EAZA News 57-12 - European Association of Zoos and Aquaria

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eaza news 57 2007 26 research Animal conservation genetics An overview with relevance to captive breeding programmes John O’Brien, Dublin Zoo and Department of Zoology, University College Dublin, Ireland Dublin Zoo has collaborated with two universities (Trinity College Dublin and University College Dublin) in recent years in developing a mutually beneficial research programme. These partnerships have allowed the zoo to fulfil its research obligations under the World Zoo and Aquarium Conservation Strategy, and additionally to contribute to effecting improvements in animal welfare and husbandry. In collaboration with University College Dublin, Dublin Zoo recently undertook its first genetic study of an endangered species, the Rodrigues fruit bat (Pteropus rodricensis). Because genetic studies are highly technical and expensive, they are often considered beyond the scope of many zoos. Inter-institutional collaborations can facilitate such projects. Some aspects relevant to genetic studies of captive animals are summarized below. Zoos act as valuable reservoirs of genetic material. Given increasing i pressures on wild populations and the need to manage them more proactively, much in the same way as captive populations, the application of conservation genetic methodologies to ex situ populations can generate considerable benefits to animal conservation in general. Conservation genetics can operate at three levels; population level, species level and individual level. All are relevant to captive breeding programmes. Species level studies The identification of taxonomic units is one of the most fundamental applications of conservation genetics. In some cases, morphological taxonomies are insufficient to quantify genetic diversity due to cryptic speciation. For example, genetic studies have supported suggestions that the two island populations of orang-utan (Pongo pygmaeus) on Borneo and Sumatra are separate sub-species and differ to such an extent that it has prompted zoos to manage them separately (Warren et al., 2001). Canis simiensis photo martin harvey Hybridisation events Genetic analyses can also reveal hybridisation events, which have important implications for conservation planning. For example, plans for an intensive breeding programme for the Ethiopian wolf (Canis simiensis) have been hampered by difficulties in finding purebred animals due to introgression of domestic dog (Canis familiaris) DNA (Gottelli et al., 1994). Phylogeography Studying the genetic relationship between species and their geographic distribution (phylogeography) enables placement of the speciation processes in an evolutionary context. This is relevant to contemporary levels of genetic variability (Avise, 2000). Zoos are sometimes able to contribute to phylogeographic studies, since captive specimens are often sourced from throughout the range of a species (e.g. jaguars Panthera onca, Eizirik et al., 2001) Population level studies The comparison of genetic variability between populations is important for both in situ and ex situ conservation. The impact of founder effects, genetic drift, gene flow and bottlenecks on variability are important questions for the effective management of captive populations, with significant implications for inbreeding and outbreeding. If a founder population becomes reproductively isolated, random genetic drift will lead to divergence, of which the extent will be determined by gene flow (i.e. mating). If regular gene flow occurs between populations, the populations will tend to follow the same evolutionary trajectory.
Thus, it is important that zoos continue to exchange individual animals, since failure to do so will ensure that each group or programme will diverge not only from each other, but also from the wild population. Bottleneck events Species with short generation times, high reproductive output and equal mating opportunities tend to retain most of their genetic variability following a population bottleneck, provided that such events are infrequent (e.g. European rabbits in Australia, Zenger et al., 2003). In contrast, species with long generation times, low reproductive output and a skew in mating opportunities tend to suffer from eroded genetic variability (Luikart et al., 1998), particularly following repeated bottleneck events (e.g. North Atlantic right whale Eubalaena glacialis, Schaeff et al., 1997). In instances where a bottlenecked population remains small, the risk of inbreeding increases (e.g. captive wolves Canis lupus, Laikre and Ryman, 1991). This tends to reduce heterozygosity (the proportion of individuals in a population possessing two variations of a gene at a given gene fragment) and increases the likelihood of the expression of recessive lethal genes. Evidence is accruing for the negative impact of inbreeding on ‘fitness’ in terms of reproductive success and offspring survival (Crnokrak and Roff, 1999), particularly in zoo populations (Ralls et al., 1979, Kalinowski and Miller, 1999 and Miller and Hedrick, 2005). Two populations may suffer lower overall ‘fitness’ if they interbreed, as this tends to disrupt linked gene complexes that may be important evolutionary adaptations to the local environment (Zschokke and Baur, 2002). Marshall and Spalton (2000) described simultaneous inbreeding and outbreeding in reintroduced Arabian oryx (Oryx leucoryx) populations. Encouraging breeding between different populations without considering their origins can be counter-productive. Individual level studies In some cases genetic parentage analysis can assist in verifying pedigrees (e.g. Asian elephants Elephas maximus, Vandebona et al., 2005), allowing studbook managers to make confident decisions regarding breeding and transfer of individuals between zoos. This ensures that genetic diversity is retained and inbreeding is limited. The assignment of individual animals to their source population based on their genetic profile is proving to be a valuable technique in the fight against poaching and the illegal movement of animals by ensuring that animals are sourced from approved populations (Manel et al., 2002). Genetic methodologies have also been successfully used in sex determination of juveniles and monomorphic species, such as many birds (Griffiths et al., 1998) and reptiles (Mohanty, 1992), which is of significant benefit to the management of studbooks. research Genetic incompatibility versus mate choice The role that genetic incompatibility plays in mate choice and the reproductive success of animals remains largely unknown, but evidence is accumulating that it is an important factor (Tregenza and Wedell, 2000). Thus, potentially, the lack of reproductive success of some animal pairs or groups may be explained by the absence of natural mate choice mechanisms in the captive environment. Further investigation of genes implicated in mate choice could prove fruitful for studbook management and techniques for artificial reproduction. Potential pitfalls There is no consensus as to what degree of genetic differentiation constitutes separate species (Ferguson, 2002), and indeed no such consensus is ever likely to be reached. Instead, genetic analysis must be used in conjunction with other evidence such as morphological or behavioural differences to confirm species or sub-species status. Evidence of genetic invariability or genetic divergence must be treated with caution. Since gene fragments may evolve differently and the choice of genes for analysis is largely subjective, the fragments selected may not be truly representative of the genome-wide genetic variability of an individual or population. This problem can be overcome somewhat by comparing results to other more distantly related species. Systematic tissue sampling A potentially significant drawback to the use of parentage analysis in pedigree confirmation for captive breeding programmes is the absence of parental genotypes in the sample set. To overcome this, zoos should consider systematically taking post-mortem tissue samples from individuals of species in current or planned captive breeding programmes. Given the limited space requirements and low cost of sampling, this process should not confer any additional financial burden (see www.protocol-online.org for sampling and storage techniques). Conclusion Despite continuing cost reductions, genetic analysis remains an expensive technique. Furthermore, the technical and laboratory requirements are restrictive, although many aspects can now be outsourced to private firms. As increasing numbers of genetic studies are being carried out on endangered species in the wild, the benefits of comparative studies with captive populations should be apparent. Such studies are prudent, given growing limitations on the size of captive breeding populations and the increasing need to offset any negative genetic effects of small population size through intensive management in zoos. • Given that zoo-based research centres are beyond the financial means of most zoos, the greatest benefits will be achieved if zoos form collaborative projects with local universities that can provide the technical and academic expertise necessary and additionally, share the financial burden. This article has been abridged and the references were left out due to space limitations. For the original full version of the article including reference details, please refer to the ‘Magazine’ section of the EAZA website. eaza news 57 2007 27
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