Identification of endangered Hawaiian ducks (Anas wyvilliana), introduced North American mallards (A. platyrhynchos) and their hybrids using multilocus genotypes

Hawaiian ducks (Anas wyvilliana), or koloa, are endemic to the Hawaiian Islands and are listed as a federal and state endangered species. Hybridization between koloa and introduced mallards (A. platyrhynchos) is believed to be a primary threat to the recovery of koloa. We evaluated the utility of two sets of nuclear markers (microsatellite loci and amplified fragment length polymorphisms) and a variable portion of the mitochondrial DNA control region to distinguish among koloa, mallards, and hybrids. We show that microsatellite and AFLP markers can be used to distinguish between koloa and mallard-koloa hybrids with a high degree of confidence. For all but one of the putative koloa in our sample, the posterior probability of belonging to the koloa category was >0.90. Similarly all but one of the mallard-koloa hybrids were assigned to the hybrid category with posterior probabilities >0.98. Subsets of markers led to poorer resolution among koloa, mallard and hybrid categories. Among a sample of 61 koloa, hybrids and mallards, we found 25 different mtDNA haplotypes, belonging to two groups of haplotypes (A and B) identified previously in mallards and their relatives. All putative koloa samples exhibited group B haplotypes, of which 65% comprised one haplotype, while the rest were divided among four haplotypes. All Hawai’i mallard samples exhibited haplotypes that belonged to group A. Hybrids and California mallards exhibited haplotypes belonging to both groups, but a majority were of group A, suggesting that hybridization may more commonly involve mating between Hawai’i mallard females and koloa males.     

Genetic structure of the critically endangered Red‐headed Wood Pigeon Columba janthina nitens and its implications for the management of threatened island populations

The Red‐headed Wood Pigeon Columba janthina nitens is endemic to the Ogasawara Islands, an oceanic island chain located 1000 km south of the main islands of Japan. The subspecies is at high risk of extinction because of its small population size and restricted habitat range. We undertook genetic analyses of this pigeon using sequences of a portion of the mitochondrial control region and five microsatellite markers to estimate the genetic characteristics of two wild populations from the Bonin and Volcano Islands, as well as one captive breeding population. The genetic diversity of the wild individuals was exceptionally low in both the mitochondria (nucleotide diversity = 0.00105) and at the microsatellite (3.2 alleles per locus and H_E = 0.12) loci. Higher numbers of microsatellite genotypes were observed in the Volcano Islands population than in the Bonin Islands population, which may be because of the relatively low impact of human disturbance. The most common mitochondrial haplotypes and microsatellite alleles observed in the two wild populations were completely fixed in the captive population. Our results suggest that the genetic diversity of the captive population needs to be increased. However, introduction of a wild individual into a captive population can lead to a decreased genetic diversity in the wild population and therefore should be done with caution. The genetic differentiation between the Bonin and the Volcano island groups was low, and the populations of the two island groups should be regarded as a single evolutionarily significant unit. However, special consideration is required for habitat conservation in the Volcano Islands, which may be functioning as a sanctuary for the Red‐headed Wood Pigeon. For the long‐term conservation of threatened bird species that live on remote oceanic islands, determination of management units considering gene flow caused by their flying capacity and maintenance of genetically suitable wild and captive populations are essential.     

Genetic analysis of natural hybrids between endemic and alien Rubus (Rosaceae) species in Hawai`i

A population of putative hybrids between theendemic Rubus hawaiensis and naturalizedR. rosifolius was discovered inKpahulu Valley, on the island of Maui inthe Hawaiian archipelago. The goal of thisstudy was to molecularly characterize thisnatural hybridization event, investigate themode of hybridization, and determine the malefertility of the hybrid individuals. Bothmorphological and RAPD marker data indicatethat the putative hybrid individuals are theprogeny of R. rosifolius and R.hawaiensis. All 39 hybrid individuals sampledhad the chloroplast DNA haplotype of R.rosifolius. Thus hybridization appears to beasymmetric, with R. rosifolius acting asthe maternal parent. All hybrid individualsassessed for pollen stainability were sterile,and there was no evidence of backcrossing toeither parent. This result suggests thathybrids are of the first filial generation andthat variation among hybrids reflectsdifferences within the parental populations.Sympatric populations of R. hawaiensisand R. rosifolius occur on four islandsand six additional alien species of Rubusare naturalized and sympatric with R. hawaiensisin Hawai`i. Further investigationis merited to assess whether hybridization maypose a threat to the long term viability ofR. hawaiensis. This study highlights theincreasing frequency and negative consequencesof native-alien hybridization and theimportance of maintaining active alien speciescontrol programs in the Hawaiian Islands.     

