POLLINATOR‐MEDIATED REPRODUCTIVE ISOLATION AMONG DIOECIOUS FIG SPECIES (FICUS,MORACEAE)

The extent of isolation among closely related sympatric plant species engaged in obligate pollination mutualisms depends on the fitness consequences of interspecies floral visitation. In figs (Ficus), interspecific gene flow may occur when pollinating wasps (Agaonidae) visit species other than their natal fig species. We studied reproductive isolation in a clade of six sympatric dioecious fig species in New Guinea. Microsatellite genotyping and Bayesian clustering analysis of the fig community indicated strong reproductive barriers among sympatric species. A total of 1–2% of fig populations consisted of hybrid individuals. A new experimental method of manipulating fig wasps investigated the reproductive consequences of conspecific and heterospecific pollinator visitation for both mutualists. Fig wasps introduced to Ficus hispidioides pollinated and oviposited in receptive figs. Seed development and seedling growth were largely comparable between conspecific and heterospecific crosses. Heterospecific pollinator fitness, however, was significantly less than that of conspecific pollinators. Heterospecific pollinators induced gall formation but offspring did not develop to maturity in the new host. Selection on pollinators maintaining host specificity appears to be an important mechanism of contemporary reproductive isolation among these taxa that could potentially influence their diversification.     

Phenotypic plasticity of mate recognition systems prevents sexual interference between two sympatric leaf beetle species

Maladaptive sexual interactions among heterospecific individuals (sexual interference) can prevent the coexistence of animal species. Thus, the avoidance of sexual interference by divergence of mate recognition systems is crucial for a stable coexistence in sympatry. Mate recognition systems are thought to be under tight genetic control. However, we demonstrate that mate recognition systems of two closely related sympatric leaf beetle species show a high level of host‐induced phenotypic plasticity. Mate choice in the mustard leaf beetles, Phaedon cochleariae and P. armoraciae, is mediated by cuticular hydrocarbons (CHCs). Divergent host plant use causes a divergence of CHC phenotypes, whereas similar host use leads to their convergence. Consequently, both species exhibit significant behavioral isolation when they feed on alternative host species, but mate randomly when using a common host. Thus, sexual interference between these syntopic leaf beetles is prevented by host‐induced phenotypic plasticity rather than by genotypic divergence of mate recognition systems.     

Primary attraction and kairomonal host discrimination in three species of Dendroctonus (Coleoptera: Scolytidae)

Abstract 1 Synthetic blends of bole and foliage volatiles of four sympatric species of conifers were released from pheromone‐baited multiple‐funnel traps to determine if three species of tree‐killing bark beetles (Coleoptera: Scolytidae): (i) exhibited primary attraction to volatiles of their hosts and (ii) discriminated among volatiles of four sympatric species of host and nonhost conifers. 2 Bole and foliage volatiles from Douglas‐fir, Pseudotsuga menziesii (Mirb.) Franco, increased the attraction of coastal and interior Douglas‐fir beetles, Dendroctonus pseudotsugae Hopkins, to pheromone‐baited traps. Primary attraction to bole volatiles was observed in interior D. pseudotsugae. Beetles were significantly less attracted to the pheromone bait when it was combined with volatiles of lodgepole pine, Pinus contorta var. latifolia Engelm. or interior fir, Abies lasiocarpa × bifolia. 3 The monoterpene myrcene synergized attraction of mountain pine beetles, Dendroctonus ponderosae Hopkins, to their aggregation pheromones, but there was no evidence of primary attraction to host volatiles or discrimination among volatiles from the four conifers. 4 There was significant primary attraction of the spruce beetle, Dendroctonus rufipennis Kirby, to bole and foliage volatiles of interior spruce, Picea engelmannii × glauca, but beetles did not discriminate among volatiles of four sympatric conifers when they were combined with pheromone baits. 5 Our results indicate that host volatiles act as kairomones to aid pioneer Douglas‐fir beetles and spruce beetles in host location by primary attraction, and that their role as synergists to aggregation pheromones is significant. For the mountain pine beetle, we conclude that random landing and close range acceptance or rejection of potential hosts would occur in the absence of aggregation pheromones emanating from a tree under attack.     

