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Figure 1 Shape variation. Outlines represent the variation in shape accounted by the first five principal components. Each outline was reconstructed from the estimated coefficients by letting the score of the corresponding principal component be equal to the mean and mean plus or minus two standard deviations (SD) and the remaining components set to zero. Black line, blue line and red line stand for mean shape, mean + 2 SD and mean - 2 SD respectively.

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Figure 2 Hybrid genital size. Mean area size (in pixels) of aedeagus of individuals reared in vials with fermenting Opuntia. Dk: Drosophila koepferae; Db: Drosophila buzzatii. Cross 3512 (●); Cross 4853 (■); Cross 4855 (◆); Cross 8832 (▲).

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Figure 3 Hybrid genital shape. Bivariate plot of mean shape scores of each genotype for the first two principal dimensions of shape variation (and percent of variance explained). The lines drawn connect genotypes involved in the same cross. Outlines below the graphic depict the genital shape variation accounted for the first principal component. Outlines to the left of the plot represent the genital shape variation accounted for the second principal component. Dk: Drosophila koepferae; Db: Drosophila buzzatii, F1: hybrids. Cross 3512 (●); Cross 4853 (■); Cross 4855 (◆); Cross 8832 (▲).

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Figure 4 Analysis of the morphological resemblance of hybrids to their corresponding parental strains. Hybrid mean Euclidean distances (and confidence intervals) to each mean parental shape of each cross. Dk: Drosophila koepferae; Db: Drosophila buzzatii. Cross 3512 (●); Cross 4853 (■); Cross 4855 (◆); Cross 8832 (▲).

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Figure 5 Aedeagus morphology. Lateral aspect of the intromittent organ in Drosophila koepferae and D. buzzatii (modified with permission from [6]). Shaded areas represent the portion excluded from the shape quantification (See [6] for details of the morphology of male genitalia).

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