Section 7 — Cytogenetics 



to the zero level — may not be detectable, because 

 of the masking action of the duplicates at other 

 loci. Harland W suggested that a duplicated gene 

 could escape this masking effect by mutating to 

 an allele with a divergent function. An example 

 of this kind of mutant may be Neatby's virescent 

 (v) on chromosome 3B (III) of common wheat, 

 a hexaploid. 



Increase in dosage of v from 2 to 3 results in 

 greater abnormality (less chlorophyll), whereas 

 increased dosage of the normal allele For either 

 of the homologous (related) chromosomes 3A 

 (XII) and 3D (XVI) shifts the phenotype toward 

 normal. This suggests that v is antimorphic to V 

 and that F(or VI) has duplicates, V2 and V3, on 

 chromosomes 3A and 3D. 



That V3, and therefore presumably VI and V2, 

 are involved in chlorophyll production was 

 shown by inducing a deficiency for vl and com- 

 bining this with nullisomes 3A and 3D, respec- 

 tively, thus reducing the dosage of V from the 

 normal 6 to only 2. Nullisomic-3D plants (V2V2) 

 had normal chlorophyll, whereas nullisomic-3A 

 plants (V3V3) were of reduced chlorophyll 

 content. Thus V2 is more potent than V3 in 

 promoting chlorophyll development. On the 

 other hand, V3 is more effective than V2 in 

 reducing the expression of v. Although v may be 

 a mutant with a divergent function, another ex- 

 planation is possible; namely, that it is less 

 efficient than V in producing chlorophyll but 

 more efficient in competing for substrate. 



1. Biol. Rev. 11, 1936. 



7.22. Induction of Autosomal Crossing Over by cc- 

 heterochromatin and Demonstration of Chro- 

 mosome Interference in Phryne cincta. B. E. 



Wolf (Berlin, Germany). 



Investigating the internal causes of crossing 

 over in the euchromatic chromosomes of Phryne 

 cincta the compact- or cc-heterochromatin was 

 recognized to be a decisive factor. Obviously the 

 so-called "a-quantum difference", i.e. the differ- 

 ence in the amount of cc-heterochromatin (or 

 DNA) between the homologues of the X-chro- 

 mosome, is causally related to the crossing-over- 

 process ("cc-differential-effect" ; Wolf, Chromo- 

 soma, 13, 646, 1963). It was supposed, that by 

 way of a potential function it effects crossing 

 over not only within the "a-differential" tetrad 

 but also in nonhomologous and unrelated pairs 

 of chromosomes in the same meiotic cell. 



This hypothesis could be ascertained in cyto- 

 logical crossover studies by means of either two 



independent inversions coupled in autosome 2 

 and in autosome 3. Both pairs of inversions per- 

 mitted to establish the influence of the "oc-quan- 

 tum-difference" in the X on crossing-over- 

 frequency in the autosomes. Brother and sister 

 males and females heterozygous for one pair of 

 the inversions or for both together, furthermore 

 characterized by particular X-chromosomes or 

 X-chromosome-combinations (a/Y, b/Y, c/Y in 

 the male and a/b, b/b, c/b in the female: a and c 

 with much cc-heterochromatin, b with less), have 

 been backcrossed to homozygous animals of the 

 standard stock (b/b resp. b/Y, free from in- 

 versions). 



In the progeny of the males only the parental 

 inversion-combinations in the autosomes 2 and 

 3 were found (n = 356 and 270 resp.). In the test 

 cross progeny of the females the expected types 

 of recombination appeared: with only one in- 

 version in autosome 2 resp. 3 or simultaneously 

 in both autosomes. There appeared a very high 

 difference in linkage dependent on the present 

 X-chromosome-combination in the mother. Fe- 

 males homozygous for the X-chromosomes (type 

 b/b) exhibited only little recombination in chro- 

 mosome 2 (3.0 per cent, n = 430) and no in chro- 

 mosome 3 (« = 318) though the tested segments 

 approximately involve a third of the total length 

 in both chromosomes. Sister-individuals due to 

 the presence of "a-differential" X-chromosomes 

 (type a/b or c/b) gave a high rate of recombina- 

 tion in chromosome 2 (31.0 per cent, /; = 434) 

 as well as in chromosome 3 (11.4 per cent, 

 n = 784). 



Interchromosomal effects have been detected 

 by observation of crossing over in the autosomes 

 2 and/or 3 and in the X-chromosome simultane- 

 ously. The frequency of contemporary crossing 

 over in both autosomes as well as in the X and in 

 one of the autosomes, based on independent 

 occurrence, is significantly reduced. The results 

 represent a cytological parallelism to genetic 

 data on Drosophila of Schultz (1933) and Schultz 

 and Redfield (1951). 



7.23. Inherited Partial Sterility in Habrobracon. 



R. C. von Borstel (Oak Ridge, U.S.A.). 



Inherited partial sterility has been shown to be 

 the consequence of meiotic segregation of normal , 

 translocated, and duplication-deficiency chro- 

 mosomes in individuals heterozygous for a trans- 

 location. In the wasp Habrobracon, these can be 

 detected by hatchability methods and confirmed 

 by outcrossing and progeny-testing surviving 

 haploid males from females giving low hatch- 

 ability. One-half of the surviving males should 



124 



