POLYMORPHIC TERMINATION OF DIAPAUSE BY CECROPIA: GENETIC AND GEOGRAPHICAL ASPECTS

¹ Department of Entomology, University of Illinois, Urbana, Illinois 61801

Hyalophora cecropia (L.) is dimorphic for the termination of diapause at Urbana and Chicago, Illinois, about 40°6' and 41°51' north latitude respectively. Under natural conditions the emergence of the adults is clearly bimodal in both places. At Urbana the early emergers (Group I) initiate development at the end of March and emerge during the second half of May; the late emergers (Group II) initiate development at the beginning of June and emerge during the second half of June and the first week of July. Members of either group produce some progeny which emerge with Group I and a majority which emerge within Group II.

The bimodality has a genetic basis. In either two or three generations we were able to line select from the Urbana population strains which averaged over 75% emergence with Group I as compared with the average of about 8% which was typical of the wild population.

Reciprocal transfers in the fall of wild-collected cocoons between Urbana and the Chicago area revealed that Group II exhibits geographic adaptation but that Group I does not. Pupae transferred in either direction produced Group I adults which emerged more or less synchronously with the local wild Group I. On the other hand, in Chicago the median emergence date of Group II Urbana moths was two weeks later than the median emergence date of the local Group II; in Urbana the median emergence date of Group II Chicago moths was twelve to fourteen days earlier than the median emergence date of the local Group II.

Rearing the progeny of Chicago moths in Urbana established that there is a genetic basis for the difference in emergence the between the Urbana and Chicago populations. Group II of the Urbana-reared F1 and F2 descendants of Chicago cecropia continued to emerge earlier than the local Group II. Furthermore, Group II of the F₂ generation emerged synchronously with wild Group II individuals collected in Chicago and transferred to Urbana the previous fall.

Whether a transfer was involved or not, the interval between the emergence of Groups I and II was shorter for the Chicago population than for the Urbana population, the time between median emergence dates averaging nineteen and twenty-eight days, respectivly, for the Chicago and Urbana populations.