traditional phylogenies inferred from chloroplast DNA fragments have not obtained a well-resolved evolutionary history for the backbone of Apioideae, the largest subfamily of Apiaceae. In this study, we applied the genome skimming approach of next-generation sequencing to address whether the lack of resolution at the tip of the Apioideae phylogenetic tree is due to limited information loci or the footprint of ancient radiation. A total of 90 complete chloroplast genomes (including 23 newly sequenced genomes and covering 20 major clades of Apioideae) were analyzed (RAxML and MrBayes) to provide a phylogenomic reconstruction of Apioideae. Dating analysis was also implemented using BEAST to estimate the origin and divergence time of the major clades. As a result, the early divergences of Apioideae have been clarified but the relationship among its distally branching clades (Group A) was only partially resolved, with short internal branches pointing to an ancient radiation scenario. Four major clades, Tordyliinae I, Pimpinelleae I, Apieae and Coriandreae, were hypothesized to have originated from chloroplast capture events induced by early hybridization according to the incongruence between chloroplast-based and nrDNA-based phylogenetic trees. Furthermore, the variable and nested distribution of junction positions of LSC (Large single copy region) and IR_B (inverted repeat region B) in Group A may reflect incomplete lineage sorting within this group, which possibly contributed to the unclear phylogenetic relationships among these clades inferred from plastome data. Molecular clock analysis revealed the chloroplast capture events mainly occurred during the middle to late Miocene, providing a geological and climate context for the evolution of Apioideae.

Molecular phylogeny and ancestral biogeographic reconstruction of Platanthera subgenus Limnorchis (Orchidaceae) using target capture methods

Platanthera is one of the largest genera of temperate orchids in the Holarctic and exemplifies a lineage that has adaptively radiated into diverse habitats within North America, Asia, Europe, North Africa, Borneo, and Sarawak. Major centers of diversity in this genus are North America and eastern Asia. Despite its diversity, a thorough phylogenetic hypothesis for the genus is lacking because no studies have yet sampled taxa exhaustively or developed a robust molecular toolkit. While there is strong evidence that suggests monophyly of subgenus Limnorchis, most taxa in this group have not been included in a phylogenetic analysis. In this study, we developed a new toolkit for Platanthera consisting of genomic information from 617 low-copy nuclear loci. Using a targeted enrichment approach, we collected high-throughput sequence data in 23 accessions of nine of the 12 diploid species of subgenus Limnorchis and outgroup species across Platanthera. A maximum likelihood analysis resolved a strongly supported monophyletic clade for subgenus Limnorchis. Ancestral biogeographic reconstruction indicated that subgenus Limnorchis originated in western North America ca. 3-4.5 Mya from an ancestor that was widespread in western North America and eastern Asia and subsequently diversified in western North America, followed by dispersal of some species to eastern North America. Our results indicate complex biogeographic connections between Asia and North America, and therefore it suggests that Platanthera is a suitable system to test biogeographic hypotheses over time and space in the Holarctic. Our results are also expected to facilitate further study of diversification and biogeographic spread across Platanthera and lay the groundwork for understanding independent origins, biogeography, and morphological diversification of polyploid species within subgenus Limnorchis.

What nature separated, and human joined together: About a spontaneous hybridization between two allopatric dogwood species (Cornus controversa and C. alternifolia)

