HETEROGENEOUS DEVELOPMENT OF HYBRIDS OF THE MIDGE CHIRONOMUS-THUMMI -A 3RD TRAIT OF THE HLE SYNDROME CONTRIBUTING TO REPRODUCTIVE ISOLATION

One abnormal trait of the HLE hybrid dysgenesis syndrome which occurs if Chironomus th. piger [E] female are crossed with Ch. th. thummi [HL ] male is the heterogeneous development of the hybrids of 61% of the e gg masses. In these cases the growth of about 25% of the larvae is del ayed, and the period of emergence of 28% of the adults lasts up to 17 days in comparison to six days in not heterogeneously developing egg m asses. The long period of emergence of the adult hybrids results in a reduction of their reproductive success because the minimum number of about 30 contemporarily occuring males which are necessary for swarm f ormation and successful mating is mostly not attained during the secon d half of the extended period of emergence. It is concluded that the h eterogeneous development of the piger female X thummi male hybrids rep resents an abnormal trait of the HLE hybrid dysgenesis syndrome which constitutes together with two other traits of this syndrome, namely em bryo death and malformations, a potent postzygotic reproductive isolat ion mechanism. This mechanism operates if the prezygotic isolation whi ch works on the basis of a different swarming behavior has failed.