HYBRIDIZATION STUDIES ON THE HOST RACES OF EUROSTA SOLIDAGINIS: IMPLICATIONS FOR SYMPATRIC SPECIATION

We studied the inheritance of survival ability in host-associated populations of the tephritid fly, Eurosta solidaginis, to test predictions of sympatric speciation models. Eurosta solidaginis induces galls on two species of goldenrod, Solidago altissima and S. gigantea. The host-associated populations have been hypothesized to be host races that originated in sympatry (Craig et al. 1993). We found evidence for disruptive selection for host use, which is a critical assumption of sympatric speciation models. Each host race had higher survival rates on their host plant than on the alternative host. F₁ and backcross hybrids also had lower survival rates than the pure host-race flies on their host plant. Since assortative mating occurs due to host-plant preference (Craig et al. 1993) this would select for divergence in host preference. Low hybrid survival could have been due to strong genetic incompatibilities of the populations or due to host adaptation by each population. Strong genetic incompatibilities would result in poor survival on all host plants, while host adaptation could result in low overall survival with high hybrid survival on some host plants with particularly “benign” environments. High survival of F₁, F₂, and backcross hybrids on some plant genotypes in some years supported the host adaptation hypothesis. F₁ flies mated and oviposited normally and produced viable F₂ and backcross hybrids indicating gene flow is possible between the host races. A few flies developed and emerged on the alternative host plant. This demonstrates that genes necessary to utilize the alternative host exist in both host races. This could have facilitated the origin of one of the populations via a host shift from the ancestral host. The inheritance of survival ability appears to be an autosomal trait. We did not find evidence that survival ability was maternally influenced or sex linked.     

Detection of ecological hybrid inviability in a pair of sympatric phytophagous ladybird beetles (Henosepilachna spp.)

Ecological speciation is a process by which reproductive isolation evolves as the result of divergent natural selection between populations inhabiting distinct environments or exploiting alternative resources. Ecological hybrid inviability provides direct evidence for ecological speciation. To detect ecological hybrid inviability, we examined survival rates to the second instar of F1 hybrids and backcross hybrids in a pair of sympatric phytophagous ladybird beetles, Henosepilachna niponica Lewis and Henosepilachna yasutomii Katakura (Coleoptera: Coccinellidae: Epilachninae), reared on their respective host plants, thistle [Cirsium alpicola Nakai (Asteraceae)] and blue cohosh [Caulophyllum robustum Maxim. (Berberidaceae)], and on a common food plant, Japanese nightshade [Solanum japonense Nakai (Solanaceae)]. Hybrid larvae reared on leaves of the Japanese nightshade always had high rates of survival, irrespective of the crossing type of their parents, suggesting a lack of intrinsic hybrid inviability between the two species. In contrast, survival rates on thistle and blue cohosh varied greatly. On blue cohosh, the survival rate of F1 hybrids was nearly as high as that of H. yasutomii, but on thistle, survival was significantly lower than of H. niponica. Survival rates of backcross hybrids on the two host plants were intermediate between those of the parents, showing a reversed rank order of different types of backcross hybrids on the two food plant species. These results suggest that ecological hybrid inviability exists between H. niponica and H. yasutomii, although the two species do not show intrinsic hybrid inviability. Thus, our study supports the hypothesis that H. niponica and H. yasutomii underwent ecological speciation by divergent selection.     

The role of natural-enemy escape in a gallmaker host-plant shift

The successful colonization of novel host-plant species by herbivorous insects may be facilitated by a reduction in natural-enemy attack on insect populations associated with the novel (derived) host plant. This is particularly true if natural enemies use host-plant or habitat cues in searching for their herbivore prey. In order to test whether the acquisition of enemy-free space could have influenced the host shift in the goldenrod ball gallmaker, Eurosta solidaginis, we estimated levels of natural-enemy attack in 25 host-race populations associated with Solidago altissima and S. gigantea (Compositae) spanning the zone of sympatry between S. altissima and S. gigantea host races in New England. Mortality due to attack by the parasitoid wasp Eurytoma obtusiventris was significantly higher for the ancestral than for the derived host race (30.5% versus 0.4%) across the transect, which is consistent with the enemy escape hypothesis. Contrary to this hypothesis, mordellid beetles caused significantly higher mortality on the derived than ancestral host race (17.1% versus 2.6%). Mortality by a second parasitoid wasp and birds showed no significant differences between the two host races. Overall, the derived host race had significantly higher survivorship across the transect (36.6% versus 20.8%). An analysis of survivorship and parasitoid mortality levels from sympatric sites in this study and previous studies showed a highly significant correlation between the levels of Eurytoma obtusiventris attack and the survivorship advantage of the derived host race. Observations of this parasitoid's searching behavior confirmed that it preferentially searches the ancestral host for fly larvae. Current patterns of host-race mortality and naturalenemy behavior and abundance are consistent with the facilitation of the host shift by escape from a specialist parasitoid.     

