Ecological barriers to gene flow between riparian and forest species of Ainsliaea (Asteraceae)

Strong plastic responses in aerenchyma formation in F1 hybrids of Imperata cylindrica under different soil moisture conditions

Hybrids can express traits plastically, enabling them to occupy environments that differ from parental environments. However, there is insufficient evidence demonstrating how phenotypic plasticity in specific traits mediates hybrid performance. Two parental ecotypes of Imperata cylindrica produce F1 hybrids. The E-type in wet habitats has larger internal aerenchyma than the C-type in dry habitats. This study evaluated relationships between habitat utilisation, aerenchyma plasticity, and growth of I. cylindrica accessions. We hypothesize that plasticity in expressing parental traits explains hybrid establishment in habitats with various soil moisture conditions. Aerenchyma formation was examined in the leaf midribs, rhizomes and roots of two parental ecotypes and their F1 hybrids in their natural habitats. In common garden experiments, we examined plastic aerenchyma formation in leaf midribs, rhizomes and roots of natural and artificial F1 hybrids and parental ecotypes and quantified vegetative growth performance. In the natural habitats where soil moisture content varied widely, the F1 hybrids showed larger variation in aerenchyma formation in rhizomes than their parental ecotypes. In the common garden experiments, F1 hybrids showed high plasticity of aerenchyma formation in rhizomes, and their growth was similar to that of C-type and E-type under drained and flooded conditions, respectively. The results demonstrate that F1 hybrids of I. cylindrica exhibit plasticity in aerenchyma development in response to varying local soil moisture content. This characteristic allows the hybrids to thrive in diverse soil moisture conditions.

Sympatric co-existence of two ecotypes of Impatiens noli-tangere (Balsaminaceae) with different morphology and flowering phenology

In angiosperms, intraspecific variation of flowering phenology may affect reproductive isolation and, consequently, speciation. This study focused on Impatiens noli-tangere (Balsaminaceae), which is distributed over broad latitudinal and altitudinal ranges in Japan. We aimed to reveal the phenotypic mixture of two ecotypes of I. noli-tangere with different flowering phenology and morphological traits in a narrow contact zone. Previous studies have shown that I. noli-tangere has early- and late-flowering types. The early-flowering type makes buds in June and is distributed at high-elevation sites. The late-flowering type makes buds in July and is distributed at low-elevation sites. In this study, we analyzed the flowering phenology of individuals at an intermediate elevation site where the early- and late-flowering types grow in sympatry (contact zone). We found no individuals showing intermediate flowering phenology at the contact zone, and early- and late-flowering types were clearly distinguishable. We also found that the differences in many other phenotypic traits between the early- and late-flowering types were maintained, including the number of flowers produced (total number of chasmogamous and cleistogamous flowers), leaf morphology (aspect ratio, number of serrations), seed traits (aspect ratio), and flower bud formation positions on the plant. This study showed that these two flowering ecotypes maintain many different traits in sympatry.

Phylogeographic and demographic modeling analyses of the multiple origins of the rheophytic goldenrod Solidago yokusaiana Makino

Understanding adaptation mechanisms is important in evolutionary biology. Parallel adaptation provides good opportunities to investigate adaptive evolution. To confirm parallel adaptation, it is effective to examine whether the phenotypic similarity has one or multiple origins and to use demographic modeling to consider the gene flow between ecotypes. Solidago yokusaiana is a rheophyte endemic to the Japanese Archipelago that diverged from Solidago virgaurea. This study examined the parallel origins of S. yokusaiana by distinguishing between multiple and single origins and subsequent gene flow. The haplotypes of noncoding chloroplast DNA and genotypes at 14 nuclear simple sequence repeat (nSSR) loci and single-nucleotide polymorphisms (SNPs) revealed by double-digest restriction-associated DNA sequencing (ddRADseq) were used for phylogeographic analysis; the SNPs were also used to model population demographics. Some chloroplast haplotypes were common to S. yokusaiana and its ancestor S. virgaurea. Also, the population genetic structures revealed by nSSR and SNPs did not correspond to the taxonomic species. The demographic modeling supported the multiple origins of S. yokusaiana in at least four districts and rejected a single origin with ongoing gene flow between the two species, implying that S. yokusaiana independently and repeatedly adapted to frequently flooding riversides.

