Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds

Interspecific competition is assumed to play an important role in the ecological differentiation of species and speciation. However, empirical evidence for competition's role in speciation remains surprisingly scarce. Here, we studied the role of interspecific competition in the ecological differentiation and speciation of two closely related songbird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia). Both species are insectivorous and ecologically very similar. They hybridize in a secondary contact zone, which is a mosaic of sites where both species co‐occur (syntopy) and sites where only one species is present (allotopy). We analysed fine‐scale habitat data for both species in both syntopic and allotopic sites and looked for associations between habitat use and bill morphology, which have been previously shown to be more divergent in sympatry than in allopatry. We found that the two nightingale species differ in habitat use in allotopic sites, where L. megarhynchos occurred in drier habitats and at slightly higher elevations, but not in syntopic sites. Birds from allotopic sites also showed higher interspecific divergence in relative bill size compared to birds from syntopic sites. Finally, we found an association between bill morphology and elevation. Our results are consistent with the view that interspecific competition in nightingales has resulted in partial habitat segregation in sympatry and that the habitat‐specific food supply has in turn very likely led to bill size divergence. Such ecological divergence may enhance prezygotic as well as extrinsic postzygotic isolation and thus accelerate the completion of the speciation process.     

An investigation of habitat occupancy by the nightingale Luscinia megarhynchos with respect to population change at the edge of its range in England

The nightingale Luscinia megarhynchos has undergone population decline and range contraction at the north-western limit of its distribution in England during the last 25 years. We examine patterns of habitat occupancy and habitat availability across sites with a range of population histories to see whether habitat loss is a plausible explanation for these declines. The number of singing males in 1999 correlated with area of primary nightingale habitat in the East Midlands (where the species has declined), but not in East Anglia (where the population has been stable). Change in population size between 1980 and 1999 and current habitat availability were weakly correlated in the East Midlands but not in East Anglia. These results are consistent with habitat loss having contributed to the decline of the nightingale in the East Midlands, but suggest that other, wider-scale, factors may be at least partially responsible for determining the abundance of the species within England as a whole.     

FEMALE HETEROGAMETY AND SPECIATION: REDUCED INTROGRESSION OF THE Z CHROMOSOME BETWEEN TWO SPECIES OF NIGHTINGALES

Several lines of evidence suggest that the X chromosome plays a large role in intrinsic postzygotic isolation. The role of the Z chromosome in speciation is much less understood. To explore the role of the Z chromosome in reproductive isolation, we studied nucleotide variation in two closely related bird species, the Thrush Nightingale ( Luscinia luscinia ) and the Common Nightingale ( L. megarhynchos ). These species are isolated by incomplete prezygotic isolation and female hybrid sterility. We sequenced introns of four Z-linked and eight autosomal loci and analyzed patterns of polymorphism and divergence using a divergence-with-gene flow framework. Our results suggest that the nightingale species diverged approximately 1.8 Mya. We found strong evidence of gene flow after divergence in both directions, although more introgression occurred from L. megarhynchos into L. luscinia . Gene flow was significantly higher on the autosomes than on the Z chromosome. Our results support the idea that the Z chromosome plays an important role in intrinsic postzygotic isolation in birds, although it may also contribute to the evolution of prezygotic isolation through sexual selection. This highlights the similarities in the genetic basis of reproductive isolation between organisms with heterogametic males and organisms with heterogametic females during the early stages of speciation.     

Evidence for divergence and adaptive isolation in post‐glacially derived bimodal allopatric and sympatric rainbow smelt populations

The roles of ecological speciation and reinforcement in the formation of contemporary diversity remain contentious. In the present study, we contrast phenotypic and molecular divergence within morphologically diverged bimodal sympatric and allopatric pairs of rainbow smelt, Osmerus mordax. We hypothesize that, in sympatry, evidence of selection associated with resource partitioning will be visible through strong divergence, reinforcement, and greater character displacement. Parallel morphological divergence was observed between the two trophic forms (macrophagous and microphagous), with several examples of greater trait divergence in sympatry than allopatry. Mitochondrial DNA sequence analysis indicated no association between historical clades and morphology; however, Bayesian clustering using microsatellites supported the isolation of these morphs under both allopatry and sympatry. Estimates of genetic isolation were one order of magnitude lower than measures of morphological divergence, consistent with a hypothesis of strong contemporary selection. Using experimental crosses, we obtained similar rates of fertilization success among the allopatric hybrid and pure crosses; whereas, in the sympatric hybrid crosses, fertilization rates dropped by 30–50%, suggesting a clear role for reinforcement through prezygotic incompatibilities. The present study supports the hypothesis that processes of post‐glacial radiation and diversification differ between sympatry and allopatry, and indicates a role for reinforcement and ecological processes in recent sympatric diversification. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 583–594.     

