SECTION 6 



CYTOLOGY 



6.1. Heteropycnosis and Sex Chromosomes in Mosses. 



Lewis E. Anderson (Durham, U.S.A.). 



Heterochromosomes have been studied and 

 figured in a large number of species of mosses. 

 Invariably these heterochromosomes are hetero- 

 pycnotic and are represented in interphase 

 nuclei as conspicuous darkstaining condensed 

 chromosomes that remain in a tightly coiled 

 condition. Heterochromosomes have been 

 postulated to represent sex chromosomes in 

 mosses, although there has been no direct 

 evidence that this is the case. The present paper 

 presents evidence that the heterochromosomes of 

 mosses are sex-associated. The heterochromo- 

 somes are represented in interphase nuclei by 

 heteropycnotic bodies of unequal size. In 

 sporophytic tissue, a large and a small hetero- 

 pycnotic body is distinguishable; in gametophytic 

 tissue of the moss plant, archegonial plants pos- 

 sess nuclei with the large heteropycnotic body 

 while antheridial plants possess nuclei with the 

 smaller heteropycnotic body. It is probable, 

 therefore, that the heterochromosomes re- 

 present X and Y chromosomes. Much of the 

 work reported upon was carried out using several 

 species of the genus Anomodon. 



6.2. A Genetic Test of the Chromosome Replication 

 Pattern in Maize. G. M. Greenblatt (Shore- 

 wood, U.S.A.). 



Previous studies of twinned red-light variegated 

 mutations in the pericarp of maize (Greenblatt 

 and Brink, 1962, 1963) have shown that Modu- 

 lator {Mp) can transpose from the P locus (cen- 

 trally located on the short arm of chromosome 

 one) at the time /'and its conjoined Mp replicate, 

 and that the receptor sites of Mp may be 

 either replicated or unreplicated. In the latter 

 case Mp replicates a second time. Each of these 

 two possible events appear as distinguishable 

 twin mutations. An analysis of the sites to which 

 Mp transposes from the P locus provides a test 

 of the pattern of replication of this chromosome 

 arm. By means of a three-point linkage test the 

 sites to which Mp transposes were measured in 

 the light variegated sector in both classes of twin 



mutations. The results show that the receptor 

 sites of Mp located on the short arm of chromo- 

 some one, at which a second replication takes 

 place, are not randomly distributed between 

 proximal and distal locations; nine cases of extra 

 replication were at distal sites while only one was 

 at a proximal site. In those cases where Mp did 

 not replicate a second time, four were at proximal 

 sites and one was at a distal site (the very end of 

 the chromosome arm). This data strongly sup- 

 ports the hypothesis that the replication pattern 

 on the short arm of chromosome one is highly, 

 but not completely, polarized; the proximal 

 regions replicate prior to the distal regions. 



6.3. Bimitosis. G. Gimenez-Martin, A. Gonzalez- 

 Fernandez and J. F. Lopez-Saez (Madrid, 

 Spain). 



In this work the mitosis within binucleate cells 

 of onion root-tip is studied. For this synchron- 

 ous mitosis the denomination of bimitosis is 

 proposed. The binucleate cells, an uncommon 

 condition, are a result of a treatment with 

 HCCH (y Hexa Chloro Cyclo Hexano). The two 

 nuclei can be either both diploid or one hypoploid 

 and the other hyperploid. At the same time, they 

 can be completely independent or united by 

 chromosomic bridges. 



Many times, these binucleate cells lack com- 

 pletely the middle lamella between the nuclei. 

 Other times, the cytoplasmic continuity is main- 

 tained only through chromosome bridges, 

 existing in incomplete middle lamella. 



Always when a connection existed between the 

 two nuclei the two mitoses were synchronous. In 

 this bimitosis both mitoses had either the same 

 axis or perpendicular directions. The two mitoses 

 were synchronous even when one nucleus was 

 hyperploid and the other hypoploid. 



The mitotic apparatus showed the same 

 characteristics as in the normal mitosis. The 

 separation between the poles is similar to the 

 normal, although the cytoplasmic space was 

 sometimes wider. 



At last, the interactions between both nuclei 

 are discussed. 



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