Climate-driven mitochondrial selection in lacertid lizards

The mitochondrion, which is an intracellular organelle responsible for most of the energy-producing pathways, can have its genome targeted for climate-driven selection. However, climate-driven mitochondrial selection remains a sparsely studied area in reptiles. Here, we reported the complete mitochondrial genome sequence of a lacertid lizard (Takydromus intermedius) and used mitogenomes from 54 species of lacertid lizards to study their phylogenetic relationships and to identify the mitochondrial genes under positive selection by climate. The length of the complete mitochondrial genome sequence of T. intermedius was 17,713 bp, which was within the range of lengths (17,224-18,943) ever reported for Takydromus species. The arrangement of mitochondrial genes in T. intermedius was the same as in other congeneric species. The 54 lacertid species could be divided into three geographically and climatically different clades. We identified three mitochondrial genes (ATP6, ATP8, and ND3) under positive selection by climate, and found that isothermality, temperature seasonality, precipitation of wettest month, and precipitation seasonality were the most important climatic variables contributing to the gene selection.

Hybridization and divergent climatic preferences drive divergence of two allopatric Gentiana species on the Qinghai-Tibet Plateau

BACKGROUND AND AIMS

Exploring how species diverge is vital for understanding the drivers of speciation. Factors such as geographical separation and ecological selection, hybridization, polyploidization and shifts in mating system are all major mechanisms of plant speciation, but their contributions to divergence are rarely well understood. Here we test these mechanisms in two plant species, Gentiana lhassica and G. hoae, with the goal of understanding recent allopatric species divergence on the Qinghai-Tibet Plateau (QTP).

METHODS

We performed Bayesian clustering, phylogenetic analysis and estimates of hybridization using 561 302 nuclear genomic single nucleotide polymorphisms (SNPs). We performed redundancy analysis, and identified and annotated species-specific SNPs (ssSNPs) to explore the association between climatic preference and genetic divergence. We also estimated genome sizes using flow cytometry to test for overlooked polyploidy.

KEY RESULTS

Genomic evidence confirms that G. lhassica and G. hoae are closely related but distinct species, while genome size estimates show divergence occurred without polyploidy. Gentiana hoae has significantly higher average FIS values than G. lhassica. Population clustering based on genomic SNPs shows no signature of recent hybridization, but each species is characterized by a distinct history of hybridization with congeners that has shaped genome-wide variation. Gentiana lhassica has captured the chloroplast and experienced introgression with a divergent gentian species, while G. hoae has experienced recurrent hybridization with related taxa. Species distribution modelling suggested range overlap in the Last Interglacial Period, while redundancy analysis showed that precipitation and temperature are the major climatic differences explaining the separation of the species. The species differ by 2993 ssSNPs, with genome annotation showing missense variants in genes involved in stress resistance.

CONCLUSIONS

This study suggests that the distinctiveness of these species on the QTP is driven by a combination of hybridization, geographical isolation, mating system differences and evolution of divergent climatic preferences.

Flower Size as an Honest Signal in Royal Irises (Iris Section Oncocyclus, Iridaceae)

Flower traits, such as flower size or color changes, can act as honest signals indicating greater rewards such as nectar; however, nothing is known about shelter-rewarding systems. Large flowers of Royal irises offer overnight shelter as a reward to Eucera bees. A black patch might signal the entrance to the tunnel (shelter) and, together with the flower size, these might act as honest signals. We hypothesize that larger flowers and black patches indicate larger tunnels, and larger tunnels will increase pollinator visits, enhancing the plants' reproductive success. We measured seven species in a controlled environment and two species from three natural populations varying in flower size. Fruit and seed sets were assessed in these natural populations. We found a positive correlation between the flower, patch size, and tunnel volume, suggesting that the flowers and patch size act as honest signals, both under controlled conditions and in the wild. However, in natural populations, this positive relationship and its effect on fitness was population-specific. Flower size increased the fitness in YER I. petrana, and interactions between flower/patch size and tunnel size increased the fitness in YER and I. atropurpurea NET populations. This suggests that the honesty of the signal is positively selected in these two populations. This study supports the hypothesis that pollinator-mediated selection leads to the honest signaling of flower advertisement.

