Absence of intrinsic post‐zygotic incompatibilities in artificial crosses between sympatric coregonid species from upper Lake Constance

A full factorial crossing experiment with five females and five males of each of two coregonid species from upper Lake Constance was used to test for intrinsic post‐zygotic incompatibilities during early ontogeny. Up until shortly before hatching, there was no difference in embryo mortality between homo and heterologous crosses. A maternal effect on mortality was found in both species, but paternal effects and female–male interactions were absent. Thus, genetic incompatibility during early ontogeny does not appear to prevent introgressive hybridization, suggesting that genetic divergence between these species is maintained primarily by pre‐zygotic barriers. The recent genetic homogenizations of coregonid species flocks in European alpine lakes may have been caused by a flattening of adaptive landscapes through eutrophication, but intensive stocking with larvae obtained in hatcheries from artificially fertilized eggs is also likely to be a contributing factor. To safeguard diversity among sympatric coregonids, it is important to re‐establish ecological conditions conducive to species divergence and to revise traditional management strategies.     

Divergence in drought‐response traits between sympatric species of Mimulus

Differential adaptation to local environmental conditions is thought to be an important driver of speciation. Plants, whose sedentary lifestyle necessitates fine‐tuned adaptation to edaphic conditions such as water availability, are often distributed based on these conditions. Populations occupying water‐limited habitats may employ a variety of strategies, involving numerous phenotypes, to prevent and withstand desiccation. In sympatry, two closely related Mimulus species—M. guttatus and M. nasutus—occupy distinct microhabitats that differ in seasonal water availability. In a common garden experiment, we characterized natural variation within and between sympatric M. guttatus and M. nasutus in the ability to successfully set seed under well‐watered and drought conditions. We also measured key phenotypes for drought adaptation, including developmental timing, plant size, flower size, and stomatal density. Consistent with their microhabitat associations in nature, M. nasutus set seed much more successfully than M. guttatus under water‐limited conditions. This divergence in reproductive output under drought was due to differences in mortality after the onset of flowering, with M. nasutus surviving at a much higher rate than M. guttatus. Higher seed set in M. nasutus was mediated, at least in part, by a plastic increase in the rate of late‐stage development (i.e., fruit maturation), consistent with the ability of this species to inhabit more ephemeral habitats in the field. Our results suggest adaptation to water availability may be an important factor in species maintenance of these Mimulus taxa in sympatry.     

Ecological speciation in sympatric palms: 2. Pre‐ and post‐zygotic isolation

We evaluated reproductive isolation in two species of palms (Howea) that have evolved sympatrically on Lord Howe Island (LHI, Australia). We estimated the strength of some pre‐ and post‐zygotic mechanisms in maintaining current species boundaries. We found that flowering time displacement between species is consistent across in and ex situ common gardens and is thus partly genetically determined. On LHI, pre‐zygotic isolation due solely to flowering displacement was 97% for Howea belmoreana and 80% for H. forsteriana; this asymmetry results from H. forsteriana flowering earlier than H. belmoreana and being protandrous. As expected, only a few hybrids (here confirmed by genotyping) at both juvenile and adult stages could be detected in two sites on LHI, in which the two species grow intermingled (the Far Flats) or adjacently (Transit Hill). Yet, the distribution of hybrids was different between sites. At Transit Hill, we found no hybrid adult trees, but 13.5% of younger palms examined there were of late hybrid classes. In contrast, we found four hybrid adult trees, mostly of late hybrid classes, and only one juvenile F1 hybrid in the Far Flats. This pattern indicates that selection acts against hybrids between the juvenile and adult stages. An in situ reciprocal seed transplant between volcanic and calcareous soils also shows that early fitness components (up to 36 months) were affected by species and soil. These results are indicative of divergent selection in reproductive isolation, although it does not solely explain the current distribution of the two species on LHI.     

