Speciation with gene flow in whiptail lizards from a Neotropical xeric biome

Coalescent simulations indicate that the São Francisco River is a biogeographic barrier for six vertebrates in a seasonally dry South American forest

The riverine barrier hypothesis has been extensively explored in Neotropical rainforests, while its importance in drier regions such as the Caatinga, a seasonally dry tropical forest in northeastern Brazil, has only recently received more attention. The Caatinga is bisected by the São Francisco River (SFR), which has long been suggested to be an important biogeographic feature in the region. However, recent studies have found mixed support for the role of the SFR as a hard barrier, most of them relying on the presence or absence of genetic breaks congruent with its course. Here, we used published multilocus and next-generation data from six vertebrate species to test the SFR’s strength as a barrier. Using model-based approaches (approximate Bayesian computation and supervised machine learning), we tested demographic models incorporating full, intermediate, and zero migration across the SFR, estimating divergence times and migration rates for each species. We found support for the SFR’s role as a barrier, allowing gene flow for some species. Estimated divergence times varied among species but are limited to the late Pleistocene, coherent with one of several proposed paleocourse changes in the river’s geological history. Contrary to the mixed results of previous studies, our study supports the SFR as an important phylogeographic barrier across different taxonomic groups, driving diversification in the Caatinga.

Genome-wide data reveal extensive gene flow during the diversification of the western rattlesnakes (Viperidae: Crotalinae: Crotalus)

Hybridization and introgression are important, but often overlooked processes when inferring phylogenies. When these processes are not accounted for and a strictly diverging phylogenetic model is applied to groups with a history of hybridization, phylogenetic inference and parameter estimation can be inaccurate. Recent developments in phylogenetic network approaches coupled with the increasing availability of genomic data allow inferences of reticulate evolutionary histories across the tree of life. The western rattlesnake species group (C. viridis species complex, C. mitchellii species complex, C. scutulutas, and C. tigris) is an iconic snake lineage that is widespread across western North America. This group is composed of several species complexes with unclear species limits, likely the result of ongoing gene flow among nascent lineages. Here I generate reduced representation genomic data and test for a history of reticulation within this group. I demonstrate that all species have undergone hybridization with at least one other lineage, suggesting introgression is widespread in this group. Topologies differ between phylogenies estimated under the multispecies coalescent and multispecies network coalescent methods, indicating that gene flow has obscured phylogenetic relationships within this group. These past introgression events are predominantly restricted to species that co-occur geographically. However, within species that have a history of introgression, this signature is detected regardless of specimen sampling across geography. Overall, my results suggest the accumulation of reproductive isolating barriers occurs slowly in rattlesnakes which likely leads to the difficulty in delimiting species, furthermore, the results of this study have implications for trait evolution in this group.

SODA: Multi-locus species delimitation using quartet frequencies

MOTIVATION

Species delimitation, the process of deciding how to group a set of organisms into units called species, is one of the most challenging problems in evolutionary computational biology. While many methods exist for species delimitation, most based on the coalescent theory, few are scalable to very large datasets, and methods that scale tend to be not accurate. Species delimitation is closely related to species tree inference from discordant gene trees, a problem that has enjoyed rapid advances in recent years.

RESULTS

In this paper, we build on the accuracy and scalability of recent quartet-based methods for species tree estimation and propose a new method called SODA for species delimitation. SODA relies heavily on a recently developed method for testing zero branch length in species trees. In extensive simulations, we show that SODA can easily scale to very large datasets while maintaining high accuracy.

AVAILABILITY

The code and data presented here are available on https://github.com/maryamrabiee/SODA.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Lizards from the Alto Sertão region of Sergipe state, northeastern Brazil

Abstract We present the first detailed inventory of the lizard fauna of Alto Sertão region, west of Sergipe state, based on three field expeditions carried out by the authors and specimens deposited at the Herpetological Collection of Universidade Federal de Sergipe. The three inventories were carried out between 2010 and 2016, in the municipalities of Canindé de São Francisco, Poço Redondo, Monte Alegre de Sergipe, Nossa Senhora da Glória, and Porto da Folha. In total, we sampled 9,485 lizards of 19 species (10 families) of which 10 are new records for the study area. The most abundant species were Ameivula ocellifera, Tropidurus hispidus and T. semitaeniatus. Thirteen species were considered as widely distributed in Caatinga, five presented a relictual distribution and one was exotic. We suggest that future research focus mainly on non-sampled areas and should prioritize the municipalities of Gararu, Nossa Senhora da Glória and Nossa Senhora de Lourdes since they are localities with little or no sampling records.

