Elliptic Fourier analysis of the form of genitalia in two Spodoptera species and their hybrids (Lepidoptera: Noctuidae)

S. latifascia and S. descoinsi are two closely related species of Noctuid moths. In laboratory conditions they hybridize, and Fl, F2 and backcross hybrids may be obtained easily. Some interspecific matings do not succeed due to anatomical incompatibilities resulting from morphological differences in the genitalia of the species. One of these differences involves the clasper, a highly sclerotized process of the male genitalia. The overall contour of the clasper in S. descoinsi, S. latifascia, Fl, F2, and backcross hybrids was analysed using elliptic Fourier analysis. All outlines were rotated along a common direction defined by two homologous landmarks, centered, and normalized for size. Fourier descriptors were used in a canonical variate analysis. Shape variations along factorial axes, or more generally along any variate of interest, such as size, were modelized by a multivariate regressive approach and visualized by inverse Fourier reconstructions of shapes. This approach allows a direct visualization of the shape transformations, negating the so-called impossibility or difficulty to interpret results of Fourier analyses. The results emphasize the specificity of the male clasper in the parental species. Most hybrids and backcrosses segregate from their parents. Many of them are displaced towards one of the parental forms. These results suggest that morphological differences may be controlled by a polygenic system with maternal effect in Fl and parental dominance in backcross.