Section 6 — Cytology 



6.31. Differential Rates of Meiotic Development in 

 Desynaptic Hordeum vulgare L. E. B. Wage- 

 naar (Ottawa, Canada). 



Cytological analysis of the first meiotic 

 division of desynaptic barley (var. Huskey, 

 2« = 14; homozygous for gene ds) produced 

 evidence that the developmental rate was not 

 equal for all metaphase cells. This differential 

 rate was apparently related to two cellular 

 characteristics which simultaneously affected 

 metaphase development. These characteristics 

 were the degree of irregularity and the chiasma 

 frequencies per bivalent. Cells with numbers of 

 bivalents approaching that of the normal con- 

 dition (7 bivalents), as well as cells with highest 

 average chiasma frequencies per bivalent, tended 

 to proceed most rapidly through metaphase I. 

 Possible underlying factors inducing the observed 

 developmental pattern are discussed. 



6.32. Cytogenetical Studies of Oryza sativa L. and 

 its Related Species. 5. Differential Conden- 

 sation and Chromosome Pairing in the Hybrid 



0. sativa X O. australiensis. H. W. Li, C. C. 



Chen, H. K. Wu and Katherine C. L. Lu 

 (Taipei, China). 



The chromosomes of O. australiensis were 

 made up partly of heterochromatin whereas 

 those of O. sativa were almost completely 

 euchromatic. 



In the Fi hybrid of O. sativa O. australiensis, 

 there was differential condensation in these two 

 morphologically different types of chromosomes. 

 The ones with partly heterochromatin and partly 

 euchromatin condensed early, starting off 

 presumably from pachynema, on till diakinesis. 

 The ones with only euchromatin seemingly 

 started their condensation later but condensed 

 more complete at first metaphase. Thus before 

 diakinesis, the australiensis chromosomes were 

 darker in staining, while at Ml-Al they were 

 2-4 times the size of the sativa chromosomes. 



At either diakinesis or MI-AI, about two 

 bivalents could be found per PMC. These 

 bivalents could be separated into two types, at 

 MI-AI by size, or by difference in having taken 

 up the stain at diakinesis: 



1. Autosyndetic. 



2. Allosyndetic (multivalents were also found 

 but very rarely). 



All these bivalents were proved to be authen- 

 tically true bivalents. The evidences were: 



1. There were loosely paired chromosomal 

 segments observed repeatedly in many PMC's at 

 pachynema. 



2. At diplonema, allosyndetically paired biva- 

 lents were found to have one chiasma mostly, or 

 two or more chiasmata in some PMC's. 



3. At diakinesis, these allosyndetically paired 

 bivalents were found to be ring-shaped as well as 

 end-to-end ones. 



4. Closed allosyndetic bivalents with two 

 chiasmata were frequently observed at MI-AI. 



Pairing of the homologous segments in these 

 allosyndetic pairs was assumed to be carried out 

 at the euchromatic regions of the two chromo- 

 somes concerned. Presumably, these euchromatic 

 regions of the chromosomes from two different 

 species might have the same rate of condensation 

 at various stages of meiosis. 



6.33. Meiosis of Luzula purpurea. H. Nordenskiold 

 (Uppsala, Sweden). 



Luzula purpurea belongs to the materials, the 

 chromosomes of which have diffuse or poly- 

 centric centromeres. Somatically it has six equal- 

 sized chromosomes. During early prophase the 

 chromosomes pair and chiasmata are formed. 

 The chiasmata are easily studied during diakine- 

 sis, and one or two per bivalent is the most 

 common number. They may be interstitial or 

 more or less terminalized at first metaphase. 

 During that stage each chromosome of the 

 bivalents is arranged in the equatorial plane, i.e. 

 is showing auto-orientation. 



The stage at which the individual chromatids 

 can best be studied is the transition between first 

 anaphase and telophase. At this stage the two 

 halves of each side of the equatorial plane always 

 form mirror-images when compared with each 

 other. This phenomenon illustrates the cytolo- 

 gically equational mode of the first meiotic 

 division. Relict chiasmata may occur between the 

 chromatids of the half-bivalents. This pheno- 

 menon demonstrates the fact that the two 

 chiasma-forming chromatids are able to move 

 towards the same pole. As chiasmata are gener- 

 ally assumed to be formed between non-sister 

 chromatids the separation of chromatids in the 

 points of the original exchange in such cases 

 ought to have occurred between the sister-chro- 

 matids. The separating half-bivalents are com- 

 posed of two chromatids which may be kept 

 together by the relict chiasmata or their cast 

 formations, and/or by the ends of the chromatids 

 belonging to the same original chromosome, the 

 terminal parts of which seem to separate at a 

 comparatively late stage in meiosis. At interki- 

 nesis the homologous chromatids of the half- 

 bivalents pair again and these pairs form two very 



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