Molecular population genetics of a host‐associated sibling species complex of phytophagous ladybird beetles (Coleoptera: Coccinellidae: Epilachninae)

To clarify evolutionary relationships and assess the intensity and persistence of reproductive isolation because of host fidelity in three closely related host‐associated phytophagous ladybird beetles (Henosepilachna pustulosa, Henosepilachna niponica and Henosepilachna yasutomii), we determined sequences of part of the nuclear DNA internal transcribed spacer‐II (ITS‐2) region (388 bp long) for 70 individuals from seven populations, and part of the mitochondrial cytochrome c oxidase subunit I (COI) gene (985 bp long) for 280 individuals from 28 populations across the three species. We failed to detect any polymorphic sites in the ITS‐2 region. We detected 59 COI haplotypes, among which two were shared by different species. While a haplotype network and a phylogenetic gene tree of the COI haplotypes did not support monophyly for any of the three species, the results of AMOVA did not invalidate the classification of these beetles into three species. The isolation‐with‐migration analytic (IMa) test suggested that gene flow between allopatric species (i.e., H. pustulosa versus H. niponica; H. pustulosa versus H. yasutomii) was well prevented, probably because of their limited dispersal power. On the other hand, the IMa test detected a low level of unidirectional gene flow between the sympatric species H. niponica and H. yasutomii, from the former to the latter. This result was consistent with a survival rate of F1 hybrids between the two species that is higher on the host plant of H. yasutomii than on the host plant of H. niponica. High F_ST values among two sympatric and three nearly sympatric population pairs of H. niponica and H. yasutomii, however, indicated that gene flow between the two species has been quite restricted even if it has occurred. Because no reproductive barriers other than the difference in host plants is known for sympatric H. niponica and H. yasutomii, our results suggest that host differentiation and associated ecological divergence function as an effective reproductive barrier between the two species.     

Host specificity in spinturnicid mites: do parasites share a long evolutionary history with their host?

Host specificity in parasites can be explained by spatial isolation from other potential hosts or by specialization and speciation of specific parasite species. The first assertion is based on allopatric speciation, the latter on differential lifetime reproductive success on different available hosts. We investigated the host specificity and cophylogenetic histories of four sympatric European bat species of the genus Myotis and their ectoparasitic wing mites of the genus Spinturnix. We sampled >40 parasite specimens from each bat species and reconstructed their phylogenetic COI trees to assess host specificity. To test for cospeciation, we compared host and parasite trees for congruencies in tree topologies. Corresponding divergence events in host and parasite trees were dated using the molecular clock approach. We found two species of wing mites to be host specific and one species to occur on two unrelated hosts. Host specificity cannot be explained by isolation of host species, because we found individual parasites on other species than their native hosts. Furthermore, we found no evidence for cospeciation, but for one host switch and one sorting event. Host‐specific wing mites were several million years younger than their hosts. Speciation of hosts did not cause speciation in their respective parasites, but we found that diversification of recent host lineages coincided with a lineage split in some parasites.     

