Concordance of genetic and phenotypic characters across a sapsucker hybrid zone

Abstract

Hybridization has presented a challenge for taxonomists and conservation biologists, since hybridizing forms could be stable evolutionary entities or ephemeral forms that are blending together. However, hybrid zones also provide a unique opportunity for evolutionary biologists who study the interaction between gene fl ow and reproductive isolation in speciation. Th ree forms of woodpeckers (sapsuckers; genus Sphyrapicus ) in North America that are mostly geographically separated but hybridize with each other where they come into contact present a remarkable system for the study of hybridization. We provide the fi rst comprehensive analysis of phenotypic and genetic variation across a hybrid zone between two of these forms, the red-breasted Sphyrapicus ruber and yellow-bellied S. varius sapsuckers. Th e objective was to infer whether selection maintains the diff erences between forms. Our analysis of eight morphometric and 20 plumage traits, and two molecular markers showed clear diff erences between the forms and roughly concordant clinal variation across a narrow hybrid zone. Th irty percent of sampled birds in the hybrid zone had mixed west/east genotypes at the genetic markers examined. Th e center of the genetic cline was located 20 km west of the crest of the Rocky Mountains. Th e width of the zone was 122 km, narrower than would be expected under neutral blending given reasonable estimates of the age of the zone and individual dispersal distances. Heterozygote defi cit and cytonuclear disequilibrium at the centre of the hybrid zone suggested nonrandom mating or limited hybridization. Given these patterns and lack of evidence for habitat segregation we conclude that this hybrid zone is maintained by selection, most likely in the form of hybrid inferiority. Th is study provides an illustrative example of extensive hybridization between stable entities, providing additional evidence against the historical practice of treating hybridizing forms as members of the same species.

Tests of multiple molecular markers for the identification of Great Spotted and Syrian Woodpeckers and their hybrids

“…Among woodpeckers (family Picidae), several species pairs are known to hybridize (Short 1982;Randler 2002). Most examples concern strictly American genera: Campephilus, Sphyrapicus, Melanerpes, Celeus, Centurus, Veniliornis, and Picumnus (Selander and Giller 1959;Johnson and Johnson 1985;Seneviratne et al 2012;Fuchs et al 2013). Examples are also known within African Campethera woodpeckers (Short 1982), Picus (viridis/canus) in Eurasia (Beuch 2012), Dryocopus (schulzi/linneatus) in South America (Madroño Nieto and Pearman 1992), and among a few Picoides species in North America (Miller 1955;Short 1982).…”

Phenotypic and genetic analysis support distinct species status of the Red-backed Woodpecker (Lesser Sri Lanka Flameback:Dinopium psarodes) of Sri Lanka

“…We measured 8 variables: culmen length, bill height and bill width to calculate the bill breadth (sum of bill height and width divided by 2); head length and head width to calculate the size of the head (sum of head length and width divided by 2); and flattened wing length, tail length, length of hind reduced digit, first claw length, and tarsus length (Baldwin et al 1931, Seneviratne et al 2012. A single observer (S.F.)…”

“…We followed the same procedures that we used to amplify and sequence DNA (Seneviratne et al 2012) with several modifications. At least 4 samples per taxon were sequenced for all flameback taxa presented in this study.…”

“…We used the program CFit-7 to estimate parameters for the best-fitting cline for the Cytb marker (as in Seneviratne et al 2012). The cline was estimated as a simple sigmoidal curve across the contact zone.…”

“…across the transition zone between the 2 forms) using optical and acoustic cues, and playback of drumming calls. When a bird was located, call playbacks and a decoy (a life-size replica made out of plastic resembling a Dinopium male) were used to entice the bird to fly to the mist net (Seneviratne et al 2012). From captured birds we measured morphology and plumage, obtained a blood sample (~50 lL), a set of photographs (to be maintained as a reference collection for later phenotypic analysis), and determined the sex (males have red crowns, females have white spots on the forward part of their crowns).…”