Hybridization among sympatric species of Rhododendron (Ericaceae) in Turkey: morphological and molecular evidence

Rhododendron (Ericaceae) is a large genus in which barriers to hybridization are especially weak, but many species are maintained in sympatry. Hybridization among four species of Rhododendron subsect. Pontica, which occur in sympatry in Turkey, was investigated. Material of R. ponticum, R. smirnovii, R. ungernii, and R. caucasicum and their putative hybrids was collected from the wild. Based on morphology, chloroplast DNA and nuclear ribosomal DNA restriction fragment length polymorphism (RFLP) profiles, each accession was identified as a species or hybrid combination. Five of the six possible hybrid combinations among the four species were detected. Rhododendron ponticum × R. smirnovii was represented by a single individual and R. caucasicum × R. smirnovii by one small group of hybrid plants. The combinations R. ponticum × R. ungernii and R. ungernii × R. smirnovii showed evidence of frequent backcrossing, while R. ponticum × R. caucasicum appeared unusual in that an intermediate hybrid type was abundant, whereas hybrids with phenotypes approaching either parent were rare. Possible explanations of this latter situation are discussed. The results suggest that natural hybridization among Rhododendron species is common and that ecological factors are important in maintaining integrity when species occur in sympatry.     

Post‐zygotic isolation in cactophilic Drosophila: larval viability and adult life‐history traits of D. mojavensis/D. arizonae hybrids

Drosophila mojavensis and Drosophila arizonae are cactophilic flies that have been used extensively in speciation studies. Incomplete premating isolation, evidence of reinforcement, and a lack of recent introgression between these species point to a potentially important role for post‐zygotic isolating barriers in this system. Other than hybrid male sterility, however, post‐zygotic isolation between D. mojavensis and D. arizonae has received little attention. In this study, we examined viability and life‐history traits of D. mojavensis/D. arizonae F₁ hybrids from sympatric crosses. Specifically, we reared hybrids and purebreds on the natural host cacti of each parental species and compared viability, development time, thorax length, and desiccation resistance between hybrids and purebreds. Interestingly, hybrid females from both crosses performed similarly or even better than purebred females. In contrast, hybrid sons of D. arizonae mothers, in addition to being sterile, had shorter average thorax length than males of both parental species, and hybrid males from both crosses had substantially lower desiccation resistance than D. mojavensis males. The probable cost to hybridization for D. mojavensis females resulting from reduced desiccation resistance of hybrid sons may have been an important selective factor in the history of reinforcement for crosses involving these females.     

Parallel polyploid speciation: distinct sympatric gene‐pools of recurrently derived allo‐octoploid Asplenium ferns

Although polyploidy is widespread, its significance to the generation of biodiversity remains unclear. Many polyploids have been derived recurrently. For a particular polyploid, gene‐flow between the products of independent origin is typical where they come into contact. Here, we use AFLP DNA‐fingerprinting and chloroplast DNA sequences to demonstrate parallel polyploid speciation within both of the ferns Asplenium cimmeriorum and A. gracillimum. Both of these taxa comprise at least two allopolyploids, recurrently derived from the same progenitor pair. Each of these allopolyploids remain genetically distinguishable even with extensive sympatry, and could therefore be considered distinct species. To our knowledge, parallel speciation on this scale amongst recurrent polyploids has not been previously reported. With their parallel origins, these ‘evolutionary replicates’ provide an unrivalled opportunity to investigate how the reproductive barriers and ecological differentiation necessary for speciation arise following polyploidy.     

Convergent evolutionary processes driven by foraging opportunity in two sympatric morph pairs of Arctic charr with contrasting post‐glacial origins