Evolution of a hybrid zone of two willow species (Salix L.) in the European Alps analyzed by RAD-seq and morphometrics

Natural hybridization of plants can result in many outcomes with several evolutionary consequences, such as hybrid speciation and introgression. Natural hybrid zones can arise in mountain systems as a result of fluctuating climate during the exchange of glacial and interglacial periods, where species retract and expand their territories, resulting in secondary contacts. Willows are a large genus of woody plants with an immense capability of interspecific crossing. In this study, the sympatric area of two diploid sister species, S. foetida and S. waldsteiniana in the eastern European Alps, was investigated to study the genomic structure of populations within and outside their contact zone and to analyze congruence of morphological phenotypes with genetic data. Eleven populations of the two species were sampled across the Alps and examined using phylogenetic network and population genetic structure analyses of RAD Seq data and morphometric analyses of leaves. The results showed that a homoploid hybrid zone between the two species was established within their sympatric area. Patterns of genetic admixture in homoploid hybrids indicated introgression with asymmetric backcrossing to not only one of the parental species but also one hybrid population forming a separate lineage. The lack of F1 hybrids indicated a long-term persistence of the hybrid populations. Insignificant isolation by distance suggests that gene flow can act over large geographical scales. Morphometric characteristics of hybrids supported the molecular data and clearly separated populations of the parental species, but showed intermediacy in the hybrid zone populations with a bias toward S. waldsteiniana. The homoploid hybrid zone might have been established via secondary contact hybridization, and its establishment was fostered by the low genetic divergence of parental species and a lack of strong intrinsic crossing barriers. Incomplete ecological separation and the ability of long-distance dispersal of willows could have contributed to the spatial expansion of the hybrid zone.     

Spatial and ecological population genetic structures within two island‐endemic Aeonium species of different niche width

The Crassulacean genus Aeonium is a well‐known example for plant species radiation on oceanic archipelagos. However, while allopatric speciation among islands is documented for this genus, the role of intra‐island speciation due to population divergence by topographical isolation or ecological heterogeneity has not yet been addressed. The aim of this study was to investigate intraspecific genetic structures and to identify spatial and ecological drivers of genetic population differentiation on the island scale. We analyzed inter simple sequence repeat variation within two island‐endemic Aeonium species of La Palma: one widespread generalist that covers a large variety of different habitat types (Ae. davidbramwellii) and one narrow ecological specialist (Ae. nobile), in order to assess evolutionary potentials on this island. Gene pool differentiation and genetic diversity patterns were associated with major landscape structures in both species, with phylogeographic implications. However, overall levels of genetic differentiation were low. For the generalist species, outlier loci detection and loci–environment correlation approaches indicated moderate signatures of divergent selection pressures linked to temperature and precipitation variables, while the specialist species missed such patterns. Our data point to incipient differentiation among populations, emphasizing that ecological heterogeneity and topographical structuring within the small scales of an island can foster evolutionary processes. Very likely, such processes have contributed to the radiation of Aeonium on the Canary Islands. There is also support for different evolutionary mechanisms between generalist and specialist species.     

Population structure of island‐associated dolphins: Evidence from mitochondrial and microsatellite markers for common bottlenose dolphins (Tursiops truncatus) around the main Hawaiian Islands

We used mitochondrial and nuclear genetic markers to investigate population structure of common bottlenose dolphins, Tursiops truncatus, around the main Hawaiian Islands. Though broadly distributed throughout the world's oceans, bottlenose dolphins are known to form small populations in coastal waters. Recent photo‐identification data suggest the same is true in Hawaiian waters. We found genetic differentiation among (mtDNA Φ_ST= 0.014–0.141, microsatellite F’_ST= 0.019–0.050) and low dispersal rates between (0.17–5.77 dispersers per generation) the main Hawaiian Island groups. Our results are consistent with movement rates estimated from photo‐identification data and suggest that each island group supports a demographically independent population. Inclusion in our analyses of samples collected near Palmyra Atoll provided evidence that the Hawaiian Islands are also occasionally visited by members of a genetically distinct, pelagic population. Two of our samples exhibited evidence of partial ancestry from Indo‐Pacific bottlenose dolphins (T. aduncus), a species not known to inhabit the Hawaiian Archipelago. Our findings have important implications for the management of Hawaiian bottlenose dolphins and raise concerns about the vulnerability to human impacts of pelagic species in island ecosystems.     