Eco-Geography and Phenology Are the Major Drivers of Reproductive Isolation in the Royal Irises, a Species Complex in the Course of Speciation

The continuous nature of speciation implies that different species are found at different stages of divergence, from no- to complete reproductive isolation. This process and its underlying mechanisms are best viewed in incipient species. Moreover, the species complex can offer unique insight into how reproductive isolation (RI) has evolved. The royal irises (Iris section Oncocyclus) are a young group of species in the course of speciation, providing an ideal system for speciation study. We quantified pre- and post-zygotic reproductive barriers between the eight Israeli species of this complex and estimated the total RI among them. We tested for both pre-pollination and post-pollination reproductive barriers. Pre-pollination barriers, i.e., eco-geographic divergence and phenological differentiation were the major contributors to RI among the Iris species. On the other hand, post-pollination barriers, namely pollen-stigma interactions, fruit set, and seed viability had negligible contributions to total RI. The strength of RI was not uniform across the species complex, suggesting that species may have diverged at different rates. Overall, this study in a young, recently diverged group of species provides insight into the first steps of speciation, suggesting a crucial role of the pre-zygotic barriers.

Dark-colored Oncocyclus irises in Israel analyzed by AFLP, whole chloroplast genome sequencing and species distribution modeling

The Haynei is one of seven species aggregates (clusters of species having similar flower morphology) recognized in section Oncocyclus of genus Iris. This aggregate, characterized by dark-colored flowers, is represented by six species in Israel and adjacent Jordan. There is, however, no knowledge of the genetic relationship of these species making verification of their taxonomic status impossible. We investigated genetic variation in this group using analysis of whole chloroplast genomes and amplified fragment length polymorphism (AFLP). We also used species distribution modeling (SDM) to predict species ranges under current climatic conditions. We found some population groups within the currently recognized species of section Oncocyclus to represent dramatically different genetic entities which devaluates a general trend of merging many previously recognized species of section Oncocyclus based on their flower morphology. Despite the importance of homoploid hybridization in this group’s evolution and some apparently sporadically happening inter-specific gene flow, the main evolutionary forces in Oncocyclus appear to be vicariance and spatial isolation. Our findings suggest that some of the currently recognized species in section Oncocyclus need revision. A revision must be based on genetic analyses allowing the reconstruction of ancestry and recognition of the importance of vicariance and spatial isolation in the evolution of this group. The implications of the present findings for conservation are discussed.

Climate warming will increase chances of hybridization and introgression between two Takydromus lizards (Lacertidae)

Coexisting species may experience population and range changes alone or jointly in response to environmental change. Here, we used six climate variables and ten modeling algorithms to predict the distribution of two Takydromus species (T. septentrionalis and T. sexlineatus) in China. We identified the sympatric and allopatric areas by comparing projections between the two species based on habitat suitability under present and future climate scenarios. We constructed the hypervolumes of six climate variables for the two species and then evaluated overlaps between hypervolumes. From this study, we know the following. First, minimum temperature of coldest month contributes the most to the prediction of habitat suitability. Second, habitats suitable for the two species will shift northward in response to climate warming. Third, the range of T. sexlineatus will expand across the four future time intervals before 2,100, namely the 2021-2040, 2041-2060, 2061-2080, and 2081-2100 intervals, under both Shared socioeconomic pathway (SSP) 245 and SSP585 scenarios, and the range of T. septentrionalis will also expand in the future except at the 2081-2100 interval under the SSP585 scenario. Fourth, the sympatric areas will contract or expand under the SSP245 scenario and expand across the four future time intervals before 2,100 under the SSP585 scenario. Fifth, the niche hypervolumes of the two species partially overlapped, and the differences in niche centroid show some degree of niche differentiation between the two species. These results allow to conclude that climate warming will not only drive the northward drift of sympatric areas but also increase the size of these areas if nothing is done to limit the emission of greenhouse gases. Given the existence of hybridization and introgression between T. septentrionalis and T. sexlineatus in the field where they coexist, we also conclude that climate warming will increase chances of hybridization and introgression between the two species.

