Patterns of reproductive isolation in a haplodiploid – strong post‐mating,prezygotic barriers among three forms of a social spider mite

In speciation research, much attention is paid to the evolution of reproductive barriers, preventing diverging groups from hybridizing back into one gene pool. The prevalent view is that reproductive barriers evolve gradually as a by‐product of genetic changes accumulated by natural selection and genetic drift in groups that are segregated spatially and/or temporally. Reproductive barriers, however, can also be reinforced by natural selection against maladaptive hybridization. These mutually compatible theories are both empirically supported by studies, analysing relationships between intensity of reproductive isolation and genetic distance in sympatric taxa and allopatric taxa. Here, we present the – to our knowledge – first comparative study in a haplodiploid organism, the social spider mite Stigmaeopsis miscanthi, by measuring premating and post‐mating, pre‐ and post‐zygotic components of reproductive isolation, using three recently diverged forms of the mite that partly overlap in home range. We carried out cross‐experiments and measured genetic distances (mitochondrial DNA and nuclear DNA) among parapatric and allopatric populations of the three forms. Our results show that the three forms are reproductively isolated, despite the absence of premating barriers, and that the post‐mating, prezygotic component contributes most to reproductive isolation. As expected, the strength of post‐mating reproductive barriers positively correlated with genetic distance. We did not find a clear pattern of prezygotic barriers evolving faster in parapatry than in allopatry, although one form did show a trend in line with the ecological and behavioural relationships between the forms. Our study advocates the versatility of haplodiploid animals for investigating the evolution of reproductive barriers.     

Pollen competition as an asymmetric reproductive barrier between two closely related Silene species

Reproductive barriers are important determinants of gene flow between divergent populations or species. We studied pollen competition as a post‐mating reproductive barrier between Silene dioica and S. latifolia. Gene flow between these species is extensive, but early‐generation hybrids are rare. In an experiment with conspecific, heterospecific and 50 : 50 mixed pollinations in the two species, pollination treatments did not significantly affect seed set and seed weight. However, molecular determination of siring success after mixed pollinations showed that fewer than expected hybrids were produced in S. latifolia (18% hybrids) but not in S. dioica (51% hybrids). This constitutes an asymmetric post‐mating reproductive barrier and likely contributes to the rarity of early‐generation hybrids. Our study shows that pollen competition can be an effective barrier to hybridization between closely related species that likely acts in concert with other reproductive barriers.     

Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

Airborne‐pollen pool and mating pattern in a hybrid zone between Pinus pumila and P. parviflora var. pentaphylla

The reproductive isolation barriers and the mating patterns among Pinus pumila, P. parviflora var. pentaphylla and their hybrids were examined by flowering phenology and genetic assays of three life stages: airborne‐pollen grains, adults and seeds, in a hybrid zone on Mount Apoi, Hokkaido, Japan. Chloroplast DNA composition of the airborne‐pollen was determined by single‐pollen polymerase chain reaction. Mating patterns were analysed by estimating the molecular hybrid index of the seed parent, their seed embryos and pollen parents. The observation of flowering phenology showed that the flowering of P. pumila precedes that of P. parviflora var. pentaphylla by about 6 to 10 days within the same altitudinal ranges. Although this prezygotic isolation barrier is effective, the genetic assay of airborne‐pollen showed that the two pine species, particularly P. pumila, still have chances to form F₁ hybrid seeds. Both parental species showed a strong assortative mating pattern; F₁ seeds were found in only 1.4% of seeds from P. pumila mother trees and not at all in P. parviflora var. pentaphylla. The assortative mating was concluded as the combined result of flowering time differentiation and cross‐incompatibility. In contrast to the parental species, hybrids were fertilized evenly by the two parental species and themselves. The breakdown of prezygotic barriers (intermediate flowering phenology) and cross‐incompatibility may account for the unselective mating. It is suggested that introgression is ongoing on Mount Apoi through backcrossing between hybrids and parental species, despite strong isolation barriers between the parental species.     