Genetic admixture despite ecological segregation in a North African sparrow hybrid zone (Aves,Passeriformes, Passer domesticus × Passer hispaniolensis)

Under different environmental conditions, hybridization between the same species might result in different patterns of genetic admixture. Particularly, species pairs with large distribution ranges and long evolutionary history may have experienced several independent hybridization events over time in different zones of overlap. In birds, the diverse hybrid populations of the house sparrow (Passer domesticus) and the Spanish sparrow (Passer hispaniolensis) provide a striking example. Throughout their range of sympatry, these two species do not regularly interbreed; however, a stabilized hybrid form (Passer italiae) exists on the Italian Peninsula and on several Mediterranean islands. The spatial distribution pattern on the Eurasian continent strongly contrasts the situation in North Africa, where house sparrows and Spanish sparrows occur in close vicinity of phenotypically intermediate populations across a broad mosaic hybrid zone. In this study, we investigate patterns of divergence and admixture among the two parental species, stabilized and nonstabilized hybrid populations in Italy and Algeria based on a mitochondrial marker, a sex chromosomal marker, and 12 microsatellite loci. In Algeria, despite strong spatial and temporal separation of urban early‐breeding house sparrows and hybrids and rural late‐breeding Spanish sparrows, we found strong genetic admixture of mitochondrial and nuclear markers across all study populations and phenotypes. That pattern of admixture in the North African hybrid zone is strikingly different from i) the Iberian area of sympatry where we observed only weak asymmetrical introgression of Spanish sparrow nuclear alleles into local house sparrow populations and ii) the very homogenous Italian sparrow population where the mitogenome of one parent (P. domesticus) and the Z‐chromosomal marker of the other parent (P. hispaniolensis) are fixed. The North African sparrow hybrids provide a further example of enhanced hybridization along with recent urbanization and anthropogenic land‐use changes in a mosaic landscape.     

Microevolutionary patterns and processes of the Native Hawaiian colonizing fern Odontosoria chinensis (Lindsaeaceae)

Abstract.— The vascular‐plant flora of the Hawaiian Islands is characterized by one of the highest rates of species endemism in the world. Among flowering plants, approximately 89% of species are endemic, and among pteridophytes, about 76% are endemic. At the single‐island level, however, rates of species endemism vary dramatically between these two groups with 80% of angiosperms and only 6% of pteridophytes being single‐island endemics. Thus, in many groups of Hawaiian angiosperms, it is possible to link studies of phylogeny, evolution, and biogeographic history at the interspecific and interisland levels. In contrast, the low level of single‐island species endemism among Hawaiian pteridophytes makes similar interspecific and interisland studies nearly impossible. Higher levels of interisland gene flow may account for the different levels of single‐island endemism in Hawaiian pteridophytes relative to angiosperms. The primary question we addressed in the present study was: Can we infer microevolutionary patterns and processes among populations within widespread species of Hawaiian pteridophytes wherein gene flow is probably common? To address this broad question, we conducted a population genetic study of the native Hawaiian colonizing species Odontosoria chinensis. Data from allozyme analyses allowed us to infer: (1) significant genetic differentiation among populations from different islands; (2) historical patterns of dispersal between particular pairs of islands; (3) archipelago‐level patterns of dispersal and colonization; (4) founder effects among populations on the youngest island of Hawaii; and, (5) that this species primarily reproduces via outcrossing, but may possess a mixed‐mating system.     

