Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone

Walsh, Jennifer; Shriver, W. Gregory; Olsen, Brian J.; Kovach, Adrienne I.

doi:10.1186/s12862-016-0635-y

Cited by 45 publications

(72 citation statements)

References 90 publications

(123 reference statements)

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“…This interpretation is bolstered by observations that traits exhibiting steep clines across hybrid zones are often involved in local adaptation (e.g., Culumber, Shepard, Coleman, Rosenthal, & Tobler, ; Culumber et al., ; Mullen & Hoekstra, ; Taylor et al., ; Walsh, Rowe, Olsen, Shriver, & Kovach, ) and that individuals with intermediate phenotypes are frequently maladapted to their environments (e.g., Delmore & Irwin, ; Hanson, Moore, Taylor, Barrett, & Hendry, ; Huber, De Leon, Hendry, Bermingham, & Podos, ), are unattractive as mates (e.g., Naisbit, Jiggins, & Mallet, ; Rundle, Nagel, Boughman, & Schluter, ; Saether et al., ) or both (e.g., Jiggins, Estrada, & Rodrigues, ). Furthermore, a particular trait is more likely to contribute to reproductive isolation if its cline is steeper than the average ancestry cline, as this suggests that the loci associated with that trait introgress across the hybrid zone at lower rates than the genome‐wide average (Baldassarre, White, Karubian, & Webster, ; Gay et al., ; Walsh, Shriver, Olsen, & Kovach, ; Wu, ). Comparing the location and width of phenotypic and genotypic clines is thus informative about the roles of different traits in contributing to isolating barriers.…”

Section: Introductionmentioning

confidence: 99%

Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry

et al. 2017

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Hybrid zones are geographic regions where isolating barriers between divergent populations are challenged by admixture. Identifying factors that facilitate or inhibit hybridization in sympatry can illuminate the processes that maintain those reproductive barriers. We analysed patterns of hybridization and phenotypic variation across two newly discovered hybrid zones between three subspecies of barn swallow (Hirundo rustica). These subspecies differ in ventral coloration and wing length, traits that are targets of sexual and natural selection, respectively, and are associated with genome-wide differentiation in allopatry. We tested the hypothesis that the degree of divergence in these traits is associated with the extent of hybridization in secondary contact. We applied measures of population structure based on >23,000 SNPs to confirm that named subspecies correspond to distinct genomic clusters, and assessed coincidence between geographic clines for ancestry and phenotype. Although gene flow was ongoing across both hybrid zones and pairwise F between subspecies was extremely low, we found striking differences in the extent of hybridization. In the more phenotypically differentiated subspecies pair, clines for ancestry, wing length and ventral coloration were steep and coincident, suggestive of strong isolation and, potentially, selection associated with phenotype. In the less phenotypically differentiated pair, gene flow and phenotypic variation occurred over a wide geographic span, indicative of weaker isolation. Traits associated with genome-wide differentiation in allopatry may thus also contribute to isolation in sympatry. We discuss potentially important additional roles for evolutionary history and ecology in shaping variation in the extent hybridization between closely related pairs of subspecies.

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Section: Introductionmentioning

confidence: 99%

Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry

et al. 2017

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“…; 0.3—Walsh et al . ). A moderate proportion of SNPs (12%) exhibited elevated levels of genetic differentiation between the two species ( F ST > 0.8).…”

Section: Discussionmentioning

confidence: 97%

“…osmotic regulation and melanic plumage; Walsh et al . ). One SNP that was identified by bayescan as being under selection when comparing A. nelsoni and A. caudacutus blasted to a Wnt protein.…”

Section: Discussionmentioning

confidence: 97%

“…; Walsh et al . , ). Average F ST between the coastal A. nelsoni subspecies (N‐SUB) and A. caudacutus was 0.22 and similar to previous F ST estimates from microsatellites (0.15—Shriver et al .…”

Section: Discussionmentioning

confidence: 99%

“…; Walsh et al . , ) differences. The initial subspecies delineation within A. nelsoni and A. caudacutus , however, was based on differences in geography (splits based on disjunct ranges) and plumage (Dwight ; Bishop ).…”

Section: Introductionmentioning

confidence: 96%

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Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird

et al. 2017

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Understanding the processes that drive divergence within and among species is a long-standing goal in evolutionary biology. Traditional approaches to assessing differentiation rely on phenotypes to identify intra- and interspecific variation, but many species express subtle morphological gradients in which boundaries among forms are unclear. This intraspecific variation may be driven by differential adaptation to local conditions and may thereby reflect the evolutionary potential within a species. Here, we combine genetic and morphological data to evaluate intraspecific variation within the Nelson's (Ammodramus nelsoni) and salt marsh (Ammodramus caudacutus) sparrow complex, a group with populations that span considerable geographic distributions and a habitat gradient. We evaluated genetic structure among and within five putative subspecies of A. nelsoni and A. caudacutus using a reduced-representation sequencing approach to generate a panel of 1929 SNPs among 69 individuals. Although we detected morphological differences among some groups, individuals sorted along a continuous phenotypic gradient. In contrast, the genetic data identified three distinct clusters corresponding to populations that inhabit coastal salt marsh, interior freshwater marsh and coastal brackish-water marsh habitats. These patterns support the current species-level recognition but do not match the subspecies-level taxonomy within each species-a finding which may have important conservation implications. We identified loci exhibiting patterns of elevated divergence among and within these species, indicating a role for local selective pressures in driving patterns of differentiation across the complex. We conclude that this evidence for adaptive variation among subspecies warrants the consideration of evolutionary potential and genetic novelty when identifying conservation units for this group.

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New extralimital breeding records of saltmarsh sparrows (Ammospiza caudacuta) and Nelson's sparrows (Ammospiza nelsoni) and their implications

Ruskin,

Clark,

Hotopp

et al. 2023

Ecology and Evolution

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Saltmarsh (Ammospiza caudacuta) and Nelson's (A. nelsoni) sparrows are sister taxa that breed in tidal marshes along the coast of the Northeastern United States and Canada. The Saltmarsh Sparrow breeds from mid‐coast Maine south to Virginia, while the Acadian Nelson's Sparrow breeds from the Canadian maritime provinces south to northern Massachusetts. Here, we present three extralimital observations of breeding Saltmarsh (n = 2) and Nelson's (n = 1) sparrows. In 2021 and 2022, we observed Saltmarsh Sparrow females attending nests at Mendall Marsh, ME, and Milbridge, ME, respectively, approximately 60 and 110 km beyond the documented northern extent of the Saltmarsh Sparrow breeding range. In 2022, we observed a breeding‐condition male Nelson's sparrow singing in the upriver portion of a marsh on Cape Cod, Massachusetts, approximately 115 km beyond the previously documented southern extent of the Nelson's Sparrow breeding range. We confirmed morphological species identification using a panel of microsatellite DNA loci. Due to both the well‐documented population declines of these species in the region and the intensity of sampling effort undertaken in recent years, we suggest that these observations likely are not indicative of range expansion. However, they do indicate that these 2 taxa have the capacity to use and successfully reproduce in marshes well beyond their established breeding limits. Our findings provide novel insight into the potential for these taxa to occur and successfully breed outside their documented breeding ranges. Given increased interest in their conservation, these results support the idea that management actions aimed at creating or maintaining nesting habitat across both species ranges could benefit both taxa.

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Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone

Cited by 45 publications

References 90 publications

Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry

Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry

Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird

New extralimital breeding records of saltmarsh sparrows (Ammospiza caudacuta) and Nelson's sparrows (Ammospiza nelsoni) and their implications

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