Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
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Captive breeding efforts consider genetic diversity

Jessica McGlynn

One of the primary concerns of captive breeding programs is the challenge of maintaining genetic diversity despite a limited number of animals in these populations. Genetic diversity is important because it is directly related to a species' fitness, which is measured by how many surviving offspring are produced. 

For example, consider the importance of genetic variation in the genes affecting the immune system.  These genes allow foreign pathogens to be recognized.  The more diversity present in these genes, the more pathogens an organism can recognize and fight against.  If a group of individuals have the same genotypes (and therefore similar immune systems), they are susceptible to same diseases.  A single disease can wipe out an entire population. However, if a population maintains a high level of diversity, it will be much more resistant to disease.

The main technique captive breeding programs employ in choosing breeding individuals is attempting to maximize heterozygosity, said Melanie Culver, a Natural Resources assistant professor at the University of Arizona and a participant in several captive breeding projects.

An organism's genotype is heterozygous if it contains both a dominant and recessive allele.  Alleles are forms of genes that can affect the same characteristic, such as eye color.  A dominant allele - such as the one for brown eyes - masks a recessive allele - in this example, the allele for blue eyes. When an individual has one of each, the dominant allele will show up as observable, but the blue eyed allele could still be passed on to future generations. So two brown-eyed parents could have a blue-eyed child, if they both carry the recessive allele.

When animals are bred with the intention of producing heterozygous genotypes, both dominant and recessive alleles and their dissimilar traits are passed on to the next generation.  This can maintain genetic diversity. 

Another method to maintain genetic variation includes the preservation of rare alleles.  Although an individual may have one or two alleles, a species population may have between 20 and 40.  An animal with an allele unique to its breeding colony is very valuable for genetic diversity and is likely to be used for breeding. 

When heritage is known, biologists can select the least-related individuals for breeding.  Another way to examine an animal’s breeding suitability is to look at the genetic relatedness and the amount of shared genetic alleles.   The fewer alleles that two animals share, the more unrelated they are.   When heritage is unknown, biologists can select the two animals with the fewest shared genetic alleles. 

One more important genetic factor to consider is adaptation to captivity.  For example, by the ninth or tenth generation, the offspring have had a lot of time to adapt to their new captive environment. Although the initial breeders taken from the wild were not adapted to captivity, the surviving offspring may have genotypes and mutations that give them an increased chance of survival in captivity.As a result, these offspring are not necessarily the individuals that would do best in the wild. 

For this reason, offspring from the second or third generations may be chosen to be reintroduced to the wild, rather than the later generations. 

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