Section 6 — Cytology 



between the amounts of DNA and RNA in the 

 chromosomal regions, the precise quantitative 

 relationship between DNA and RNA in the 

 nucleoli themselves is still under investigation. 



1. McLeish, J. (1963) Proc. Roy. Soc. B (in the 

 press). 



6.28. Chiasma Distribution at Normal and Elevated 

 Temperatures in a Locust. S. A. Henderson 



(Cambridge, Great Britain). 



Using the technique of directly measuring 

 chiasma position along each chromosome, an 

 analysis of the normal chiasma distribution has 

 been made for all members of the complement of 

 the desert locust, Schistocerca gregaria, where a 

 wide range of chromosome size is present. On 

 subjecting this species to high temperature 

 treatment the pattern of chiasma distribution is 

 greatly modified. Reduction in chiasma frequency 

 is accompanied by a change in the position occu- 

 pied by those chiasmata which do form. This 

 shift, to the chromosome ends, may culminate in 

 univalence. Chromosomes of different sizes differ 

 in their responses to heat-treatment. 



very typical configurations ensue whereby the 

 tips and often also the proximal regions attach, 

 the more movable parts being strongly displaced 

 in the centrifugal direction. The more indifferent 

 parts can be displaced at very low speeds, e.g. 

 1000 g, especially in interphase and early pro- 

 phase, less well in later prophase when condens- 

 ing chromosomes become rigid but in prometa- 

 phase they are deposited in toto. Breaks of chro- 

 mosomes with tips still anchored are found and 

 also indentations in the NM at points where tips, 

 especially many of them, attach. In most plants 

 studied there is no bouquet orientation. Just 

 prior to pairing the chromosomes detach them- 

 selves and then pair immediately. In early pachy- 

 tene the "standard" situation is resumed, i.e. 

 reattachment. The bouquet of Tradescantia and 

 Liliaceae is complicated: the chromosomes be- 

 have as if they were totally loose. Yet, there 

 seems to be a very weak bond — they seem to glide 

 towards the bouquet arrangement as has been 

 observed by others before. Pachytene shows 

 attachment of the tips. Condensed chromosomes 

 in diakinesis are peripheral in toto, as is well 

 known. The attachment is firm in pachytene but 

 in most cases will become strikingly labilized at 

 diplotene though they seldom detach themselves. 

 Nonhomologous associations, if present, to some 

 extent interfere with the pachytene reattachment. 



6.29. Some New Principles Governing Chromosome 

 Pairing — The Spatial Relations of Chromosomes 

 and Nuclear Membrane. K. Pusa (Helsinki, 

 Finland). 



It has been shown by others that in many 

 instances in the soma the chromosomes attach 

 very firmly to the nuclear membrane (NM) by 

 their (1) heteropycnotic regions or (2) proximal 

 or (3) distal parts. This has been demonstrated 

 by micromanipulation or ultracentrifugation 

 which thereby produces characteristic configura- 

 tions. There are many observations on mere 

 attachment by direct inspection of parts (2) and 

 (3) and the situation of (1) is well known. The 

 questions are: Can those findings be generalized? 

 How is it just before and at chromosome pairing? 



A study was undertaken on a number of flower- 

 ing plants, mainly Cruciferae (conspicuous heter- 

 opycnosis at centromeric regions), Tradescantia, 

 Liliaceae and Plantago as well as scattered 

 samples from various families. Centrifugation, 

 both normal and ultraspeeds (up to 150,000 g) 

 were employed. In all cases, in the archespore 

 and early PMC's before pairing the situation is 

 the same as in the surrounding parenchyma: 



6.30. Chromosome Pairing at Meiosis. C. R. Burn- 

 ham (St. Paul, U.S.A.). 



Previous studies (M. Tabata in maize and K. 

 J. Kasha in barley) indicate that pairing is initia- 

 ted at or near the ends of chromosomes and not 

 at the centromeres. This information has come 

 from hybrids between stocks of interchanges in 

 which the breaks in both chromosomes were in 

 opposite arms in the two parental interchanges. 

 In the previous studies in maize the total length 

 of the "between-breaks" segments was equal to 

 or less than the total length of the interchanged 

 segments. In the combinations now planned the 

 total length of the "between-breaks" segments 

 will be greater than that of the interchanged 

 segments. 



Observations (O. L. Miller) on asynaptic (as 

 as) plants in maize show that the members of 

 partially synapsed pairs are associated at the 

 centromeres. The two seemingly divergent obser- 

 vations can be reconciled if it assumed that in 

 as as maize plants pairing at the ends of the chro- 

 mosomes is prevented, but that this may or may 

 not extend to the remainder of the two homo- 

 logues. 



110 