Mating Preferences of the Gynogenetic Amazon Molly Differ Between Populations Sympatric with Different Host Species

Gynogenetic species rely on sperm from heterospecifics for reproduction but do not receive genetic benefits from mating because none of the paternal genome is incorporated into offspring. The gynogenetic Amazon molly (Poecilia formosa) is a species of hybrid origins that are sympatric with one of the two parent species that provide sperm for reproduction, P. latipinna or P. mexicana. Amazons should not prefer to mate with one species over the other because sperm from both species will trigger embryogenesis, but mating preferences may be present in Amazons through other mechanisms. Amazons may prefer the more familiar species (males found in sympatry), or Amazons may prefer males with the greatest lateral projection area (LPA), a preference that is present in the parent species and may be retained within the Amazon hybrid genome. We tested association preferences of two populations of Amazons sympatric with either P. mexicana or P. latipinna. We first performed live trials to test whether Amazons preferred one host species over the other and found that neither population of Amazons showed a preference. We then tested whether Amazons preferred sympatric male (familiar) host or the male with the greatest lateral projection area (LPA) using four animated male models that varied in host species and manipulation of LPA. We found Amazons from a population sympatric with P. latipinna showed no variation in their association preference across the different models. In contrast, Amazons from a population sympatric with P. mexicana (naturally small LPA) spent more time associating with the male models that had smaller LPA, which is more familiar to this population of Amazons. We suggest that Amazons may have population differences in mating preferences, where Amazons sympatric with P. latipinna may not show mating preference for host species, but Amazons sympatric with P. mexicana may show preferences for more familiar‐shaped males.     

Reproductive barriers between two sympatric beetle species specialized on different host plants

Knowledge on interspecific pre‐ and post‐zygotic isolation mechanisms provides insights into speciation patterns. Using crosses (F₁ and backcrosses) of two closely related flea beetles species, Altica fragariae and A. viridicyanea, specialized on different hosts in sympatry, we measured: (a) the type of reproductive isolation and (b) the inheritance mode of preference and host‐specific performance, using a joint‐scaling test. Each species preferred almost exclusively its host plant, creating strong prezygotic isolation between them, and suggesting that speciation may occur at least partly in sympatry. Reproductive isolation was intrinsic between females of A. fragariae and either A. viridicyanea or F₁ males, whereas the other crosses showed ecologically dependent reproductive isolation, suggesting ecological speciation. The genetic basis of preference and performance was at least partially independent, and several loci coded for preference, which limits the possibility of sympatric speciation. Hence, both ecological and intrinsic factors may contribute to speciation between these species.     

Phoretic Poecilochirus mites specialize on their burying beetle hosts

Recurring species interactions can cause species to adapt to each other. Specialization will increase the fitness of symbionts in the coevolved association but may reduce the flexibility of symbiont choice as it will often decrease fitness in interactions with other than the main symbiont species. We analyzed the fitness interactions between a complex of two cryptic mite species and their sympatric burying beetle hosts in a European population. Poecilochirus mites (Mesostigmata, Parasitidae) are phoretic on burying beetles and reproduce alongside beetles, while these care for their offspring at vertebrate carcasses. While Poecilochirus carabi is typically found on Nicrophorus vespilloides beetles, P. necrophori is associated with N. vespillo. It has long been known that the mites discriminate between the two beetle species, but the fitness consequences of this choice remained unknown. We experimentally associated both mite species with both beetle species and found that mite fitness suffered when mites reproduced alongside a nonpreferred host. In turn, there is evidence that one of the beetle species is better able to cope with the mite species they are typically associated with. The overall fitness effect of mites on beetles was negative in our laboratory experiments. The Poecilochirus mites studied here are thus specialized competitors or parasites of burying beetles.     

Species overlap, speciation and the evolution of aggregation pheromones in bark beetles

Theoretical predictions suggest that species‐specific signals used in the attraction of mates should evolve to reduce the risk of mismating and hybridization. These predictions lead to the hypothesis that the signals of spatially overlapping (i.e. sympatric or syntopic) species should differ more substantially than those of non‐overlapping species. Earlier studies have tested this prediction for auditory and visual signals. Here we test the hypothesis using olfactory signals, specifically the aggregation pheromones of species from two genera of bark beetles, Dendroctonus and Ips. We found no direct evidence from within these genera regarding the fact that the chemical blends that make up these pheromones differ more substantially in species that overlap in their geographical ranges and/or host‐tree use than in allopatric taxa. However, when comparing between genera, the pheromones of overlapping species appear to be more similar than non‐overlapping species. We hypothesize that the species of host tree utilized by the beetles may have some influence on their pheromone blends. Additionally, within genera, species that overlap in host use tend to be more closely related than species that use different hosts. These results may provide indirect evidence for an effect of species overlap on the evolution of bark beetle pheromones.     

