THEPONTIA DAPLIDICE-ED USAHYBRID ZONE IN NORTHWESTERN ITALY

Abstract

Abstract.-The pierid butterflies Pontia daplidice and P. edusa, parapatrically distributed in southern Europe, have very similar morphologies and life histories, but show fixed differences at four allozyme markers. We sampled these allozymes in a 28-population transect north of Genoa in Italy, through the hybrid zone where these taxa meet. We used the numerical techniques developed for hybrid zone analysis to study the patterns of genetic differentiation and their underlying evolutionary causes. The hybrid zone is characterized by a very short and steep central region, flanked by broad tails of introgression extended up to 100 km in either direction. From mean two-locus disequilibium of D = 0.148 (maximum-likelihood two-unit support limits 0.139-0.153), and after accounting for minor differences in the center locations of the single-locus clines, which act to bias the dispersal estimate, we estimated a dispersal rate of a = 4.4 (3.7-5.5) km/gen 1/2. The effective selection needed to maintain the steep central portion is strong, 0.47 -s s' :s; 0.64, when combined over potential intrinsic (genetic background) and extrinsic (ecological) sources of selection. The clines in allozyme loci showed variation that was significantly different between the most divergent shapes, and the differences are attributable to different degrees of introgression on the edusa side of the zone. The average selection acting on individual allozyme loci was high at s, = 1.5%, but because of the narrowness of the central region of the cline, we suspect that this estimate is somewhat biased by selection on loci closely linked to the allozyme markers. A common question for taxa that show fixed allozyme differences in para patry is whether or not they are genetically isolated. A fairly general measure of genetic isolation across hybrid zones is the time, T, that it takes a neutral allele to cross the hybrid zone and recombine into the opposite genetic background, given by T = (13/ rr)2, where 13 is the barrier strength of the hybrid zone. Genetic isolation in the Pontia zone is weak, with T = 25 generations for most allozyme markers. By this measure, populations of daplidice and edusa on opposite sides of the hybrid zone share more identical-by-descent alleles than do populations of phenotypically pure daplidice in, say, France and Morocco. Accordingly, we think it best for systematists to consider edusa as a well-marked subspecies of P. daplidice.

Quaternary history, population genetic structure and diversity of the cold-adapted Alpine newt Ichthyosaura alpestris in peninsular Italy

“…Moreover, secondary contacts between closely related lineages have previously been observed within this area (e.g. refs 49, 50 ), and it marks the range boundary of several other taxa, including amphibians (e.g. Hyla meridionalis , Pelodytes punctatus , and Rana italica ).…”

Hybridization and extensive mitochondrial introgression among fire salamanders in peninsular Italy

“…Interestingly, both mtDNA and nuclear contact zones are located within well-known suture zones (sensu 33 ), where interspecific and intraspecific hybrid zones and range edges have been previously reported for various taxa, including amphibians (e.g. 4,34–37 ). Therefore, lines of cross-taxon concordance cannot be used here as inferential clues in the attempt to reconcile such discordance into a population history.…”

Origin and cross-century dynamics of an avian hybrid zone

“…In contrast to other studies [ 23 ], our work was not systematically designed to examine changes in the structure of the Ramphocelus hybrid zone over time, with repeated sampling of multiple specimens at fixed locations; rather, specimens were collected by numerous researchers, often opportunistically, for different purposes, and with no common sampling schemes at different times. Therefore, we did not feel confident in assigning individuals to discrete localities as required for cline-fitting algorithms which rely on calculating means and variances for samples of specimens from the same sites [ 66 – 68 ]. Our reanalysis of a previously published data set suggests that cline parameters estimated using Hill functions mirror closely those estimated by software commonly employed in studies of hybrid zones, suggesting such functions are useful alternatives when methods designed to study clines cannot be employed due to the nature of the available data.…”

“…Hybrid zones are often studied using cline-fitting algorithms that employ Bayesian or maximum-likelihood methods to estimate parameters like cline center and width (e.g. HZAR [ 66 ], Analyse [ 67 ], ClineFit [ 68 ]), and are based on population genetic models. These approaches typically require data from multiple individuals per sampling site to properly characterize variation within and among localities; accordingly, individuals need to be sampled at (or assigned to) a set of discrete sites.…”