DEPARTMENT OF GENETICS. 95 



DETAILED REPORTS ON INVESTIGATIONS IN PROGRESS. 

 INTERCHROMOSOMAL MUTATION. , 



The work of the past year has extended the studies on mutations in the 

 jimson weed (Datura) due to variations in chromosome number and probably- 

 due in turn to abnormahties in the process of formation of the gametes. 

 This work has become so broad as to require the cooperative studies of a 

 number of persons and we have been so fortunate as to secure the collabora- 

 tion during the summer time of investigators who are connected primarily 

 with other institutions. Besides Dr. A. F. Blakeslee, who has charge of 

 these experiments, Dr. John Belling, who is making the principal cytological 

 studies and Mr. M. E. Farnham, who has immediate charge of most of the 

 work in the greenhouse and the field, we have had the cooperation of Professor 

 E. W. Sinnott, of the Connecticut Agricultural College at Storrs, who has 

 been studying the internal anatomy of the Daturas and as a result of that 

 study finds he is able to recognize the majority of the mutants from an exami- 

 nation of their tissues; of Dr. John T. Buckholz, who has been studying, with 

 the use of a newly perfected method, the growth of pollen-tubes and abortion 

 of ovules as problems in developmental selection. Miss Dorothy Bergner 

 and Miss Lois Lampe have helped respectively in making chromosome counts, 

 in counting pollen, and assisting in hybridization. Mr. J. L, Cartledge has 

 again acted as summer recorder. 



In addition to the securing of further data upon the breeding behavior of 

 types mentioned in the Year Book for 1921, pages 104 to 107, work has been 

 done on certain new types which have been discovered. These may be 

 treated collectively under the heads of balanced and unbalanced types. 



Balanced Types of Datura Mutants. 



One of the most fundamental doctrines of genetical biology is that the 

 germ-cells before union contain half of the number of chromosomes of the 

 fertilized egg or developing embryo (zygote). It has been assumed that, 

 except in the case of parthenogenetic species, the soma must contain chromo- 

 somes in sets of two, one of paternal and one of maternal origin. One of the 

 most notable discoveries of the year was the finding of five haploid or In 

 plants, which appeared in the offspring of parents treated with cold in an 

 attempt to induce mutations by external stimuli. 



They were early recognized as new forms by external appearance, but more 

 definitely by the condition of the pollen. The 12 chromosomes in their 

 pollen mother-cells undergo a pseudo-reduction into 8+4, 7 + 5, etc., a process 

 which suggests that a paired condition of the homologous chromosomes is 

 not necessary for the reduction division in gametogenesis. Occasionally a 

 pollen mother-cell fails to undergo reduction, and normal In. grains are pro- 

 duced which apparently function like pollen from a normal diploid (2n). 

 The few seeds we have obtained this season from selfing our haploids (In) 

 have given rise to diploids (2n) which are of considerable genetic interest, 

 since they furnish a new method of rendering a stock homozygous without 

 long inbreeding. Coming from plants with but a single chromosome in each 

 chromosomal set, the paired chromosomes in each set of these diploids must 

 be identical. Hence, barring new mutations, diploids derived from a given 

 haploid must be completely homozygous, as alike as identical twins. 