Behavioural mechanisms of sexual isolation involving multiple modalities and their inheritance

Speciation research dissects the genetics and evolution of reproductive barriers between parental species. Hybrids are the “gatekeepers” of gene flow, so it is also important to understand the behavioural mechanisms and genetics of any potential isolation from their parental species. We tested the role of multiple behavioural barriers in reproductive isolation among closely related field crickets and their hybrids (Teleogryllus oceanicus and Teleogryllus commodus). These species hybridize in the laboratory, but the behaviour of hybrids is unusual and there is little evidence for gene flow in the wild. We found that heterospecific pairs exhibited reduced rates of courtship behaviour due to discrimination by both sexes, and that this behavioural isolation was symmetrical. However, hybrids were not sexually selected against and exhibited high rates of courtship behaviour even though hybrid females are sterile. Using reciprocal hybrid crosses, we characterized patterns of interspecific divergence and inheritance in key sexual traits that might underlie the mating patterns we found: calling song, courtship song and cuticular hydrocarbons (CHCs). Song traits exhibited both sex linkage and transgressive segregation, whereas CHCs exhibited only the latter. Calculations of the strength of isolation exerted by these sexual traits suggest that close‐range signals are as important as long‐distance signals in contributing to interspecific sexual isolation. The surprisingly weak mating barriers observed between hybrids and parental species highlight the need to examine reproductive isolating mechanisms and their genetic bases across different potential stages of introgressive hybridization.     

Evaluation of reproductive barriers and realisation of interspecific hybridisations depending on genetic distances between species in the genus Helleborus

The genus Helleborus comprises 22 species, which are allocated to six sections. H. x hybridus and H. niger, which belong to different Helleborus sections, are economically important ornamentals. Several other species with minor impact exhibit interesting features, e.g. flower size, flower colour, foliage, scent and disease resistance, which should be introgressed into H. x hybridus or H. niger through interspecific hybridisation. The aims of this study were to investigate whether and which kind of hybridisation barriers occur in crosses between Helleborus species and if they differ in their manifestations, depending on the genetic distance of the respective partners. In order to obtain interspecific hybrids despite crossing barriers, a method to overcome these barriers should be developed. Crossing barriers in Helleborus were localised as predominantly post‐zygotic according to in situ pollen tube staining with aniline blue. For certain crosses, pre‐zygotic barriers could also be assumed, but pollen tube growth was not totally inhibited. Therefore, embryo rescue techniques via ovule culture were established to overcome the post‐zygotic barriers. Ovules were isolated from maternal plants 5–7 weeks after pollination in most cases and then cultured in vitro. Overall, 219 hybrids were successfully obtained, of which 16 were derived from inter‐sectional crosses. Hybrids were verified by flow cytometry and/or by molecular DNA markers.     

Evidence for sexual isolation as a prezygotic barrier to gene flow between morphologically divergent species of Rhagoletis fruit flies

1. Certain groups of fruit flies in the genus Rhagoletis (Diptera: Tephritidae) are exemplars for sympatric speciation via host plant shifting. Flies in these species groups are morphologically similar and overlap in their geographic ranges, yet attack different, non‐overlapping sets of host plants. Ecological adaptations related to differences in host choice and preference have been shown to be important prezygotic barriers to gene flow between these taxa, as Rhagoletis flies mate on or near the fruit of their respective host plants. Non‐host‐related assortative mating is generally absent or present at low levels between these sympatrically diverging fly populations. 2. However, some Rhagoletis taxa occasionally migrate to ‘non‐natal’ plants that are the primary hosts of other, morphologically differentiated fly species in the genus. These observations raise the question of whether sexual isolation may reduce courtship and copulation between morphologically divergent species of Rhagoletis flies, contributing to their prezygotic isolation along with host‐specific mating. 3. Using reciprocal multiple‐choice mating trials, we measured sexual isolation among nine species pairs of morphologically differentiated Rhagoletis flies. Complete sexual isolation was observed in eight of the nine comparisons, while partial sexual isolation was observed in the remaining comparison. 4. We conclude that sexual isolation can be an effective prezygotic barrier to gene flow contributing to substantial reproductive isolation between many morphologically distinct Rhagoletis species, even in the absence of differential host plant choice and host‐associated mating.     

