Wing shape, a tool to discriminate Trichogramma species

Morphological identification of the most species of Trichogramma is difficult due to their small size and overlap of potentially diagnostic characters. Almost all of the diagnostic characters for Trichogramma are found on the antennal flagellum or on the genitalia of males. Female Trichogramma have been usually considered “unidentifiable”. This can be a problem regarding that most species of Trichogramma have a biased sex-ratio (females more common than males) or are thelytokous.

In the context of COLBICS, the mixed INRA-IAS-ANASAC R&D team carried out an exploratory study aiming to detect variations on the shape of wing between several Trichogramma species using geometric morphometrics approaches. In fact, landmark-based geometric morphometrics is a powerful approach to quantifying biological shape, shape variation, and covariation of shape with biotic or abiotic variables or factors. More precisely, landmark-based geometric involves summarizing shape in terms of a landmarks configuration (constellation of discrete anatomical loci, each described by 2 ou 3 dimensional Cartesian coordinates). Thus, this method allows drawing evocative diagrams of morphological transformations or differences, and offering an immediate visualization of shape and the spatial localization of shape variation.

The differences in wing shape between 4 species of Trichogramma: T. brassicae (N=20), T. semblidis (N= 11), T. achaeae (N=9), T. cacoeciae (N=15) were studied. Initially, 7 landmarks (LM) and 12 semi-landmarks (SL) were defined and Cartesian coordinates (x and y) for each LM and SL of the wings were obtained. After, a Procrustes Superimposition on the Cartesian coordinates, and a Principal Component analysis on the superimposed coordinates, a UPGMA cluster analysis based on squared Mahalonobis distances derived from the first axes of the PCA were done. The UPGMA cluster analysis revealed high significantly dissimilarities in wing shape between T. brassicae, T. semblids, T. achaeae and T. caceociae.

This preliminary study suggest that wing geometry can be a good tool for discriminate some Trichogramma species. In fact, comparing to wing size, wing shape have different genetic properties, size heritability being generally low, while wing shape is less sensitive to environmental changes and highly heritable. Moreover, assuming that wing shape can be related to flight ability for example, we can imagine that using geometric morphometrics on Trichogramma wings can also be a good tool for correlate dispersion efficacy of strains on field and their wing shape for example.