Guest post by Lara Semple
Increasingly, the world's ecosystems are exploited and fragmented, and
many wildlife communities are declining and increasingly vulnerable to
extinction. As a consequence of these changes, small genetically isolated
populations are at risk of losing genetic diversity and becoming inbred (Ralls
et al., 2017).
The International Union for the Conservation of Nature (IUCN), and some countries’ legalisation advise preservation of genetic diversity. However, fundamental steps towards considering genetic diversity in conservation decision making are rare, and conservation genetics have only been incorporated in a minority of governmental policies (Laikre et al. 2010, Cook & Sgrò 2017). Recently, Pierson et al. (2016) demonstrated that just 36 of 110 European species' recovery plans give explicit consideration to genetic factors related to population recovery. These European percentages are lower than species recovery plans compiled in the USA and Australia (Pierson et al., 2016).
Genetic factors are also often overlooked in monitoring of post-reintroductions or translocations, but there is a need to consider such factors to better assess whether populations have successfully colonised (Ottewell et al., 2014). One indicator of genetic diversity is heterozygosity, which refers to the presence of two different alleles (e.g. Aa) at a given locus - one being recessive (a) and the other dominant (A). Reduced heterozygosity in threatened species can suggest lowered evolutionary potential and reduced reproductive fitness. For example, in a meta-analysis, Spielman et al. (2004) found that heterozygosity was lower for the majority of threatened when compared with taxonomically related non-threatened taxa; threatened taxa had, on average, 35% lower heterozygosity than comparative species. Spielman et al. (2004) found that species like Eurasian Otter, a near threatened species on the IUCN Red List, had a lower heterozygosity compared to two closely related species that are listed as Least Concern on the IUCN Red List.
Without the explicit consideration of genetics in threatened species recovery plans and monitoring, a tool known as genetic rescue is being overlooked (Frankham, 2015; Ralls et al., 2017). Genetic rescue involves mixing of new genotypes into a population of 'at-risk' members to increase individual and population fitness (Waller, 2015). One example of a natural genetic rescue is the isolated Scandinavian metapopulation of Grey Wolf (Canis lupus) which originated from a single pair (Vila et al., 2002). At the time of writing this there were approximately 30 published examples where genetic rescue was used in conservation (e.g., Frankham 2015; Stowell et al., 2017; Ralls et al., 2017).
The rarity of genetic rescue in conservation practice, despite being talked about in literature for more than 20 years, appears to be driven by perceived biological, cultural and political barriers (Frankham, 2015; Stowell et al., 2017). Biological barriers seem to be primarily concerned with out-breeding depression (reduced reproductive fitness), and to do with the loss of local adaptation and doubt as to the scale of the consequences (Ralls et al., 2017). It is critical to be able to predict the probability of out-breeding depression in out-crossing between fragmented populations than were once found in continuous habitat.
Dr. Richard Frankham has focused on exploring the professed biological barriers of genetic rescue. Frankham shows that the chance of out-breeding depression occurring increases when crosses are between distinct species, which have not exchanged genes for ≥500 years or inhabit different environments. Therefore, the probability of out-breeding depression being a consequence of two populations of the same species is low for those of the same karyotype that have been isolated for < 500 years and occupy similar habitats. Incredibly, it has recently been shown that genetic rescue can benefit fitness and evolutionary potential in F2 and F3 generations (Frankham, 2016) and even up to F10 (Bijlsma et al., 2010). Loss of local adaptation is usually a minor issue, as an isolated population experiencing genetic drift will not be fully equipped to adapt to changing environmental changes anyway (Ralls et al., 2017). With thorough planning, local adaptations can be sustained into the new population as seen with the Florida Panther (Puma concolor) (Johnson et al., 2010).
