Hybridization in geese: a review
© Ottenburghs et al. 2016
Received: 16 March 2016
Accepted: 10 May 2016
Published: 12 May 2016
The high incidence of hybridization in waterfowl (ducks, geese and swans) makes this bird group an excellent study system to answer questions related to the evolution and maintenance of species boundaries. However, knowledge on waterfowl hybridization is biased towards ducks, with a large knowledge gap in geese. In this review, we assemble the available information on hybrid geese by focusing on three main themes: (1) incidence and frequency, (2) behavioural mechanisms leading to hybridization, and (3) hybrid fertility. Hybridization in geese is common on a species-level, but rare on a per-individual level. An overview of the different behavioural mechanisms indicates that forced extra-pair copulations and interspecific nest parasisitm can both lead to hybridization. Other sources of hybrids include hybridization in captivity and vagrant geese, which may both lead to a scarcity of conspecifics. The different mechanisms are not mutually exclusive and it is currently not possible to discriminate between the different mechanisms without quantitative data. Most hybrid geese are fertile; only in crosses between distantly related species do female hybrids become sterile. This fertility pattern, which is in line with Haldane’s Rule, may facilitate interspecific gene flow between closely related species. The knowledge on hybrid geese should be used, in combination with the information available on hybridization in ducks, to study the process of avian speciation.
KeywordsHybridization Introgression Behaviour Nest parasitism Extra-pair copulations Fertility Anatidae Captivity
Hybridization, interbreeding of species, has always intrigued ornithologists. Ernst Mayr  pointed out that “In birds, we have a fair amount of information, since some collectors, sensing their scarcity value, have specialized in the collecting of hybrids, and amateur observers have always been fascinated by them.” The first attempt to compile the numerous scattered references and reports of avian hybrids was undertaken by Suchetet . Later on, many more checklists and compilations of avian hybrids have been published [3–10]. The incidence of hybridization varies among bird orders, with the Anseriformes (waterfowl: ducks, geese and swans) showing the highest propensity to hybridize. Over 60 % of waterfowl species has hybridized with at least one other species and this figure increases to almost 77 % when including captive hybrids .
The high incidence of hybridization in waterfowl makes this bird group an excellent study system to answer questions related to the origin and preservation of species. For example, how do waterfowl species remain distinct despite high levels of hybridization? Does hybridization lead to the exchange of genetic material (i.e., introgression) and if so, does this provide individuals with an adaptive advantage or disadvantage? Indeed, there are still many open questions in speciation and hybridization research that could be answered by studying hybridization in waterfowl [11, 12]. These questions, however, are not the focus of this review.
The knowledge on waterfowl hybridization is biased towards ducks, as illustrated by an extensive inventory of hybrid ducks , an analysis of hybrid duck fertility patterns  and several genetic studies documenting interspecific gene flow due to introgressive hybridization (e.g., [15–17]). The knowledge of goose hybrids is clearly lagging behind. Several studies reported goose hybrids [18–21] or provided a description of local records of hybrid geese [22–24], but no study has been dedicated to the incidence of goose hybrids or their fertility. The differences in species discrimination and social structure between ducks and geese provide the opportunity to formulate and test research questions that will broaden our understanding on the origin and preservation of waterfowl species. For instance, how does sexual selection (as measured by the degree of sexual dimorphism) relate to the frequency of hybridization? Does hybridization accelerate of slow down the speciation process? Which behavioural and morphological characateristics determine conspecific or heterospecific mate choice? Is there strong selection against hybrids?
In this review, we address the knowledge gaps on hybrid geese by focusing on three main themes: (1) incidence and frequency, (2) behavioural mechanisms leading to hybridization, and (3) hybrid fertility.
