Who’s Your Alpha Wolf Now? Genomic Sweep in Isle Royale Wolf Packs

Isle Royale in Lake Superior has been a laboratory for the study of wolf biology for over half a century (Figure 1). Wolves (Canis lupus) first colonized the island in 1950 by walking across 24 kilometers of frozen ice from the Canadian mainland. Since the arrival of the first three wolves, occasional ice bridges have brought others and those on the island have flourished, producing three to four wolf packs. Because Isle Royale is isolated and immigration is very rare, inbreeding among the island wolves is common. By the late 1990s the inbreeding coefficient (percentage of the chances two alleles are identical by descent) was 0.81 indicating that the population was highly inbred. Indeed, the wolves on Isle Royale had very high rates of congenital bone deformities compared to mainland wolf populations.

wolfpack
Figure 1. An alpha wolf (foreground top) making a subordinate submit (foreground bottom). (From Flickr/Angell Williams)

Everything changed in 1997. That year a single male wolf immigrated across the frozen channel to Isle Royale. This new male went unnoticed by researchers studying the wolves until genetic analyses revealed that one male (wolf 93) was genetically different from the other wolves on the island. DNA collected from scats, blood, and dead wolf carcasses over several decades was analyzed using
microsatellite markers. These data allowed the researchers to create pedigree relationships and understand the genetic structure of the island’s wolf population. A total of 94 wolves were included in the pedigree analysis (Figure 2).

pedigree
Figure 2. The wolf pedigree for Isle Royale from 1999 to 2009. The circles represent females, squares represent males, and diamonds represent several non-breeding individuals. The numbers in circles and squares represent wolf IDs and the numbers inside the diamonds represent number of wolves. Single lines represent matings and double lines represent matings between related individuals. Since 1999, all wolves are descended from only 7 wolves (represented by shaded or hatched symbols). Wolf 93 is the immigrant male. (From Adams et al., 2011)

The results were startling. The immigrant male (wolf 93) produced 34 pups after his arrival on the island, and his offspring have contributed another 45 pups over the 9 years since his arrived. Not surprisingly, the inbreeding coefficient dropped to 0.09 and average heterozygosity increased. This phenomenon is known as
genetic rescue. However, this pattern was short-lived. Wolf 93 was larger and he quickly became the dominant breeder in his pack. He began mating with his daughter (wolf 58), and they produced offspring that would then go on to breed among themselves (sibling-sibling mating) and later establish the Paduka Pack (Figure 2). By 2002 “two out of three breeding pairs were first-degree relatives and five of the population’s six breeders were either the immigrant or an offspring of the immigrant” according to the authors of the study (Adams et al., 2011).

Interestingly, wolf 93 was so successful that by 2009 every wolf could trace 56 % (on average) of their ancestry to the immigrant, wolf 93. More surprising was that this
genomic sweep took place in roughly one generation (generation time averages 4.2 years). Although gene flow from wolf 93 into a highly inbred population resulted in a genomic sweep, the benefits were short-lived. The mating success of wolf 93 ultimately led to more inbreeding and the inbreeding coefficient began to rise again. Thus a genomic sweep may not necessarily result in genetic rescue in the long term. Obviously, this has important implications for the conservation and genetic restoration of small isolated populations.

References

Adams, J., Vucetich, L., Hedrick, P., Peterson, R., & Vucetich, J. (2011). Genomic sweep and potential genetic rescue during limiting environmental conditions in an isolated wolf population Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.0261