Cultural barriers related to genetic rescue seem to originate from people's fear that genetic rescue could result in the loss of genetic purity or integrity after an out-crossing is implemented (Stowell et al., 2017). However, if we consistently choose to view nature anthropomorphically, categorising life into discrete species, and are reluctant to consider genetic rescue, extinctions could result that could otherwise be reduced or avoided by employing such methods in conservation programs.
Political barriers are often related to logistics, such as the movement of biological material across jurisdictions, and the lack of hybridisation or sub-species definitions in current endangered species legalisation (Frankham et al., 2015; Stowell et al., 2017). Repositioning taxa across boundaries is regular practice in zoos and botanic gardens, and such approaches could be extended to cross-jurisdiction genetic rescue programs. In addition, incorporation of the dynamic nature of species into conservation policy will provide population managers greater flexibility to make decisions about genetic rescued in the future (Stowell et al., 2017).
The International Union for the Conservation of Nature (IUCN), and some countries’ legalisation advise preservation of genetic diversity. However, fundamental steps towards considering genetic diversity in conservation decision making are rare, and conservation genetics have only been incorporated in a minority of governmental policies (Laikre et al. 2010, Cook & Sgrò 2017). Recently, Pierson et al. (2016) demonstrated that just 36 of 110 European species' recovery plans give explicit consideration to genetic factors related to population recovery. These European percentages are lower than species recovery plans compiled in the USA and Australia (Pierson et al., 2016).
Genetic factors are also often overlooked in monitoring of post-reintroductions or translocations, but there is a need to consider such factors to better assess whether populations have successfully colonised (Ottewell et al., 2014). One indicator of genetic diversity is heterozygosity, which refers to the presence of two different alleles (e.g. Aa) at a given locus - one being recessive (a) and the other dominant (A). Reduced heterozygosity in threatened species can suggest lowered evolutionary potential and reduced reproductive fitness. For example, in a meta-analysis, Spielman et al. (2004) found that heterozygosity was lower for the majority of threatened when compared with taxonomically related non-threatened taxa; threatened taxa had, on average, 35% lower heterozygosity than comparative species. Spielman et al. (2004) found that species like Eurasian Otter, a near threatened species on the IUCN Red List, had a lower heterozygosity compared to two closely related species that are listed as Least Concern on the IUCN Red List.
Without the explicit consideration of genetics in threatened species recovery plans and monitoring, a tool known as genetic rescue is being overlooked (Frankham, 2015; Ralls et al., 2017). Genetic rescue involves mixing of new genotypes into a population of 'at-risk' members to increase individual and population fitness (Waller, 2015). One example of a natural genetic rescue is the isolated Scandinavian metapopulation of Grey Wolf (Canis lupus) which originated from a single pair (Vila et al., 2002). At the time of writing this there were approximately 30 published examples where genetic rescue was used in conservation (e.g., Frankham 2015; Stowell et al., 2017; Ralls et al., 2017).
The rarity of genetic rescue in conservation practice, despite being talked about in literature for more than 20 years, appears to be driven by perceived biological, cultural and political barriers (Frankham, 2015; Stowell et al., 2017). Biological barriers seem to be primarily concerned with out-breeding depression (reduced reproductive fitness), and to do with the loss of local adaptation and doubt as to the scale of the consequences (Ralls et al., 2017). It is critical to be able to predict the probability of out-breeding depression in out-crossing between fragmented populations than were once found in continuous habitat.
Dr. Richard Frankham has focused on exploring the professed biological barriers of genetic rescue. Frankham shows that the chance of out-breeding depression occurring increases when crosses are between distinct species, which have not exchanged genes for ≥500 years or inhabit different environments. Therefore, the probability of out-breeding depression being a consequence of two populations of the same species is low for those of the same karyotype that have been isolated for < 500 years and occupy similar habitats. Incredibly, it has recently been shown that genetic rescue can benefit fitness and evolutionary potential in F2 and F3 generations (Frankham, 2016) and even up to F10 (Bijlsma et al., 2010). Loss of local adaptation is usually a minor issue, as an isolated population experiencing genetic drift will not be fully equipped to adapt to changing environmental changes anyway (Ralls et al., 2017). With thorough planning, local adaptations can be sustained into the new population as seen with the Florida Panther (Puma concolor) (Johnson et al., 2010).