Current taxonomy for the True Geese (tribe Anserini)
Taiga Bean Goose
A. f. fabalis
A. f. johanseni
A. f. middendorffii
Tundra Bean Goose
A. serrirostris rossicus
A. serrirostris serrirostris
Greater White-fronted Goose
A. a. albifrons (Eurasian)
A. a. flavirostris (Greenland)
A. a. gambeli (Western)
A. a. frontalis (Western)
A. a. elgasi (Tule)
Lesser White-fronted Goose
A. a. anser (European)
A. a. rubrirostris (Siberian)
A. c. caerulescens
A. c. atlantica
B. b. bernicla (Dark-bellied)
B. b. hrota (Pale-bellied or Atlantic)
B. b. nigricans (Black)
B. b. orientalis
B. h. leucopareia (Aleutian)
B. h. hutchinsii (Richardson’s)
B. h. minima (Minima)
B. h. taverneri (Taverner’s)
B. c. moffitti
B. c. maxima
B. c. occidentalis
B. c. fulva
B. c. canadensis
B. c. interior
B. c. parvipes
Incidence and frequency of goose hybrids
Barnacle Goose x Canada Goose
Barnacle Goose x Lesser White-fronted Goose
Barnacle Goose x Greylag Goose
Barnacle Goose x Bar-headed Goose
Barnacle Goose x Emperor Goose
Barnacle Goose x Greater White-fronted Goose
Barnacle Goose x Red-breasted Goose
Barnacle Goose x Ross’ Goose
Barnacle Goose x Snow Goose
Lesser x Greater White-fronted Goose
Greylag Goose x Canada Goose
Greylag Goose x Bar-headed Goose
Greylag Goose x Greater White-fronted Goose
Greylag Goose x Snow Goose
Greylag Goose x Swan Goose
Canada Goose x Bar-headed Goose
Canada Goose x Greater White-fronted Goose
Canada Goose x Swan Goose
Bar-headed Goose x Emperor Goose
Swan Goose x Bar-headed Goose
Origin of goose hybrids
Several behavioural mechanisms have been called upon to explain the production of hybrid offspring in birds [35–37]. Here, we discuss four mechanisms that are relevant for the occurrence of goose hybrids, namely (1) nest parasitism, (2) extra-pair copulations, (3) rarity of conspecifics, and (4) captive birds.
Occurrence of intra- and interspecific nestparastism and extra-pair copulations in all goose species
Greater White-fronted Goose
Lesser White-fronted Goose
Some goose species adopt conspecific young [43–47]. Whether geese also adopt heterospecific goslings and if this adoption can affect sexual imprinting and future mate choice is unknown. Heterospecific adoption has been documented between several distantly related bird species, but seems to be a rare phenomenon .
Forced extra-pair copulations (often called “rapes”) have been reported in several species of waterfowl . Trivers  suggested that such extrapair copulations could be functional; he noted that “a mixed strategy will be the optimal male course – to help a single female raise young, while not passing up opportunities to mate with other females whom he will not aid.” Males of several goose species engage in forced extra-pair copulations, such as Greater White-fronted Goose , Brent Goose  and Canada Goose . But this behaviour has been studied most extensively in Snow Goose and Ross’ Goose [54–56]. In the Canadian Karrak Lake Colony, Dunn et al.  observed that among successful copulations, 33 and 38 % were extra-pair in Ross’ and Snow Geese, respectively. Despite this high precentage of extra-pair copulations, only 2–5 % of the goslings had another father than the male guarding the nest. A similar low percentage of extrapair paternity (2–4 %) was also reported for Snow Geese in northern Manitoba, Canada . Based on these low ferilization percentages, forced extra-pair copulations appear to be a relatively inefficient reproductive tactic for males of these goose species. However, offspring resulting from successful extrapair copulations do provide a fitness benefit to males.
