The population bottleneck of the Iberian wolf impacted genetic diversity but not admixture with domestic dogs: A temporal genomic approach

After decades of intense persecution, the Iberian wolf subspecies faced a severe bottleneck in the 1970s that considerably reduced its range and population size, nearly leading to its extinction in central and southern Iberian Peninsula. Such population decline could have impacted the genetic diversity of Iberian wolves through different processes, namely genetic drift and dynamics of hybridization with domestic dogs. By contrasting the genomes of 68 contemporary with 54 historical samples spanning the periods before and immediately after the 1970s bottleneck, we found evidence of its impact on genetic diversity and dynamics of wolf-dog hybridization. Our genome-wide assessment revealed that wolves and dogs form two well-differentiated genetic groups in Iberia and that hybridization rates did not increase during the bottleneck. However, an increased number of hybrid individuals was found over time during the population re-expansion, particularly at the edge of the wolf range. We estimated a low percentage of dog ancestry (~1.4%) in historical samples, suggesting that dog introgression was not a key driver for wolf extinction in central and southern Iberia. Our findings also unveil a significant decline in genetic diversity in contemporary samples, with the highest proportion of homozygous segments in the genome being recently inherited. Overall, our study provides unprecedented insight into the impact of a sharp decline on the Iberian wolf genome and refines our understanding of the ecological and evolutionary drivers of wolf-dog hybridization in the wild.

Consequences of Hybridization in Mammals: A Systematic Review

Hybridization, defined as breeding between two distinct taxonomic units, can have an important effect on the evolutionary patterns in cross-breeding taxa. Although interspecific hybridization has frequently been considered as a maladaptive process, which threatens species genetic integrity and survival via genetic swamping and outbreeding depression, in some cases hybridization can introduce novel adaptive variation and increase fitness. Most studies to date focused on documenting hybridization events and analyzing their causes, while relatively little is known about the consequences of hybridization and its impact on the parental species. To address this knowledge gap, we conducted a systematic review of studies on hybridization in mammals published in 2010-2021, and identified 115 relevant studies. Of 13 categories of hybridization consequences described in these studies, the most common negative consequence (21% of studies) was genetic swamping and the most common positive consequence (8%) was the gain of novel adaptive variation. The total frequency of negative consequences (49%) was higher than positive (13%) and neutral (38%) consequences. These frequencies are biased by the detection possibilities of microsatellite loci, the most common genetic markers used in the papers assessed. As negative outcomes are typically easier to demonstrate than positive ones (e.g., extinction vs hybrid speciation), they may be over-represented in publications. Transition towards genomic studies involving both neutral and adaptive variation will provide a better insight into the real impacts of hybridization.