DEPARTMENT OF GENETICS. 91 



"The effects of chromosome differences upon the biochemical product 

 should soon be studied. For such work we are getting together what we 

 believe is the most nearly comparable material ever used in plant breeding. 

 From our first haploid (plant A) in line 1, we have obtained diploids by non- 

 reduction which (barring new mutations) must be absolutely homozygous. 

 It has been our effort ultimately to get all our mutants into this homozygous 

 lA hne and we have already succeeded with the tetraploid (4n) and the 

 majority of the main (2?r4-l) mutants. We are now crossing the 4n with 2n 

 plants in this Hne and should within a few months have triploids (3n) which 

 should throw the main mutants. The latter, with sufficiently large numbers 

 of offspring, should give rise to the mutant varieties. 



"We are obviously in need of Mendelian characters with which to tag the 

 individual chromosomes and parts of chromosomes and are trying to locate 

 in their respective chromosomes some 6 or 8 Mendelian factors which we have 

 isolated from our main lines or from new mutations. We have succeeded in 

 hybridizing our Datura stramonium with the distinct species Datura ferox, and 

 in later generations hope to isolate a number of Mendelian characters from 

 this species cross. In addition, we are planning to sow seed from a large 

 number of selfed parents in line lA, with the hope of getting new gene 

 mutations recognizable in the seedpan," 



Other Mutant Types in Datura. 



The occurrence of haploids has been shown to be probably due to a process 

 of true parthenogenesis of the reduced egg. An inermis haploid was obtained 

 from a cross between a female parent heterozygous for armed and inermis 

 and a pollen parent homozygous for armed. Further, two haploids have 

 arisen from the cross D. stramonium X D. ferox and resemble in all visible 

 respects the D. stramonium parent. In these three cases apparently the male 

 gamete did not enter in any way into the constitution of the haploid offspring. 

 Parthenogenesis has been observed not infrequently in tetraploids when they 

 are crossed with pollen from diploids. By using recessive white tetraploids 

 as female parents and pollen from purple diploids, the white diploids which 

 resulted could be assumed to have originated from the tetraploid parent 

 alone. The purple offspring from this cross have all been triploid. 



It has previously been shown that in addition to the usual disomic ratios 

 obtained when the chromosomes are in twos in the set heterozygous for a 

 given factor, distinct trisomic ratios are obtained when these chromosomes 

 are in threes, as in (2n+l) mutants; and tetrasomic ratios when they are in 

 fours, as in tetraploids. This season we have obtained the first pentasomic 

 ratio from a (4/1+1) Poinsettia parent heterozygous for purple and white 

 flower-color. 



The findings of the past year show that chromosomal aberrations are a 

 not unexpected cause of sectorial chimeras in Datura and may account for 

 the reported bud sports in other species. Four plants have been found in 

 our cultures with a branch which bore leaves of a distinct character. In two 

 of these cases. Dr. Belling has found the abnormal branch to be a chromosome 

 deficiency of the type (2n— 1), so far as the pollen mother-cells is concerned. 

 The two other plants have not as yet been studied cytologically, but from 

 appearance can be provisionally placed in the same category. The deficiency 

 does not appear to be transmitted to the offspring. The deficiencies just 

 mentioned have been parts of a plant otherwise apparently normally diploids. 

 A single plant has been discovered, however, all the branches of which bear 