Disentangling the complex evolutionary history of the Western Palearctic blue tits (Cyanistes spp.) – phylogenomic analyses suggest radiation by multiple colonization events and subsequent isolation

Isolated islands and their often unique biota continue to play key roles for understanding the importance of drift, genetic variation and adaptation in the process of population differentiation and speciation. One island system that has inspired and intrigued evolutionary biologists is the blue tit complex (Cyanistes spp.) in Europe and Africa, in particular the complex evolutionary history of the multiple genetically distinct taxa of the Canary Islands. Understanding Afrocanarian colonization events is of particular importance because of recent unconventional suggestions that these island populations acted as source of the widespread population in mainland Africa. We investigated the relationship between mainland and island blue tits using a combination of Sanger sequencing at a population level (20 loci; 12 500 nucleotides) and next‐generation sequencing of single population representatives (>3 200 000 nucleotides), analysed in coalescence and phylogenetic frameworks. We found (i) that Afrocanarian blue tits are monophyletic and represent four major clades, (ii) that the blue tit complex has a continental origin and that the Canary Islands were colonized three times, (iii) that all island populations have low genetic variation, indicating low long‐term effective population sizes and (iv) that populations on La Palma and in Libya represent relicts of an ancestral North African population. Further, demographic reconstructions revealed (v) that the Canary Islands, conforming to traditional views, hold sink populations, which have not served as source for back colonization of the African mainland. Our study demonstrates the importance of complete taxon sampling and an extensive multimarker study design to obtain robust phylogeographical inferences.     

Diversification of Hawaiian Cyrtandra (Gesneriaceae) under the influence of incomplete lineage sorting and hybridization

Cyrtandra (Gesneriaceae) is a genus of flowering plants with over 800 species distributed throughout Southeast Asia and the Pacific Islands. On the Hawaiian Islands, 60 named species and over 89 putative hybrids exist, most of which are identified on the basis of morphology. Despite many previous studies on the Hawaiian Cyrtandra lineage, questions regarding the reconciliation of morphology and genetics remain, many of which can be attributed to the relatively young age and evidence of hybridization between species. We utilized targeted enrichment, high‐throughput sequencing, and modern phylogenomics tools to test 31 Hawaiian Cyrtandra samples (22 species, two putative hybrids, four species with two samples each, one species with four samples) and two outgroups for species relationships and hybridization in the presence of incomplete lineage sorting (ILS). Both concatenated and species‐tree methods were used to reconstruct species relationships, and network analyses were conducted to test for hybridization. We expected to see high levels of ILS and putative hybrids intermediate to their parent species. Phylogenies reconstructed from the concatenated and species‐tree methods were highly incongruent, most likely due to high levels of incomplete lineage sorting. Network analyses inferred gene flow within this lineage, but not always between taxa that we expected. Multiple hybridizations were inferred, but many were on deeper branches of the island lineages suggesting a long history of hybridization. We demonstrated the utility of high‐throughput sequencing and a phylogenomic approach using 569 loci to understanding species relationships and gene flow in the presence of ILS.     

Assessing changes in genomic divergence following a century of human‐mediated secondary contact among wild and captive‐bred ducks

Along with manipulating habitat, the direct release of domesticated individuals into the wild is a practice used worldwide to augment wildlife populations. We test between possible outcomes of human‐mediated secondary contact using genomic techniques at both historical and contemporary timescales for two iconic duck species. First, we sequence several thousand ddRAD‐seq loci for contemporary mallards (Anas platyrhynchos) throughout North America and two domestic mallard types (i.e., known game‐farm mallards and feral Khaki Campbell's). We show that North American mallards may well be becoming a hybrid swarm due to interbreeding with domesticated game‐farm mallards released for hunting. Next, to attain a historical perspective, we applied a bait‐capture array targeting thousands of loci in century‐old (1842–1915) and contemporary (2009–2010) mallard and American black duck (Anas rubripes) specimens. We conclude that American black ducks and mallards have always been closely related, with a divergence time of ~600,000 years before present, and likely evolved through prolonged isolation followed by limited bouts of gene flow (i.e., secondary contact). They continue to maintain genetic separation, a finding that overturns decades of prior research and speculation suggesting the genetic extinction of the American black duck due to contemporary interbreeding with mallards. Thus, despite having high rates of hybridization, actual gene flow is limited between mallards and American black ducks. Conversely, our historical and contemporary data confirm that the intensive stocking of game‐farm mallards during the last ~100 years has fundamentally changed the genetic integrity of North America's wild mallard population, especially in the east. It thus becomes of great interest to ask whether the iconic North American mallard is declining in the wild due to introgression of maladaptive traits from domesticated forms. Moreover, we hypothesize that differential gene flow from domestic game‐farm mallards into the wild mallard population may explain the overall temporal increase in differentiation between wild black ducks and mallards, as well as the uncoupling of genetic diversity and effective population size estimates across time in our results. Finally, our findings highlight how genomic methods can recover complex population histories by capturing DNA preserved in traditional museum specimens.     

