Genetic Studies on Selective Segregation of Chromosomes in Sciara Coprophila Lintner.

HE unusual behavior of the chromosomes in Sciara has been studied T in considerable detail both cytologically and genetically. Although the main facts regarding the distribution and behavior of the chromosomes (as revealed by cytological examination) , have been known for some time, the genetic data have been incomplete because of the paucity of mutant characters with which to follow the history of the individual chromosomes in any one species. The present paper is designed to supply additional genetic data derived from an intensive study of Sciara coprophila Lintner, over a period of five years. Because of the great difficulty of securing mutant characters in Sciara, only a few have been obtained, even with the aid of X-rays, and some of these are unsatisfactory. Even the latter have been included, however, in order to make the account as complete as possible. The main results of this study were presented by SMITH (1932) but publication of the present account has been delayed in the hope of securing more and better characters-a hope that has not been realized. Previous genetic studies on this species have dealt with one pair of autosomes (METZ 1927, truncate wings), and the sex chromosomes (METZ, ULLIAN, SCHMUCK, SMITH 1929-3 I). The new characters considered here make it possible to present the essential facts concerning the behavior of the remaining pairs of autosomes, as well as additional data concerning the first pair. Before analyzing the genetic results in detail, i t is necessary to review briefly certain aspects of chromosome behavior in Sciara. One feature of importance is the difference in chromosome number between the two sexes, and between the soma and germ line in each sex (METZ 1931). These differences are brought about by a process of chromosome elimination during cleavage in the developing egg (DUBOIS 1932). The typical number of chromosomes in somatic groups is eight in the female (one pair of V’s, three pairs of rods) ; and seven in the male (one pair of V’s, two pairs of rods, one single rod). These are shown in figure Ia and Ib. The male group is similar to the female group but lacks one rod, presumably a sex chromosome. There are present in the germ-line of both sexes, one, two, or three