Hybridization and hybrid zone stability between two lizards explained by population genetics and niche quantification

Understanding the factors that affect hybridization is an important issue in the study of species evolution. In this work, we analyse the genetic structure of two lizard species, Salvator merianae and Salvator rufescens, at a microscale within a climatic niche analysis framework, to reveal the main factors that contribute to the stability of their hybrid zone. We assess the effect of climate in hybridization by quantifying and decomposing the niche overlap of both species. Using a mitochondrial and a nuclear marker, we find that hybridization is frequent and is not restricted to the sympatric region. The gene flow is mainly from S. rufescens to S. merianae, with introgression into the range of S. merianae. Also, S. merianae would have long been present in the area, while S. rufescens appears to be a recent colonizer. The climate contributes to the population structure of S. merianae, but not to that of S. rufescens. The niches occupied by S. rufescens in the hybrid zone and the non-hybrid zone are similar, while the niches of S. merianae are different. Our results do not fit previous models of hybrid zone stability, suggesting the need to develop new models that consider the evolutionary factors that can differentially affect parental species and hybrids.

Strict allopatric speciation of sky island Pyrrhula erythaca species complex

Increasing evidence of post-divergence gene flow between taxa is shifting our understanding on the mode of speciation. A fundamental question arises concerning the circumstances under which strict allopatric speciation still holds true. Sky island populations might undergo reduced gene flow by niche conservatism to highland habitats and follow divergence in an allopatric manner. In this study, we tested this hypothesis in the sky island Grey-headed Bullfinch (Pyrrhula erythaca) species complex via statistical analyses of both genetic and ecological data. Results of coalescent-based analysis of multiple nuclear loci suggested that P. e. owstoni likely colonized Taiwan island during the severe mid-Pleistocene glacial climate followed by strictly allopatric divergence from P. e. erythaca distributed in Himalayas-Hengduan mountains and central North China. Results of ecological niche modeling suggested that their speciation may be attributed to the niche conservatism of these birds and the lack of a suitable ecological corridor during subsequent milder glacial episodes. In addition, we delimited the traditionally defined P. erythaca into two full species, P. erythaca in the Asian mainland and P. owstoni on the island of Taiwan, based on both genetic and behavioural evidences. These results suggest that ecology can have a dynamic role in allowing highland populations to expand their ranges and isolated by habitat barriers to diversify in a strictly allopatric manner.

Genetic diversity of toll-like receptor genes in the vulnerable Chinese egret (Egretta eulophotes)

Toll-like receptor (TLR) genes have recently been employed to assess genetic diversity, as they can be used to infer both demographic history and adaptation to environments with different pathogen pressure. Here, we sampled 120 individuals of the Chinese egret (Egretta eulophotes), a globally vulnerable species, from four breeding populations across China. We assessed the levels of genetic diversity, selection pressure, and population differentiation at seven TLR loci (TLR1LB, TLR2A, TLR3, TLR4, TLR5, TLR7, and TLR15). Using a variety of metrics (SNPs, heterozygosity, nucleotides, haplotypes), our analyses showed that genetic diversity was lower at 4 of the 7 TLR loci in the vulnerable Chinese egret compared to the more common little egret (Egretta garzetta). The selection test indicated TLRs, except for TLR5, were under purifying selection in TLR evolution, suggesting that low TLR genetic diversity in the Chinese egret may be caused by purifying selection. Moreover, analysis of molecular variance indicated low but significant population differentiation among four populations at all of the TLR loci in this egret. However, some comparisons based on fixation index analyses did not show significant population differentiation, and Bayesian clustering showed admixture. Our finding suggested that these four populations of the Chinese egret in China may be considered a single unit for conservation planning. These results, the new report of TLR genetic diversity in a long-distance migratory vulnerable Ardeid species, will provide fundamental TLR information for further studies on the conservation genetics of the Chinese egret and other Ardeids.