Tests of reproductive isolation among species in the <Emphasis Type="Italic">Fundulus notatus</Emphasis> (Cyprinodontiformes: Fundulidae) species complex

We assessed prezygotic (probability of spawning) and postzygotic (hatching success) reproductive isolation among the three ecologically and morphologically similar species in the Fundulus notatus species complex. We employed a multi-generation breeding experiment to test the hypotheses that karyotypic differences, body size differences, or geographic isolation among populations will increase pre or postzygotic reproductive barriers. Overall, prezygotic barriers were strong and postzygotic barriers weak in crosses of non-hybrid heterospecifics (F1 hybrid crosses) while prezygotic barriers were weaker and postzygotic barriers stronger in crosses involving hybrid individuals (F2 hybrid crosses and backcrosses). Prezygotic barriers among the two smaller species (Fundulus notatus and F. euryzonus) broke down rapidly; first generation hybrids spawned (F2 hybrid crosses and backcrosses) as frequently as parental forms in intraspecific crosses. There was no increase in postzygotic barriers among species with cytogenetic differences. There were increased prezygotic, but not postzygotic, barriers among geographically isolated populations of one species. While pure males and females were just as likely to spawn with hybrids, some types of hybrid females suffered from increased sterility, but not inviability, over hybrid males. Female sterility was only seen in hybrids with a Fundulus euryzonus parent, while other female hybrids produced viable eggs.     

Reproductive isolating mechanisms contributing to asymmetric hybridization in Killifishes (Fundulus spp.)

When species hybridize, one F1 hybrid cross type often predominates. Such asymmetry can arise from differences in a variety of reproductive barriers, but the relative roles and concordance of pre-mating, post-mating prezygotic, and post-zygotic barriers in producing these biases in natural animal populations have not been widely investigated. Here, we study a population of predominantly F1 hybrids between two killifish species (Fundulus heteroclitus and F. diaphanus) in which >95% of F1 hybrids have F. diaphanus mothers and F. heteroclitus fathers (D♀ × H♂). To determine why F. heteroclitus × F. diaphanus F1 hybrids (H♀ × D♂) are so rare, we tested for asymmetry in pre-mating reproductive barriers (female preference and male aggression) at a common salinity (10 ppt) and post-mating, pre-zygotic (fertilization success) and post-zygotic (embryonic development time and hatching success) reproductive barriers at a range of ecologically relevant salinities (0, 5, 10, and 15 ppt). We found that F. heteroclitus females preferred conspecific males, whereas F. diaphanus females did not, matching the observed cross bias in the wild. Naturally rare H♀ × D♂ crosses also had lower fertilization success than all other cross types, and a lower hatching success than the prevalent D♀ × H♂ crosses at the salinity found in the hybrid zone centre (10 ppt). Furthermore, the naturally predominant D♀ × H♂ crosses had a higher hatching success than F. diaphanus crosses at 10 ppt, which may further increase their relative abundance. The present study suggests that a combination of incomplete mating, post-mating pre-zygotic and post-zygotic reproductive isolating mechanisms act in concert to produce hybrid asymmetry in this system.     

The importance of pre-mating barriers and the local demographic context for contemporary mating patterns in hybrid zones of Eucalyptus aggregata and Eucalyptus rubida

