Figures
Abstract
Interspecific hybridization is common in plants and animals, particularly in waterfowl (Anatidae). One factor shown to contribute to hybridization is restricted mate choice, which can occur when two species occur in sympatry but one is rare. The Hubbs principle, or “desperation hypothesis,” states that under such circumstances the rarer species is more likely to mate with heterospecifics. Here we report interspecific hybridization between two waterfowl species that coexist in broad sympatry and mixed flocks throughout southern South America. Speckled teal (Anas flavirostris) and yellow-billed pintails (Anas georgica) are abundant in continental South America, but in the Falkland Islands speckled teal outnumber yellow-billed pintails approximately ten to one. Using eight genetic loci (mtDNA and 7 nuclear introns) coupled with Bayesian assignment tests and relatedness analysis, we identified a speckled teal x yellow-billed pintail F1 hybrid female and her duckling sired by a male speckled teal. Although our sample in the Falkland Islands was small, we failed to identify unequivocal evidence of hybridization or introgression in a much larger sample from Argentina using a three-population “isolation with migration” coalescent analysis. While additional data are needed to determine if this event in the Falkland Islands was a rare singular occurrence, our results provide further support for the “desperation hypothesis,” which states that scarcity in one population and abundance of another will often lead to hybridization.
Citation: McCracken KG, Wilson RE (2011) Gene Flow and Hybridization between Numerically Imbalanced Populations of Two Duck Species in the Falkland Islands. PLoS ONE 6(8): e23173. https://doi.org/10.1371/journal.pone.0023173
Editor: Vincent Laudet, Ecole Normale Supérieure de Lyon, France
Received: May 5, 2011; Accepted: July 11, 2011; Published: August 26, 2011
Copyright: © 2011 McCracken, Wilson. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided by the Institute of Arctic Biology at the University of Alaska, Fairbanks, Experimental Program to Stimulate Competitive Research (National Science Foundation [NSF] EPS-0092040 and EPS-0346770), grants from the Delta Waterfowl Foundation and Frank M. Chapman Fund at the American Museum of Natural History to REW, and NSF grants DEB-0444748, DEB-0746365, and IOS-0949439. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Interspecific hybridization is an important mechanism of lineage diversification and adaptation in plants [1], [2], [3], and it has also been shown to be an important evolutionary force in animals [4], [5], [6]. Birds are no exception; at least one in ten species is known to hybridize [7], [8], [9], [10]. The waterfowl (Anatidae) comprise more than half of known avian hybrids [11], [12], [13]. Numerous factors have been implicated in the ability of the Anatidae to hybridize [12], [14], including Haldane's [15] rule. One factor in particular is that hybridization is encouraged by restricted mate choice, and is therefore common in areas where two species occur in sympatry but one species is rare [14]. This concept was first formalized by Hubbs [16]: “Great scarcity of one species coupled with the abundance of another often leads to hybridization: the individuals of the sparse species seem to have difficulty in finding their proper mates.” Hubbs referred to this principle as the “desperation hypothesis,” for which empirical support has now been found among numerous species of birds, including waterfowl [14].
Here, using multi-locus genetic data, we report an example of interspecific hybridization between two waterfowl species that exist in widespread sympatry throughout southern South America, but which show hybridization in a numerically imbalanced situation on the Falkland Islands. Both species, speckled teal (Anas flavirostris) and yellow-billed pintails (Anas georgica) are common throughout continental South America, but in the Falkland Islands one species is common and the other is rare [17], [18], [19]. Speckled teal are estimated to number approximately 6,000–11,000 breeding pairs in the Falkland Islands, whereas yellow-billed pintail breeding pairs likely number 600–1,000 [20]. The order-of-magnitude numerical imbalance in speckled teal and yellow-billed pintail population sizes in the Falkland Islands thus stands in contrast to continental South America, where each species is common and populations likely exceed 1,000,000 individuals.
Using data from eight genetic loci and Bayesian assignment tests and coalescent models, we identified an F1 female hybrid and her duckling in a small sample of 15 speckled teal banded in the Falkland Islands, but we found no evidence of hybridization or introgression among 56 speckled teal and 64 yellow-billed pintails collected over an area of sympatry in southern Argentina. Our results provide further support for Hubbs's [16] principle, the “desperation hypothesis.” Our study also revealed significant haplotype and allele frequency differences between speckled teal populations in the Falkland Islands and Argentina, suggesting that gene flow is restricted.
Materials and Methods
Specimen Collection
Speckled teal (n = 56) and yellow-billed pintails (n = 64) were collected at widespread localities in Argentina between 2001 and 2005, and blood samples were obtained from speckled teal banded on East Falkland Island (n = 15) in 2002. Localities of speckled teal are illustrated in Figure 1. Localities for yellow-billed pintails are illustrated in figure 1 of McCracken et al. [21]. All specimens in Argentina were collected at elevations <2,100 meters, and all of the speckled teal in our study were identified as the nominate subspecies Anas f. flavirostris. Vertebrate collecting activities were approved by the University of Alaska Fairbanks Institutional Animal Care and Use Committee (IACUC 02-01, 05-05) and by federal and provincial governments in Argentina and the Falkland Islands (D.F.S. No. 3209/01, 13168/03, 13169/03, 20419/05, 20420/05).
