LOCAL CYTONUCLEAR EXTINCTION OF THE GOLDEN-WINGED WARBLER

I compared the mtDNA compositions of two adjacent populations of Vermi vora chrysoptera (golden-winged warbler) at different stages of transi ent hybridization with its sister species V. pinus (blue-winged warble r). Pinus mtDNA introgresses asymmetrically and perhaps rapidly into c hrysoptera phenotypes without comparable reverse introgression of chry soptera mtDNA into replacing pinus populations. Pinus mtDNA was virtua lly fixed (98%) in an actively hybridizing lowland population with var ied phenotypes. Pinus mtDNA increased from 27% (n = 11) in 1988 to 70% (n = 10) in 1992 in successive samples of a highland population in th e initial stages of hybridization. This population comprised mostly pu re and slightly introgressed chrysoptera phenotypes. The rapid pace of asymmetrical introgression may be the result of initial invasion of c hrysoptera populations by pioneering female pinus and/or an unknown co mpetitive advantage of pinus females and their daughters over chrysopt era females.