Selective pollination by fungus gnats potentially functions as an alternative reproductive isolation among five Arisaema species

BACKGROUND AND AIMS

Interspecific difference in pollinators (pollinator isolation) is important for reproductive isolation in flowering plants. Species-specific pollination by fungus gnats has been discovered in several plant taxa, suggesting that they can contribute to reproductive isolation. Nevertheless, their contribution has not been studied in detail, partly because they are too small for field observations during flower visitation. To quantify their flower visitation, we used the genus Arisaema (Araceae) because the pitcher-like spathe of Arisaema can trap all floral visitors.

METHODS

We evaluated floral visitor assemblage in an altitudinal gradient including five Arisaema species. We also examined interspecific differences in altitudinal distribution (geographic isolation) and flowering phenology (phenological isolation). To exclude the effect of interspecific differences in altitudinal distribution on floral visitor assemblage, we established ten experimental plots including the five Arisaema species in high- and low-altitude areas and collected floral visitors. We also collected floral visitors in three additional sites. Finally, we estimated the strength and contribution of these three reproductive barriers using a unified formula for reproductive isolation.

KEY RESULTS

Each Arisaema species selectively attracted different fungus gnats in the altitudinal gradient, experimental plots and additional sites. Altitudinal distribution and flowering phenology differed among the five Arisaema species, whereas the strength of geographic and phenological isolations were distinctly weaker than those in pollinator isolation. Nevertheless, the absolute contribution of pollinator isolation to total reproductive isolation was weaker than geographic and phenological isolations, because pollinator isolation functions after the two early-acting barriers in plant life history.

CONCLUSIONS

Our results suggest that selective pollination by fungus gnats potentially contributes to reproductive isolation. Since geographic and phenological isolations can be disrupted by habitat disturbance and interannual climate change, the strong and stable pollinator isolation might compensate for the weakened early-acting barriers as an alternative reproductive isolation among the five Arisaema species.

Comparative analysis of spatial genetic structures in sympatric populations of two riparian plants, Saxifraga acerifolia and Saxifraga fortunei

PREMISE

The genetic structure between plant populations is facilitated by the spatial population arrangement and limited dispersal of seed and pollen. Saxifraga acerifolia, a local endemic species in Japan, is a habitat specialist that is confined to waterfalls in riparian environments. Its sister species, Saxifraga fortunei, is a generalist that is widely distributed along riverbanks. Here, we examined sympatric populations of the two Saxifraga species to test whether the differences in habitat preference and colonization process influenced regional and local genetic structures.

METHODS

To reveal genetic structures, we examined chloroplast microsatellite variations and genome-wide nucleotide polymorphisms obtained by genotyping by sequencing. We also estimated the gene flow among and within populations and performed landscape genetic analyses to evaluate seed and pollen movement and the extent of genetic isolation related to geographic distance and/or habitat differences.

RESULTS

We found strong genetic structure in the specialist S. acerifolia, even on a small spatial scale (<1 km part); each population on a different waterfall in one river system had a completely different predominant haplotype. By contrast, the generalist S. fortunei showed no clear genetic differentiation.

CONCLUSIONS

Our findings suggest that the level of genetic isolation was increased in S. acerifolia by the spatially fragmented habitat and limited seed and pollen dispersal over waterfalls. Habitat differentiation between the sister taxa could have contributed to the different patterns of gene flow and then shaped the contrasting genetic structures.

Mechanisms of the Morphological Plasticity Induced by Phytohormones and the Environment in Plants

Plants adapt to environmental changes by regulating their development and growth. As an important interface between plants and their environment, leaf morphogenesis varies between species, populations, or even shows plasticity within individuals. Leaf growth is dependent on many environmental factors, such as light, temperature, and submergence. Phytohormones play key functions in leaf development and can act as molecular regulatory elements in response to environmental signals. In this review, we discuss the current knowledge on the effects of different environmental factors and phytohormone pathways on morphological plasticity and intend to summarize the advances in leaf development. In addition, we detail the molecular mechanisms of heterophylly, the representative of leaf plasticity, providing novel insights into phytohormones and the environmental adaptation in plants.

Pre-pollination barriers between two sympatric Arisaema species in northern Shikoku Island, Japan

PREMISE

The genus Arisaema (Araceae) has rapidly diversified in Japan, and multiple species often coexist in the field. Although Japanese Arisaema species hybridize from artificial crossing, hybrid individuals are rare in mixed populations; suggesting the presence of effective pre-pollination barriers. We examined the following reproductive barriers between A. sikokianum and A. tosaense: habitat, phenology, and pollinator isolations.