One adult female from the Falkland Islands (REW 325) and her duckling (REW 324) were identified in the field as possible hybrids. An unsampled male speckled teal was observed tending the brood with the female. Yellow-billed pintails are larger than speckled teal, and REW 325 was more similar to yellow-billed pintail in wing chord, culmen, and tarsus measurements. Along with body size, the female had some plumage characteristics typical of a yellow-billed pintail (Fig. 2). The back was uniform brown with large, dark scales, and the brown head feathers seemed more like pintail feathers. However, the speculum was bright green as found in speckled teal, but extended further out on the secondaries like yellow-billed pintail. The body, head, and neck shapes were more similar to a speckled teal, which is stockier than the more slender appearance of the yellow-billed pintail.
DNA Extraction, PCR, and Sequencing
DNA was isolated from frozen muscle or blood using standard protocols with DNeasy Tissue Kits (QIAGEN, Valencia, California). Eight gene regions including the mitochondrial DNA (mtDNA) control region and seven nuclear loci were sequenced (Table 1). Methods describing PCR and DNA sequencing protocols are described in McCracken et al. [21]. Sequences and specimen voucher information, including geo-referenced localities, are available in GenBank (accession numbers FJ617587–FJ617592, FJ617597–FJ617598, FJ617634–FJ617670, FJ617677–FJ617694, FJ617702, FJ617703–FJ617708, FJ617713–FJ617714, FJ617750–FJ617784, FJ617791–FJ617808, FJ617816, FJ617817–FJ617822, FJ617827–FJ617828, FJ617864–FJ617900, FJ617907–FJ617924, FJ617932, FJ617933–FJ617938, FJ617943–FJ617944, FJ617980–FJ618016, FJ618023–FJ618040, FJ618048, FJ618049–FJ618054, FJ618059–FJ618060, FJ618096–FJ618132, FJ618139–FJ618156, FJ618164, FJ618165–FJ618170, FJ618175–FJ618176, FJ618212–FJ618248, FJ618255–FJ618272, FJ618280, FJ618281–FJ618286, FJ618291–FJ618292, FJ618328–FJ618364, FJ618371–FJ618388, FJ618396, FJ618397–FJ618402, FJ618407–FJ618408, FJ618444–FJ618480, FJ618487–FJ618504, FJ618512, GQ269874–GQ269943, GQ270014–GQ270084, GQ270155–GQ270225, GQ270296–GQ270372, GQ270476–GQ270546, GQ271325–GQ271395, GQ272063–GQ272132, and JN223305–JN223375).
Allelic Phase Determination
The allelic phase of each nuclear sequence that was heterozygous at two or more nucelotide positions was determined independently for each species using allele-specific priming and the software PHASE 2.1 [22]. PHASE uses a Bayesian method to infer haplotypes from diploid genotypic data while incorporating recombination and the decay of linkage disequilibrium (LD) with distance. We first analyzed each composite sequence of both alleles using the default software settings (100 main iterations, 1 thinning interval, 100 burn-in) followed by 1,000 main iterations and 1,000 burn-in (−×10 option) for the final iteration. The PHASE algorithm was run five times (−×5 option) from different starting points, selecting the result with the best overall goodness of fit. For individuals with allele pair probabilities <80%, we then designed allele-specific primers to selectively amplify a single allele [23], [24]. The resulting haploid allele sequence was then subtracted from the diploid composite sequence to obtain the gametic phase of the second allele. Each data set was then analyzed five more times using PHASE and the additional known allele sequences (−k option). PHASE analyses and allele specific priming were performed for the complete sequences of the ODC1, ENO1, FGB, GRIN1, and PCK1 introns and the HBA2 gene. For the HBB gene, which was shown to exhibit high levels of recombination in speckled teal [25], we analyzed a subset of the 3′ HBB sequence consisting of the last 291 bp of intron two and exon three. The gametic phase of each autosomal sequence was identified experimentally or with >95% posterior probability for approximately 95% of the inviduals included in each data set.
Analysis of Genetic Differentiation between and within Species
To characterize genetic differentiation between speckled teal and yellow-billed pintails and examine how these patterns varied among loci, we calculated ΦST between species using the software Arlequin 3.5 [26] and illustrated networks for each locus using the median-joining algorithm in the software NETWORK 4.6 ([27]; Fluxus Technology, Ltd.). Additionally, we calculated ΦST between the Falkland Islands and Argentina for speckled teal, as well as other measures of genetic diversity such as the total number of polymorphic positions, frequency of the most common allele in each population, and nucleotide diversity (π/site). Because speckled teal sampling was unbalanced between the island and mainland sites, allelic richness was standardized to the smallest sample size (30 alleles for autosomal loci and 15 haplotypes for mtDNA).
Identification of Admixed Individuals
We used the software Structure 2.2 [28] to identify speckled teal and yellow-billed pintail individuals with admixed ancestry and compute their probability of assignment to respective populations. The eight-locus sequence data were first converted to numerical genotypic data using the software Collapse [29], with missing data coded in place of a second allele for the mtDNA. In the first step of the analysis, a simple two-population model (K = 2) was conducted using the admixture model (α = 1) with independent allele frequencies (λ = 1) and no a priori population information (POPFLAG = 0). In the second step, individuals with assignment probabilities >0.99 as determined in the first analysis were pre-assigned to their respective clusters corresponding to populations 1 and 2 (POPFLAG = 1). The ancestry of individuals with assignment probabilities <0.99 in the first analysis was then re-estimated (POPFLAG = 0) using allele frequencies defined by individuals previously determined to have posterior probabilities >0.99. Information about the allele frequencies from pre-defined individuals was thus used to improve the accuracy of inference about the admixture of unknown individuals. We used RE-RAT [30] to estimate Queller and Goodnight's [31] pairwise, symmetric relatedness statistic (rxy) and compute the average relatedness for each population.