Extra-pair copulations can lead to hybridization when males copulate with females of another species. This has been observed for ducks: for instance, Seymour  reports three occasions of an extra-pair copulation attempt by a male Mallard (Anas platyrhynchos) on a female Black Duck (A. rubripes). However, interspecific extra-pair copulations have not been documented in geese. This can be due to the limited number of behavioural studies of geese during the period when copulations are most likely, and may also reflect differences in species discrimination and social structure between ducks and geese . Male ducks often seem unable or indifferent to discriminate between females of different species (which look very similar) as many studies report male ducks displaying to heterospecific females [36, 58, 59]. The social structure of geese, with long-term pairbonds and nest guarding by males, limits the opportunties for males to seek extra-pair copulations . Although interspecfic extra-pair copulations can potentially result in hybrid offspring, this behavioural mechanism seems of minor importance in the origin of hybrid geese, because of its low frequency and the low fertilization rate of such extra-pair copulations. This conclusion is in line with the study by Randler , who showed that “interspecific brood amalgamation has a stronger impact on natural hybridization in wildfowl than forced extra-pair copulations.”
Scarcity of conspecifics
Hubbs’ Principle or the Desparation Hypothesis states that the rarer species is more likely to mate with heterospecifics . There are several situations in which individual birds can be confronted with a scarcity of conspecifics, such as range expansion, vagrant birds or the release/escape of captive birds in a non-native environment. With regard to geese, range expansion should include an expansion of the wintering grounds, where mate choice occurs . Some birds will “make the best out of a bad job” and pair with a heterospecific mate: hybridizing with a closely related species may be a better solution than remaining unpaired [37, 64]. For instance, Indigo Buntings (Passerina cyanea) and Lazuli Buntings (P. amoena) switched to heterospecifics when no conspecific mates were available . Another good example of the Desparation Hypothesis concerns two duck species on the Falkland Islands, where Speckled Teals (Anas flavirostris) outnumber Yellow-billed Pintails (A. georgica) about ten to one. This numerical imbalance leads to hybridization . The Desperation Hypothesis is not restricted to natural situations, in captivity birds are often confronted with a scarcity of conspecifics and might choose to mate with the available heterospecifics.
Cockrum  already noted that “If hybrids resulting from birds in captivity were listed, the list would be much larger, especially among ducks and geese.” Indeed, numerous hybrids have been produced in captivity . The occurrence of hybridization in captivity can be explained by the mechanisms discussed above, namely extra-pair copulations, nest parasitism and scarcity of conspecifics. When these hybrids escape, they can be mistakenly reported as wild hybrids. However, it may be possible to deduce the captive origin of hybrids when one of the parent species is not native by examining the range of occurrence. Table 2 shows that many hybrid geese probably have captive origin; for instance, some of the most common hybrids in Europe are between Greylag Goose and two introduced species, Canada Goose and Swan Goose. However, there is also the possibility that vagrant geese enter the range of other species. For example, North American Snow Geese are occasionally observed in Europe during migration  and hybrids between Snow Goose and several European species have been reported .
Randler  introduced the “captivity effect” to account for the high rates of Anser hybrids in released populations. He argued that domestication of Greylag Goose and Swan Goose has resulted in genetical impoverishment and unnatural behaviour, leading to a relatively strong tendency for hybridization. For example, in Greylag Geese, the frequency of hybrids was higher in naturalised compared to natural populations [23, 67, 68]. The effects of captivity on hybridization should thus be taken into account.
Fertility of goose hybrids
In The Origin of Species, Darwin  discussed the fertility of hybrids between two domesticated goose species, the Greylag Goose and the Swan Goose:
“The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred inter se. This was effected by Mr Eyton, who raised two hybrids from the same parents but from different hatches; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far more fertile; for I am assured by two eminently capable judges, namely Mr Blyth and Capt. Hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly fertile.”
Later, he repeated the experiment of Mr. Eyton by crossing “a brother and sister hybrid from the same hatch” that he received from Rev. Goodacre . He only managed to rear five hybrids (several eggs did not hatch or remained unfertilized), but he was still startled by “the fact that these two species of geese [are] breeding so freely together.” He attributed the fertility of these hybrids to the long history of goose domestication. We now know that, irrespective of domestication, the potential for hybridization is lost slowly on an evolutionary timescale in birds,  and that many bird species are capable of producing fertile hybrids .