METHODS

Habitat isolation was examined by interspecific comparisons of microhabitat conditions at a mixed site and of altitude at the sampling site of herbarium specimens. Phenological isolation was evaluated by comparing seasonal transition in apparent spathe condition and frequency of insect visitation. Pollinator isolation was examined by comparing floral visitor assemblages between the two Arisaema species. To avoid overestimation of pollinator isolation due to seasonal changes in insect assemblages, we also compared visitor assemblages between natural and late-flowering A. sikokianum, where the latter was experimentally introduced and blooming with a natural A. tosaense population.

RESULTS

Microhabitat conditions and sampling elevations of herbarium specimens overlapped between the two Arisaema species. At the population level, A. sikokianum and A. tosaense flowered for 39 and 52 days, respectively, with 13 days overlap. Insect visitation in A. sikokianum decreased before the seasonal overlap. Floral visitor assemblages differed between the two Arisaema species, while the difference between natural and late-flowering A. sikokianum was less distinct.

CONCLUSIONS

Phenological and pollinator isolation contribute to reproductive isolation between the two Arisaema species and should enable the two species to coexist in this area.

Varieties of the highly dispersible and hypervariable tree, Metrosideros polymorpha, differ in response to mechanical stress and light across a sharp ecotone

PREMISE

The drivers of isolation between sympatric populations of long-lived and highly dispersible conspecific plants are not well understood. In the Hawaiian Islands, the landscape-dominant tree, Metrosideros polymorpha, displays extraordinary phenotypic differences among sympatric varieties despite high dispersibility of its pollen and seeds, thereby presenting a unique opportunity to investigate how disruptive selection alone can maintain incipient forms. Stenophyllous M. polymorpha var. newellii is a recently evolved tree endemic to the waterways of eastern Hawai'i Island that shows striking neutral genetic differentiation from its ancestor, wet-forest M. polymorpha var. glaberrima, despite sympatry of these forms. We looked for evidence for, and drivers of, differential local adaptation of these varieties across the range of M. polymorpha var. newellii.

METHODS

For paired populations of these varieties, we compared seedling performance under contrasting light conditions and a strong water current characteristic of the riparian zone. We also conducted a reciprocal transplant experiment and contrasted adult leaf anatomy.

RESULTS

Results suggest that the riparian zone is harsh and that selection involving the mechanical stress of rushing water, and secondarily, light, led to significant reciprocal immigrant inviability in adjacent forest and riparian environments. The strongest adaptive divergence between varieties was seen in leaves and seedlings from the site with the sharpest ecotone, coincident with the strongest genetic isolation of M. polymorpha var. newellii observed previously.

CONCLUSIONS

These findings suggest that disruptive selection across a sharp ecotone contributes to the maintenance of an incipient riparian ecotype from within a continuous population of a long-lived and highly dispersible tree species.

Convergent origin of the narrowly lanceolate leaf in the genus Aster-with special reference to an unexpected discovery of a new Aster species from East China

Narrowly lanceolate leaves occur frequently in the genus Aster. It was often employed as a distinguishing character in the taxonomy of this genus. The origin of this particular leaf shape, however, has never been investigated using comparative methods. In this study, we reconstructed a comprehensive phylogeny that includes most species of Aster with narrowly lanceolate leaf. We then gathered data on riparian habitats and the presence or absence of narrowly lanceolate leaves, and investigated the evolutionary association between them in a phylogenetic context. Our analysis indicated that the species with narrowly lanceolate leaves are nested in unrelated lineages of the genus Aster, implying that they originated independently several times. Using Pagel’s comparative method of discrete data, we demonstrated a significant correlation between riparian habitats and narrowly lanceolate leaves. We further inferred the sequence of transition of the two characters. This analysis indicated that the sequence of evolution of riparian habitat and narrowly lanceolate leaf form was usually uncertain, but some positive results showed that the occurrence of riparian habitats may not precede the evolution of narrowly lanceolate leaf form. This study provided new insights into the adaptive evolution in a mega-diverse family. In addition, Aster tonglingensis, an unexpected new species with narrowly lanceolate leaves, was discovered and established based on the evidence from morphology, micromorphology and molecular phylogeny.