The mechanism of pollinator specificity between two sympatric fig varieties: a combination of olfactory signals and contact cues

Background and Aims

Pollinator specificity facilitates reproductive isolation among plants, and mechanisms that generate specificity influence species boundaries. Long-range volatile attractants, in combination with morphological co-adaptations, are generally regarded as being responsible for maintaining extreme host specificity among the fig wasps that pollinate fig trees, but increasing evidence for breakdowns in specificity is accumulating. The basis of host specificity was examined among two host-specific Ceratosolen fig wasps that pollinate two sympatric varieties of Ficus semicordata, together with the consequences for the plants when pollinators entered the alternative host variety.

Methods

The compositions of floral scents from receptive figs of the two varieties and responses of their pollinators to these volatiles were compared. The behaviour of the wasps once on the surface of the figs was also recorded, together with the reproductive success of figs entered by the two Ceratosolen species.

Key Results

The receptive-phase floral scents of the two varieties had different chemical compositions, but only one Ceratosolen species displayed a preference between them in Y-tube trials. Specificity was reinforced at a later stage, once pollinators were walking on the figs, because both species preferred to enter figs of their normal hosts. Both pollinators could enter figs of both varieties and pollinate them, but figs with extra-varietal pollen were more likely to abort and contained fewer seeds. Hybrid seeds germinated at normal rates.

Conclusions

Contact cues on the surface of figs have been largely ignored in previous studies of fig wasp host preferences, but together with floral scents they maintain host specificity among the pollinators of sympatric F. semicordata varieties. When pollinators enter atypical hosts, post-zygotic factors reduce but do not prevent the production of hybrid offspring, suggesting there may be gene flow between these varieties.     

The effects of foods consumed after adult eclosion on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Coleoptera: Cerambycidae)

The effects of changes in host plants on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Thomson) (Coleoptera: Cerambycidae) were examined. All individuals were raised on the same artificial diet until they became pupae. Analysis of the mate-searching behavior of the males showed that many more newly emerged males were attracted to the odor of the artificial diet than to an unbaited control. We prepared three groups of beetles and fed each group on different host plants for one week. The host plants used included the following: an artificial diet (containing Morus alba Linné), Citrus unshiu Marc. branches, and Vaccinium spp. branches. The mate-searching behavior of the males changed in relation to the plant supplied for feeding. Simultaneously, the preference among the three host plants was tested. The newly emerged males preferred the artificial diet. After a week of feeding on one of the three plants, however, the adult males selected and consumed significantly more of the plant that they had just experienced than the other plants. These results suggest that the male mate-location cue can be acquired after adult eclosion. In addition, the male beetles are capable of changing their host-plant preference. If they do so, they use different odor cues for mate location. Newly emerged A. malasiaca females showed no preference for their first choice of food among the three host plants presented, whereas the consumption was significantly larger on C. unshiu branches. After one week of feeding on different host plants, females chose their host plant after the adult stage as well as C. unshiu, but consumed mostly C. unshiu. These results suggest that the food preferences of females are different from those of males. The behavior of females may not be affected by chemical signals from their original host-plant species (as pupae) or from the host-plant species acquired when they emerge as adults.     

Differences in morphology and reproductive traits of Galerucella nymphaeae from four host plant species

The water lily beetle Galerucella nymphaeae L. (Coleoptera: Chrysomelidae) exploits different hosts, including Nuphar lutea Sm. and Nymphaea alba L. (both Nymphaeaceae), as well as Polygonum amphibium L. and Rumex hydrolapathum Hudson (both Polygonaceae). The present study investigates whether within-species differences in morphological and reproductive traits are associated with differences in host species exploitation. A total of 1103 adult beetles were collected from 11 localities in The Netherlands, one of which contained all four hosts and three other localities contained hosts from both families (sympatric localities). Adults originating from Nuphar and Nymphaea were on average darker in colour and larger in size and had disproportionally bigger mandibles than beetles originating from Polygonum and Rumex across the 11 localities. Head capsules of first instar larvae from Nymphaeaceae hosts were between 17% and 28% larger than those of larvae from Polygonaceae hosts. Furthermore, beetles from Nuphar and Nymphaea laid larger sized eggs, but fewer eggs per clutch than beetles originating from Polygonum and Rumex. Although host related variation was less pronounced at the sympatric localities than in the allopatric localities, differences in larval and adult size were still highly significant at the sympatric localities. It is not clear whether the observed differences are genetically based, as opposed to host induced. However, leaf toughness varied among species in a way suggesting that leaf toughness may be partly responsible for host related differences in G. nymphaeae.     