The expression of two or more discrete phenotypes amongst individuals within a species (morphs) provides multiple modes upon which selection can act semi‐independently, and thus may be an important stage in speciation. In the present study, we compared two sympatric morph systems aiming to address hypotheses related to their evolutionary origin. Arctic charr in sympatry in Loch Tay, Scotland, exhibit one of two discrete, alternative body size phenotypes at maturity (large or small body size). Arctic charr in Loch Awe segregate into two temporally segregated spawning groups (breeding in either spring or autumn). Mitochondrial DNA restriction fragment length polymorphism analysis showed that the morph pairs in both lakes comprise separate gene pools, although segregation of the Loch Awe morphs is more subtle than that of Loch Tay. We conclude that the Loch Awe morphs diverged in situ (within the lake), whereas Loch Tay morphs most likely arose through multiple invasions by different ancestral groups that segregated before post‐glacial invasion (i.e. in allopatry). Both morph pairs showed clear trophic segregation between planktonic and benthic resources (measured by stable isotope analysis) but this was significantly less distinct in Loch Tay than in Loch Awe. By contrast, both inter‐morph morphological and life‐history differences were more subtle in Loch Awe than in Loch Tay. The strong ecological but relatively weak morphological and life‐history divergence of the in situ derived morphs compared to morphs with allopatric origins indicates a strong link between early ecological and subsequent genetic divergence of sympatric origin emerging species pairs. The emergence of parallel specialisms despite distinct genetic origins of these morph pairs suggests that the effect of available foraging opportunities may be at least as important as genetic origin in structuring sympatric divergence in post‐glacial fishes with high levels of phenotypic plasticity. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Pre‐ and post‐mating reproductive barriers drive divergence of five sympatric species of Naryciinae moths (Lepidoptera: Psychidae)

The biological species concept suggests that species can be separated on the basis of reproductive isolation. However, because natural interbreeding capabilities are often unknown, differences in morphology are generally used to separate species. Alternatively, genetic dissimilarity is used to separate morphologically similar species. Many genetic markers, including the maternally inherited mitochondrial cytochrome oxidase I sequence, cannot show interbreeding and therefore species status of groups may remain unresolved. In species of the genera Dahlica and Siederia (Lepidoptera: Psychidae: Naryciinae) the lack of morphological distinction and unknown interbreeding has led to unclear and unresolved taxonomic status. Mitochondrial DNA sequences suggest five sexual species to occur in Finland. However, their species status remains unconfirmed, due to a lack of knowledge on interbreeding, unclear morphological distinction and the limited variation in mitochondrial DNA. We combine three methods, a cross‐mating experiment, an analysis of mitochondrial and nuclear DNA, and a detailed male genital morphological examination, to establish the species status of the five suspected species. All suspected species exhibit intraspecies mating preference, although several interspecies pairs readily produce offspring. The genetic analysis, however, fails to show hybrids or introgression, suggesting that both pre‐ and post‐copulation mechanisms isolate the species reproductively. Morphological analysis of the male genitalia confirms that the species have diverged. Our results highlight the need of combining behavioural, morphological and genetic methods to determine species status in challenging taxonomic groups. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 584–605.     

Climate‐driven build‐up of temporal isolation within a recently formed avian hybrid zone

Divergence in the onset of reproduction can act as an important source of reproductive isolation (i.e., allochronic isolation) between co‐occurring young species, but evidence for the evolutionary processes leading to such divergence is often indirect. While advancing spring seasons strongly affect the onset of reproduction in many taxa, it remains largely unexplored whether contemporary spring advancement directly affects allochronic isolation between young species. We examined how increasing spring temperatures affected onset of reproduction and thereby hybridization between pied and collared flycatchers (Ficedula spp.) across habitat types in a young secondary contact zone. We found that both species have advanced their timing of breeding in 14 years. However, selection on pied flycatchers to breed earlier was weaker, resulting in a slower response to advancing springs compared to collared flycatchers and thereby build‐up of allochronic isolation between the species. We argue that a preadaptation to a broader niche use (diet) of pied flycatchers explains the slower response to raising spring temperature, but that reduced risk to hybridize may contribute to further divergence in the onset of breeding in the future. Our results show that minor differences in the response to environmental change of co‐occurring closely related species can quickly cause allochronic isolation.     

