Linkage and cytogenetic studies in the swallowtail butterflies Papilio polyxenes Fab. and Papilio machaon L. and their hybrids

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Abstract
By the hand-pairing technique, F1 hybrids, back crosses and, for the first time, F2 hybrids have been obtained between Papilio polyxenes Fab. (black North American swallowtail) and Papilio machaon L. (yellow old world swallowtail). Three types of investigation were carried out in parent species and hybrids: (1) linkage, (2) the presence of heteropyknotic bodies, and (3) pachytene chromosomes. (1) It was conclusively demonstrated that the loci controlling adult wing pattern and larval spot colour are not linked. (2) The two species differ with respect to the heteropyknotic body in somatic cells. In P. polyxenes, a body is present in cells from females but absent in those of males, whereas in P. machaon the body is present also in a high proportion of males. This difference was studied in the various hybrids. (3) The morphology of the W (= Y) chromosome was found to differ in the two species and this chromosome can, therefore, be traced in crosses. The data also suggest that in P. machaon the W consists of two parts which have been tentatively designated as W1 and W2, both pairing with the Z ( = X) chromosome. An additional nucleolar bivalent is present in some P. machaon individuals and this may be responsible for the polymorphism of the heteropyknotic body in the males. The fact that there are about 700 species of Papilios, many of which can be hybridized, means that comparative studies on heterochromatin and chromosomes are possible, and the findings may be relevant to hetero­pyknotic bodies in other orders.