Phylogeographic analysis suggests two origins for the riparian azalea Rhododendron indicum (L.) Sweet

Ecological speciation is an important factor in the diversification of plants. The distribution of the woody species Rhododendron indicum, which grows along rivers and is able to withstand water flow when rivers flood (i.e. it is a rheophyte), is disjunct, in contrast to the widespread distribution of its relative, Rhododendron kaempferi. This study aimed to elucidate the phylogenetic relationships between R. indicum and R. kaempferi and the evolutionary processes that gave rise to them. The sequences of three non-coding chloroplast DNA regions (total length 1977 bp) were obtained from 21 populations covering the ranges of the two species. In addition, genome-wide SNPs were genotyped from 20 populations using a genotyping by sequencing method. Leaf morphologies were measured for eight representative populations. Two chloroplast DNA haplotypes, which were detected in R. indicum, were shared between the two species. Genome-wide SNPs identified two lineages in R. indicum and these lineages did not constitute a monophyletic group. Each of these two lineages was related to geographically close populations of R. kaempferi. Leaf morphology, which is a characteristic feature in rheophytes, was not differentiated between the two lineages in R. indicum. The morphological similarity between the two heterogeneous lineages may be a result of parallel evolution from R. kaempferi or of introgressive hybridization between the species due to strong selective pressure imposed by flooding.

Tight species cohesion among sympatric insular wild gingers (Asarum spp. Aristolochiaceae) on continental islands: Highly differentiated floral characteristics versus undifferentiated genotypes

The Amami Island group of the Ryukyu Archipelago, Japan, harbors extensive species diversity of Asarum in a small landmass. The fine-scale population genetic structure and diversity of nine insular endemic Asarum species were examined using nuclear DNA microsatellite loci and ITS sequences. High population genetic diversity (HS = 0.45-0.79) was estimated based on the microsatellites, implying outcrossing of Asarum species within populations accompanied by inbreeding. Bayesian clustering analyses revealed that species were divided into three robust genetic clusters and that the species within each cluster had a homogeneous genetic structure, indicating incomplete lineage sorting. This conclusion was supported by an ITS phylogeny. The degree of genetic differentiation among species was very low both within and between clusters (FST = 0.096-0.193, and 0.096-0.266, respectively). Although species can be crossed artificially to produce fertile hybrids, our results indicate that there is very little evidence of hybridization or introgression occurring among species in the wild, even within stands composed of multiple sympatric species. The highly differentiated floral morphology of the studied species is likely to impose reproductive isolation between them and maintain their integrity in the wild. A lack of genetic differentiation between sympatric species suggests that speciation within this group occurred rapidly and recently.

Why is population information crucial for taxonomy? A case study involving a hybrid swarm and related varieties

Hybridization has become a focal topic in evolutionary biology, and many taxonomists are aware that the process occurs more frequently than previously assumed. Nonetheless many species and varieties are still described without explicitly considering the possibility of hybridization, especially in countries that have relatively short scientific histories, but which often possess the highest species diversities. Furthermore, new taxa are often described based only on herbarium specimens, not taking into account information from wild populations, significantly decreasing the potential to detect morphologies arising from hybridization at this crucial descriptive stage. We used morphological data from a hybrid swarm involving two Rhododendron species to showcase possible character combinations in intermediates. Certain characters used to distinguish taxa were more variable within the same individual than between species, emphasizing the importance of population information for an adequate choice of characters. Most described varieties of the two species fell within the spectrum of hybrid morphology, suggesting that these taxa would be unlikely to have merited formal description if contemporary standards had been employed. In all investigated cases the hybrid nature of described varieties seems to have been detectable with adequate morphological data alone, if populations had been assessed. A post hoc assessment of taxa is often complicated, especially if certain types of information are not provided. To avoid accumulation of such invalid taxa, careful scrutiny should be employed for new descriptions. Hybrids (not hybrid species) described as taxa obscure valuable information about natural processes and impact negatively on further research that depends on taxonomic data.

The Effects of Rainfall on the Population Dynamics of an Endangered Aquatic Plant, Schoenoplectus gemmifer (Cyperaceae)

The conservation of aquatic plants in river ecosystems should consider the wash-out (away) problem resulting from severe rainfall. The aquatic plant Schoenoplectus gemmifer is an endangered species endemic to Japan. Our previous study reported that the population size of S. gemmifer in Hamamatsu city, Japan, had decreased by one-tenth because many individuals had been washed out by a series of heavy rains in 2004. However, there is insufficient information on the ecological nature of this endangered aquatic plant for adequate conservation. In this paper, we report the population dynamics of one population in Hamamatsu city from 2004 to 2012 in relation to rainfall. We surveyed the number and growing location of all living individuals in the population 300 times during the study period. To examine the temporal changes of individual plants, we also counted the number of culms for 38 individuals in four observations among 300 records. Decreases and increases in the population size of this plant were associated with washing out and the settlement of gemmae (vegetative propagation), respectively. The major cause of the reduction in the population size was an increase in the number of washed-out individuals and not the decreased settlement of gemmae. The wash-out rates for small and large individuals were not significantly different. Small individuals having a stream form with linear leaves resisted flooding, and large individuals were often partially torn off by flooding events. Modification of river basins to reduce the flow velocity may be effective for the conservation of S. gemmifer.