The evolution of hybrid sterility and inviability (both caused by postzygotic incompatibilities) has been studied in Drosophila , frogs , butterflies  and birds . These studies showed an increase of postzygotic isolation between species with divergence time. Furthermore, the evolution of postzygotic incompatibility follows Haldane’s Rule , which states that “when in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [or heterogametic] sex”. In birds, where sex is determined by a ZZ/ZW system, females are the heterogametic sex and hybrid females are thus expected to show greater fitness reductions compared to male hybrids. This expectation has been confirmed for birds in general , but also for specific bird groups, including ducks , galliform birds  and pigeons and doves .
One of the possible mechanisms that has been invoked to explain Haldane’s rule is dominance theory, which is based on the Dobzhansky-Muller incompatibility model [79, 80]. Dominance theory states that hybrid sterility and unviability are the outcome of the interaction of two (or more) genes that have developed incompatible alleles in allopatry. If these alleles are recessive and located on the Z-chromosome, their effect will be much larger in female birds because this sex lacks another Z-chromosome that could hold a dominant version of the incompatible allele, which would nullify the negative effect of the recessive one. Moreover, it has been suggested that the Z-chromosome plays a disproportionately large role in the development of intrinsic incompatibilities . Several lines of evidence support this “Large Z-effect.” First, Z-linked genes evolve faster compared to autosomal loci (“Faster Z-effect”), thereby speeding up the accumulation of incompatible alleles on this sex chromosome [82, 83]. Second, if genes involved in premating and postzygotic isolation both occur on the Z-chromosome and thus become physically linked, it is expected that this facilitates the evolution of isolation barriers by means of reinforcement . This situation has been described for Ficedula flycatchers, where genes for female preference and low hybrid fitness are located on the Z-chromosome [85, 86].
We tested whether geese also conform to Haldane’s Rule. We obtained cytochrome b sequences from GENBANK and calculated genetic distances between taxa using the Maximum Composite Likelihood model with Gamma Distribution in MEGA6 . Reports on hybrid goose fertility were collected from the Handbook of Avian Hybrids of the World . We performed a logistic regression in SPSS (version 19.0) with hybrid fertility as dependent variable (0 = both sexes fertile, 1 = only males fertile) and genetic distance as independent variable. To our knowledge, there are no reports of goose hybrids where only the females are fertile.
Hybridization in geese is common on a species-level, but rare on a per-individual level. The origin of the occasional hybrids is difficult to determine. An overview of the different mechanisms shows that, in theory, interspecific nest parasisitm or forced extra-pair copulations could lead to hybridization. Other sources of hybrids include a scarcity of conspecifics, hybridization in captivity and vagrant geese. The different mechanisms are not mutually exclusive, for instance, certain hybrids might be the result of extra-pair copulations in captivity. Currently, it is not possible to discriminate between the different mechanisms without quantitative data. To unravel the relative importance of these mechanisms, field data should be collected and experiments could be conducted in captivity. For example, the frequency of interspecific nest parasitism and extra-pair copulations may be documented in mixed breeding colonies. The occurrence of possible hybrids (which can be identified by means of genetic tests) in such colonies can then be related to the frequency of these behaviours. In captivity, experiments can be set up to observe how different goose species react to a scarcity of conspecifics and the availability of diverse heterospecifics. Most goose hybrids are fertile; only at high genetic distances do female hybrids become sterile. This fertility pattern provides the opportunity for interspecific gene flow between closely related species.
The overview of hybridization in geese presented here can be used, in combination with the knowledge available on duck hybrids, to study the process of avian speciation. Moreover, the differences in species discrimination and social structure between ducks and geese provide the opportunity to formulate and test research questions that will broaden our understanding on the origin and preservation of species.
We are grateful to Milena Holmgren and Kevin D. Matson for their inspiring discussions during the development of this manuscript. We would like to thank two anonymous reviewers for their useful comments.
We have read and understood BioMed Central policy on declaration of interests and declare that we have no competing interests.
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