Autumn migration direction of juvenile willow warblers (Phylloscopus t. trochilus and P. t. acredula) and their hybrids assessed by qPCR SNP genotyping

Abstract

Backgrounds: Geographic regions, where two closely related taxa with different migration routes come into contact, are known as migratory divides. Hybrids originating from migratory divides are hypothesized to migrate intermediately relative to the parental populations. Few studies have tested this hypothesis in wild birds, and only in hybrids that have completed the migration back to the breeding grounds. Here, we make use of the wellestablished migration routes of willow warblers (Phylloscopus trochilus), for which the subspecies trochilus and acredula have migration-associated genetic markers on chromosomes 1 and 5. The genetic approach enabled us to analyze the geographic distribution of juveniles during their first autumn migration, predicting that hybrids should be more frequent in the central flyway over Italy than along the typical SW routes of trochilus and SE routes of acredula. Methods: Blood and feather samples were collected from wintering birds in Africa (n = 69), and from juveniles during autumn migration in Portugal (n = 33), Italy (n = 38) and Bulgaria (n = 32). Genotyping was carried out by qPCR SNP assays, on one SNP each on chromosome 1 (SNP 65) and chromosome 5 (SNP 285). Both these SNPs have alternative alleles that are highly fixed (> 97%) in each of the subspecies. Results: The observed combined genotypes of the two SNPs were associated with the known migration routes and wintering distributions of trochilus and acredula, respectively. We found hybrids (HH) among the juveniles in Italy (5/38) and in Portugal (2/33). The proportion of hybrids in Italy was significantly higher than expected from a background rate of hybrid genotypes (1.5%) in allopatric populations of the subspecies.

Transposable elements mark a repeat‐rich region associated with migratory phenotypes of willow warblers (Phylloscopus trochilus)

“…For the analysis of possible associations between derived copy numbers and genotypes on the divergent regions on chromosomes 1 and 5, we selected the samples based on previous genotyping done by a single nucleotide ppolymorphism (SNP)-array (Lundberg et al, 2017) or qPCR-SNP (Zhao et al, 2020). In total, we analysed 189 birds consisting of 105 pure trochilus (SS-southern homozygous for chromosome 1 and 5 respectively), 47 pure acredula (NN-northern homozygous for chromosome 1 and 5 respectively) and 37 F 2 or later generation hybrids (17 NS and 20 SN), sampled between the years 1996 and 2019.…”

Climate in Africa sequentially shapes spring passage of Willow WarblerPhylloscopus trochilusacross the Baltic coast

“…Ph. t. acredula winters mainly in eastern Africa ( Zhao et al, 2020 ), where IOD reflects rainfall and temperatures ( Marchant et al, 2006 ), and in southern Africa ( Herremans, 1999 ; Zhao et al, 2020 ), where SOI reflects these climate features ( McPhaden, Santoso & Cai, 2020 ). But Ph.…”

“…t. acredula flies 5,000–12,000 km from wintering grounds mainly in central, eastern and southern Africa to breeding grounds in northern Sweden, Finland, eastern Poland and far northeastern Europe ( Fig. 1 ) ( Hedenström & Pettersson, 1987 ; Cramp & Brooks, 1992 ; Bensch, Andersson & Åkesson, 1999 ; Tomiałojć & Stawarczyk, 2003 ; Bensch, Bengtsson & Åkesson, 2006 ; Fransson & Hall-Karlsson, 2008 ; Bensch et al, 2009 ; Zwarts et al, 2009 ; Valkama et al, 2014 ; Lerche-Jørgensen et al, 2017 ; Maciąg et al, 2017 ; Zhao et al, 2020 ). The proportion of Willow Warblers that winter in eastern and southern Africa increases along a west-east axis across the breeding grounds ( Zwarts et al, 2009 ).…”

“…The ranges of months are the main periods when Willow Warblers occur within their breeding, migration and wintering areas. The westernmost route in the Sahel region reflects geolocator tracks of birds breeding in Denmark ( Lerche-Jørgensen et al, 2017 ), the eastern route considers genotyping ( Zhao et al, 2020 ). This figure is based on Remisiewicz & Underhill (2020 , modified), and is derived from “ Phylloscopus trochilus Range Map.png” by Keith W. Larson, licensed under CC-BY-SA-3.0 by Magdalena Remisiewicz.…”

“…The species winters throughout Africa south of the Sahara Desert and two subspecies migrate across the Polish coast of the Baltic Sea past the bird ringing station at Bukowo, Poland (54°20′–54°27′N, 16°14′–16°24′E), so we chose Willow Warblers as a representative long-distance migrant to determine how widespread patterns of climate might influence the timing of migration in complex ways. The two subspecies tend to occupy different winter quarters in Africa, but both breed in northern Europe ( Bensch, Bengtsson & Åkesson, 2006 ; Bensch et al, 2009 ; Zhao et al, 2020 ); thus we expected that the pattern of their spring arrivals at stopovers on the Baltic Sea coast might be related to climate factors operating in different areas in Africa.…”