GENETIC STUDIES ON DROSOPIIILA VIRILIS. 



I. INTRODUCTORY. 



GENERAL INTRODUCTION. 



In most groups of animals and plants, including those susceptible 

 to genetic study through intensive breeding, the chromosome groups 

 of related species show a high degree of uniformity. The different 

 members of a genus, for instance, usually differ relatively little in 

 this regard. In some cases the uniformity may extend even to sub- 

 families or families, as, for example, in the Acridida? among the 

 Orthoptera, where all of approximately 40 genera studied agree 

 in having essentially the same chromosome group (cf. McClung, 

 1914; Harvey, 1916). It would seem probable, a priori, that 

 where many related species exhibit such a constancy in chromosome 

 groups, the apparent homology of chromosomes is real, i. e., similar 

 chromosomes in different species are essentially alike in genetic 

 make-up. On the other hand, there are various exceptions to the 

 general rule of constancy, and a number of cases are known in which 

 closely related species have very dissimilar chromosome groups. 

 Hence it may be argued that even where a constancy exists it may 

 be superficial and not dependent upon, or significant of, a likeness 

 in genetic constitution of similar chromosomes. In fact, very little 

 is known as to how far morphological criteria are trustworthy as 

 indications of homology between chromosomes. In one case recently 

 investigated by Lancefield and Metz (1921, 1922), the results 

 indicate that genetic homology does not correspond to morpho- 

 logical similarity as regards two pairs of chromosomes in Drosophila 

 willistoni compared with two similar pairs in D. vielanog aster. 



These and other considerations serve to emphasize the necessity 

 of learning something of the genetic constitution of chromosomes 

 before they can safely be compared from the evolutionary standpoint. 

 It is believed that the only method of obtaining rehable information 

 on chromosome evolution is by means of genetic analysis combined 

 with cytological observation. 



Ideal material for such a study would be provided by a group of 

 species satisfying the following four requirements: In the first 

 place, it should exhibit among its members a series of different 

 chromosome groups; secondly, the species should be susceptible to 

 intensive breeding under controlled conditions; in the third place, 

 one or more of the species should be favorable for genetic analysis 

 through the study of mutant races; and lastly, the species should 

 hybridize with one another and give fertile hybrids. 



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D. H. HILL LIBRARY 

 North Carolina State College 



