Section 8 — Cytatoxonomy and experimental taxonomy 



cific D and E chromosomes in each derivative 

 has not been determined. 



If this phenomenon has occurred in nature it 

 may explain the difficulty encountered in deter- 

 mining the exact parentage of such naturally 

 occurring allopolyploids as Triticum aestivum. 



8.15. DNA and Wheat Ancestry. H. Rees and W. I. C. 



Davies (Aberystwyth, Great Britain). 



It is generally agreed that the AA chromo- 

 somes found in the AABB tetraploid and 

 AABBDD hexaploid wheats are derived from 

 Triticum monococcum or a closely related diploid 

 species, and that the DD chromosomes are de- 

 rived from Aegilops squarrosa. There is some 

 doubt concerning the source of the BB chromo- 

 somes. The chief candidates are Ae. speltoides, 

 Ae. bicornis and Agropyron triticeum. From an 

 analysis of the DNA content of nuclei in the 

 polyploid and diploid species our evidence shows 

 that Ae. speltoides is the most likely contributor 

 of the BB chromosomes. Its nuclear DNA con- 

 tent added to that of T. monococcum (AA) agrees 

 with the values found in AABB (T. durum), and 

 in combination with AA and DD values agrees 

 exactly with the DNA value for AABBDD (T. 

 aestivum). The DNA values for Ae. bicornis and 

 A. triticeum are, compared with that for Ae. spel- 

 toides, too high and too low respectively. 



8.16. Cytotaxonomic Studies in the Genus Agropyron 

 Gaertn. Jurgen Schulz-Schaeffer, Peter 

 Jurasits, Helmut Lorenz and Penelope W. 

 Allderdice (Montana, U.S.A.). 



Within the framework of a biosystematic in- 

 vestigation in the genus Agropyron studies of 

 cytological characteristics are being carried out 

 for taxonomic purposes. Karyotypes of 33 Agro- 

 pyron species were determined from root tip 

 cells and were compared. In general, centro- 

 meres are median or submedian. Agropyron 

 triticeum (2/7 = 14) and A. orientate (2n = 28) 

 are exceptions to this rule. Nuclear organizer 

 chromosomes were compared. According to an 

 evolutionary concept, one nucleolar organizer 

 chromosome should be present for each basic 

 genome. This was almost averaged in previous 

 studies of the genus Bromus. Only about half of 

 the expected number were observed in Agro- 

 pyron. There seems to be a considerable difficulty 

 involved in detecting certain nucleolar organizer 



chromosomes in mitotic metaphase. Investiga- 

 tions in meiosis are performed to supplement. 

 Eighty-eight nucleolar organizer chromosomes 

 were observed in 160 genomes. They can be 

 classified as 11 different types. One type (F-l, 

 F-2) occurs in 19 of the 33 species. Type F seems 

 to represent one basic genome which probably is 

 common to a great number of Agropyron species. 

 The nucleoli as a possible measure of nucleolar 

 organizer chromosome number were determined 

 by the Rattenbury-method for 5 species. The 

 average number was, in general, corresponding 

 with the number of basic genomes. An average 

 of 4 nucleoli was found for tetraploid A. deser- 

 torum. This may confirm unpublished evidence 

 from coworkers that A. desertorum has a certain 

 number of nucleolar organizer chromosomes. 



8.17. Karyotypic Studies of South American Snakes. 

 W. Becak:, M. L. Becak: and H. R. S. Naza- 

 reth (Sao Paulo, Brazil). 



The sex elements can be readily identified by 

 cytological means in the chromosome comple- 

 ments of most mammalian and avian species. In 

 the lower vertebrates, however, the Z and W, or 

 X and Y, may still be so undifferentiated as to be 

 morphologically indistinguishable. 



Karyotypic studies have been made of two 

 species of South American snakes, Boa constric- 

 tor amarali and Bothrops jararaca. Both have 36 

 chromosomes: 16 macrochromosomes and 20 

 microchromosomes. 



In Boa constrictor amarali the centromere is 

 almost median in the first, third and fourth 

 pairs, submedian in the second pair and sub- 

 terminal in the fifth, sixth, seventh and eighth 

 pairs. Neither quantitative evaluation nor direct 

 observation of the karyotypes revealed the pres- 

 ence of a heteromorphic sex pair in either the 

 male or female of this species. 



In the male of Bothrops jararaca, the first, 

 third and fourth pairs have submedian centro- 

 meres and the sixth and eighth pairs have sub- 

 terminal centromeres. In the female, however, 

 the fourth pair is distinctly heteromorphic. One 

 element is much smaller than the other and has 

 a subterminal centromere as well. Thus in this 

 species the fourth largest pair of chromosomes 

 appears to be the sex pair; the female is the 

 heterogametic sex, and the sex-determining mech- 

 anism is the ZZ-ZW type. The fact that the W 

 chromosome appearing only in the heterogametic 

 female is seven-tenths the size of the Z, permits 

 cytological identification of the sex chromosomes 

 in Bothrops jararaca. 



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