Returning animals to the wild after extirpation is often an attractive option for managers, especially in "pristine" localities where the native fauna is protected and maintenance of the existing flora and fauna is mandated (e.g. national parks and nature reserves). Such programs include repatriation of rescued wildlife, translocation of wildlife from a more prolific region, or reintroduction of offspring that have been raised in captivity.
Reintroductions of amphibians have had mixed success [101
]. The programs can be expensive and labor-intensive, and complicated by potential adaptation to captivity, and the presence of disease in the captive population or at the release site [104
]. Many of these challenges, however, can be addressed. For example, to prevent genetic adaptation to captivity, breeding programs can minimize the number of generations produced before release, delay reproduction, or cryopreserve eggs and sperm if release is not imminent [107
]. When properly executed and monitored [103
] reintroductions have potential for success (e.g. the natterjack toad, Bufo calamita
reintroduction in the U.K.[101
]). Artificial selection has been successful to improve resistance to viral and bacterial pathogens in livestock [111
] and in many fish species, i.e
]. Incorporating disease resistance into amphibian reintroduction programs may be desirable for species threatened by chytridiomycosis.
Similar to fish, high fecundity and short generation times of many amphibians may make them well-suited to selective challenge with Bd, using survivors as breeding stock for the next generation. Many amphibian species, however, produce very few eggs or their captive husbandry remains obscure. In these cases, a possible alternative to selection by pathogen exposure is to select for specific, measurable immunological characters that have the potential to impart resistance. Gaining an in-depth understanding of amphibian immunity is critical.
A strong candidate for this type of experiment would be selection for effective antimicrobial peptides (AMPs; [116
]). Large quantities of AMPs are produced in the skin granular glands of many amphibians as an investment in the innate immune system. The ability of amphibian AMPs to inhibit Bd
growth in vitro
has been shown to positively correlate with resistance to chytridiomycosis [117
] and has been used to predict disease susceptibility among species and populations [118
]. Because AMPs can be collected by noninvasive techniques and the amount and effectiveness of the peptides produced by each individual can be assessed [120
], developing a screening process for individuals with the most effective peptide repertoires has potential for use with selective breeding.
This approach hinges on whether enhanced AMP expression reduces susceptibility to chytridiomycosis and whether the effectiveness or quantity of AMPs produced among individuals is variable and heritable. Evidence is mounting to demonstrate these prerequisites: An increase in Bd
infection intensity resulted from reducing AMPs in young African clawed frogs, Xenopus laevis
]. AMP production changes little upon entry into captivity [119
], AMP expression is induced upon pathogen exposure in some disease resistant species (D.C. Woodhams, unpublished), and AMP expression is both heritable and variable among individuals [119
]. Immune defense genes such as those encoding AMPs that allow for tolerance of Bd
may be rapidly fixed in a population [123
]; whereas, the frequency of genes allowing resistance to infection may fluctuate [116
]. Although AMPs may have a role in both tolerance and resistance, some species such as Panamanian golden frogs, Atelopus zeteki
, and boreal toads, Bufo boreas
, among others, do not appear to produce anti-Bd
skin peptides [125
]. Other heritable defenses including both innate and adaptive defenses [61
] may be better targets in these species, and such defenses may be best identified in remnant populations (Figure ).
In some cases, reintroduction programs can also benefit from natural selection for disease resistance by focusing on populations that have persisted beyond initial outbreaks of chytridiomycosis. From such a population at Peñalara Natural Park in Spain, founders from relict metapopulations of midwife toads, Alytes obstetricans
, were captured after 10 years of successive and severe mass mortality events. Natural selection has been shown to occur even in such short time frames (e.g. [129
]) and surviving toads appear less susceptible to disease (J. Bosch, unpublished). Similarly, in the Rocky Montains, USA, some populations of boreal toad, Bufo boreas
, persist with disease [19
]; the mechanism is unknown, but some genetic lines have survival advantages [57
]. By attenuating disease-induced population declines long enough for natural selection to produce disease resistance, captive colonies and the problems associated with artificial selection can be avoided. This strategy is being employed in Australia for the critically endangered Corroboree Frog, Pseudophryne corroboree
. Field-collected egg masses are raised in predator-free mesocosms to head-start populations [130
]. Preserving the full range of amphibian habitats is essential for this strategy because environmental conditions that allow hosts an advantage over disease may occur in only a subset of habitats.