We also used Structure's two-population model (K = 2) to compute the probability of assignment for speckled teal individuals inhabiting the Falkland Islands and Argentina (yellow-billed pintails excluded). No prior population information was incorporated into this analysis, and the same run parameters described above were used, with the analysis repeated five times.
Isolation with Migration Analysis
We further assessed evidence for hybridization between speckled-teal and yellow-billed pintails using “Isolation with Migration” in IMa2 [32], which allows for analysis of divergence and gene flow between two or more populations. In this case, we estimated the effective population size parameters (θ), time since divergence (t), and gene flow rates (M) in both directions between two populations of speckled teal and one population of yellow-billed pintails (Table 2).
Because the IM model assumes that all sequences are free from intra-locus recombination, we tested for recombination using the four-gamete test [33] implemented in DNAsp 4.10 [34]. Evidence of recombination was detected for all nuclear loci except FGB. The other loci were therefore truncated to include the longest fragment with no apparent recombination. For ODC1 this included positions 1–151, ENO1 positions 1–172, GRIN1 positions 75–178, PCK1 positions 1–254, and HBA2 positions 412–678. HBB was not included in the analysis as few segregating sites were retained after removing recombining blocks of sequence. The mtDNA was omitted because it was highly divergent and reciprocally monophyletic between speckled teal and yellow-billed pintails; no mtDNA haplotypes were shared between species (see below). A total of six nuclear loci were thus included in the IMa2 analysis. The HKY [35] substitution model was used in the IMa2 analysis, as opposed to the infinite-sites model, because all six loci possessed three or more alleles at one or more sites.
IMa2 was first run with wide priors to explore the sensitivity of parameter estimates to different upper bounds. The analyses were then conducted with uniform priors that encompassed the full posterior distribution of each parameter from the preliminary runs (θ = 5, t = 2, and M = 100). The upper bound for t was selected based on the assumption that time since divergence could not exceed TMRCA (time to most recent common ancestor of all sequences). The Markov chain Monte Carlo was run for 15 million steps, sampling the posterior distribution every 50 steps for a total of 300,000 sampled genealogies, with a burn-in of 500,000 steps. All runs included 20 chains with a geometric heating scheme. Autocorrelation was monitored during the run, and analyses were repeated five times with different random number seeds to ensure that parameter estimates converged.
Parameter estimates for θ and t were converted to biologically meaningful values using published estimates of generation time and the geometric mean of the substitution rate (μ per locus) calculated for the same six loci in Anas ducks using fossil dates calibrated to the duck/snow goose (Anser caerulescens) split (see McCracken et al. [21], [25]). The number of effective migrant individuals per generation was obtained by multiplying θ (4Neμ) by M (m/μ) to obtain 4Nem.
To determine whether yellow-billed pintail alleles had introgressed into speckled teal or vice versa, we examined the resulting posterior distribution for the four pertinent gene flow estimates shown in Table 2. Estimates of MAR>YP, MYP>AR, MFK>YP, or MYP>FK (see Table 2 for definitions) with a lower 95% confidence interval that did not overlap zero were interpreted as quantitatively strong evidence for hybridization, whereas estimates of M that overlapped zero could not be decisively interpreted as evidence of gene flow. Finally, the timing of each inferred interspecific gene flow event was recorded for all loci in each sampled genealogy, and the posterior distribution of timing of these events was compared to the posterior distribution of t between the speckled teal and pintail lineages.
Results
Genetic Differentiation between Speckled Teal and Yellow-billed Pintails
Speckled teal and yellow-billed pintails were significantly differentiated at all loci, with ΦST values ranging from 0.11 to 0.94 (Table 3). MtDNA haplotype groups were reciprocally monophyletic (ΦST = 0.94), and uncorrected divergence between species clusters was 5.6% (Fig. 3). The two aforementioned individuals (REW 325, 324) possessed speckled teal mtDNA haplotypes, not yellow-billed pintail haplotypes. ODC1 (ΦST = 0.81) was quasi-reciprocally monophyletic; two apparent species clusters were separated by 2.6% uncorrected sequence divergence. One yellow-billed pintail (KGM 750) that was morphologically indistinguishable from other yellow-billed pintails possessed a private singleton ODC1 allele that was one to three bases divergent from three other alleles in the speckled teal cluster, and REW 325 and 324 each possessed one identical allele that was shared with eight yellow-billed pintails, but not speckled teal. PCK1 (ΦST = 0.77) possessed no shared alleles, except for REW 325, which possessed the most common allele found in yellow-billed pintail on one chromosome and an allele shared by speckled teal on the other chromosome. Among the other loci, ENO1 and FGB had low allelic diversity and two and three alleles shared between species, respectively, whereas GRIN1, HBA2, and HBB had higher allelic diversity and two to six shared alleles. In total, REW 325 had five alleles at five loci that were shared exclusively with yellow-billed pintails, and REW 324 had four such alleles at three loci.
Speckled teal alleles are illustrated in black (Argentina) and grey (Falkland Islands), and yellow-billed pintail alleles are illustrated in white. Circle area is proportional to the number of shared alleles. Asterisks indicate the positions of putatively introgressed yellow-billed pintail alleles for two individuals (REW 325 and REW 325) in the Falkland Islands.
