A first demonstration of interspecific hybridization in Myrmica ants by geometric morphometrics (Hymenoptera: Formicidae)

Bagherian Yazdi, A.; Münch, W.; Seifert, B.

Abstract: A case of hybridization between Myrmica scabrinodis Nylander, 1846 and M. vandeli Bondroit, 1920 is demonstrated by means of geometric morphometrics in a nest sample found in Baden-Württemberg / Germany. 41 landmarks and 252 semilandmarks were fixed in four anatomical aspects in 299 worker ants – dorsal head, frontodorsal clypeus, dorsal mesosoma and lateral petiole. 316 relative warps (RWs) were extracted from the shape variables. Shape differences between these very similar species are visualized by mean deformation grids exaggerated by the factor of three. Among a number of differences already known, geometric morphometrics revealed also differences not discovered so far by conventional morphometrics or subjective character assessment. A full separation of the parental species was provided by the first two RWs of the head and clypeus aspect but this approach was not sufficient to reliably demonstrate hybrid identities on individual level. A stepwise linear discriminant analysis (LDA) using the RWs as characters and reducing character number from 316 to 25, offered very powerful separation of the 291 workers of the parental species (F = 6735.1, ANOVA) and placed the eight workers of the hybrid sample in a coherent cluster exactly in the empty space between the parental species. Basically similar results were achieved with conventional linear morphometrics considering 16 characters but this system was less powerful (F = 3108.0, ANOVA) and placed two specimens of the parental species close to the hybrids. There was no directional asymmetry detectable in the whole material and the hybrid sample did not show increased fluctuating asymmetry. With the current methodology applied, data acquisition time was 48 minutes per specimen in conventional linear morphometrics but as much as 160 minutes in geometric morphometrics. The main problems of the latter method are the slowness of the automatic z-stack imaging methods and the absent software assisting landmark fixation. A considerable reduction of total data acquisition time to 84 (one-fold digitizing) or to 108 minutes (two-fold digitizing) per specimen is predicted if a grid-and-sector system assisting landmark fixation is automatically projected on the screen. The overall hybridization frequency for the region of Baden-Württemberg is estimated as 0.44% from the side of M. vandeli and 0.03% from the side of M. scabrinodis. Despite apparently strong reproductive barriers between M. scabrinodis and M. vandeli, rare hybridization may occur because of the constant and extremely close spatial association, largely overlapping swarming times and some aspects of male mating behavior. Myrmica vandeli is very likely a facultative temporary social parasite of M. scabrinodis. As much as 17% of the 225 M. vandeli nests found in Baden-Württemberg still contained M. scabrinodis workers.

Myrmecol. News 17: 121-131; printable