Variation in complex courtship traits across a hybrid zone between the grasshopper
species of the Chorthippus albomarginatus group
Institute for Information Transmission Problems, Russian Academy of Sciences, B. Karetnyi per. 19, Moscow 127994, Russia
Closely related grasshopper species of the Chorthippus albomarginatus group are notable for their extremely complex courtship songs accompanied by a visual display. Two species of this group, Ch. albomarginatus and Ch. oschei, were previously shown to hybridize in a wide mosaic hybrid zone in Ukraine and Moldova. In this paper, variation in five courtship song characters, one character of visual display and the number of stridulatory pegs was analyzed across the hybrid zone to estimate selection against hybrids and strength of assortative mating. Comparison of cline width and position across the hybrid zone showed concordant and coincident clines in four traits, such as three song and one morphological characters, and discordant and non-coincident clines in two other song characters and the character of visual display. Concordance of clines in different characters suggests an equal strength of selection acting on underlying loci. Increase of variance and covariance between phenotypic traits at the cline centre could more likely result from assortative mating than from selection against hybrids. Most pairwise cases showed the highest covariance for the oschei-like, than for the albomarginatus-like hybrid populations. This indicates that introgression of the oschei genes into albomarginatus genome is stronger than vice versa, and may be the evidence of the movement of the hybrid zone in favour of Ch. albomarginatus. Analysis of associations between phenotype and local vegetation showed that mosaic structure of the hybrid zone is explained to a great extent by habitat-phenotype associations. It is discussed whether all courtship traits studied in the albomarginatus / oschei hybrid zone are under strong sexual selection, and whether this may explain a relatively weak selection against hybrids acting on some courtship traits.