Identification of Admixed Individuals
In the Structure analysis with no prior population information, one yellow-billed pintail (KGM 1250) that was morphologically indistinguishable from other yellow-billed pintails was assigned to population cluster 1 with <0.99% posterior probability (P = 0.978). Two speckled teal (REW 325 and 324) were assigned to cluster 2 with <0.99% posterior probability (P = 0.812 and 0.978, respectively). In the second analysis in which prior assignments were used to preassign all but these three individuals, KGM 1250 was assigned to the yellow-billed pintail population with P = 0.961, and REW 325 and 324 were assigned to the speckled teal population with P = 0.630 and P = 0.860, respectively. The KGM 1250 individual was thus assigned to the yellow-billed pintail cluster with high probability, whereas REW 325 and REW 324 exhibited admixed ancestry.
REW 325 and 324 shared the same mtDNA haplotype as expected and one allele each at five of seven nuclear loci. Both pairs of alleles were identical at the other two loci. Based on this finding and the above referenced results, REW 325 was likely an F1 hybrid sired by a male yellow-billed pintail and female speckled teal that then successfully backcrossed to a male speckled teal and produced the REW 324 duckling. Queller and Goodnight [31] relatedness (rxy) for REW 325 and 324 was 0.564. Average relatedness for the speckled teal population as a whole was 0.009±0.004 (SE) and 0.013±0.002 (SE) for the yellow-billed pintail population.
Gene Flow between Speckled Teal and Yellow-billed Pintails
The three-population IMa2 coalescent analysis corroborated the results of the assignment tests. In proceeding further, it is useful to note that all estimates of the gene flow rate parameter M in coalescent genealogy samplers like IM are scaled to the mutation rate (M = m/μ), so M is thus the ratio of gene flow to mutation. The number of effective immigrants (4Nem) is obtained by multiplying the scaled gene flow rate M by θ. Confidence intervals are reported for the 95% highest posterior density (HPD).
Gene flow from yellow-billed pintails into the Falkland Islands speckled teal population was statistically greater than zero (MFK>YP = 4.75, HPD95% = 0.25–98.65; Fig. 4A), suggesting that yellow-billed pintails have introduced new mutations into the Falkland Islands specked teal population at rate equivalent to approximately five times the substitution rate. Furthermore, the timing of inferred gene flow events (t) between Falkland Islands speckled teal and pintails peaked sharply at zero time before present (HPD95% = 0.000–0.049). Interspecific gene flow therefore postdated the confidence intervals for the timing of divergence (t) between the speckled teal and pintail lineages (Fig. 5) by a wide margin. Identical alleles therefore could not be attributed to ancient coalescence of ancestral polymorphisms but could only be explained by recent gene flow. By contrast, gene flow between yellow-billed pintails and speckled teal in Argentina could not be distinguished from zero (MAR>YP = 0.25, HPD95% = 0.00–14.55; Fig. 4B), and has likely occurred at a rate lower than the substitution rate. It is difficult to estimate the total number of yellow-billed pintails hybridizing with speckled teal in the Falkland Islands from this type of data because variance in M and θ must be considered jointly. Multiplying the point estimate of MFK>YP (4.75) by the point estimate of θFK (0.0075) suggests that the number of effective yellow-billed pintail immigrants is less than one per generation. But if the upper 95% HPD of both M (98.65) and θ (0.2875) are considered, the number could be as high as 28 immigrants per generation.
A) Effective population size parameter θ. B) Time since divergence index (t) between speckled teal in the Falkland Islands and Argentina. C) Time since divergence index (t) between speckled teal and yellow-billed pintails. The 95% highest posterior densities (HPD) for each parameter are shown in parentheses (see Table 2 for definitions). Arrows depict the six gene flow parameters (M = m/μ). HPD95% estimates of M that overlap zero are shown with dashed lines (as shown in Figure 4).
Gene flow between speckled teal in Argentina and the Falklands Islands was asymmetric. Gene flow from Argentina to the Falkland Islands (MFK>AR) peaked at the upper prior (M = 100), so the true value is likely higher (Fig. 4C). Gene flow in the opposite direction, from the Falkland Islands to Argentina could not be distinguished from zero, but could just as well be greater as the tail did not asymptotically approach the x-axis (MAR>FK = 0.05, HPD95% = 0.00–99.45). The posterior density distributions for MFK>AR and MAR>FK were not smooth, which is a common outcome in IM when low ΦST values are observed for most loci (see below).
Given equal substitution rates among populations, which is likely a valid assumption for closely related species, the effective population size (Ne) of the Argentine population was 17 times greater than the Falkland Islands, and Ne for yellow-billed pintails was more than ten times greater than speckled teal (Fig. 5A). Time since divergence between the Argentine and Falklands Islands populations of speckled teal is likely very recent, the posterior probability of t peaked sharply at zero (t = 0.001, HPD95% = 0.00–0.495; Fig. 5B). The peak was bimodal, however, suggesting that divergence could be older (t = 0.007). By contrast, divergence between speckled teal and yellow-billed pintails was much deeper (t = 0.585, HPD95% = 0.301–1.121; Fig. 5C). Based on a point estimate substitution rate of 3.93×10−7 substitutions/locus/year obtained for the same six loci from five species of Anas ducks [25], divergence between speckled teal and yellow-billed pintails might date between approximately 0.77 and 2.85 million years.