In this study, possible hybridization between two allopatric species, Cornus controversa and Cornus alternifolia, was explored using molecular and morphological approaches. Scanning electron microscope analyses of the adaxial and the abaxial leaf surfaces yielded a few new not yet described characters typical for the particular species and intermediate for hybrids. With the use of 14 Random Amplified Polymorphic DNA and 5 Amplified Fragment Length Polymorphism primer combinations, 44 fragments species specific to C. controversa and 51 species specific to C. alternifolia were obtained. Most of these bands were also found in putative hybrids. All clustering analyses based on binary data combined from both methods confirmed a separate and intermediate status of the hybrids. Hybrid index estimates for hybrids C1-C5 indicated that all were the first generation of offspring (F₁). Chloroplast intergenic spacers (trnF-trnL and psbC-trnS) were used to infer the hybridization direction. Based on the assumption of maternal inheritance of chloroplast DNA, C. controversa seems to be the maternal parent of the hybrid. Internal transcribed spacer sequences of the five hybrids analyzed here indicated higher similarity with the sequences of C. controversa (all shared the majority of its single nucleotide polymorphisms). Sequence analysis of PI-like genes fully confirmed the hybrid origin of C1-C5 hybrids. Our results also showed that two specimens in the C. alternifolia group, A1 and A3, are not free of introgression. They are probably repeated backcrosses toward C. alternifolia. Furthermore, molecular data seem to point not only to unidirectional introgression toward C. controversa (the presence of hybrids) but to bidirectional introgression as well, since the presence of markers specific for C. controversa in the profiles of C. alternifolia specimen A3 was observed.

Concealed truth: Modeling reveals unique Quaternary distribution dynamics and refugia of four related endemic keystone Abies taxa on the Tibetan Plateau

Understanding the factors driving the Quaternary distribution of Abies in the Tibetan Plateau (TP) is crucial for biodiversity conservation and for predicting future anthropogenic impacts on ecosystems. Here, we collected Quaternary paleo-, palynological, and phylogeographical records from across the TP and applied ecological niche models (ENMs) to obtain a profound understanding of the different adaptation strategies and distributional changes in Abies trees in this unique area. We identified environmental variables affecting the different historical biogeographies of four related endemic Abies taxa and rebuilt their distribution patterns over different time periods, starting from the late Pleistocene. In addition, modeling and phylogeographic results were used to predict suitable refugia for Abies forrestii, A. forrestii var. georgei, A. fargesii var. faxoniana, and A. recurvata. We supplemented the ENMs by investigating pollen records and diversity patterns of cpDNA for them. The overall reconstructed distributions of these Abies taxa were dramatically different when the late Pleistocene was compared with the present. All Abies taxa gradually receded from the south toward the north in the last glacial maximum (LGM). The outcomes showed two well-differentiated distributions: A. fargesii var. faxoniana and A. recurvata occurred throughout the Longmen refuge, a temporary refuge for the LGM, while the other two Abies taxa were distributed throughout the Heqing refuge. Both the seasonality of precipitation and the mean temperature of the driest quarter played decisive roles in driving the distribution of A. fargesii var. faxoniana and A. recurvata, respectively; the annual temperature range was also a key variable that explained the distribution patterns of the other two Abies taxa. Different adaptation strategies of trees may thus explain the differing patterns of distribution over time at the TP revealed here for endemic Abies taxa.

A Phytogeographic Divide Along the 500 mm Isohyet in the Qinghai-Tibet Plateau: Insights From the Phylogeographic Evidence of Chinese Alliums (Amaryllidaceae)

The Qinghai-Tibet Plateau (QTP) has been biogeographically divided into the eastern monsoonal and the western continental climatic zones along the 500 mm isohyet. However, this biogeographic hypothesis has been rarely tested using a phylogeographic approach. The members of the genus Allium subgenus Cyathophora coincidentally distribute across this biogeographical divide. Intriguingly, Allium fasciculatum of subgenus Amerallium co-occurs in the distribution range of subgenus Cyathophora. To illuminate the role of this biogeographic divide on the genetic divergence, we genotyped 466 individuals of 52 populations of subgenus Cyathophora and 110 individuals of 19 populations of A. fasciculatum using three chloroplast DNA fragments, whole nrITS and nine nuclear microsatellite loci, supplemented with the present environmental space and paleo-distribution modeling. Our phylogeographical evidence recovered the concordant east-west genetic breaks both for subgenus Cyathophora and A. fasciculatum along the 500 mm isohyet. The divergence time estimations and environmental niche differentiations suggested this east-west genetic breaks could have been triggered by the climatic-induced vicariance during the early Pleistocene. Noticeably, this split within subgenus Cyathophora could have been deepened by the morphological vicariance from the eastern umbel to the western spicate, while that within A. fasciculatum could have been obscured due to the pollen flows from the east to west caused by the postglacial expansion. The genetic structures and ecological niche modelings (ENMs) recovered the distinct responses to the Quaternary climatic oscillations for species constricted to different climatic zones, further highlighting the profound effect of the climatic differences and tectonic uplifts on the genetic diversification. Overall, our findings offer strong evidence for the existence of a biogeographic divide between the eastern monsoonal and the west continental climatic zones of the QTP nearly along the 500 mm isohyet.