Competition‐driven build‐up of habitat isolation and selection favoring modified dispersal patterns in a young avian hybrid zone

Competition‐driven evolution of habitat isolation is an important mechanism of ecological speciation but empirical support for this process is often indirect. We examined how an on‐going displacement of pied flycatchers from their preferred breeding habitat by collared flycatchers in a young secondary contact zone is associated with (a) access to an important food resource (caterpillar larvae), (b) immigration of pied flycatchers in relation to habitat quality, and (c) the risk of hybridization in relation to habitat quality. Over the past 12 years, the estimated access to caterpillar larvae biomass in the habitat surrounding the nests of pied flycatchers has decreased by a fifth due to shifted establishment possibilities, especially for immigrants. However, breeding in the high quality habitat has become associated with such a high risk of hybridization for pied flycatchers that overall selection currently favors pied flycatchers that were forced to immigrate into the poorer habitats (despite lower access to preferred food items). Our results show that competition‐driven habitat segregation can lead to fast habitat isolation, which per se caused an opportunity for selection to act in favor of future “voluntarily” altered immigration patterns and possibly strengthened habitat isolation through reinforcement.     

Phylogenomic inference of two widespread European leaf miner species complexes suggests mechanisms for sympatric speciation (Lepidoptera: Nepticulidae: Ectoedemia)

Leafmining insects have extraordinarily intimate relationships with their host plants and are therefore prime candidates for potential ecological speciation in sympatry, but how commonly this mode of speciation occurs in any group of life remains debated. Prior research on the pygmy moth (Nepticulidae) genus Ectoedemia using morphology, DNA barcoding, and multi-marker phylogenetic approaches left two unresolved species complexes, each with each four described species. In this study, we thoroughly sampled each complex across a range of host plants and localities across Europe. We used a double digest restriction-site associated DNA (ddRAD) sequencing approach to clarify species boundaries. In the E. rubivora complex, ddRAD data resolved all four species, contrary to morphological and COI data, which supports a potential scenario of host plant-driven speciation where the host plant specialization provides an ecological barrier to hybridization. However, we found no indication of host race formation within the oligophagous E. atricollis (Stainton). In the E. subbimaculella complex, SNP data only partly distinguishes between the parapatric E. subbimaculella (Haworth) and E. heringi (Toll), but with some statistical overlap, suggesting incomplete lineage sorting which may represent early phases of host-based ecological speciation, or admixture following a period of isolation.     

Natural selection for salt tolerance quantitative trait loci (QTLs) in wild sunflower hybrids: implications for the origin of Helianthus paradoxus,a diploid hybrid species

For a new diploid or homoploid hybrid species to become established, it must diverge ecologically from parental genotypes. Otherwise the hybrid neospecies will be overcome by gene flow or competition. We initiated a series of experiments designed to understand how the homoploid hybrid species, Helianthus paradoxus, was able to colonize salt marsh habitats, when both of its parental species (H. annuusxH. petiolaris) are salt sensitive. Here, we report on the results of a quantitative trait locus (QTL) analysis of mineral ion uptake traits and survivorship in 172 BC2 hybrids between H. annuus and H. petiolaris that were planted in H. paradoxus salt marsh habitat in New Mexico. A total of 14 QTLs were detected for mineral ion uptake traits and three for survivorship. Several mineral ion QTLs mapped to the same position as the survivorship QTLs, confirming previous studies, which indicated that salt tolerance in Helianthus is achieved through increased Ca uptake, coupled with greater exclusion of Na and related mineral ions. Of greater general significance was the observation that QTLs with effects in opposing directions were found for survivorship and for all mineral ion uptake traits with more than one detected QTL. This genetic architecture provides an ideal substrate for rapid ecological divergence in hybrid neospecies and offers a simple explanation for the colonization of salt marsh habitats by H. paradoxus. Finally, selection coefficients of +0.126, -0.084 and -0.094 for the three survivorship QTLs, respectively, are sufficiently large to account for establishment of new, homoploid hybrid species.     