Genetic Differentiation between the Falkland Islands and Argentina
Speckled teal exhibited highly significant mtDNA differentiation between the Falkland Islands and Argentina (ΦST = 0.45; Table 4). Sixteen mtDNA haplotypes were found in Argentina, whereas six haplotypes were sampled in the Falkland Islands (Fig. 3). Two haplotypes were shared between the Falkland Islands and Argentina, and the other four, including the most common haplotype in the Falklands, were not observed in Argentina (Fig. 6, Table 4). MtDNA allelic richness was greater in Argentina, but nucleotide diversity (π/site) was greater in the Falklands.
In contrast to the mtDNA, most nuclear loci showed less population differentiation. Three loci (ODC1, PCK1, and HBA2) yielded significant ΦST values ranging from 0.02 to 0.23 (P<0.05; Table 4). Eight private alleles were observed for HBA2 in the Falklands, and three and one private alleles were found in GRIN1 and HBB, respectively. In all but one case (ODC1), the most common allele occurred in Argentina at lower frequency than in the Falklands (Fig. 6). Nuclear allelic richness either did not differ or was greater in Argentina.
For speckled teal, the average assignment probabilities (±SD) to the Falkland Islands and Argentina were 0.59±0.04 and 0.55±0.03, respectively. Despite strong differentiation in the mtDNA, low differentiation and evidence of high gene flow in the nuclear DNA resulted in little power to discriminate between island and continental populations of speckled teal.
Discussion
Interspecific hybridization is not uncommon in birds, especially the waterfowl. Indeed, the majority of known avian hybrids are represented by the Anatidae [13], and hybridization has been studied previously in a variety of waterfowl species using genetic assays [36], [37], [38], [39], [40], [41], [42]. Key factors contributing to hybridization in the waterfowl have been shown to include a tendency for hybridization to occur among more closely related species and between species that coexist in sympatry [12]. Randler [14], likewise, found that hybridization occurs more frequently when one species is common and the other species is rare, thus indicating that “scarcity of conspecifics facilitates hybridization in general.” But in the same study Randler [14] found no evidence to suggest that females prefer mates of a larger species.
Haldane's [15] rule states that hybrid inviability occurs more frequently in the heterogametic sex. This phenomenon likely applies to all species that have sex chromosomes, and support for Haldane's rule has been demonstrated for F1 hybrid waterfowl. Kirby et al. [43] found evidence of post-mating isolation mechanisms in American black ducks (Anas rubripes) and mallards (A. platyrhynchos). Sixty-five percent of captive F1 hybrids were male, but the sex ratio did not differ for F1×F1 offspring or for F1 individuals backcrossed to parentals. Similar patterns are observed among domestic ducks [44]. Haldane's rule thus likely reduces the proportion of F1 female hybrids, but its effects may have only a minimal effect retarding interspecific gene flow, and identifying hybrid individuals other than F1 is generally not possible [45]. Tubaro and Lijtmaer [12] found that hybrid males outnumbered hybrid females, but this pattern was not found to be significant after correcting for male-biased sex ratios observed in adult duck populations.
The capacity for genes to introgress from one species to another has also been investigated in a variety of waterfowl species. Muñoz-Fuentes et al. [40] found that ruddy ducks (Oxyura jamaicensis) and white-headed ducks (O. leucocephala) hybridized frequently and produced viable offspring, but like our study in Argentina they did not find evidence of extensive introgression. On the other hand, Mank et al. [37] found evidence of extensive introgression between American black ducks and mallards. Peters et al. [38] likewise found an interesting example of ancient hybridization in the gadwall (Anas strepera), in which 5.5% of individuals sampled from North America possess heterospecific, falcated duck (Anas falcata) mtDNA haplotypes. Several other instances of mtDNA haplotype sharing have also been attributed to hybridization in waterfowl [24], [42], [46]. Such patterns are common in both mtDNA and nuclear DNA for a wide variety of other species in which hybridization has been examined using molecular methods [47], [48], [49], [50], [51], [52], [53].
In this regard our discovery of an F1 hybrid speckled teal x pintail and her duckling is not unusual. It is possible that it was just an isolated rare occurrence, but what is noteworthy is where it occurred, under what circumstances, and additionally where evidence of hybridization was not detected. Speckled teal and yellow-billed pintails are not sister taxa [54]. Nonetheless, they occur in widespread sympatry and are abundant thoughout their range in South America. Moreover, they frequently occur in mixed flocks, and to the casual observer, they are similar in appearance. Unlike any other waterfowl in South America, both species have yellow bills and brown plumage. In the Falkland Islands, however, yellow-billed pintails are outnumbered by speckled teal by approximately ten to one. It is not known whether yellow-billed pintails were formerly more abundant or if they always existed on the islands at low densities. Woods and Woods [19] reported that they have been uncommon since at least 1860. Only a couple pintails were observed in the Falkland Islands during our study period, and none were captured or banded. Nonetheless, we identified an F1 hybrid and her duckling with only a small sample of speckled teal (n = 15), compared to a much larger sample of both yellow-billed pintails and speckled teal (n = 120) in Argentina, in which no such hybrids were detected. Nor was firm evidence of introgression detected in Argentina with the three-population “isolation with migration” coalescent analysis. Our results thus provide further support for Hubbs' [16] “desperation hypothesis,” which states that scarcity in one population and abundance of another will often lead to hybridization. Our findings are also relevant to Haldane's [15] rule, because the F1 hybrid that was identified in this case was female (the heterogametic sex), and she successfully hatched a duckling. Based on her mtDNA she resulted from a pairing between a male yellow-billed pintail and female speckled teal. Yellow-billed pintails are among the two largest Anas dabbling duck species in South America, whereas speckled teal is the smallest. So although Randler [14] found no support for the hypothesis that females should prefer bigger males when mating heterospecifically, this was not necessarily the case in our study.