Maintenance of genetic and morphological identity in two sibling Syrrhopodon species (Calymperaceae,Bryopsida) despite extensive introgression

Bryophytes are a group of land plants in which the role of hybridization has long been challenged. Using genotyping by sequencing to circumvent the lack of molecular variation at selected loci previously used for phylogeny and morphology, we determine the level of genetic and morphological divergence and reproductive isolation between the sibling Syrrhopodon annotinus and S. simmondsii (Calymperaceae, Bryopsida) that occur in sympatry but in different habitats in lowland Amazonian rainforests. A clear morphological differentiation and a low (0.06), but significant Fst derived from the analysis of 183 single nucleotide polymorphisms were observed between the two species. Conspecific pairs of individuals consistently exhibited higher average kinship coefficients along a gradient of geographic isolation than interspecific pairs. The weak, but significant genetic divergence observed is consistent with growing evidence that ecological specialization can lead to genetic differentiation among bryophyte species. Nevertheless, the spatial genetic structures of the two species were significantly correlated, as evidenced by the significant slope of the Mantel test based on kinship coefficients between pairs of interspecific individuals and the geographic distance separating them. Interspecific pairs of individuals are thus more closely related when they are geographically closer, suggesting that isolation‐by‐distance is stronger than the interspecific reproductive barrier and pointing to interspecific gene flow. We conclude that interspecific introgression, whose role has long been questioned in bryophytes, may take place even in species wherein sporophyte production is scarce due to dioicy, raising the question as to what mechanisms maintain differentiation despite weak reproductive isolation.     

Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Rapid diversification of sexual traits is frequently attributed to sexual selection, though explicit tests of this hypothesis remain limited. Spermatozoa exhibit remarkable variability in size and shape, and studies report a correlation between sperm morphology (sperm length and shape) and sperm competition risk or female reproductive tract morphology. However, whether postcopulatory processes (e.g., sperm competition and cryptic female choice) influence the speed of evolutionary diversification in sperm form is unknown. Using passerine birds, we quantified evolutionary rates of sperm length divergence among lineages (i.e., species pairs) and determined whether these rates varied with the level of sperm competition (estimated as relative testes mass). We found that relative testes mass was significantly and positively associated with more rapid phenotypic divergence in sperm midpiece and flagellum lengths, as well as total sperm length. In contrast, there was no association between relative testes mass and rates of evolutionary divergence in sperm head size, and models suggested that head length is evolutionarily constrained. Our results are the first to show an association between the strength of sperm competition and the speed of sperm evolution, and suggest that postcopulatory sexual selection promotes rapid evolutionary diversification of sperm morphology.     

Non-parallel morphological divergence following colonization of a new host plant

Adaptation to new ecological niches is known to spur population diversification and may lead to speciation if gene flow is ceased. While adaptation to the same ecological niche is expected to be parallel, it is more difficult to predict whether selection against maladaptive hybridization in secondary sympatry results in parallel divergence also in traits that are not directly related to the ecological niches. Such parallelisms in response to selection for reproductive isolation can be identified through estimating parallelism in reproductive character displacement across different zones of secondary contact. Here, we use a host shift in the phytophagous peacock fly Tephritis conura, with both host races represented in two geographically separate areas East and West of the Baltic Sea to investigate convergence in morphological adaptations. We asked (i) if there are consistent morphological adaptations to a host plant shift and (ii) if the response to secondary sympatry with the alternate host race is parallel across contact zones. We found surprisingly low and variable, albeit significant, divergence between host races. Only one trait, the length of the female ovipositor, which serves an important function in the interaction with the hosts, was consistently different between host races. Instead, co-existence with the other host race significantly affected the degree of morphological divergence, but the divergence was largely driven by different traits in different contact zones. Thus, local stochastic fixation or reinforcement could generate trait divergence, and additional evidence is needed to conclude whether divergence is locally adaptive.

     

Sequential organization in the song of thrush nightingale (Luscinia luscinia): clustering and sequential order of the song types

Sequential organization (or syntax) is widely recognized as one of the most intriguing traits of avian song. We examined the singing of thrush nightingale (Luscinia luscinia) with emphasis on the clustering and sequencing of different song types. The individual repertoire size averaged 12.2 ± 3.7 song types (range 7–23 song types). The song types most common in the population were mutually associated within individual repertoires in non-random combinations. The vast majority of transitions between these song types were predominantly unidirectional. During singing, different song types were mutually associated in the fixed vocal programmes. Each programme was composed of 2–6 song types, which were usually delivered in a relatively fixed order. The compositions of some programmes were found to be identical across the whole of Moscow and even far outside it. We suggest that, during song learning, thrush nightingales memorize the programme not as a set of separate song types but as a unitary vocal pattern. Most frequently, the performance of the programs started from the very beginning. The number of complete and incomplete renditions that began from initial song types exceeded considerably the number of renditions that began from any intermediate song types. In the conclusion, we discuss some differences and similarities between song systems of the thrush and common nightingale (Luscinia megarhynchos).     