The frequency of hybridization in plants is context dependent and can be influenced by the local mating environment. We used progeny arrays and admixture and pollen dispersal analyses to assess the relative importance of pre‐mating reproductive barriers and the local demographic environment as explanations of variation in hybrid frequency in three mapped hybrid zones of Eucalyptus aggregata and E. rubida. A total of 731 open‐pollinated progeny from 36 E. aggregata maternal parents were genotyped using six microsatellite markers. Admixture analysis identified substantial variation in hybrid frequency among progeny arrays (0–76.9%). In one hybrid zone, hybrid frequency was related to pre‐mating barriers (degree of flowering synchrony) and demographic components of the local mating environment (decreasing population size, closer proximity to E. rubida and hybrid trees). At this site, average pollen dispersal distance was less and almost half (46%) of the hybrid progeny were sired by local E. rubida and hybrid trees. In contrast, at the other two sites, pre‐mating and demographic factors were not related to hybrid frequency. Compared to the first hybrid zone where most of the E. rubida (76%) and all hybrids flowered, in the remaining sites fewer E. rubida (22–41%) and hybrid trees (0–50%) flowered and their reproductive success was lower (sired 0–23% of hybrids). As a result, most hybrids were sired by external E. rubida/hybrids located at least 2–3 km away. These results indicate that although pre‐mating barriers and local demography can influence patterns of hybridization, their importance can depend upon the scale of pollen dispersal.     

Hybrid bridges to gene flow: a case study in milkweeds (Asclepias)

Natural hybridization occurs throughout areas of sympatry for the North American milkweeds Asclepias exaltata and A. syriaca (Asclepiadaceae), even though the formation of F1 hybrid seed is a rare event. For introgressive hybridization to proceed, F1 and advanced hybrids must be released from reproductive barriers and successfully mate with one or both parental species. I investigated the mating system of natural hybrids between A. exaltata and A. syriaca in three populations in Shenandoah National Park, Virginia. Allozyme data and a maximum-likelihood procedure were used to estimate the frequency of six genotypic classes (parentals, F1, F2, and backcrosses) of the hybridizing populations, the pollinia received by hybrid plants, and the paternal parents of seeds produced by hybrids. F1 hybrids, backcross A. syriaca, and parental A. syriaca individuals were common in three hybrid populations. Even though self-pollinations and interhybrid pollinations were common, F2 seed production and the occurrence of F2 individuals were rare in hybrid populations. Hybrid plants received more pollen from A. syriaca than A. exaltata, which resulted in the production of more backcross-A. syriaca seed than backcross-A. exaltata seed. Asclepias exaltata was rare in the hybrid populations, but A. exaltata pollinia were received by hybrids and this species sired between 15% and 36% of the seeds produced on hybrids. The potential for introgression with A. exaltata populations is lower because this species is unsuccessful as the maternal parent in interspecific and backcross hand-pollinations. The asymetry of hybridization with A. syriaca as the maternal parent is further supported by the incorporation of maternally inherited chloroplast DNA markers in hybrids. Hybrid milkweeds frequently backcross with both parental species and may be released from the reproductive barriers that limit the formation of F1 hybrids in natural populations. The direction of interspecific gene flow and introgression in milkweeds is influenced by the reproductive biology of hybrids, the constituency of the surrounding population, and failure of some crosses to produce seeds. Finally, introgressive hybridization remains an important evolutionary force even when the initial formation of F1 hybrids in natural populations is rare.     

The evolution of intrinsic reproductive isolation in the genus Cakile (Brassicaceae)

In theory, adaptive divergence can increase intrinsic post‐zygotic reproductive isolation (RI), either directly via selection on loci associated with RI, or indirectly via linkage of incompatibility loci with loci under selection. To test this hypothesis, we measured RI at five intrinsic post‐zygotic reproductive barriers between 18 taxa from the genera Cakile and Erucaria (Brassicaceae). Using a comparative framework, we tested whether the magnitude of RI was associated with genetic distance, geographic distance, ecological divergence and parental mating system. Early stages of post‐zygotic RI related to F₁ viability (i.e. initial seed set) tended to be stronger than later stages related to F₁ fecundity (i.e. flower number, fruit number). Mating system significantly influenced early stages of RI, such that RI was lowest when the mother was selfing and father was outcrossing, consistent with an imbalance between sink strength and resistance to provisioning. We found little evidence that adaptive divergence accelerates the evolution of intrinsic post‐zygotic RI, consistent with a nonecological model of evolution that predicts the nonlinear accumulation of RI and RI asymmetry with time (i.e. genetic distance), irrespective of adaptive divergence. Thus, although certain aspects of ecological divergence do not appear to have contributed strongly to the evolution of RI in this system, divergence in mating system actually reduced RI, suggesting that mating system evolution may play a significant role in speciation dynamics.     

