NOTES AND LITERATURE 



SWINGLE 1 ON VAKIATION IN F x CITRUS HYBRIDS 

 AND THE THEORY OF ZYGOTAXIS 



Swingle in two recent papers has published some very inter- 

 esting observations on Citrus species and their F x hybrids. On 

 the basis of these observations, the somewhat startling statement 

 is made that current theories of heredity and variation give no 

 adequate explanation of variability in F x hybrid generations 

 from ' 1 pure bred" parent strains. Swingle assumes this vari- 

 ability to be so great that qualitative differences in chromosomes 

 can not account for it, As the chromosomes in the F x hybrid 

 remain unfused until synapsis, there is said to be no opportunity 

 for quantitative exchange of hereditary substance, so that this 

 variation can not be accounted for on this basis. Hence, 

 if proof can be given to show that in certain specific cases, pairs of 

 gametes of identical hereditary composition 2 give rise to very diverse 

 organisms, the way has been opened for a general reinvestigation of the 

 validity of our modern theories of heredity. 



The term "pure bred" as used by Swingle implies that cer- 

 tain Citrus species reproduce themselves in a relatively faithful 

 manner from seed, there being no overlapping of distinguishing 

 specific characters and very little variation of these characters 

 intraspecifically. C. aurantium and C. trifoliata are examples 

 of such widely separated species. The former has been grown 

 from seed in Florida for two hundred years, and though varia- 

 tions have appeared, they are said to differ but little from the 

 general type of C. aurantium, and in no way to approximate 

 that of C. trifoliata. 



On the basis of evidence of this kind, Swingle believes the 

 various Citrus species (C. aurantium, C. trifoliata, C. medica 

 Umonum, etc.) breed true in nearly all their characters and 

 especially in those which differentiate them from one another. 

 Hence, for genetic studies, the germ cells of these species are 



Swingle, W. T., "Variation in First Generation Hybrids (Imperfect 

 Dominance): Its Possible Explanation through Zygotaxis," IV Conf. In- 

 ternal de Genetique, Paris, 1911, pp. 381-394; "Some New Citrus Fruits," 

 Amer. Breed. Ma#., 4: 83-95, 1913. 



2 The italics are my own. 



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