Host location and discrimination mediated through olfactory stimuli in two species of Encyrtidae

The role of olfactory stimuli in host detection and evaluation was studied in two encyrtid Hymenoptera. The first, Epidinocarsis lopezi De Santis, is a monophagous parasitoid of the cassava mealybug Phenacoccus manihoti Matile-Ferrero, itself feeding exclusively on cassava, Manihot esculenta Crantz. The second, Leptomastix dactylopii Howard, is a monophagous parasitoid of the Citrus mealybug, Planococcus citri Risso, but this latter is highly polyphagous. The behaviour of females of both parasitoids (attaction and locomotion) was compared in a tubular olfactometer for the odours of their respective hosts on cassava and poinsettia. Tests were made using: 1) healthy host-plant alone; 2) host-plant infested with unparasitized mealybugs; 3) unparasitized mealybugs only; 4) host-plant infested with parasitized mealybugs and 5) parasitized mealybugs only. Only E. lopezi was attracted by the odour of the host-plant alone, but both species were attracted by the odour of an infested host-plant and that of unparasitized mealybugs. The odour of parasitized mealybugs, alone or on host-plant, induced an undirected activity. The attraction of E. lopezi to the odour of the host-plant alone could be linked to the monophagous diet of its host, whereas the attraction of the two species of parasitoids to the odours of infested host-plants and unparasitized mealybugs could be due to the fact that both parasitoids are specialists. The behavioural response of both species to the odour of parasitized mealybugs revealed a new aspect in host discrimination: the identification of parasitized hosts could be partly mediated through olfactory stimuli, and not only through gustatory stimuli.     

Recent speciation in three closely related sympatric specialists: inferences using multi-locus sequence, post-mating isolation and endosymbiont data

Shifting between unrelated host plants is relatively rare for phytophagous insects, and distinct host specificity may play crucial roles in reproductive isolation. However, the isolation status and the relationship between parental divergence and post-mating isolation among closely related sympatric specialists are still poorly understood. Here, multi-locus sequence were used to estimate the relationship among three host plant-specific closely related flea beetles, Altica cirsicola, A. fragariae and A. viridicyanea (abbreviated as AC, AF and AV respectively). The tree topologies were inconsistent using different gene or different combinations of gene fragments. The relationship of AF+(AC+AV) was supported, however, by both gene tree and species tree based on concatenated data. Post-mating reproductive data on the results of crossing these three species are best interpreted in the light of a well established phylogeny. Nuclear-induced but not Wolbachia-induced unidirectional cytoplasmic incompatibility, which was detected in AC-AF and AF-AV but not in AC-AV, may also suggest more close genetic affinity between AC and AV. Prevalence of Wolbachia in these three beetles, and the endosymbiont in most individuals of AV and AC sharing a same wsp haplotype may give another evidence of AF+(AC+AV). Our study also suggested that these three flea beetles diverged in a relative short time (0.94 My), which may be the result of shifting between unrelated host plants and distinct host specificity. Incomplete post-mating isolation while almost complete lineage sorting indicated that effective pre-mating isolation among these three species should have evolved.     