Biogeographic barriers, Pleistocene refugia, and climatic gradients in the southeastern Nearctic drive diversification in cornsnakes (Pantherophis guttatus complex)

The southeastern Nearctic is a biodiversity hotspot that is also rich in cryptic species. Numerous hypotheses (e.g., vicariance, local adaptation, and Pleistocene speciation in glacial refugia) have been tested in an attempt to explain diversification and the observed pattern of extant biodiversity. However, previous phylogeographic studies have both supported and refuted these hypotheses. Therefore, while data support one or more of these diversification hypotheses, it is likely that taxa are forming within this region in species-specific ways. Here, we generate a genomic data set for the cornsnakes (Pantherophis guttatus complex), which are widespread across this region, spanning both biogeographic barriers and climatic gradients. We use phylogeographic model selection combined with hindcast ecological niche models to determine regions of habitat stability through time. This combined approach suggests that numerous drivers of population differentiation explain the current diversity of this group of snakes. The Mississippi River caused initial speciation in this species complex, with more recent divergence events linked to adaptations to ecological heterogeneity and allopatric Pleistocene refugia. Lastly, we discuss the taxonomy of this group and suggest there may be additional cryptic species in need of formal recognition.

Dwarf geckos and giant rivers: the role of the São Francisco River in the evolution of Lygodactylus klugei (Squamata: Gekkonidae) in the semi-arid Caatinga of north-eastern Brazil

Species diversification can be strongly influenced by geomorphological features, such as mountains, valleys and rivers. Rivers can act as hard or soft barriers to gene flow depending on their size, speed of flow, historical dynamics and regional topographical characteristics. The São Francisco River (SFR) is the largest perennial river in the Caatinga biome in north-eastern Brazil and has been considered a barrier to gene flow and dispersal. Herein, we evaluated the role of the SFR on the evolution of Lygodactylus klugei, a small gecko from the Caatinga. Using a single-locus species delimitation method (generalized mixed Yule coalescent), we defined lineages (haploclades). Subsequently, we evaluated the role of the SFR in structuring genetic diversity in this species using a multilocus approach to quantify migration across margins. We also evaluated genetic structure based on nuclear markers, testing the number of populations found through an assignment test (STRUCTURE) across the species distribution. We recovered two mitochondrial lineages structured with respect to the SFR, but only a single population was inferred from nuclear markers. Given that we detected an influence of the SFR only on mitochondrial markers, we suggest that the current river course has acted as a relatively recent geographical barrier for L. klugei, for ~450 000 years.

Patterns, Mechanisms and Genetics of Speciation in Reptiles and Amphibians

In this contribution, the aspects of reptile and amphibian speciation that emerged from research performed over the past decade are reviewed. First, this study assesses how patterns and processes of speciation depend on knowing the taxonomy of the group in question, and discuss how integrative taxonomy has contributed to speciation research in these groups. This study then reviews the research on different aspects of speciation in reptiles and amphibians, including biogeography and climatic niches, ecological speciation, the relationship between speciation rates and phenotypic traits, and genetics and genomics. Further, several case studies of speciation in reptiles and amphibians that exemplify many of these themes are discussed. These include studies of integrative taxonomy and biogeography in South American lizards, ecological speciation in European salamanders, speciation and phenotypic evolution in frogs and lizards. The final case study combines genomics and biogeography in tortoises. The field of amphibian and reptile speciation research has steadily moved forward from the assessment of geographic and ecological aspects, to incorporating other dimensions of speciation, such as genetic mechanisms and evolutionary forces. A higher degree of integration among all these dimensions emerges as a goal for future research.

