Lock-and-Key Hypothesis: Lack of Mechanical Isolation in a Butterfly (Lepidoptera: Pieridae) Hybrid Zone

Abstract

The “lock-and-key” hypothesis of mechanical isolation predicts little if any natural hybridization between genitalically differentiated taxa. Chilean Tatochila mercedis (Eschscholtz) differs from Argentine T. sterodice Staudinger and T. vanvolxemii (Capronnier) in the degree and direction of a twist of the aedeagus. We report data on the genetic bases for these traits and use these data for an analysis of the degree of hybridization and introgression of genitalic traits in contact zones in the field. The direction of twist is based on one or a few genes, whereas the degree of twist is probably polygenic; the two characters are statistically independent in the laboratory. The ease of laboratory pairing and the high incidence of hybridization in nature clearly indicate that mechanical isolation based on these traits is untenable. There is a lack of strict concordance of clines involving these characters in the field; introgression of twist direction occurs farther eastward than twist degree, but this is a likely consequence of the different genetic bases for these traits. We reject the lock-and-key model for this group and suggest that arthropod systematists be cautious in recognizing species based on simple genitalic characters in parapatry.