Multiple sources of reproductive isolation in a bimodal butterfly hybrid zone

An important evolutionary question concerns whether one or many barriers are involved in the early stages of speciation. We examine pre‐ and post‐zygotic reproductive barriers between two species of butterflies (Heliconius erato chestertonii and H. e. venus) separated by a bimodal hybrid zone in the Cauca Valley, Colombia. We show that there is both strong pre‐ and post‐mating reproductive isolation, together leading to a 98% reduction in gene flow between the species. Pre‐mating isolation plays a primary role, contributing strongly to this isolation (87%), similar to previous examples in Heliconius. Post‐mating isolation was also strong, with absence of Haldane’s rule, but an asymmetric reduction in fertility (< 11%) in inter‐specific crosses depending on maternal genotype. In summary, this is one of the first examples of post‐zygotic reproductive isolation playing a significant role in early stages of parapatric speciation in Heliconius and demonstrates the importance of multiple barriers to gene flow in the speciation process.     

REPRODUCTIVE ISOLATION BETWEEN SYMPATRIC RACES OF PEA APHIDS. I. GENE FLOW RESTRICTION AND HABITAT CHOICE

Determining the extent and causes of barriers to gene flow between genetically divergent populations or races of single species is an important complement to post facto analyses of the causes of reproductive isolation between recognized species. Sympatric populations of pea aphids (Acyrthosiphon pisum Harris, Homoptera: Aphididae) on alfalfa and red clover are highly genetically divergent and locally adapted. Here, hierarchical estimates of population structure based on F_st suggest that gene exchange between closely adjacent aphid populations on the two hosts is highly restricted relative to that among fields of the same host plant. Although these host-associated races are presently considered to be the same subspecies, they appear to be significantly reproductively isolated, suggesting incipient speciation. Habitat (host) choice was investigated as the first in a temporal series of factors that could reduce gene exchange between these sympatric populations. Field studies of winged colonists to newly planted fields of each host suggest pronounced habitat fidelity. This result was verified using replicated observations of the host choice behavior of different aphid genotypes for which the relative demographic performance on each host was known. These laboratory observations of behavior revealed a strong genetic correlation between habitat choice (or acceptance) and the relative performance in each habitat. Because mating occurs on the host plant, habitat choice in this system leads to assortative mating and is therefore a major cause of reproductive isolation between the sympatric pea aphid populations on alfalfa and clover. However, the extent of dispersal between hosts estimated from the field study of winged colonists (9–11%) is too great to be consistent with the genetic divergence estimated between the races. This suggests that barriers to gene flow other than host choice also exist, such as selection against migrants or hybrids in the parental environments, hybrid sterility, or hybrid breakdown.     

Hybridization dynamics between sympatric species of trout: loss of reproductive isolation

Although reinforcement should enhance reproductive barriers in sympatric species, sympatric trout species do hybridize. Using mitochondrial and nuclear species markers, we investigated hybridization directionality, hybrid mating biases, and selection against hybrids in 13 sympatric cut-throat and rainbow trout populations on Vancouver Island, Canada. Approximately 50% of the genotyped fish were hybrid (F1 or higher-order) and populations ranged from very recent (all F1 hybrids) to extremely advanced higher-order hybridization. Overall, interbreeding was reciprocal, although some populations showed directional hybridization. Pronounced cytonuclear disequilibrium in post-F1 hybrids indicated a remarkable mating bias not previously reported, which is most likely because of behavioural reproductive preferences. Selection against hybrids was observed in only two populations, indicative of extrinsic selection. Two populations were 'hybrid swarms', with a complete loss of reproductive isolation. The complex hybridization dynamics in this system represent a valuable natural experiment of the genetic and evolutionary implications of recent and on-going interspecific hybridization.     