The Origins and Diversification of the Exceptionally Rich Gemsnakes (Colubroidea: Lamprophiidae: Pseudoxyrhophiinae) in Madagascar

Processes leading to spectacular diversity of both form and species on islands have been well-documented under island biogeography theory, where distance from source and island size are key factors determining immigration and extinction resistance. But far less understood are the processes governing in situ diversification on the world’s mega islands, where large and isolated land masses produced morphologically distinct radiations from related taxa on continental regions. Madagascar has long been recognized as a natural laboratory due to its isolation, lack of influence from adjacent continents, and diversification of spectacular vertebrate radiations. However, only a handful of studies have examined rate shifts of in situ diversification for this island. Here, we examine rates of diversification in the Malagasy snakes of the family Pseudoxyrhophiinae (gemsnakes) to understand if rates of speciation were initially high, enhanced by diversification into distinct biomes, and associated with key dentition traits. Using a genomic sequence-capture data set for 366 samples, we determine that all previously described and newly discovered species are delimitable and therefore useful candidates for understanding diversification trajectories through time. Our analysis detected no shifts in diversification rate between clades or changes in biome or dentition type. Remarkably, we demonstrate that rates of diversification of the gemsnake radiation, which originated in Madagascar during the early Miocene, remained steady throughout the Neogene. However, we do detect a significant slowdown in diversification during the Pleistocene. We also comment on the apparent paradox where most living species originated in the Pleistocene, despite diversification rates being substantially higher during the earlier 15 myr.

The Biogeography of Deep Time Phylogenetic Reticulation

Most phylogenies are typically represented as purely bifurcating. However, as genomic data have become more common in phylogenetic studies, it is not unusual to find reticulation among terminal lineages or among internal nodes (deep time reticulation; DTR). In these situations, gene flow must have happened in the same or adjacent geographic areas for these DTRs to have occurred and therefore biogeographic reconstruction should provide similar area estimates for parental nodes, provided extinction or dispersal has not eroded these patterns. We examine the phylogeny of the widely distributed New World kingsnakes (Lampropeltis), determine if DTR is present in this group, and estimate the ancestral area for reticulation. Importantly, we develop a new method that uses coalescent simulations in a machine learning framework to show conclusively that this phylogeny is best represented as reticulating at deeper time. Using joint probabilities of ancestral area reconstructions on the bifurcating parental lineages from the reticulating node, we show that this reticulation likely occurred in northwestern Mexico/southwestern US, and subsequently, led to the diversification of the Mexican kingsnakes. This region has been previously identified as an area important for understanding speciation and secondary contact with gene flow in snakes and other squamates. This research shows that phylogenetic reticulation is common, even in well-studied groups, and that the geographic scope of ancient hybridization is recoverable.

Comparison of Methods for Molecular Species Delimitation Across a Range of Speciation Scenarios

Species are fundamental units in biological research and can be defined on the basis of various operational criteria. There has been growing use of molecular approaches for species delimitation. Among the most widely used methods, the generalized mixed Yule-coalescent (GMYC) and Poisson tree processes (PTP) were designed for the analysis of single-locus data but are often applied to concatenations of multilocus data. In contrast, the Bayesian multispecies coalescent approach in the software Bayesian Phylogenetics and Phylogeography (BPP) explicitly models the evolution of multilocus data. In this study, we compare the performance of GMYC, PTP, and BPP using synthetic data generated by simulation under various speciation scenarios. We show that in the absence of gene flow, the main factor influencing the performance of these methods is the ratio of population size to divergence time, while number of loci and sample size per species have smaller effects. Given appropriate priors and correct guide trees, BPP shows lower rates of species overestimation and underestimation, and is generally robust to various potential confounding factors except high levels of gene flow. The single-threshold GMYC and the best strategy that we identified in PTP generally perform well for scenarios involving more than a single putative species when gene flow is absent, but PTP outperforms GMYC when fewer species are involved. Both methods are more sensitive than BPP to the effects of gene flow and potential confounding factors. Case studies of bears and bees further validate some of the findings from our simulation study, and reveal the importance of using an informed starting point for molecular species delimitation. Our results highlight the key factors affecting the performance of molecular species delimitation, with potential benefits for using these methods within an integrative taxonomic framework.