Asymmetrical reproductive barriers in sympatric jewelflowers: are floral isolation, genetic incompatibilities and floral trait displacement connected?

Floral visitors influence reproductive interactions among sympatric plant species, either by facilitating assortative mating and contributing to reproductive isolation, or by promoting heterospecific pollen transfer, potentially leading to reproductive interference or hybridization. We assessed preference and constancy of floral visitors on two co-occurring jewelflowers [Streptanthus breweri and Streptanthus hesperidis (Brassicaceae)] using field arrays, and quantified two floral rewards potentially important to foraging choice – pollen production and nectar sugar concentration – in a greenhouse common garden. Floral visitors made an abundance of conspecific transitions between S. breweri individuals, which thus experienced minimal opportunities for heterospecific pollen transfer from S. hesperidis. In contrast, behavioural isolation for S. hesperidis was essentially absent due to pollinator inconstancy. This pattern emerged across multiple biotic environments and was unrelated to local density dependence. S. breweri populations that were sympatric with S. hesperidis had higher nectar sugar concentrations than their sympatric congeners, as well as allopatric conspecifics. Previous work shows that S. breweri suffers a greater cost to hybridization than S. hesperidis, and here we find that it also shows asymmetrical floral isolation and floral trait displacement in sympatry. These findings suggest that trait divergence may reduce negative reproductive interactions between sympatric but genetically incompatible relatives.

Predicting Suitable Habitats of Camptotheca acuminata Considering Both Climatic and Soil Variables

Camptotheca acuminata is considered a natural medicinal plant with antitumor activity. The assessment of climate change impact on its suitable habitats is important for cultivation and conservation. In this study, we applied a novel approach to build ecological niche models with both climate and soil variables while the confounding effects between the variables in the two categories were avoided. We found that the degree-days below zero and mean annual precipitation were the most important climatic factors, while the basic soil saturation, soil gravel volume percentage, and clay content were the main soil factors, determining the suitable habitats of C. acuminata. We found that suitable habitats of this species would moderately increase in future climates under both the RCP4.5 and RCP8.5 climate change scenarios for the 2020s, 2050s, and 2080s. However, substantial shifts among levels of habitat suitability were projected. The dual high-suitable habitats would expand, which would be favorable for commercial plantations. Our findings contribute to a better understanding of the impact of climate change on this species and provide a scientific basis for the cultivation and conservation purposes.

Within-day temporal isolation of two species of Iris (Iridaceae) sharing the same pollinator

Knowledge of factors driving reproductive isolation is essential to understand the process of speciation. To study the reproductive isolation of two closely related species with overlapping flowering seasons, Iris domestica and Iris dichotoma, we compared their reproductive system, floral biology and pollination biology. The results indicated that I. domestica was facultatively xenogamous, whereas I. dichotoma was facultatively autogamous. Although the two species differed significantly in floral colour, floral diameter, floral structure, nectar volume, flower opening and closing times, they shared the same diurnal pollinator, the honeybee Apis cerana. The frequency of pollination by A. cerana did not differ significantly between the two species, but honeybee pollination of I. domestica was more efficient compared with that of I. dichotoma. Despite the difference in floral structure between the two species, both species deposited pollen on the same parts of the body of honeybees. The temporal partitioning of within-day flowering times between I. domestica (from 07.15 to 08.15 h to 18.00 to 19.00 h) and I. dichotoma (from 15.45 to 16.15 h to 22.00 to 23.00 h), together with the time memory of honeybees, meant that the two species did not overlap in the time of their pollination, thus leading to temporal isolation as a major driver of reproductive isolation between the two species.