Gene flow across a chromosomal tension zone. I. Relicts of ancient hybridization

Abstract

Two parapatric subspecies of grasshopper with extensive karyotypic differences form a hybrid zone in which the change-over of chromosomal characters occurs over a distance of 800 m. Asymmetrical introgression of restriction-fragment markers of the nuclear ribosomal RNA genes, and mitochondrial DNA, and also of four enzyme electromorphs is reported. These markers were found to have introgressed for varying distances (100-300 km) to the north of the present-day hybrid zone. It is proposed that these markers are relicts of ancient hybridization between the Moreton and Torresian subspecies in an area where only the Torresian form (as defined by karyotype) is now found, and that the two taxa have maintained their chromosomal distinction despite prolonged hybridization and the geographic displacement of the Moreton subspecies by the Torresian form. © 1988, The Genetical Society of Great Britain.