Genetic diversity and distribution patterns of diploid and polyploid hybrid water frog populations (Pelophylax esculentus complex) across Europe

Polyploidization is a rare yet sometimes successful way for animals to rapidly create geno‐ and phenotypes that may colonize new habitats and quickly adapt to environmental changes. In this study, we use water frogs of the Pelophylax esculentus complex, comprising two species (Pelophylax lessonae, genotype LL; Pelophylax ridibundus, RR) and various diploid (LR) and triploid (LLR, LRR) hybrid forms, summarized as P. esculentus, as a model for studying recent hybridization and polyploidization in the context of speciation. Specifically, we compared the geographic distribution and genetic diversity of diploid and triploid hybrids across Europe to understand their origin, maintenance and potential role in hybrid speciation. We found that different hybrid and parental genotypes are not evenly distributed across Europe. Rather, their genetic diversity is structured by latitude and longitude and the presence/absence of parental species but not of triploids. Highest genetic diversity was observed in central and eastern Europe, the lowest in the northwestern parts of Europe. This gradient can be explained by the decrease in genetic diversity during postglacial expansion from southeastern glacial refuge areas. Genealogical relationships calculated on the basis of microsatellite data clearly indicate that hybrids are of multiple origin and include a huge variety of parental genomes. Water frogs in mixed‐ploidy populations without any parental species (i.e. all‐hybrid populations) can be viewed as evolutionary units that may be on their way towards hybrid speciation. Maintenance of such all‐hybrid populations requires a continuous exchange of genomes between diploids and triploids, but scenarios for alternative evolutionary trajectories are discussed.     

Fine-scale population genetic structure of a wildlife disease vector: the southern house mosquito on the island of Hawaii

The southern house mosquito, Culex quinquefasciatus, is a widespread tropical and subtropical disease vector. In the Hawaiian Islands, where it was introduced accidentally almost two centuries ago, it is considered the primary vector of avian malaria and pox. Avian malaria in particular has contributed to the extinction and endangerment of Hawaii's native avifauna, and has altered the altitudinal distribution of native bird populations. We examined the population genetic structure of Cx. quinquefasciatus on the island of Hawaii at a smaller spatial scale than has previously been attempted, with particular emphasis on the effects of elevation on population genetic structure. We found significant genetic differentiation among populations and patterns of isolation by distance within the island. Elevation per se did not have a limiting effect on gene flow; however, there was significantly lower genetic diversity among populations at mid elevations compared to those at low elevations. A recent sample taken from just above the predicted upper altitudinal distribution of Cx. quinquefasciatus on the island of Hawaii was confirmed as being a temporary summer population and appeared to consist of individuals from more than one source population. Our results indicate effects of elevation gradients on genetic structure that are consistent with known effects of elevation on population dynamics of this disease vector.     

The effect of hybrid transgression on environmental tolerance in experimental yeast crosses

Evidence is rapidly accumulating that hybridization generates adaptive variation. Transgressive segregation in hybrids could promote the colonization of new environments. Here, we use an assay to select hybrid genotypes that can proliferate in environmental conditions beyond the conditions tolerated by their parents, and we directly compete them against parental genotypes in habitats across environmental clines. We made 45 different hybrid swarms by crossing yeast strains (both Saccharomyces cerevisiae and S. paradoxus) with different genetic and phenotypic divergence. We compared the ability of hybrids and parents to colonize seven types of increasingly extreme environmental clines, representing both natural and novel challenges (mimicking pollution events). We found that a significant majority of hybrids had greater environmental ranges compared to the average of both their parents’ ranges (mid‐parent transgression), but only a minority of hybrids had ranges exceeding their best parent (best‐parent transgression). Transgression was affected by the specific strains involved in the cross and by the test environment. Genetic and phenotypic crossing distance predicted the extent of transgression in only two of the seven environments. We isolated a set of potentially transgressive hybrids selected at the extreme ends of the clines and found that many could directly outcompete their parents across whole clines and were between 1.5‐ and 3‐fold fitter on average. Saccharomyces yeast is a good model for quantitative and replicable experimental speciation studies, which may be useful in a world where hybridization is becoming increasingly common due to the relocation of plants and animals by humans.     