Host-associated differences in fitness within and between populations of a seed beetle (Bruchidae): effects of plant variability

Summary Experiments were conducted with the sexually reproducing seed beetle Stator limbatus and its hosts in north-central Arizona to determine if it was substructured into units, each specialized for higher fitness on a specific host species. Unlike many studies, we incorporated scale, i.e., conducting experiments between and within beetle populations on seeds from within and between plant species. Of particular interest was whether intraspecific plant variability prevented beetle specialization within beetle populations. Results suggest that S. limbatus is specialized to certain hosts. On the palo verde Cercidium floridum, beetles originally reared from this host had significantly higher emergence compared to beetles transferred from other hosts. We did not test directly for a genetic basis for this. Alternative hypotheses of variation in symbiotic microorganisms in larval guts and maternal effects were assessed. Essentially no bacteria, yeast or protozoa were found, and maternal effects as expressed by varying egg weights were not detected; however, other microorganisms might have been present and maternal effects through inducible enzymes was possible. Caution, then, is needed in any genetic interpretations of our results. The differences on C. floridum were detected from tests between and within beetle populations. Evidence for specialization was not detected on the other hosts, Cercidium microphyllum and Acacia greggii. On the other hosts, beetles performed well regardless of their source. Significant differences were detected among individual plants of C. floridum as to the suitability of their seeds for deveoopment of S. limbatus. No such differences were detected among the other host plants. These patterns of conspecific plant variability are opposite of what is expected if plant variability prevents specialization of beetles to particular species of hosts. Thus, the data suggest seed variability among plants does not prevent specialization to host species in this system. We discuss how the patterns of host use in this study relate to the hypothesis of sympatric host race formation.     

Sympatric speciation in a genus of marine reef fishes

Sympatric speciation has been contentious since its inception, yet is increasingly recognized as important based on accumulating theoretical and empirical support. Here, we present a compelling case of sympatric speciation in a taxon of marine reef fishes using a comparative and mechanistic approach. Hexagrammos otakii and H. agrammus occur in sympatry throughout their ranges. Molecular sequence data from six loci, with complete sampling of the genus, support monophyly of these sister species. Although hybridization occurrs frequently with an allopatric congener in an area of slight distributional overlap, we found no F₁ hybrids between the focal sympatric taxa throughout their coextensive ranges. We present genetic evidence for complete reproductive isolation based on SNP analysis of 382 individuals indicating fixed polymorphisms, with no shared haplotypes or genotypes, between sympatric species. To address questions of speciation, we take a mechanistic approach and directly compare aspects of reproductive isolation between allopatric and sympatric taxa both in nature and in the laboratory. We conclude that the buildup of reproductive isolation is strikingly different in sympatric vs. allopatric taxa, consistent with theoretical predictions. Lab reared hybrids from allopatric species crosses exhibit severe fitness effects in the F₁ or backcross generation. No intrinsic fitness effects are observed in F₁ hybrids from sympatric species pairs, however these treatments exhibited reduced fertilization success and complete pre‐mating isolation is implied in nature because F₁ hybrid adults do not occur. Our study addresses limitations of previous studies and supports new criteria for inferring sympatric speciation.     

Ecological speciation in anemone‐associated snapping shrimps (Alpheus armatus species complex)

Divergent natural selection driven by competition for limited resources can promote speciation, even in the presence of gene flow. Reproductive isolation is more likely to result from divergent selection when the partitioned resource is closely linked to mating. Obligate symbiosis and host fidelity (mating on or near the host) can provide this link, creating ideal conditions for speciation in the absence of physical barriers to dispersal. Symbiotic organisms often experience competition for hosts, and host fidelity ensures that divergent selection for a specific host or host habitat can lead to speciation and strengthen pre‐existing reproductive barriers. Here, we present evidence that diversification of a sympatric species complex occurred despite the potential for gene flow and that partitioning of host resources (both by species and by host habitat) has contributed to this diversification. Four species of snapping shrimps (Alpheus armatus, A. immaculatus, A. polystictus and A. roquensis) are distributed mainly sympatrically in the Caribbean, while the fifth species (A. rudolphi) is restricted to Brazil. All five species are obligate commensals of sea anemones with a high degree of fidelity and ecological specificity for host species and habitat. We analysed sequence data from 10 nuclear genes and the mitochondrial COI gene in 11–16 individuals from each of the Caribbean taxa and from the only available specimen of the Brazilian taxon. Phylogenetic analyses support morphology‐based species assignments and a well‐supported Caribbean clade. The Brazilian A. rudolphi is recovered as an outgroup to the Caribbean taxa. Isolation–migration coalescent analysis provides evidence for historical gene flow among sympatric sister species. Our data suggest that both selection for a novel host and selection for host microhabitat may have promoted diversification of this complex despite gene flow.