Egg size and hybrid syndrome-dependent embryo mortality in Chironomus hybrids (Diptera : Chironomidae)

Female hybrids of the cross Chironomus t. thummi female x Ch. t. piger male which are largely affected by the sterility inducing Rud syndrome were bac kcrossed with males of both parental strains. The aim of the study was to p rovide information about those egg volumes that are insufficient for a norm al embryogenesis and to ascertain whether in the hybrids the lethally small egg size represents a new abnormal trait of the Rud syndrome. The egg mass es obtained contain eggs of very different sizes with volumes ranging from 0.5 nl to 3.49 nl. Embryo mortality is unusually frequent in those eggs of the backcrosses and of the parental strains that have volumes smaller than 1.5 nl. An egg volume of 1.5 nl represents in Ch. thummi the lower limit fo r those volumes that are sufficient for a normal embryogenesis. Mortality i ncreases with decreasing egg size, reaching 100% in backcross eggs with vol umes of 0.99 nl and smaller. Small egg size is a new trait of the Rud syndr ome affected thummi female x piger male hybrids. This trait is part of a po stzygotic reproductive isolation barrier between thummi and piger and manif ests first in the backcrosses. Most backcross eggs show volumes between 1.5 nl and 2.99 nl. Within this volume range the amount of mortality does not depend upon egg volume. Here, embryo death is great in the backcrosses but normal in the parental strains. The high frequency of embryo death in the b ackcrosses must be predominantly due to the action of the Rud syndrome and a second hybrid syndrome, called HLE syndrome. Since further characteristic traits of these syndromes could be detected in surviving backcross individ uals, the study demonstrates the occurrence of the syndromes in this genera tion also. Therefore, the postzygotic reproductive isolation mechanism of b oth hybrid syndromes is effective in the hybrids and in their progeny as we ll.