Finally, our study raises questions about small populations inhabiting oceanic islands. Island endemic populations are well known for their demographic properties and unique adaptations to insular environments [55], [56], [57], [58]. Island populations have additionally been shown to have lower genetic diversity, which may contribute to empirically higher rates of extinction [59], [60]. Speckled teal in the Falkland Islands were found to be significantly differentiated from populations in Argentina in their mtDNA and at three nuclear loci (Table 4, Fig. 3). Similar patterns of allelic endemism have recently been found among other island duck populations, including several species that have excellent dispersal capabilities and otherwise exhibit minimal geographic structure at continental scales: mallards [61], red-breasted mergansers (Mergus serrator; [62]), and green-winged teal (Anas crecca; J. Peters, pers. comm.). Speckled teal and yellow-billed pintails have excellent dispersal capabilities, and our study found that gene flow from Argentina to the Falkland Islands, in the direction of the strong prevailing winds in Patagonia [57], [63], is likely >100-fold greater than background rates of mutation. But given the small effective population size on the Falkland Islands, this may still equate to a small number of annual immigrants. Even if M were 1,000-fold greater than the mutation rate, the number of immigrants would be less than ten per generation. Gene flow in the opposite direction is likely occuring at substantially lower levels relative to mutation. Given that the number of effective immigrants is not high, it is thus no surprise that differention was found in mtDNA and other loci, particularly as female waterfowl have been repeatedly shown to exhibit high levels of philopatry and breeding site fidelity [64], [65]. In sum, speckled teal inhabiting the Falkland Islands likely comprise a distinct demographic unit, and while significant numbers of immigrants probably arrive from Argentina annually, gene flow is likely restricted and it may be that the Falkland Islands population is predominantly resident and non-migratory.
Similar information about yellow-billed pintails in the Falkland Islands is not yet available because of their scarcity. It is not known whether the species experienced a bottleneck prior to the first acounts in 1860 or always occurred at low densities. Regardless, the effect of hybridization on the yellow-billed pintail population would be of interest because of their small population size in the Falkland Islands. Hybridization has been shown to be an important factor leading to population declines [3], [66]. Its effects may be exacerbated in island ecosystems, particulary when one species is common but another is rare, and when pre- and post-zygotic barriers are porous or weakly developed [67]. Hybridization and backcrossing to speckled teal such as we observed here could potentially be one factor that has contributed to persistently low population numbers of yellow-billed pintails. More work in the Falkland Islands is clearly needed to answer these questions and determine the full extent of introgression.
Acknowledgments
We thank Mariana Bulgarella, Alan Eagle, Sonia Felton, Sergio Goldfeder, Kevin Johnson, Cecilia Kopuchian, Dario Lijtmaer, Pamela McCracken, Daniel Ramadori, Pablo Tubaro, Thomas Valqui, and the staff of Patagonia Outfitters for their superb logistical support and/or assistance in the field. Field sampling was made possible by the Falkland Islands Government and the provinces of Salta, San Juan, Mendoza, Cordoba, Buenos Aires, Neuquén, Chubut, and Santa Cruz in Argentina. Shawn Houston provided support for high-performance computing through the Life Science Informatics Portal. John Pearce, Sarah Sonsthagen, Kevin Winker, and two anonymous reviewers provided helpful comments on drafts of the manuscript.
Author Contributions
Conceived and designed the experiments: KGM REW. Performed the experiments: KGM REW. Analyzed the data: KGM. Contributed reagents/materials/analysis tools: KGM REW. Wrote the paper: KGM REW.
References
- 1. Anderson E, Stebbins GL (1954) Hybridization as an evolutionary stimulus. Evolution 8: 378–88.
- 2.
Grant V (1981) Plant Speciation. New York: Columbia University Press.
- 3. Mallet J (2005) Hybridization as an invasion of the genome. Trends Ecol Evol 20: 229–237.
- 4. Dowling TE, Secor CL (1997) The role of hybridization and introgression in the diversification of animals. Annu Rev Ecol Syst 28: 593–619.
- 5. Grant PR, Grant BR, Petren K (2005) Hybridization in the recent past. Am Nat 166: 56–57.
- 6. Mallet J (2007) Hybrid speciation. Nature 466: 279–283.
- 7.
Gray AP (1958) Bird Hybrids. A Check-List with Bibliography. Technical Communication No. 13 of the Commonwealth Bureau of Animal Breeding and Genetics. Edinburgh: Farnham Royal.
- 8.
Panov E (1989) Natural Hybridisation and Ethological Isolation in Birds. Moscow: Nauka.
- 9. Grant PR, Grant BR (1992) Hybridization of bird species. Science 256: 193–197.
- 10.
McCarthy EM (2006) Handbook of Avian Hybrids of the World. Oxford: Oxford University Press.
- 11. Johnsgard PA (1960) Hybridization in the Anatidae and its taxonomic implications. Condor 62: 25–33.
- 12. Tubaro PL, Lijtmaer D (2002) Hybridization patterns and the evolution of reproductive isolation in ducks. Biol J Linn Soc 77: 193–200.
- 13. Randler C (2008) Hybrid wildfowl in Central Europe - an overview. Waterbirds 31: 143–146.