Introgression and selection shaped the evolutionary history of sympatric sister‐species of coral reef fishes (genus: Haemulon)

Closely related marine species with large overlapping ranges provide opportunities to study mechanisms of speciation, particularly when there is evidence of gene flow between such lineages. Here, we focus on a case of hybridization between the sympatric sister‐species Haemulon maculicauda and H. flaviguttatum, using Sanger sequencing of mitochondrial and nuclear loci, as well as 2422 single nucleotide polymorphisms (SNPs) obtained via restriction site‐associated DNA sequencing (RADSeq). Mitochondrial markers revealed a shared haplotype for COI and low divergence for CytB and CR between the sister‐species. On the other hand, complete lineage sorting was observed at the nuclear loci and most of the SNPs. Under neutral expectations, the smaller effective population size of mtDNA should lead to fixation of mutations faster than nDNA. Thus, these results suggest that hybridization in the recent past (0.174–0.263 Ma) led to introgression of the mtDNA, with little effect on the nuclear genome. Analyses of the SNP data revealed 28 loci potentially under divergent selection between the two species. The combination of mtDNA introgression and limited nuclear DNA introgression provides a mechanism for the evolution of independent lineages despite recurrent hybridization events. This study adds to the growing body of research that exemplifies how genetic divergence can be maintained in the presence of gene flow between closely related species.     

Satellite species in lampreys: a worldwide trend for ecological speciation in sympatry?

Amongst several theories of speciation, sympatric speciation has been the most controversial but it is now widely accepted that populations can become reproductively isolated without being separated geographically. One problem with the acceptance of the theory of sympatric speciation, however, has been the lack of supporting empirical data and it is still believed that geographical isolation is responsible for the majority of speciation events. Here the example of species pairs in lampreys suggests that sympatric speciation in a whole taxonomic group could occur throughout its worldwide range. Lampreys occur in two ecologically distinct forms: parasitic mostly anadromous species that forage on tissue and body fluids of host fishes, and non‐parasitic forms that, apart from a short adult life when they cease feeding, spend their entire life as filter feeders in the substratum of stream beds. Both forms occur in sympatric species pairs throughout the range of lampreys that occur in Eurasia, North America and Australia and it is widely acknowledged that non‐parasitic forms derive from parasitic forms. The larvae of both forms can be distinguished by their potential fecundity and therefore, it is argued that the mode of life is not a consequence of different ecological conditions. Furthermore, as lampreys prefer to choose mates of similar sizes and fertilization success decreases with increasing difference in body size, there is a strong disruptive selection between the two forms and they are therefore reproductively isolated. Besides theoretical aspects, the similarity of the species pairs, together with their occurrence in sympatry, the occurrence of forms with intermediate characteristics, and examples where speciation might be in progress, hints at the possibility that speciation also occurred in sympatry. The difference between lampreys and other examples of sympatric speciation is that there seems to be a trend towards sympatric speciation events throughout the worldwide range of lampreys which is neither restricted to relatively small localities nor caused by human disturbance. Species pairs in lampreys therefore offer a unique possibility of studying the process of sympatric speciation on a large scale.     

Morphological and genetic analysis of sympatric dace within the Rhinichthys cataractae species complex: a case of isolation lost

The Nooksack dace (Pisces: an undescribed putative taxon within Rhinichthys) and longnose dace (Rhinichthys cataractae) are two forms within the R. cataractae species complex that are distinguishable from one another by mitochondrial (mt) DNA divergence of 2–3%, as well as by subtle morphological differences. The two forms are found in allopatry in south‐eastern British Columbia (BC), Canada, and adjacent areas of western Washington, USA, and are sympatric in three streams in the lower Fraser River valley, BC, and may represent cryptic species. We assayed 12 morphometric traits and two meristic characters (N = 582; 23 sampling locations) to test for diagnosability of the two dace, as well as to test for morphological differentiation by mtDNA type in sympatry. We then employed a 10‐locus microsatellite DNA assay (N = 374; 12 sampling locations) to test for genetic distinction between Nooksack dace and longnose dace in sympatry. We found that the two dace could not be reliably differentiated morphologically: there was overlap in all characters measured, and sampling location had a much larger effect on morphology than mtDNA group. Microsatellite analysis showed no distinction by mtDNA type in localities with sympatric dace, indicating complete admixture between the sympatric Nooksack dace and longnose dace. The Nooksack dace and longnose dace provide an example of ‘ephemeral speciation’: two lineages that, despite an estimated 1.1 Myr of isolation, have developed no apparent barriers to reproduction and appear to have collapsed into a single interbreeding population where they come into secondary contact. Nonetheless, the zone of secondary contact and the diagnosability of the Nooksack dace in terms of mtDNA represent significant aspects of the evolutionary legacy within R. cataractae and support its conservation importance.     

