Kontrolle des crossing over vom temperatur-bedingten Allozykliegrad und vom α-Heterochromatin des X-Chromosoms bei Phryne cincta

Summary

• 1. In populations of the dipteran Phryne cincta (2n = 6+XX♀, XY♂) several X-chromosomes of different structure are present. These are allocyclic in contrast to the (eucyclic) autosomes, as seen in their behaviour in mitosis and meiosis and in their varied appearance in polytene nuclei (comp. Wolf 1950, 1957).

• 2. The polytene autosomes (2–4 s. Pig. 7) differ from each other in easily recognizable characters and form no chromocenter. The polytene X-chromosome is always relatively much shorter than the autosomes and more or less β-heterochromatic (granular or indistinctly banded) in larvae, raised at room temperature. In larvae raised at low temperature (0–4° C) it passes through a morphogenesis, growing to normal or nearly normal length and assuming the aspect of an euchromatic element (comp. X _a in Figs. 3 and 4).

• 3. Two stocks, characterized by different X-chromosomes, symbolized by X _a and X _b or a and b respectively, were chosen for the experiments. In mitosis both these X-chromosomes are rod-like, a is as long as autosome 4, b has only half its length (Figs. 1, 2a–d).

• 4. Just before pupation, in polytene nuclei of larvae raised at low temperature a and b are of almost equal length. They differ from each other in several inversions and in the appearance of two large α-heterochromatic bands in a (α₂ and α₃, Fig. 5; “α-quantum-difference”).

• 5. The α-bands, especially the large block α₂, show a considerable intercellular variation in their relative DNA-content (measured as length of the α-element). The mean DNA-content is negatively correlated with the degree of stretching of the chromosome (Table 1).

• 6. In polytene nuclei of female hybrids from the crosses aaxbY or bbxaY, a and b are unpaired or incompletely paired in variable degrees (Figs. 7, 8a–e). The heterozygotic chromosome pair can be devided into 5 segments (I–V, Fig. 6), three of which are marked by clearly defined structural differences (I, III, V).

• 7. The pairing-segments II and IV, exhibiting an identical and parallel sequence of discs in a and b, allow six crossing-over recombinations (cb, db, eb, fb, gb, hb; comp. Figs. 9, 10). These new types of X-chromosomes occur in very different frequencies in the progeny of the hybrid ♀♀ when backcrossed to standard ♂♂ bY. The types eb and fb occur relatively rarely, while gb and hb are extremely rare (Figs. 9, 10, Table 2); in 70% of the progenies they are not present. The recombination-process is mainly restricted to pairing segment IV.

• 8. In a and in the crossover-chromosomes c, e, g very often a threadlike constriction appears in the region of α ₂, resulting mostly in a pseudofragmentation with more or less independent location of the two chromosome parts (Figs. 3d, 8f-i, 11a). A variety of kinds of fusion between the parts and the autosomes, especially in their α-heterochromatin, is possible (Figs. 3e, 8g–i).

• 9. Lowering the breeding-temperature increases the duration of development, the size of the larvae, and the lengthening and euchromatization of the β-heterochromatic parts of the X-chromosomes. In the hybrids it also results in increasing the recombination rate (Table 3 and 4).

• 10. In the main experiments, hybrid offspring from the same parental pair, were raised at different temperatures: some individuals at high (20–26° C), others at low temperatures (2–8° C). The resulting two series of imagos were kept in optimal or room temperatures (14–22° C). The female progeny of 23 hybrids raised at elevated temperature and of 27 hybrids raised at low temperature, i.e. 1766 and 2102 X-chromosomes respectively, have been tested (Table 5, Fig. 12). No or only little recombination occurred in hybrids raised at 20–26° C (mean = 2.4%) in contrast to their sisters raised at 2–8° C, which gave a relative high frequency (mean = 18,2%).

• 11. In these cases only the first egg batches of each hybrid ♀ have been evaluated. Later progeny exhibits a reduction in crossing-over frequency in cold-raised hybrids with growing age. The mean rate of recombination of 14 hybrids is lowered from 16,6% in the first to 7,1% in the following progeny (n=1813; Table 6, Fig. 15) presumably as a consequence of keeping the imago in higher temperature.

• 12. The results from several larger groups of cold-raised sister hybrids more or less clearly demonstrate a bimodal distribution of recombination values (Fig. 13) suggesting a difference in recombination potentiality between the two X-chromosomes in the mother of the hybrids. Some data indicate the recombination rate in the X-chromosome as negatively correlated with the rate of development (Table 7).

• 13. The Mendelian ratio, 1∶1, of the non-recombinant types, ab and bb, is found in the progeny of the warm-raised hybrids (1769∶1778). This ratio is shifted considerably in favour of type ab in the case of low temperature hybrids (1996∶1626). In the recombinant types, cb and db, the ratio is always markedly shifted in favour of cb, irrespective of temperature (712:343). There must exist a causal relation between the Mendelian anomaly and the recombination process the nature of which is yet unknown.

• 14. The temperature at which a larva is raised effects both the frequency of crossing over and the stretching and euchromatization of β-heterochromatin in the polytene X-chromosome. This parallele suggests that structural changes comparable to those in the polytene chromosomes are induced by low temperature in meiotic chromosomes as well, and that these changes are responsible for the effect on the crossing-over frequency. These changes, which are reversible by age or temperature, are supposed to cause the extention of the “effective pairing” (as in euchromatic segments) on β-heterochromatic elements, which normally are unpaired.

• 15. The so-called “α-quantum-difference”, i.e. the difference in the amount of α-heterochromatin (or in the “relative DNA-content”) between the X-chromosomes is considered to be causally related to the crossing-over process (“α-differential-effect”). By way of a potential function it effects crossing over not only within the “α-differential” tetrad but also crossing over in non-homologous and unrelated pairs of chromosomes in the same meiotic cell.

• 16. This hypothesis was ascertained by two independent inversions in autosome 2 (In 2a and In 2b, Fig. 16) which permitted to test the influence of the “α-quantum-difference” in the X-chromosomes on crossing over in the autosome. In preliminary studies the progenies of 8 females (ab, bb, cb) and 3 males (aY, cY) of the heterozygous F₁ backcrossed to homozygous b-stock have been investigated. In the progeny of the males only the parental combinations in autosome 2 were found. In the test cross progeny of the females the two expected types of recombination appeared (with only one inversion, In 2a or In 2b), but there existed a very high difference in linkage between bb- ♀♀ on the one side (3.6% crossing over) and ba- and bc-♀♀ on the other (40.2% crossing over).