- 14. Randler C (2002) Avian hybridization, mixed pairing and female choice. Anim Behav 63: 103–119.
- 15. Haldane JBS (1922) Sex ratio and unisexual sterility in hybrid animals. J Genet 12: 101–109.
- 16. Hubbs CL (1955) Hybridization between fishes in nature. Syst Zool 4: 1–20.
- 17. Calkell EM, Hamilton JE (1961) The birds of the Falkland Islands. Ibis 103: 1–27.
- 18. Weller MW (1972) Ecological studies of Falkland Islands' waterfowl. Wildfowl 23: 25–44.
- 19.
Woods RW, Woods A (1997) Atlas of Breeding Birds of the Falkland Islands. Shropshire, England: Anthony Nelson.
- 20.
Wetlands International (2006) Waterbird Population Estimates, 4th Edition. Devon, United Kingdom: Wetlands International.
- 21. McCracken KG, Bulgarella M, Johnson KP, Kuhner MK, Trucco J, et al. (2009) Gene flow in the face of countervailing selection: adaptation to high-altitude hypoxia in the βA hemoglobin subunit of yellow-billed pintails in the Andes. Mol Biol Evol 26: 815–827.
- 22. Stephens M, Smith NJ, Donnelly P (2001) A new statistical method for haplotype reconstruction from population data. Am J Hum Genet 68: 978–989.
- 23. Bottema CDK, Sarkar G, Cassady JD, Ii S, Dutton CM, et al. (1993) Polymerase chain-reaction amplification of specific alleles: a general method of detection of mutations, polymorphisms, and haplotypes. Meth Enzymol 218: 388–402.
- 24. Peters JL, McCracken KG, Zhuravlev YN, Lu Y, Wilson RE, et al. (2005) Phylogenetics of wigeons and allies (Anatidae: Anas): the importance of sampling multiple loci and multiple individuals. Mol Phylogenet Evol 35: 209–224.
- 25. McCracken KG, Barger CP, Bulgarella M, Johnson KP, Kuhner MK, et al. (2009) Signatures of high-altitude adaptation in the major hemoglobin of five species of Andean dabbling ducks. Am Nat 174: 631–650.
- 26. Excoffier L, Lischer HEL (2010) Arlequin suite ver 3.5: a new series of programs to perform population genetics analyses under Linux and Windows. Mol Ecol Res 10: 564–567.
- 27. Bandelt HJ, Forster P, Rohl A (1999) Median-joining networks for inferring intraspecific phylogenies. Mol Biol Evol 16: 37–48.
- 28. Pritchard JK, Stephens M, Donnelly P (2000) Inference of population structure using multilocus genotype data. Genetics 155: 945–959.
- 29.
Posada D (2006) Collapse: describing haplotypes from sequence alignments. Available: http://darwin.uvigo.es/software/collapse.html. Accessed 2011 Mar.
- 30.
Schwacke L, Schwacke J, Rosel P (2005) RE-RAT: relatedness estimation and rarefaction analysis tool. Available: http://people.musc.edu/~schwaclh/. Accessed 2011 Mar.
- 31. Queller DC, Goodnight KF (1989) Estimation of genetic relatedness using allozyme data. Evolution 43: 258–275.
- 32. Hey J (2010) Isolation with migration models for more than two populations. Mol Biol Evol 27: 905–920.
- 33. Hudson RR, Kaplan NL (1985) Statistical properties of the number of recombination events in the history of a sample of DNA sequences. Genetics 111: 147–164.
- 34. Rozas J, Sanchez-DelBarrio JC, Messeguer X, Rozas R (2003) DnaSP, DNA polymorphism analyses by the coalescent and other methods. Bioinformatics 19: 2496–2497.
- 35. Hasegawa M, Kishino H, Yano T (1985) Dating of the human-ape splitting by a molecular clock of mitochondrial DNA. J Mol Evol 22: 160–174.
- 36. Rhymer JM, Williams MJ, Braun MJ (1994) Mitochondrial analysis of gene flow between New Zealand mallards (Anas platyrhynchos) and grey ducks (A. superciliosa). Auk 111: 970–978.
- 37. Mank JE, Carlson JE, Brittingham MC (2004) A century of hybridization: decreasing genetic distance between American black ducks and mallards. Conserv Genet 5: 395–403.
- 38. Peters JL, Zhuravlev Y, Fefelov I, Logie A, Omland KE (2007) Nuclear loci and coalescent methods support ancient hybridization as cause of mitochondrial paraphyly between gadwall and falcated duck (Anas spp.). Evolution 61: 1992–2006.
- 39. Williams CL, Brust RC, Fendley TT, Tiller GR, Rhodes OE (2005) A comparison of hybridization between mottled ducks (Anas fulvigula) and mallards (A. platyrhynchos) in Florida and South Carolina using microsatellite DNA analysis. Conserv Genet 6: 445–453.
- 40. Muñoz-Fuentes V, Vila C, Green AJ, Negro JJ, Sorenson MD (2007) Hybridization between white-headed ducks and introduced ruddy ducks in Spain. Mol Ecol 16: 629–638.
- 41. Fowler AC, Eadie JM, Engilis A (2009) Identification of endangered Hawaiian ducks (Anas wyvilliana), introduced North American mallards (A. platyrhynchos) and their hybrids using multilocus genotypes. Conserv Genet 10: 1747–1758.
