Selection of thummi specific chromosome regions in the progeny of Chironomus thummi thummi × Chironomus thummi piger hybrids after long term culturing as a consequence of gonadal sterility

C-band differentiation between the chromosomes of two subspecies of the chironomid midge Chironomus thummi

The metaphase chromosomes of Chironomus th. thummi contain approximately 17% more pericentric C-band heterochromatin than the chromosomes of Chironomus th. piger with 11% heterochromatin. In Ch. th. thummi, the proportion of heterochromatin appeared to be much larger in metaphase chromosomes than in polytene chromosomes. This discrepancy is interpreted as being due to the specific chromosome organization and not as the result of an underreplication of heterochromatin during polytenization.

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.

Nonreciprocal gonadal dysgenesis in Chironomus thummi hybrids: temperature sensitivity of female sterility

In nonreciprocal hybrids of Chironomus thummi an environmental factor has been detected which, along with genetic factors, determines gonadal dysgenesis. Female hybrids of the cross Ch' thummi thummi female female x Ch. thummi piger male male show various degrees of rudimentary developed ovaries and sterility. The extent of these abnormalities is dependent on the developmental temperature of the hybrids. At a temperature of 21 degrees C approximately 90% of the females are completely sterile and at 16 degrees C only 30%. The curative effect of a temperature of 16 degrees C on sterility occurs, however, only in those hybrid females which hatch from a specific type of egg mass (class A). Females of another type of egg mass (class B) show nearly as many dysgenic ovaries as do those developed at 21 degrees C. At a developmental temperature of 21 degrees C no such differentiation between the A and B class of egg masses is possible. Ovarian dysgenesis and sterility is induced during a temperature-sensitive period which extends from the beginning of embryonic development through the first two-thirds of the first larva instar stage. The abnormalities observed must be due to a failure in the early development of the germ line and are probably initiated by an inhibition of primordial germ cell divisions.

Identification of a polytene chromosome band containing a male sex determiner of Chironomus thummi thummi

Hybrid males of Chironomus thummi piger female x Ch. th. thummi male crosses were backcrossed with females of both parental stocks. Fourth-instar larvae of these backcrosses showed sex specific differences in the pairing behavior of region D3d-g in chromosome arm F of salivary gland chromosome III. Analysis of the banding pattern of region D3d-g after RB and quinacrine staining demonstrated that in piger female x thummi male hybrid males a single selectively stained band occurs within this region in the heterozygous condition at map position D3e1. This band could only be found in the thummi chromosome partner, it is heterochromatic and contains AT-rich DNA. In female hybrid larvae, however, such a selectively stained band is present in neither the thummi nor the piger chromosome region D3d-g. From these results it is concluded that the selectively stained band D3e1 represents the male sex determiner of our Ch. th. thummi stock and that the male is the heterogametic sex.