Most polar bears are born in litters of fraternal twins, such as these two cubs. A new genetic study shows that identical twins are rare among polar bears.

© René Malenfant

1/11/2016 7:37:27 PM

Building a Polar Bear Family Tree: Adoptions, Inbreeding, and Identical Twins

A pedigree is a family tree. Pedigrees are extremely useful in wildlife biology, because they can tell us about reproductive success, changes in population size, and migration. Pedigrees can also be used to estimate inbreeding (i.e., mating between relatives, which can be bad for the genetic health of a bear or a population), and to determine how genetics affects variation in body size among individuals.

Recently, we created a large pedigree of polar bears from Western Hudson Bay, near Churchill, Manitoba, Canada. This pedigree is among the most extensive ever created for a large mammal. In total, it includes 4,449 bears born over six generations, most of which were sampled between 1966 and 2011 by Environment and Climate Change Canada. By studying genetic markers, we were able to determine the mothers of 2,957 individuals and the fathers of 1,861 individuals in the data set. 

We created the pedigree to help address some key questions about polar bear biology, and we made some surprising discoveries along the way:

  • First, we discovered six new cases of cub adoption-that is, six cubs that were captured together with a female who could not be their genetic mother. The adopted cubs were not closely related to the adopting mothers, and we think the adoptions may have happened because of cub misidentification. Many animals recognize their relatives by scent, and polar bears have low variation in some of the genes that affect scent. So polar bears may not be very good at recognizing their relatives, and females may sometimes walk away with the wrong cub. This could occur when polar bears come together in high densities, such as when they are ashore during Western Hudson Bay's ice-free season. 
  • Second, inbreeding appears to be very rare in this subpopulation. In the entire pedigree, we detected only one instance of mating between close relatives. In this case, a female mated with her half-brother and later gave birth to two cubs. But if polar bears are bad at recognizing their relatives (as we suspect), how can they avoid inbreeding? One possibility is that there is just not much opportunity for inbreeding. Polar bears exist at low densities, and mating occurs while polar bears are out on the ice when there may also be some mixing with bears from neighboring subpopulations. So they may encounter close relatives only very rarely during the mating season.
  • Third, we discovered the first-ever reported case of identical bear twins. Most polar bears are born as fraternal twins, which develop from two separate fertilized eggs. Identical twins develop from a single fertilized egg, which later splits to form two individuals. Whereas identical twins have exactly the same genes, fraternal twins are no more closely related than "regular" siblings. We found two two-year-old males with identical data for all 25 of the microsatellites we tested (more about microsatellites below). The probability that these two males would have exactly the same genetic data if they were fraternal twins is less than 1 in 50 billion; therefore, we assume they were identical twins.

These are just the first of many discoveries to come from our pedigree for Western Hudson Bay polar bears.

You are probably wondering-so, how do you build a pedigree? First, you need tissue samples and genetic data.

When scientists capture a polar bear, they give it a pair of ear tags marked with a unique ID. They keep the small circles of tissue left over from ear tagging for genetic analysis. The type of genetic marker that is usually used in studying wildlife is called a "microsatellite". This is a short segment of DNA with a repeated sequence at a particular place in the genome. The length of each microsatellite can differ.. For instance, one version of a microsatellite might be “ATATATAT” (=4 repeats of “AT”, or AT4); another version might be “ATATATATAT” (=5 repeats of “AT”, or AT5).

Everybody-and every bear-has two copies of each microsatellite: one from their mother and one from their father. Usually, we already know the identity of the mother (e.g., from observation) and we want to determine who the father is. For example, imagine I am trying to figure out who my own father is. If my two copies of a microsatellite are AT4 / AT7, and my mother has AT4 / AT4, then I must have inherited my AT4 from my mother and my AT7 must have come from my father. Therefore, any male who does not have at least one copy of AT7 for this microsatellite cannot be my father. We say that these males have been "excluded". If we examine a large enough number of different microsatellites from across the genome, then we can hopefully exclude all males in the population but one: my true father.

The real method is more complicated, but this is the basic idea behind the paternity testing on popular television shows like Maury. And it's how we built a huge polar bear family tree, one that gives us insights that add to our understanding of polar bears and can aid in their conservation.

This blog post was based on a paper published in the journal, Polar Biology. Reference: Malenfant et al. 2016. Evidence of adoption, monozygotic twinning, and low inbreeding rates in a large genetic pedigree of polar bears. Polar Biology, doi:10.1007/s00300-015-1871-0.

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