Deep mitochondrial introgression and hybridization among ecologically divergent vole species

The completion of speciation is typically difficult to ascertain in rapidly diverging taxa but the amount of hybridization and gene flow in sympatry or parapatry contains important information about the level of reproductive isolation achieved. Here, we examined the progress in speciation between the Mediterranean (Microtus duodecimcostatus) and the Lusitanian pine vole (M. lusitanicus), which are part of the most rapid radiation of species known in mammals. These two Iberian pine voles are classified as separate species because of differences in morphology and ecology, but relatively many ambiguous individuals can be found in sympatric conditions. Our phylogenetic analyses of rangewide data from the mitochondrial cytochrome b gene (mtDNA) demonstrated high levels of diversity and a basal separation in two parapatric lineages. However, mtDNA affiliation was at odds with morphological classification or geographical distribution of the taxa. In contrast, statistical analyses of microsatellites (nucDNA) showed two clear genetic clusters in allopatry and sympatry generally matching morphological classification. This cytonuclear discordance over a large geographic area suggests historical introgression of mtDNA from M. duodecimcostatus to M. lusitanicus. There was statistical evidence for at least two recent hybrids in the sympatry zone but gene flow is apparently low given clear‐cut differences in nucDNA. Our results indicate a relatively advanced speciation process in these Iberian pine voles without fully established reproductive isolation. This situation enables use of combined population genomic and experimental approaches for the separation of patterns and mechanisms in the ongoing explosive diversification of these and other Arvicoline rodents in the future.     

The strength of reproductive isolation in two hybridizing food-deceptive orchid species

Reproductive isolation is of fundamental importance for maintaining species boundaries in sympatry. In orchids, the wide variety of pollination systems and highly diverse floral traits have traditionally suggested a prominent role for pollinator isolation, and thus for prezygotic isolation, as an effective barrier to gene flow among species. Here, we examined the nature of reproductive isolation between Anacamptis morio and Anacamptis papilionacea, two sister species of Mediterranean food-deceptive orchids, in two natural hybrid zones. Comparative analyses of the two hybrid zones that are located on soils with volcanic origin and have different and well-dated ages consistently revealed that all hybrid individuals were morphologically and genetically intermediate between the parental species, but had strongly reduced fitness. Molecular analyses based on nuclear ITS1 and (amplified fragment length polymorphism) AFLP markers clearly showed that all examined hybrids were F1 hybrids, and that no introgression occurred between parental species. The maternally inherited plastid DNA markers indicated that hybridization between A. morio and A. papilionacea was bidirectional, as confirmed by the molecular analysis of seed families. The genetic architecture of the two hybrid zones suggests that the two parental species easily and frequently hybridize in sympatry as a consequence of partial pollinator overlap but that strong postzygotic barriers reduce hybrid fitness and prevent gene introgression. These results corroborate that chromosomal divergence is instrumental for reproductive isolation between these food-deceptive orchids and suggest that hybridization is of limited importance for their diversification.     

Differences in style length confer prezygotic isolation between two dioecious species of Silene in sympatry

One fundamental signature of reinforcement is elevated prezygotic reproductive isolation between related species in sympatry relative to allopatry. However, this alone is inadequate evidence for reinforcement, as traits conferring reproductive isolation can occur as a by‐product of other forces. We conducted crosses between Silene latifolia and S. diclinis, two closely related dioecious flowering plant species. Crosses with S. latifolia mothers from sympatry exhibited lower seed set than mothers from five allopatric populations when S. diclinis was the father. However, two other allopatric populations also exhibited low seed set. A significant interaction between style length and sire species revealed that seed set declined as style length increased when interspecific, but not intraspecific, fathers where used. Moreover, by varying the distance pollen tubes had to traverse, we found interspecific pollen placement close to the ovary resulted in seed set in both long‐ and short‐styled S. latifolia mothers. Our results reveal that the long styles of S. latifolia in sympatry with S. diclinis contribute to the prevention of hybrid formation. We argue that forces other than reinforcing selection are likely to be responsible for the differences in style length seen in sympatry.     

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single‐nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer‐winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among‐male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone.     

The evolution of postpollination reproductive isolation in Costus

Reproductive isolation is critical to the diversification of species. Postpollination barriers may be important in limiting gene flow between closely related species, but they are relatively cryptic and their evolution is poorly understood. Here, we review the role of postpollination reproductive isolation in plants, including the various stages at which it operates and the hypotheses for how it may evolve. We then review empirical studies in the plant genus Costus, evaluating documented postpollination barriers in light of these hypotheses. We summarize isolation due to parental style length differences and present evidence supporting the hypothesis that the differences are in part a by-product of selection on floral morphology. Additionally, we show that reduced pollen adhesion, germination, and tube growth contribute to reproductive isolation between two closely related sympatric species of Costus. Geographic variation in the strength of these crossing barriers supports the hypothesis that they evolved under reinforcement, or direct natural selection to strengthen isolation.     