Nuclear introgression without mitochondrial introgression in two turtle species exhibiting sex-specific trophic differentiation

Despite the presence of reproductive barriers between species, interspecific gene introgression has been documented in a range of natural systems. Comparing patterns of genetic introgression in biparental versus matrilineal markers can potentially reveal sex-specific barriers to interspecific gene flow. Hybridization has been documented in the freshwater turtles Graptemys geographica and G. pseudogeographica, whose ranges are largely sympatric. Morphological differentiation between the species is restricted to females, with female G. geographica possessing large heads and jaws compared to the narrow heads of G. pseudogeographica females. If hybrid females are morphologically intermediate, they may be less successful at exploiting parental feeding niches, thereby limiting the introgression of maternally inherited, but not biparental, molecular markers. We paired sequence data with stable isotope analysis and examined sex-specific genetic introgression and trophic differentiation in sympatric populations of G. geographica and G. pseudogeographica. We observed introgression from G. pseudogeographica into G. geographica at three nuclear loci, but not at the mitochondrial locus. Analysis of ∂(15)N and ∂(13)C was consistent with species differences in trophic positioning in females, but not males. These results suggest that ecological divergence in females may reduce the opportunity for gene flow in this system.

Widespread host-dependent hybrid unfitness in the pea aphid species complex

Linking adaptive divergence to hybrid unfitness is necessary to understand the ecological factors contributing to reproductive isolation and speciation. To date, this link has been demonstrated in few model systems, most of which encompass ecotypes that occupy relatively early stages in the speciation process. Here we extend these studies by assessing how host-plant adaptation conditions hybrid fitness in the pea aphid, Acyrthosiphon pisum. We made crosses between and within five pea aphid biotypes adapted to different host plants and representing various stages of divergence within the complex. Performance of F1 hybrids and nonhybrids was assessed on a "universal" host that is favorable to all pea aphid biotypes in laboratory conditions. Although hybrids performed equally well as nonhybrids on the universal host, their performance was much lower than nonhybrids on the natural hosts of their parental populations. Hence, hybrids, rather than being intrinsically deficient, are maladapted to their parents' hosts. Interestingly, the impact of this maladaptation was stronger in certain hybrids from crosses involving the most divergent biotype, suggesting that host-dependent postzygotic isolation has continued to evolve late in divergence. Even though host-independent deficiencies are not excluded, hybrid maladaptation to parental hosts supports the hypothesis of ecological speciation in this complex.

Demographic histories of adaptively diverged riparian and non-riparian species of Ainsliaea (Asteraceae) inferred from coalescent analyses using multiple nuclear loci

BACKGROUND

Understanding demographic histories, such as divergence time, patterns of gene flow, and population size changes, in ecologically diverging lineages provide implications for the process and maintenance of population differentiation by ecological adaptation. This study addressed the demographic histories in two independently derived lineages of flood-resistant riparian plants and their non-riparian relatives [Ainsliaea linearis (riparian) and A. apiculata (non-riparian); A. oblonga (riparian) and A. macroclinidioides (non-riparian); Asteraceae] using an isolation-with-migration (IM) model based on variation at 10 nuclear DNA loci.

RESULTS

The highest posterior probabilities of the divergence time parameters were estimated to be ca. 25,000 years ago for A. linearis and A. apiculata and ca. 9000 years ago for A. oblonga and A. macroclinidioides, although the confidence intervals of the parameters had broad ranges. The likelihood ratio tests detected evidence of historical gene flow between both riparian/non-riparian species pairs. The riparian populations showed lower levels of genetic diversity and a significant reduction in effective population sizes compared to the non-riparian populations and their ancestral populations.

CONCLUSIONS

This study showed the recent origins of flood-resistant riparian plants, which are remarkable examples of plant ecological adaptation. The recent divergence and genetic signatures of historical gene flow among riparian/non-riparian species implied that they underwent morphological and ecological differentiation within short evolutionary timescales and have maintained their species boundaries in the face of gene flow. Comparative analyses of adaptive divergence in two sets of riparian/non-riparian lineages suggested that strong natural selection by flooding had frequently reduced the genetic diversity and size of riparian populations through genetic drift, possibly leading to fixation of adaptive traits in riparian populations. The two sets of riparian/non-riparian lineages showed contrasting patterns of gene flow and genetic differentiation, implying that each lineage showed different degrees of reproductive isolation and that they had experienced unique evolutionary and demographic histories in the process of adaptive divergence.