Invoking adaptation to decipher the genetic legacy of past climate change

Persistence of natural populations during periods of climate change is likely to depend on migration (range shifts) or adaptation. These responses were traditionally considered discrete processes and conceptually divided into the realms of ecology and evolution. In a milestone paper, Davis and Shaw (2001) Science 292:673 argued that the interplay of adaptation and migration was central to biotic responses to Quaternary climate, but since then there has been no synthesis of efforts made to set up this research program. Here we review some of the salient findings from molecular genetic studies assessing ecological and evolutionary responses to Quaternary climate change. These studies have revolutionized our understanding of population processes associated with past species migration. However, knowledge remains limited about the role of natural selection for local adaptation of populations to Quaternary environmental fluctuations and associated range shifts, and for the footprints this might have left on extant populations. Next-generation sequencing technologies, high-resolution paleoclimate analyses, and advances in population genetic theory offer an unprecedented opportunity to test hypotheses about adaptation through time. Recent population genomics studies have greatly improved our understanding of the role of contemporary adaptation to local environments in shaping spatial patterns of genetic diversity across modern-day landscapes. Advances in this burgeoning field provide important conceptual and methodological bases to decipher the historical role of natural selection and assess adaptation to past environmental variation. We suggest that a process called "temporal conditional neutrality" has taken place: some alleles favored in glacial environments become selectively neutral in modern-day conditions, whereas some alleles that had been neutral during glacial periods become under selection in modern environments. Building on this view, we present a new integrative framework for addressing the interplay of demographic and adaptive evolutionary responses to Quaternary climate dynamics, the research agenda initially envisioned by Davis and Shaw (2001) Science 292:673.

The extent and meaning of hybridization and introgression between Siberian spruce (Picea obovata) and Norway spruce (Picea abies): cryptic refugia as stepping stones to the west?

Boreal species were repeatedly exposed to ice ages and went through cycles of contraction and expansion while sister species alternated periods of contact and isolation. The resulting genetic structure is consequently complex, and demographic inferences are intrinsically challenging. The range of Norway spruce (Picea abies) and Siberian spruce (Picea obovata) covers most of northern Eurasia; yet their geographical limits and histories remain poorly understood. To delineate the hybrid zone between the two species and reconstruct their joint demographic history, we analysed variation at nuclear SSR and mitochondrial DNA in 102 and 88 populations, respectively. The dynamics of the hybrid zone was analysed with approximate Bayesian computation (ABC) followed by posterior predictive structure plot reconstruction and the presence of barriers across the range tested with estimated effective migration surfaces. To estimate the divergence time between the two species, nuclear sequences from two well-separated populations of each species were analysed with ABC. Two main barriers divide the range of the two species: one corresponds to the hybrid zone between them, and the other separates the southern and northern domains of Norway spruce. The hybrid zone is centred on the Urals, but the genetic impact of Siberian spruce extends further west. The joint distribution of mitochondrial and nuclear variation indicates an introgression of mitochondrial DNA from Norway spruce into Siberian spruce. Overall, our data reveal a demographic history where the two species interacted frequently and where migrants originating from the Urals and the West Siberian Plain recolonized northern Russia and Scandinavia using scattered refugial populations of Norway spruce as stepping stones towards the west.