Weak premating isolation between Clitarchus stick insect species despite divergent male and female genital morphology

Documenting natural hybrid systems builds our understanding of mate choice, reproductive isolation and speciation. The stick insect species Clitarchus hookeri and C. tepaki differ in their genital morphology and hybridize along a narrow peninsula in northern New Zealand. We utilize three lines of evidence to understand the role of premating isolation and species boundaries: (a) genetic differentiation using microsatellites and mitochondrial DNA; (b) variation in 3D surface topology of male claspers and 2D morphometrics of female opercular organs; and (c) behavioural reproductive isolation among parental and hybrid populations through mating crosses. The genetic data show introgression between the parental species and formation of a genetically variable hybrid swarm. Similarly, the male and female morphometric data show genital divergence between the parental species as well as increased variation within the hybrid populations. This genital divergence has not resulted in reproductive isolation between species, instead weak perimating isolation has enabled the formation of a hybrid swarm. Behavioural analysis demonstrates that the entire mating process influences the degree of reproductive isolation between species undergoing secondary contact. Mechanical isolation may appear strong, whereas perimating isolation is weak.     

Hybridization patterns between two marine snails,Littorina fabalis and L. obtusata

Characterizing the patterns of hybridization between closely related species is crucial to understand the role of gene flow in speciation. In particular, systems comprising multiple contacts between sister species offer an outstanding opportunity to investigate how reproductive isolation varies with environmental conditions, demography and geographic contexts of divergence. The flat periwinkles, Littorina obtusata and L. fabalis (Gastropoda), are two intertidal sister species with marked ecological differences compatible with late stages of speciation. Although hybridization between the two was previously suggested, its extent across the Atlantic shores of Europe remained largely unknown. Here, we combined genetic (microsatellites and mtDNA) and morphological data (shell and male genital morphology) from multiple populations of flat periwinkles in north‐western Iberia to assess the extent of current and past hybridization between L. obtusata and L. fabalis under two contrasting geographic settings of divergence (sympatry and allopatry). Hybridization signatures based on both mtDNA and microsatellites were stronger in sympatric sites, although evidence for recent extensive admixture was found in a single location. Misidentification of individuals into species based on shell morphology was higher in sympatric than in allopatric sites. However, despite hybridization, species distinctiveness based on this phenotypic trait together with male genital morphology remained relatively high. The observed variation in the extent of hybridization among locations provides a rare opportunity for future studies on the consequences of different levels of gene flow for reinforcement, thus informing about the mechanisms underlying the completion of speciation.     

Males,but not females,contribute to sexual isolation between two sympatric species of <Emphasis Type="Italic">Gambusia</Emphasis>

Reproductive isolation restricts genetic exchange between species. Various pre- and post-mating barriers, such as behavior, physiology and gametic incompatibility, have been shown to evolve in sympatry. In certain scenarios, isolation can be asymmetrical, where species differentially prefer conspecifics. We examined sexual isolation via conspecific mate preference between Gambusia affinis and G. geiseri in both sexes. To investigate male contribution to sexual isolation, we compared the number of mating attempts (gonopodial thrusts) directed at either a conspecific or a heterospecific female, in both species. We also examined sperm priming and expenditure in males in the presence of conspecific or heterospecific females. We then measured female preference for either a conspecific or heterospecific male, in both species. We found that males of both species preferred to mate with conspecific females, but showed no difference in sperm production or expenditure between conspecific and heterospecific females. Females of both species did not prefer conspecific over heterospecific males. Our results suggest that sexual isolation might be mediated by male mate choice in this system and not female choice, suggesting that there is asymmetrical reproductive isolation between the sexes in G. affinis and G. geiseri, but symmetrical species isolation.     

Intraspecific divergence in sperm morphology of the green sea urchin, <Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>: implications for selection in broadcast spawners

Background  

Sperm morphology can be highly variable among species, but less is known about patterns of population differentiation within species. Most studies of sperm morphometric variation are done in species with internal fertilization, where sexual selection can be mediated by complex mating behavior and the environment of the female reproductive tract. Far less is known about patterns of sperm evolution in broadcast spawners, where reproductive dynamics are largely carried out at the gametic level. We investigated variation in sperm morphology of a broadcast spawner, the green sea urchin (Strongylocentrotus droebachiensis), within and among spawnings of an individual, among individuals within a population, and among populations. We also examined population-level variation between two reproductive seasons for one population. We then compared among-population quantitative genetic divergence (Q _ST) for sperm characters to divergence at neutral microsatellite markers (F _ST).