94 CARNEGIE INSTITUTION OF WASHINGTON. 



Non-disjunction in Tetraploid Plants. 



The occurrence of non-disjunction in tetraploid plants offers an opportunity 

 for great variation in chromosome number in the progeny. On this matter 

 Dr. Belling reports as follows: 



"The chromosome distribution has now been examined in over 100 true 

 tetraploid Daturas, including special studies of 55 plants of one sibship and 17 

 plants of another. No plants give a constant distribution of 24+24 chromo- 

 somes. All show more or less of a segregation into 23+25, etc. The propor- 

 tions in which this apparent non-disjunction takes place conform, in different 

 individuals, more or less to the laws of random sampling varying around a 

 mean of approximately 25 per cent of the 23+25 distribution and about 2 per 

 cent of the 22+26. The most reliable results agree with a ratio of regular 

 disjunction (2+2) to non-disjunction (1+3) of 35 : 1, for each quadrivalent or 

 set of four. 



"Hence a regular tetraploid, with 12 sets of 4 homologous chromosomes each, 

 should give a certain proportion of 47 and 49 chromosome plants in its 

 progeny. The two sibships above mentioned gave 73 plants with 48 chromo- 

 somes, 6 plants with 49 chromosomes, and 3 plants with 47. This is consider- 

 ably less than the numbers of 47 and 49 chromosome plants which would be 

 produced if all the 23 and 25 chromosome gametes were functional, and 

 points to an abortion or failure in competition of those gametes or zygotes 

 with excesses or deficiencies of chromosomes. 



"If two gametes with 23 and 25 chromosomes, respectively, combine, the 

 resulting 48-chromosome plant will usually have 10 sets of 4, 1 set of 3, and 

 1 set of 5 homologous chromosomes. Two such plants have been tested by 

 growing their progeny. By calculation they should give 7 pollen mother- 

 cells dividing 24+24, to 8 dividing 23+25, to 1 dividing 22+26, etc. This 

 they did approximately, giving the proportion 6 : 9 : 1. The progeny, sup- 

 posing only 24-chromosome pollen functional, and all egg-cells viable, should 



give plants with 46, 47, 50 chromosomes in the proportion 



1 : 8 : 14 : 8 : 1. The proportion found was 1 : 12 : 20 : 3 : 1. 



"In the progeny of such a pseudo-tetraploid, there should be some cases 

 where two gametes meet whose sum amounts to 48, but which produce a 

 plant having 8 sets of 4 chromosomes, 2 sets of 3 chromosomes, and 2 sets of 5 

 chromosomes. It may be calculated that such a plant would have a distribution 

 of chromosomes in the pollen mother-cells of about 14 cases of 24+24, 17 of 

 23+25, 8 of 22+26, 1 of 21+27, etc. Two such plants have been found. In 

 one case the chromosomes have been investigated, and 2 sets of 3 and 2 sets 

 of 5 were found. These two plants gave the expected high ratios of the 22+26 

 and 21+27 distribution. 



"Plants with 47 chromosomes that have originated from selfing a pure 

 tetraploid should have usually 11 sets of 4 homologous chromosomes, and 1 

 set of 3. These plants should give a distribution of chromosomes, at the first 

 division, of about 7 cases of 23+24, to 1 case of 22+25, etc. The progeny of 

 one such plant was studied. It gave among its gametes the proportion of 7.5 

 cases of 23+24 to 1 case of 22-f-25. The progeny, if the parent's 24-chro- 

 some pollen is solely or mainly functional, should give 1 plant with 46, 7 

 plants with 47, 7 plants with 48, and 1 plant with 49 chromosomes, out of 16 

 progeny. The actual ratio was 1:7:9:0. 



"A plant with 50 chromosomes, in which, there was reason to think from 

 appearance, breeding, and microscopical evidence, there were 11 sets of 4 