Hybridisation and lack of prezygotic barriers between Phymata pennsylvanica and americana

1. The persistence of both geographical and reproductive boundaries between related species poses a fundamental puzzle in biology. Reproductive interactions between species can have a substantial impact on the maintenance of a boundary, potentially contributing to its collapse (e.g. via hybridisation) or facilitating reproductive isolation (e.g. via reinforcement). 2. The degree to which two parapatric insect species in the genus Phymata are reproductively isolated was evaluated and several mechanisms that could contribute to the maintenance of species boundaries were assessed. 3. Behavioural assays showed no indication of species‐assortative mating, nor any fecundity costs associated with heterospecific mating. Thus, there was no evidence of prezygotic mechanisms of reproductive isolation between the two species. 4.In laboratory crosses, it was found that the two species were indeed capable of producing viable F1 hybrids. Morphologically, these hybrids were phenotypically intermediate to the two parental species, and similar to the phenotypes seen in natural populations thought to occur in a hybrid zone. F1 hybrids did not show reduced viability, although there was some suggestion of ‘hybrid breakdown’, evident from the lower viability observed for progeny of ‘natural hybrids’. 5. Collectively, we show that despite genetically based morphological differences between species, P. americana and pennsylvanica can, and probably do hybridise. More studies are needed to understand the mechanisms that maintain the distinct phenotypes and geographical ranges of these species, despite the considerable potential for introgression.     

Evolution of pre‐zygotic and post‐zygotic barriers to gene flow among three cryptic species within the Anastrepha fraterculus complex

Tropical tephritids are ideally suited for studies on population divergence and speciation because they include species groups undergoing rapid radiation, in which morphologically cryptic species and sister species are abundant. The fraterculus species group in the Neotropical genus Anastrepha is a case in point, as it is composed of a complex of up to seven A. fraterculus morphotypes proposed to be cryptic species. Here, we document pre‐ and post‐zygotic barriers to gene flow among adults of the Mexican A. fraterculus morphotype and three populations (Argentina, Brazil, and Peru) belonging to two separate morphotypes (Brazilian 1 and Peruvian). We unveiled three forms of pre‐zygotic reproductive isolation resulting in strong assortative mating. In field cages, free‐ranging male and female A. fraterculus displayed a strong tendency to form couples with members of the opposite sex belonging to their own morphotype, suggesting that male pheromone emission, courtship displays, or both intervene in shaping female choice before actual contact and coupling. In addition, males and females of the Peruvian morphotype became receptive and mated significantly later than adults of the Mexican and Brazilian 1 morphotypes. After contact, Mexican females exhibited greater mating discrimination than males when facing adults of the opposite sex belonging to either the Peruvian or the Brazilian 1 morphotype as evidenced by vigorous resistance to penetration once they had been forcefully mounted by heterotypic males. Forced copulations resulted in production of F1 hybrids that were either less viable (and partially fertile) than parental crosses or even sterile. Our results suggest that the Mexican morphotype is a distinct biological entity and that pre‐zygotic reproductive isolation through divergence in courtship or male‐produced pheromone and other mechanisms appear to evolve faster than post‐zygotic isolation in the fraterculus species group.     

