Abstract
Hybridization is common in bird populations but can be challenging for management, especially if one of the two parent species is of greater conservation concern than the other. King rails (Rallus elegans) and clapper rails (R. crepitans) are two marsh bird species with similar morphologies, behaviors, and overlapping distributions. The two species are found along a salinity gradient with the king rail in freshwater marshes and the clapper in estuarine marshes. However, this separation is not absolute; they are occasionally sympatric, and there are reports of interbreeding. In Virginia, USA, both king and clapper rails are identified by the state as Species of Greater Conservation Need, although clappers are thought to be more abundant and king rails have a higher priority ranking. We used a mitochondrial DNA marker and 13 diagnostic nuclear single nucleotide polymorphisms (SNPs) to identify species, classify the degree of introgression, and explore the evolutionary history of introgression in two putative clapper rail focal populations along a salinity gradient in coastal Virginia. Genetic analyses revealed cryptic introgression with site‐specific rates of admixture. We identified a pattern of introgression where clapper rail alleles predominate in brackish marshes. These results suggest clapper rails may be displacing king rails in Virginia coastal waterways, most likely as a result of ecological selection. As introgression can result in various outcomes from outbreeding depression to local adaptation, continued monitoring of these populations would allow further exploration of hybrid fitness and inform conservation management.
Highlights
Hybridization is the result of a breakdown in interspecific mating barriers, and can result in the genetic introgression of two species
We identified 13 diagnostic single nucleotide polymorphism (SNP) loci that displayed high allele frequency differences between king and clapper rails
This study shows the utility of both nuclear and mtDNA to explore species identity and genotypic distribution in a hybrid zone
Summary
Hybridization is the result of a breakdown in interspecific mating barriers, and can result in the genetic introgression of two species. The two species appear to hybridize readily, but only in areas of intermediate salinity or where freshwater and saltwater marshes are found in close proximity (Maley, 2012; Meanley, 1969; Olson, 1997) This spatial arrangement can occur naturally or as the result of anthropogenic manipulation such as the creation of dikes and ditches for impoundment or infrastructure. Clapper rails are abundant, listed as least concern globally (IUCN 2018) and appear as a low-ranked Species of Greatest Conservation Need on only a few state Wildlife Action Plans Both species are considered game species and are hunted in parts of their range (Raftovich, Chandler, & Wilkins, 2015). Our specific objectives were to: (a) identify individuals to species using mitochondrial and nuclear genetic markers, and (b) characterize genotypic distribution and examine patterns of introgression
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