THE PONTIA-DAPLIDICE-EDUSA HYBRID ZONE IN NORTHWESTERN ITALY

The pierid butterflies Pontia daplidice and P. edusa, parapatrically d istributed in southern Europe, have very similar morphologies and life histories, but show fixed differences at four allozyme markers. We sa mpled these allozymes in a 28-population transect north of Genoa in It aly, through the hybrid zone where these taxa meet. We used the numeri cal techniques developed for hybrid zone analysis to study the pattern s of genetic differentiation and their underlying evolutionary causes. The hybrid zone is characterized by a very short and steep central re gion, flanked by broad tails of introgression extended up to 100 km in either direction. From mean two-locus disequilibrium of D = 0.148 (ma ximum-likelihood two-unit support limits 0.139-0.153), and after accou nting for minor differences in the center locations of the single-locu s dines, which act to bias the dispersal estimate, we estimated a disp ersal rate of sigma = 4.4 (3.7-5.5) km/gen(1/2). The effective selecti on needed to maintain the steep central portion is strong, 0.47 less t han or equal to s less than or equal to 0.64, when combined over pote ntial intrinsic (genetic background) and extrinsic (ecological) source s of selection. The dines in allozyme loci showed variation that was s ignificantly different between the most divergent shapes, and the diff erences are attributable to different degrees of introgression on the edusa side of the zone. The average selection acting on individual all ozyme loci was high at s(e) approximate to 1.5%, but because of the na rrowness of the central region of the dine, we suspect that this estim ate is somewhat biased by selection on loci closely linked to the allo zyme markers. A common question for taxa that show fixed allozyme diff erences in parapatry is whether or not they are genetically isolated. A fairly general measure of genetic isolation across hybrid zones is t he time, T, that it takes a neutral allele to cross the hybrid zone an d recombine into the opposite genetic background, given by T = (beta/s igma)(2), where beta is the barrier strength of the hybrid zone. Genet ic isolation in the Pontia zone is weak, with T approximate to 25 gene rations for most allozyme markers. By this measure, populations of dap lidice and edusa on opposite sides of the hybrid zone share more ident ical-by-descent alleles than do populations of phenotypically pure dap lidice in, say, France and Morocco. Accordingly, we think it best for systematists to consider edusa as a well-marked subspecies of P. dapli dice.