Non-reciprocal gonadal dysgenesis in hybrids of the chironomid midge Chironomus thummi. III. Germ line specific abnormalities.

In male hybrids of the cross Chironomus thummi thummi (stock Hl) females x Ch. th. piger (stock E) males, but not the reciprocal cross, rudimentary testes develop at a growth temperature of 21 degrees C. Within the dysgenic testes of these hybrids a number of abnormalities are observed which are restricted to the germ line. Approximately 60% of the hybrid males show allocyclic chromosome behaviour in spermatogonia and spermatocyte I nuclei. Within these nuclei two groups of four chromosomes are formed which differ from one another in their state of condensation. Each chromosome group consists of three long and one short chromosome. In cases where allocycly is very pronounced, the chromosomes of both groups disintegrate into numerous unequally sized fragments at meiotic prometaphase I, and gametes are not produced. In individuals, in which the allocycly is less pronounced or absent the nuclear divisions appear to be normal but chromosome and chromatid aberrations are frequent, and the number of viable sperm is reduced. In these males, the chiasma frequency is also decreased more than 12-fold in comparison with the reciprocal, unaffected piger females x thummi males hybrids.