Model‐based demographic inference of introgression history in European whitefish species pairs'

Parallel phenotypic differentiation is generally attributed to parallel adaptive divergence as an evolutionary response to similar environmental contrasts. Such parallelism may actually originate from several evolutionary scenarios ranging from repeated parallel divergence caused by divergent selection to a unique divergence event followed by gene flow. Reconstructing the evolutionary history underlying parallel phenotypic differentiation is thus fundamental to understand the relative contribution of demography and selection on genomic divergence during speciation. In this study, we investigate the divergence history of replicate European whitefish (Coregonus lavaretus), limnetic and benthic species pairs from two lakes in Norway and two lakes in Switzerland. Demographic models accounting for semi‐permeability and linked selection were fitted to the unfolded joint allele frequency spectrum built from genome‐wide SNPs and compared to each other in each species pair. We found strong support for a model of asymmetrical post‐glacial secondary contact between glacial lineages in all four lakes. Moreover, our results suggest that heterogeneous genomic differentiation has been shaped by the joint action of linked selection accelerating lineage sorting during allopatry, and heterogeneous migration eroding divergence at different rates along the genome following secondary contact. Our analyses reveal how the interplay between demography, selection and historical contingency has influenced the levels of diversity observed in previous whitefish phylogeographic studies. This study thus provides new insights into the historical demographic and selective processes that shaped the divergence associated with ecological speciation in European whitefish.     

Asymmetric Assortative Mating Behaviour Reflects Incomplete Pre‐zygotic Isolation in the Nasonia Species Complex

Preference of con‐ over heterospecific mates leading to assortative mating can substantially contribute to pre‐zygotic reproductive isolation and prevent fitness losses if post‐zygotic hybridization barriers already exist. The jewel wasp genus Nasonia displays quite strong and well‐studied post‐zygotic reproductive isolation due to a ubiquitous Wolbachia infection causing cytoplasmic incompatibility between different species. Pre‐zygotic isolation, however, has received far less research attention in this model organism, especially concerning the mechanisms and criteria of mate choice. In the present study, we analysed mate rejection and mate acceptance rates in cross‐comparisons between all four Nasonia species. We put emphasis on observing which sex is more likely to interrupt interspecific matings and how discriminatory behaviour varies across the different species in all possible combinations. We found an asymmetric distribution of assortative mating among the four Nasonia species that appears to be highly influenced by the respective combinations of sex and species. Females appeared to be the main discriminators against heterospecific mating partners, but interestingly, we could also detect mate discrimination and rejection behaviour in males, a widely neglected factor in research on mating behaviour in general and on Nasonia in particular. Moreover, the asymmetry in the assortative mating behaviour was partially reflective of sym‐ or allopatric distributions of natural Nasonia populations.     

Testing models of speciation from genome sequences: divergence and asymmetric admixture in Island South‐East Asian Sus species during the Plio‐Pleistocene climatic fluctuations

In many temperate regions, ice ages promoted range contractions into refugia resulting in divergence (and potentially speciation), while warmer periods led to range expansions and hybridization. However, the impact these climatic oscillations had in many parts of the tropics remains elusive. Here, we investigate this issue using genome sequences of three pig (Sus) species, two of which are found on islands of the Sunda‐shelf shallow seas in Island South‐East Asia (ISEA). A previous study revealed signatures of interspecific admixture between these Sus species (Genome biology, 14 , 2013, R107). However, the timing, directionality and extent of this admixture remain unknown. Here, we use a likelihood‐based model comparison to more finely resolve this admixture history and test whether it was mediated by humans or occurred naturally. Our analyses suggest that interspecific admixture between Sunda‐shelf species was most likely asymmetric and occurred long before the arrival of humans in the region. More precisely, we show that these species diverged during the late Pliocene but around 23% of their genomes have been affected by admixture during the later Pleistocene climatic transition. In addition, we show that our method provides a significant improvement over D‐statistics which are uninformative about the direction of admixture.