Range dynamics,rather than convergent selection,explain the mosaic distribution of red‐winged blackbird phenotypes

Geographic distributions of genetic and phenotypic characters can illuminate historical evolutionary processes. In particular, mosaic distributions of phenotypically similar populations can arise from parallel evolution or from irregular patterns of dispersal and colonization by divergent forms. Two phenotypically divergent forms of the red‐winged blackbird (Agelaius phoeniceus) show a mosaic phenotypic distribution, with a “bicolored” form occurring disjunctly in California and Mexico. We analyzed the relationships among these bicolored populations and neighboring typical populations, using ~600 bp of mitochondrial DNA sequence data and 10 nuclear short tandem repeat loci. We find that bicolored populations, although separated by ~3000 km, are genetically more similar to one other than they are to typical populations separated by ~400 km. We also find evidence of ongoing gene flow among populations, including some evidence of asymmetric gene flow. We conclude that the current distribution of bicolored forms represents incomplete speciation, where recent asymmetric hybridization with typical A. phoeniceus is dividing the range of a formerly widespread bicolored form. This hypothesis predicts that bicolored forms may suffer extinction by hybridization. Future work will use fine‐scaled geographical sampling and nuclear sequence data to test for hybrid origins of currently typical populations and to more precisely quantify the directionality of gene flow.     

Conservation implications of hybridization in Hawaiian picture-winged Drosophila

In this review, we discuss the importance of hybridization among species for the conservation of Hawaiian picture-winged Drosophila. Hybridization can be a positive evolutionary process that creates new species and increases the adaptation of populations and species through the spread of adaptive alleles and traits. Conversely, hybridization can disrupt the genetic integrity of species or populations and this may be most detrimental among taxa that are recently hybridizing due to recent ecological changes. The loss of biodiversity in Hawaiian Drosophila through hybridization may be facilitated by habitat alteration and introduced species that reduce population sizes and alter geographic distributions of native species. We briefly review the evidence for hybridization in the genus Drosophila and then focus on hybridization in the Hawaiian picture-winged Drosophila. We examine three general approaches for identifying hybrids and for assessing the factors that appear to contribute to hybridization and the potential ecological and evolutionary outcomes of hybrids in natural populations. Overall, the potential for hybridization among species will likely increase the risk of extinction for Hawaiian picture-winged Drosophila species. Thus, it is important to consider the potential for hybridization among species when developing plans for the conservation of Hawaiian Drosophila.     

Genomic signatures of geographic isolation and natural selection in coral reef fishes

The drivers of speciation remain among the most controversial topics in evolutionary biology. Initially, Darwin emphasized natural selection as a primary mechanism of speciation, but the architects of the modern synthesis largely abandoned that view in favour of divergence by geographic isolation. The balance between selection and isolation is still at the forefront of the evolutionary debate, especially for the world's tropical oceans where biodiversity is high, but isolating barriers are few. Here, we identify the drivers of speciation in Pacific reef fishes of the genus Acanthurus by comparative genome scans of two peripheral populations that split from a large Central‐West Pacific lineage at roughly the same time. Mitochondrial sequences indicate that populations in the Hawaiian Archipelago and the Marquesas Islands became isolated approximately 0.5 Ma. The Hawaiian lineage is morphologically indistinguishable from the widespread Pacific form, but the Marquesan form is recognized as a distinct species that occupies an unusual tropical ecosystem characterized by upwelling, turbidity, temperature fluctuations, algal blooms and little coral cover. An analysis of 3737 SNPs reveals a strong signal of selection at the Marquesas, with 59 loci under disruptive selection including an opsin Rh2 locus. While both the Hawaiian and Marquesan populations indicate signals of drift, the former shows a weak signal of selection that is comparable with populations in the Central‐West Pacific. This contrast between closely related lineages reveals one population diverging due primarily to geographic isolation and genetic drift, and the other achieving taxonomic species status under the influence of selection.     