Cultural barriers related to genetic rescue seem to originate from people's fear that genetic rescue could result in the loss of genetic purity or integrity after an out-crossing is implemented (Stowell et al., 2017). However, if we consistently choose to view nature anthropomorphically, categorising life into discrete species, and are reluctant to consider genetic rescue, extinctions could result that could otherwise be reduced or avoided by employing such methods in conservation programs.
Political barriers are often related to logistics, such as the movement of biological material across jurisdictions, and the lack of hybridisation or sub-species definitions in current endangered species legalisation (Frankham et al., 2015; Stowell et al., 2017). Repositioning taxa across boundaries is regular practice in zoos and botanic gardens, and such approaches could be extended to cross-jurisdiction genetic rescue programs. In addition, incorporation of the dynamic nature of species into conservation policy will provide population managers greater flexibility to make decisions about genetic rescued in the future (Stowell et al., 2017).
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| European Bison. Image courtesy of Arkive. |
These current barriers are compelling inaction, which is indeed a consequential action. One subspecies which has suffered from inaction is the Dusky Seaside Sparrow (Ammodramus maritimus nigrescens) found in marshes in Florida, USA (Stowell et al., 2017). The United State's Fish and Wildlife Service would not allow any dilution of the genetics by out-crossing the last males with females of a different sparrow subspecies. In turn, the Dusky Seaside Sparrow subspecies became extinct in 1987. Despite the United State's Fish and Wildlife Service's hesitation, many subspecies and closely related species hybridise in nature (Stowell et al., 2017). For example, the European bison is a hybrid of two extinct species; the steppe bison (Bison priscus) and the auroch (Bos primigenius) (Soubrier et al., 2016). Hybridisation between these species occurred >120 kya years ago, and demonstrating that hybridisation is certainly a naturally occurring phenomenon and it is a consequence of out-crossing which we shouldn’t necessarily avoid within conservation practices.
With increasing access to genetic data for a diversity of species it
will soon be possible to further reduce the
perceived barriers of genetic rescue through improved management
guidelines. Simply put,
the more we know, the less risky the decisions will be! Frankham and an
increasing number of researchers believe that the current genetic rescue
examples represents a “miniscule proportion of the populations that might
benefit from out-crossing”. Given the continued decline of plant and animal
populations, conservationists should regard genetic rescue as a welcome
addition to their toolbox. My goal is for this blog post to promote
consideration of genetic rescue as one of the tools in the conservation toolbox
to help us to prevent species extinctions.
Literature cited: Bijlsma, R et al. 2010. Conservation Genetics 11: 449-462; Cook, CN and Sgrò, CM. 2017. Conservation Biology 31: 501–512; Frankham, R. 2015. Molecular Ecology 24: 2610–2618; Frankham, R. 2016. Biological Conservation 195: 33-36; Johnson, WE et al. 2010. Science 329: 1641-1645; Laikre, L et al. Conservation Biology 24: 86–88; Ottewell, K et al. 2014. Biological Conservation 171: 209-219; Pierson, JC et al. 2016. Frontiers in Ecology and the Environment 14: 433-440; Ralls, K et al. 2017 Conservation Letters DOI: 10.1111/conl.12412; Soubrier, J et al. 2016 Nature communications 7; Spielman, D et al. 2004. PNAS 101:15261-15264; Stowell, SML et al. 2017. Biodiversity and Conservation 26:1753–1765; Vilà, C et al. 2013. Proc Roy Soc B 270: 91-97; Walker, CW et al. 2001. Molecular Ecology 10: 53-63; Waller, DM 2015. Molecular ecology 24: 2595-2597.
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