The converse experiment, namely, red (not-fused) females by vermilion fused males also gave, when the wild-type daughters wereback-crossed to vermilion fused males, a linkage value of 27 units. Two 10-day broods were reared from each female. The data given in table 38 show that the percentage of crossing-over does not change as the flies get older. The locus of fused on the basis of all of the data is at 59.5.
Table 37.—P1vermilion ♀ ♀ × fused ♂ ♂. F1wild-type ♀ ♀ × F1vermilion ♂ ♂.
Table 38.—P1wild ♀ ♀ × vermilion fused ♂ ♂. F1wild-type ♀ × F1wild-type ♂ ♂.
FORKED.
On November 19, 1912 there appeared in a stock of a double recessive eye-color, vermilion maroon, a few males which showed a novel form of the large bristles (macrochætæ) upon the head and thorax. In this mutation (text-fig. E) the first of several which affect the shape and distribution of the bristles, the macrochætæ, instead ofbeing long, slender, and tapered (see Plate 1, fig.I), are greatly shortened and crinkled as though scorched. The ends are forked or branched, bent sharply, or merely thickened. The bristles which are mostdistortedare those upon the scutellum, where they are sometimes curled together into balls.
LINKAGE OF VERMILION AND FORKED.
Fig. E. Forked bristlesFig. E.—Forked bristles.
Since forked arose in vermilion stock, the double recessive for these two sex-linked factors could be used in testing the linkage relations of the mutation. Vermilion forked males were crossed to wild females and gave wild-type males and females, which inbred gave in F2the results shown in table 39. Forked reappeared only in the males in the following proportion: not-forked ♀, 742; not-forked ♂, 346; forked ♂, 301. The result shows that the character is a sex-linked recessive.
Table 39.—P1wild♀ ♀ ×vermilion-forked♂ ♂.F1wild-type♀ ♀ ×F1wild-type♂ ♂.
In table 39 vermilion forked and wild-type are non-cross-overs, and vermilion and forked are cross-overs, giving a cross-over value of 25 units. The locus, therefore, is 25 units to the right or to the left of vermilion, that is, either about 58 or 8 units from the yellow locus.
LINKAGE OF CHERRY AND FORKED.
Forked males were crossed to cherry females (cherry has the same locus as white, which is about 1 unit from yellow) and gave wild-type females and cherry males. These gave in F2the results shown in table 40. The non-cross-overs (cherry and forked) plus the cross-overs (cherry forked and wild type) divided into the cross-overs give a cross-over value of 46 units, which shows that the locus lies to the right of vermilion, because if it had been to the left, the value would have been 8 (i. e., 33-25) instead of 33+25=58. The difference between 58and 46 is due to the expected amount of double crossing-over. In fact, for a distance as long as 58 an almost independent behavior of linked gens is to be expected.
Table40.—P1cherry♀ ♀ ×forked♂ ♂.F1wild-type♀ ♀ ×F1cherry♂ ♂.
LINKAGE OF FORKED, BAR, AND FUSED.
This value of 58 gave the furthest locus to the right obtained up to that time, since forked is slightly beyond rudimentary. Later, the locus for bar-eye was found still farther to the right, and the locus for fused even farther to the right than bar. A cross was made involving these three gens. A forked (not-bar) fused male was bred to a (not-forked) bar (not-fused) female and gave bar females and males. The F1females were back-crossed singly to forked fused males with the result shown in table 41.
Table41.—P1bar♀ ♀ ×forked fused♂ ♂.B. C. F1bar♀ ×forked fused♂ ♂.
The same three points were combined in a different way, namely, by mating forked females to bar fused males. The bar daughters were back-crossed to forked fused males and gave the results shown in table 42.
Table 42.—P1forked♀ ♀ ×bar fused♂ ♂.B.C. F1bar♀ ×forked fused♂ ♂.
By combining the results of tables 41 and 42 data are obtained for cross-over values from which (by balancing the inviable classes, as explained in table 43) the element of inviability is reduced to a minimum.
Table 43.
The linkages involved in these data are very strong. The cross-overs between forked and bar number only 5 in a total of 1,201, which gives less than 0.5 per cent of crossing-over. There are 32 cross-overs or 2.7 per cent between bar and fused. The value for forked fused is the sum of the two other values, or 3.1 per cent.
LINKAGE OF SABLE, RUDIMENTARY, AND FORKED.
Rudimentary, forked, bar, and fused form a rather compact group at the right end of the chromosome, as do yellow, lethal 1, white, abnormal, etc., at the zero end. The following two experiments were made to determine more accurately the interval between rudimentary and the other members of this group. A sable rudimentary forkedmale mated to a wild female gave wild-type sons and daughters. These inbred give the results shown in table 44.
Table44.—P1sable rudimentary forked♂ ×wild♀.F1wild-type♀ ×F1wild-type♂ ♂.
There were 265 males, of which 42 were cross-overs between sable and rudimentary and 4 between rudimentary and forked. The values found are: sable rudimentary, 16; rudimentary forked, 1.5; sable forked, 17.
LINKAGE OF RUDIMENTARY, FORKED, AND BAR.
The three gens, rudimentary, forked, and bar, form a very compact group. A rudimentary forked male was crossed to bar females and the daughters (bar) were back-crossed singly to rudimentary forked males, the results being shown in table 45.
Table45.—P1rudimentary forked♂ ×bar♀.B.C. F1bar♀ ×rudimentary forked♂ ♂.