Genetic admixture supports an ancient hybrid origin of the endangered Hawaiian duck

Speciation is regarded primarily as a bifurcation from an ancestral species into two distinct taxonomic units, but gene flow can create complex signals of phylogenetic relationships, especially among different loci. We evaluated several hypotheses that could account for phylogenetic discord between mitochondrial DNA (mtDNA) and nuclear DNA (nuDNA) within Hawaiian duck (Anas wyvilliana), including stochastic lineage sorting, mtDNA capture and widespread genomic introgression. Our results best support the hypothesis that the contemporary Hawaiian duck is descended from an ancient hybridization event between the mallard (Anas platyrhynchos) and Laysan duck (Anas laysanensis). Whereas mtDNA clearly shows a sister relationship between Hawaiian duck and mallard, nuDNA is consistent with a genetic mosaic with nearly equal contributions from Laysan duck and mallard. In addition, coalescent analyses suggest that gene flow from either mallard or Laysan duck, depending on the predefined tree topology, is necessary to explain contemporary genetic diversity in Hawaiian ducks, and these estimates are more consistent with ancient, rather than contemporary, hybridization. Time since divergence estimates suggest that the genetic admixture event occurred around the Pleistocene–Holocene boundary, which is further supported by circumstantial evidence from the Hawaiian subfossil record. Although the extent of reproductive isolation from either putative parental taxon is not currently known, these species are phenotypically, genetically and ecologically different, and they meet primary criteria used in avian taxonomy for species designation. Thus, the available data are consistent with an admixed origin and support the hypothesis that the Hawaiian duck may represent a young hybrid species.     

ARTIFICIAL HYBRIDIZATION IN HAWAIIAN WIKSTROEMIA (THYMELAEACEAE)

Crosses were performed within and between 19 Hawaiian populations of the mostly dioecious woody shrub Wikstroemia. These populations are from a variety of habitats on the six major islands and are morphologically diverse. The populations are referrable to eight of the 12 species recognized by Peterson. Almost all hybrid combinations produced seeds, and seed-set was not significantly different among three categories of hybrids (within population, between populations on the same island, and between populations on different islands). F₁s from many of the crosses were vigorous and fertile, leading to the conclusion that these populations are not reproductively isolated from each other and the assumption that they are genetically very similar. Natural hybridization was difficult to detect because the species described by Peterson intergrade morphologically and ecologically. The crossability and interfertility of populations of Wikstroemia may be explained by one or more of the following hypotheses: 1) recency of colonization, 2) the founder effect, 3) lack of selection for barriers to interbreeding, and 4) gross morphological changes without associated large genetic changes.     

Reproductive success of Mariana Swiftlets (Aerodramus bartschi) on the Hawaiian island of O'ahu

Mariana Swiftlets (Aerodramus bartschi) are federally listed as endangered, with populations currently limited to just three islands in the Mariana Islands plus an introduced population on the Hawaiian island of O'ahu. Before efforts are made to reintroduce Mariana Swiftlets to other islands in the Mariana archipelago, additional information is needed concerning their breeding biology. Therefore, our objective was to examine the reproductive biology of Mariana Swiftlets over five annual cycles on the Hawaiian island of O'ahu. This introduced population used a human‐made tunnel for roosting and nesting, and was studied as a surrogate to negate interference with endangered populations in the Mariana Islands. Active nests (N = 478) were observed in every month of the year, with peak nesting activity between May and September. All clutches consisted of one egg. Mean duration of incubation and nestling periods were 23.9 d (range = 18–30 d, N = 233) and 55.0 d (range = 41–84 d, N = 228), respectively. Estimated nest success was 63%. Over half (52%) of nest failures were attributed to eggs found on the tunnel floor. Predation by rats (Rattus spp.) was also an important cause of nest failure and often resulted in the loss of most active nests. However, Mariana Swiftlets did re‐nest after these predation events. Our results suggest that rat predation of both nests and adults may limit growth of the Mariana Swiftlet population on O'ahu, and could also affect the chances for successful establishment of relocated populations in the Mariana Islands. Another limiting factor on O'ahu is that only one nesting site is apparently available on the island. Current goals for downlisting Mariana Swiftlets from endangered to threatened include establishing populations on Guam, Rota, Aguiguan, and Saipan. To meet these goals, the population of Mariana Swiftlets on O'ahu can be important for testing reintroduction techniques, learning more about the natural history of these swiftlets, and providing individuals for reintroduction efforts in the Mariana Islands.