Low Genetic Diversity and Low Gene Flow Corresponded to a Weak Genetic Structure of Ruddy-Breasted Crake (Porzana fusca) in China

The Ruddy-breasted Crake (Porzana fusca) is an extremely poorly known species. Although it is not listed as globally endangered, in recent years, with the interference of climate change and human activities, its habitat is rapidly disappearing and its populations have been shrinking. There are two different life history traits for Ruddy-breasted Crake in China, i.e., non-migratory population in the south and migratory population in the north of China. In this study, mitochondrial control sequences and microsatellite datasets of 88 individuals sampled from 8 sites were applied to analyze their genetic diversity, genetic differentiation, and genetic structure. Our results indicated that low genetic diversity and genetic differentiation exit in most populations. The neutrality test suggested significantly negative Fu's Fs value, which, in combination with detection of the mismatch distribution, indicated that population expansion occurred in the interglacier approximately 98,000 years ago, and the time of the most recent common ancestor (TMRCA) was estimated to about 202,705 years ago. Gene flow analysis implied that the gene flow was low, but gene exchange was frequent among adjacent populations. Both phylogenetic and STRUCTURE analyses implied weak genetic structure. In general, the genetic diversity, gene flow, and genetic structure of Ruddy-breasted Crake were low.

Climatic suitability, isolation by distance and river resistance explain genetic variation in a Brazilian whiptail lizard

Spatial patterns of genetic variation can help understand how environmental factors either permit or restrict gene flow and create opportunities for regional adaptations. Organisms from harsh environments such as the Brazilian semiarid Caatinga biome may reveal how severe climate conditions may affect patterns of genetic variation. Herein we combine information from mitochondrial DNA with physical and environmental features to study the association between different aspects of the Caatinga landscape and spatial genetic variation in the whiptail lizard Ameivula ocellifera. We investigated which of the climatic, environmental, geographical and/or historical components best predict: (1) the spatial distribution of genetic diversity, and (2) the genetic differentiation among populations. We found that genetic variation in A. ocellifera has been influenced mainly by temperature variability, which modulates connectivity among populations. Past climate conditions were important for shaping current genetic diversity, suggesting a time lag in genetic responses. Population structure in A. ocellifera was best explained by both isolation by distance and isolation by resistance (main rivers). Our findings indicate that both physical and climatic features are important for explaining the observed patterns of genetic variation across the xeric Caatinga biome.

Female Brazilian whiptail lizards (Cnemidophorus ocellifer) prefer males with high ultraviolet ornament reflectance

Conspicuous colouration is an important way of social communication in many taxa. The role of ultraviolet (UV) signals in intraspecific communication has only recently been studied in lizards, and there is not a general understanding of the adaptive role of UV colouration. Colour ornaments can signal male quality in mate choice and are therefore suitable for reliably predicting the outcome of female preference. Here, we tested the potential role of UV colouration in female spatial preference in a non-territorial teiid lizard, Cnemidophorus ocellifer. We experimentally manipulated the UV reflectance of size-matched male pairs and tested the effects of our treatment on females' spatial distribution. We found that females associated with males of higher UV reflectance, suggesting that UV colour can be an important clue during mate preference decisions. Our results provide the first empirical evidence for the importance of UV colouration in female preference in a mutually ornamented lizard species.

Absence of population structure across elevational gradients despite large phenotypic variation in mountain chickadees (Poecile gambeli)

Montane habitats are characterized by predictably rapid heterogeneity along elevational gradients and are useful for investigating the consequences of environmental heterogeneity for local adaptation and population genetic structure. Food-caching mountain chickadees inhabit a continuous elevation gradient in the Sierra Nevada, and birds living at harsher, high elevations have better spatial memory ability and exhibit differences in male song structure and female mate preference compared to birds inhabiting milder, low elevations. While high elevation birds breed, on average, two weeks later than low elevation birds, the extent of gene flow between elevations is unknown. Despite phenotypic variation and indirect evidence for local adaptation, population genetic analyses based on 18 073 single nucleotide polymorphisms across three transects of high and low elevation populations provided no evidence for genetic differentiation. Analyses based on individual genotypes revealed no patterns of clustering, pairwise estimates of genetic differentiation (F_ST, Nei's D) were very low, and AMOVA revealed no evidence for genetic variation structured by transect or by low and high elevation sites within transects. In addition, we found no consistent evidence for strong parallel allele frequency divergence between low and high elevation sites within the three transects. Large elevation-related phenotypic variation may be maintained by strong selection despite gene flow and future work should focus on the mechanisms underlying such variation.