RECIPROCAL INTROGRESSION BETWEEN GOLDEN-WINGED WARBLERS (VERMIVORA CHRYSOPTERA) AND BLUE-WINGED WARBLERS (V. PINUS) IN EASTERN NORTH AMERICA

Auk, The, Oct 2004 by Shapiro, Leo H, Canterbury, Ronald A, Stover, Dollie M, Fleischer, Robert C

ABSTRACT.-

Golden-winged Warblers (Vermivora chrysoptera) and Blue-winged Warblers (V. pinus) are small, brightly colored Neotropical migrant birds that breed in eastern North America. Wherever the two species occur together, they hybridize to a limited degree, producing distinctive hybrid phenotypes. In recent decades, chrysoptera has experienced dramatic population declines across much of its range. Those declines have often been correlated with establishment and increase of pinus in the same areas, but it remains uncertain what, if any, role pinus has played in driving the decline of chrysoptera. In a first attempt at molecular genetic analysis of chrysoptera-pinus population dynamics, Gill (1997) reported cryptic, completely asymmetric, and possibly very rapid introgression of pinus mitochondrial DNA (mtDNA) into chrysoptera, causing what he termed "local cytonuclear extinction" of chrysoptera. As Gill (1997) noted, however, those results were based on relatively small samples from a single area in Pennsylvania. To begin to investigate the generality of Gill's findings and to establish a baseline for long-term genetic and ecological studies, we intensively sampled one new study area (in southern West Virginia) and also sampled more broadly across two other areas (in Michigan and Ohio) that have experienced pinus invasions and chrysoptera declines. In southern West Virginia, introgression of mtDNA appeared to be roughly symmetrical: 15% (11 of 72) of pinus phenotypes possessed chrysoptera mtDNA, and 12% (17 of 137) of chrysoptera phenotypes possessed pinus mtDNA. Results from much smaller samples from Michigan and Ohio also failed to show any evidence of asymmetric mitochondrial introgression. The results we report here, based on mtDNA and plumage phenotype information for 337 birds representing much of the range of the two species, indicate that previous genetic results and inferences from Pennsylvania may not be broadly applicable to the many areas of contact between chrysoptera and pinus in eastern North America. Received 29 August 2003, accepted 24 June 2004.

RESUMEN. - Las reinitas Vermivora chrysoptera y V. pinus son aves migratorias peque�as de colores brillantes que se reproducen en el este de Norte Am�rica. En lugares donde las dos especies se encuentran juntas, �stas se hibridizan de forma limitada, produciendo fenotipos hibridos distintivos. En d�cadas recientes, las poblaciones de chrysoptera han disminuido dram�ticamente a trav�s de gran parte de su rango de distribuci�n. Estas disminuciones a menudo han estado correlacionadas con el establecimiento e incremento de poblaciones de pinus en las mismas �reas, pero a�n es incierto si pinus ha jugado alg�n papel causando la disminuci�n de chrysoptera. En un primer an�lisis basado en gen�tica molecular de la din�mica poblacional de chrysoptcra-pinus, Gill (1997) document� la ocurrencia de introgresi�n criptica, completamente asim�trica y posiblemente muy r�pida, del ADN mitocondrial (ADNmt) de pinus a chrysoptera, causando lo que �l llam� "extinci�n citonuclear local" de chrysoptera. Sin embargo, como lo mencion� Gill (1997), sus resultados se basaron en muestras relativamente peque�as de una sola �rea ubicada en Pennsylvania. En este trabajo muestreamos intensivamente un �rea de estudio nueva (en el sur de West Virginia) y muestreamos m�s ampliamente a trav�s de otras dos �reas (en Michigan y Ohio) en donde han ocurrido invasiones de pinus y disminuciones de chrysoptera para comenzar a investigar la generalidad de los hallazgos de Gill y para establecer una base para estudios gen�ticos y ecologicos de largo plazo. En el sur de West Virginia, la introgresi�n del ADNmt pareci� ser aproximadamente sim�trica: el 15% (11 de 72) de las aves con fenotipo de pinus presentaron ADNmt de chrysoptera, y el 12% (17 de 137) de las aves con fenotipo de chrysoptera presentaron ADNmt de pinus. Los resultados basados en muestras mucho m�s peque�as de Michigan y Ohio tampoco mostraron evidencia alguna de introgresi�n mitocondrial asim�trica. Nuestros resultados, basados en informaci�n sobre ADNmt y plumaje para 337 aves de buena parte del rango de distribuci�n de las dos especies, indican que los resultados gen�ticos previos y las inferencias de Pennsylvania no ser�an ampliamente aplicables a las m�ltiples zonas de contacta entre chrysoptera y pinus en el este de Norte Am�rica.

GOLDEN-WINGED WARBLERS (Vermivora chrysoptera) and Blue-winged Warblers (V. pi nus) are small, brightly colored Neotropical migrants that breed in eastern North America. During the past several decades, chrysoptera populations have declined precipitously in many regions, disappearing entirely from some areas (Confer 1992, Canterbury et al. 1993, Gill et al. 2001, Klaus and Buehler 2001). The cause of the decline remains uncertain, but it has been associated, in time and space, with newly initiated contact with piniis resulting from an ongoing pinus range expansion (the range of chrysoptera has also been expanding, but that expansion has been mainly northward into regions not yet inhabited by pinus). Despite intensive study of ecological and behavioral interactions between sympatric chrysoptera and pinus, there is still no clear evidence of competition between the two (e.g. Confer 1992, Confer and Larkin 1998, Gill et al. 2001). Interspecific pairings are relatively uncommon even in actively hybridizing populations, but hybrids appear to be fully fertile and to backcross readily with parental phenotypes-though Confer and Tupper (2000) suggest that hybrid males may have less success obtaining mates.