- 42. Kulikova IV, Zhuravlev YN (2010) Genetic structure of the Far Eastern population of Eurasian wigeon (Anas Penelope) inferred from sequencing of the mitochondrial DNA control region. Russ J Genet 46: 976–981.
- 43. Kirby RE, Sargeant GA, Schluter D (2004) Haldane's rule and American black duck x mallard hybridization. Can J Zool 82: 1827–1831.
- 44. Phillips JC (1914) A further study of size inheritance in ducks with observations on the sex ratio of hybrid birds. J Exp Zool 16: 131–145.
- 45. Kirby RE, Reed A, Dupuis P, Obrecht HH 3rd, Quist WJ (2000) Description and identification of American black duck, mallard, and hybrid wing plumage. U. S. Geological Survey, Biological Resources Division, Biological Science Report USGS/BRD/BSR-2000-0002.
- 46. Kulikova IV, Zhuravlev YN, McCracken KG (2004) Asymmetric hybridization and sex-biased gene flow between eastern spot-billed ducks (Anas zonorhyncha) and mallards (Anas platyrhynchos) in the Russian Far East. Auk 121: 930–949.
- 47. Masta SE, Sullivan BK, Lamb T, Routman EJ (2002) Molecular systematics, hybridization, and phylogeography of the Bufo americanus complex in Eastern North America. Mol Phylogenet Evol 24: 302–314.
- 48. Donnelly MJ, Pinto J, Girod R, Besansky NJ, Lehmann T (2004) Revisiting the role of introgression vs shared ancestral polymorphisms as key processes shaping genetic diversity in the recently separated sibling species of the Anopheles gambiae complex. Heredity 92: 61–68.
- 49. Morando M, Avila LJ, Baker J, Sites JW Jr (2004) Phylogeny and phylogeography of the Liolaemus darwinii complex (Squamata: Liolaemidae): evidence for introgression and incomplete lineage sorting. Evolution 58: 842–861.
- 50. Sefc KM, Payne RB, Sorenson MD (2005) Genetic continuity of brood parasitic indigobird species. Mol Ecol 14: 1407–1419.
- 51. Carling MD, Brumfield RT (2008) Haldane's Rule in an avian system: using cline theory and divergence population genetics to test for differential introgression of mitochondrial, autosomal, and sex-linked loci across the Passerina bunting hybrid zone. Evolution 62: 2600–2615.
- 52. Balakrishnan CN, Sefc KM, Sorenson MD (2009) Incomplete reproductive isolation following host shift in brood parasitic indigobirds. Proc Roy Soc Lond B 276: 219–228.
- 53. Steeves TE, Maloney RF, Hale ML, Tylianakis JM, Gemmell NJ (2010) Genetic analyses reveal hybridization but no hybrid swarm in one of the world's rarest birds. Mol Ecol 19: 5090–5100.
- 54. Johnson KP, Sorenson MD (1999) Phylogeny and biogeography of dabbling ducks (genus: Anas): a comparison of molecular and morphological evidence. Auk 116: 792–805.
- 55.
MacArthur RH, Wilson EO (1967) The Theory of Island Biogeography. Princeton, , NJ: Princeton University Press.
- 56. Lomolino MV (1985) Body size of mammals on islands: the island rule re-examined. Am Nat 125: 310–316.
- 57.
Weller MW (1980) The Island Waterfowl. Ames, , IA: Iowa State University Press.
- 58. Pruett CL, Winker K (2005) Northwestern song sparrow populations show genetic effects of sequential colonization. Mol Ecol 14: 1421–1434.
- 59. Frankham R (1997) Do island populations have less genetic variation than mainland populations? Heredity 78: 311–327.
- 60. Frankham R (1998) Inbreeding and extinction: island populations. Conserv Biol 12: 665–675.
- 61. Kulikova IV, Drovetski SV, Gibson DD, Harrigan RJ, Rohwer S, et al. (2005) Phylogeography of the mallard (Anas platyrhynchos): hybridization, dispersal, and lineage sorting contribute to complex geographic structure. Auk 122: 949–965.
- 62. Pearce JM, McCracken KG, Christensen TK, Zhuravlev YN (2009) Migratory patterns and population structure among breeding and wintering red-breasted (Mergus serrator) and common mergansers (M. merganser). Auk 126: 784–798.
- 63.
Simpson GG (1934) Attending Marvels. New York: Macmillan.
- 64. Rohwer FC, Anderson MG (1988) Female-biased philopatry, monogamy, and the timing of pair formation in migratory waterfowl. Curr Ornithol 5: 187–221.
- 65. Sonsthagen SA, Talbot SL, Lanctot RB, McCracken KG (2010) Do waterfowl nest in kin groups? Microgeographic genetic structure in a philopatric sea duck. Mol Ecol 19: 647–657.
- 66. Wolf DE, Takebayashi N, Rieseberg LH (2001) Predicting the risk of extinction through hybridization. Conserv Biol 15: 1039–1053.
- 67. Rhymer JM, Simberloff D (1996) Extinction by hybridization and introgression. Annu Rev Ecol Syst 27: 83–109.
- 68. Hillier LW, Miller W, Birney E, Warren W, Hardison RC, et al. (2004) Sequence and comparative analysis of the chicken genome provide unique perspectives on vertebrate evolution. Nature 432: 695–716.
- 69. Tamura K, Nei M (1993) Estimation of the number of nucleotide substitutions in the control region of mitochondrial DNA in humans and chimpanzees. Mol Biol Evol 10: 512–526.