Ants exhibit asymmetric hybridization in a mosaic hybrid zone

Research on hybridization between species provides unparalleled insights into the pre‐ and postzygotic isolating mechanisms that drive speciation. In social organisms, colony‐level incompatibilities may provide additional reproductive barriers not present in solitary species, and hybrid zones offer an opportunity to identify these barriers. Here, we use genotyping‐by‐sequencing to sequence hundreds of markers in a hybrid zone between two socially polymorphic ant species, Formica selysi and Formica cinerea. We characterize the zone, determine the frequency of hybrid workers, infer whether hybrid queens or males are produced and investigate whether hybridization is influenced by colony social organization. We also compare cuticular hydrocarbon profiles and aggression levels between the two species. The hybrid zone exhibits a mosaic structure. The asymmetric distribution of hybrids skewed towards F. cinerea suggests a pattern of unidirectional nuclear gene flow from F. selysi into F. cinerea. The occurrence of backcrossed individuals indicates that hybrid queens and/or males are fertile, and the presence of the F. cinerea mitochondrial haplotype in 97% of hybrids shows that successful F1 hybrids will generally have F. cinerea mothers and F. selysi fathers. We found no evidence that social organization contributes to speciation, because hybrids occur in both single‐queen and multiple‐queen colonies. Strongly differentiated cuticular hydrocarbon profiles and heightened interspecific aggression further reveal that species recognition cues are both present and perceived. The discovery of fertile hybrids and asymmetrical gene flow is unusual in ants, and this hybrid zone will therefore provide an ideal system with which to investigate speciation in social insects.     

The effects of heterospecific mating frequency on the strength of cryptic reproductive barriers

Heterospecific mating frequency is critical to hybrid zone dynamics and can directly impact the strength of reproductive barriers and patterns of introgression. The effectiveness of post‐mating prezygotic (PMPZ) reproductive barriers, which include reduced fecundity via heterospecific matings and conspecific sperm precedence, may depend on the number, identity and order of mates. Studies of PMPZ barriers suggest that they may be important in many systems, but whether these barriers are effective at realistic heterospecific mating frequencies has not been tested. Here, we evaluate the strength of cryptic reproductive isolation in two leaf beetles (Chrysochus auratus and C. cobaltinus) in the context of a range of heterospecific mating frequencies observed in natural populations. We found both species benefited from multiple matings, but the benefits were greater in C. cobaltinus and extended to heterospecific matings. We found that PMPZ barriers greatly limited hybrid production by C. auratus females with moderate heterospecific mating frequencies, but that their effectiveness diminished at higher heterospecific mating frequencies. In contrast, there was no evidence for PMPZ barriers in C. cobaltinus females at any heterospecific mating frequency. We show that integrating realistic estimates of cryptic isolation with information on relative abundance and heterospecific mating frequency in the field substantially improves our understanding of the strong directional bias in F1 production previously documented in the Chrysochus hybrid zone. Our results demonstrate that heterospecific mating frequency is critical to understanding the impact of cryptic post‐copulatory barriers on hybrid zone structure and dynamics, and that future studies of such barriers should incorporate field‐relevant heterospecific mating frequencies.     

Host use and developmental instability in the cactophilic sibling species Drosophila gouveai and D. antonietae

The Drosophila repleta group encompasses an ensemble of species that inhabit desertic areas that are inhospitable to other drosophilids. These species have a tractable ecology, as they breed and feed on necrotic tissues of a wide diversity of species of Cactaceae, with a certain degree of host specificity, which makes them suitable models to investigate the role of host plant shifts in diversification. Most species have their own primary host plant, which may be shared with a closely related species. However, the consequences of host plant shifts from primary to secondary hosts have not been thoroughly studied so far. We investigated the effects of the cactus host on developmental instability and performance of D. gouveai Tidon‐Sklorz & Sene and D. antonietae Tidon‐Sklorz & Sene (Diptera: Drosophilidae), a pair of closely related sibling species, and of their F1 interspecific hybrids reared in primary and secondary host plants. Fluctuating asymmetry (FA) for wing size was significantly greater in flies of both species reared in their respective secondary cactus host than in those grown in the primary host. Interspecific hybrids also exhibited host‐dependent levels of FA. However, hybrids did not show greater FA than the parental species, suggesting that hybridization per se did not affect developmental stability. Even though cactus host shifts affected different measures of fitness (larval viability and adult size) both in D. antonietae and D. gouveai, we did not detect a consistent relationship between FA and fitness. Our results suggest that FA cannot be used as a sensitive indicator of genetic and phenotypic quality and we discuss its application as a fitness predictor.