The cross-over values are: rudimentary forked, 1; forked bar, 0.6; rudimentary bar, 1.6. The order of factors is rudimentary, forked, bar. On the basis of the total data the locus of forked is at 56.5.
SHIFTED.
Shifted appeared (January 1913) in a stock culture of vermilion dot. The chief characteristic of this mutant is that the third longitudinal vein (see text-fig.F) does not reach the margin as it does in the normal fly. The vein is displaced toward the fourth throughout its length, and only very rarely does it extend far enough to join the marginal vein. The cross-vein between the third and the fourth veins is often absent because of the shifting. The flies themselves are smaller than normal. The wings are held out from the body at a wide angle. The two posterior bristles of the scutellum are much reduced in size and stick straight up—a useful landmark by which just-hatched shifted flies may be recognized, even though the wings are not expanded.
LINKAGE OF SHIFTED AND VERMILION.
Since shifted arose in vermilion, the double recessive shifted vermilion was available for the following linkage experiment: shifted vermilion males by wild females gave wild-type males and females which inbred gave the data shown in table 46.
Fig. F. Shifted venation.Fig. F.—Shifted venation. The third longitudinal vein is shifted toward the fourth and fails to reach the margin. Cross-vein between third and fourth longitudinal veins is lacking.
Fig. F.—Shifted venation. The third longitudinal vein is shifted toward the fourth and fails to reach the margin. Cross-vein between third and fourth longitudinal veins is lacking.
Disregarding the eye-color, the following is a summary of the preceding results: wild-type ♀, 1,001; wild-type ♂, 437; shifted ♂, 328. The result shows that shifted is a sex-linked recessive. The data of table 46 show that the locus of shifted lies about 15 units on one side or the other of vermilion, which from the calculated position of vermilion at 33 would give a position for shifted at either 18 or 48 from yellow.
Table 46.—P1shifted vermilion ♂ ♂ × wild ♀ ♀. F1wild-type ♀ × F1wild-type ♂ ♂.
LINKAGE OF SHIFTED, VERMILION, AND BAR.
In order to determine on which side of vermilion shifted lies, a shifted vermilion (not-bar) female was crossed to a (not-shifted red) bar male. Three factors are involved, of which one, bar, is dominant. The shifted vermilion (not-bar) stock is a triple recessive, and a three-point back-cross was therefore possible. The daughters were bar and the sons were shifted vermilion (the triple recessive). Inbred these gave the results shown in table 46. The smallest classes (double cross-overs) are shifted and vermilion bar, which places shifted to the left of vermilion at approximately 17.8 units from yellow.
Table47.—P1shifted vermilion♀ ×bar♂ ♂.F1bar♀ ×F1shifted vermillion♂ ♂.
The stock of shifted has been thrown away, since too great difficulty was encountered in maintaining it, because, apparently, of sterility in the females.
LETHALS SA AND SB.
The first lethal found by Miss Rawls was in a stock that had been bred for about 3 years. While there was noa priorireason that could be given to support the view that lethal mutations would occur more frequently among flies inbred in confinement, nevertheless a hundred females from each of several newly caught and from each of several confined stocks were examined for lethals (Stark, 1915). No lethals were found among the wild stocks, but 4 were found among the confined stocks. Whether this difference is significant is perhaps open to question. The first lethal was found in January 1913, in a stock that had been caught at Falmouth, Massachusetts, in 1911, and had been inbred for 18 months,i.e., for about 50 generations. This lethal, lethalsa, was recessive and behaved like the former lethals, being transmitted by half the females and causing the death of half the sons. The position of this lethal to the X chromosome was found as follows, by means of the cross-over value white lethalsa. Lethal-bearing females were mated to white males and the lethal-bearing daughters were again mated to white males. The white sons (894) were non-cross-overs and the red sons (256) were cross-overs. The percentage of crossing-overis 22.2. A correction of 0.4 unit should be added for double crossing-over, indicating that the locus is 22.6 units from white, or at 23.7.
When the work on lethalsahad been continued for 3 months, the second lethal, lethalsb, was found (April 1913) to be present in a female which was already heterozygous for lethalsa. It is probable that this second lethal arose as a mutation in the father, and that a sperm whose X carried lethalsbfertilized an egg whose X carried lethalsa. As in the cases of lethals 1 and 1aand lethals 3 and 3a, this lethal, lethalsb, was discovered from the fact that only a very few sons were produced, there being 82 daughters and only 3 sons. If, as in the other cases, the number of daughters is taken as the number of non-cross-overs and twice the number of sons as the cross-overs, it is found that the two lethals are about 7 units apart. Since the two lethals were in different X chromosomes, all the daughters should receive one or the other lethal, except in those few cases in which crossing over had taken place. Of the daughters 19 were tested and every one was found to carry a lethal. Again, if the cross-over values of the lethals with some other character, such as white eyes, be found and plotted, the curve should show two modes corresponding to the two lethals. This test was applied, but the curve failed to show two modes clearly,[7]the two lethals being too close together to be differentiated by the small number of determinations that were made. It seems probable that lethalsaand lethalsbare about 5 units apart.
The position of lethalsbwas accurately found by continuing the determinations with a white lethal cross-over. A white female was found which had only one of the two lethals and the linkage of this lethal with eosin and miniature was found as follows: A female carrying white and lethal in one chromosome and no mutant factor in the homologous chromosome was bred to an eosin miniature male. The white eosin daughters carried lethal, and their sons show the amount of crossing-over between white and lethal (15.6), between lethal and miniature (19.9), and between white and miniature (32.9). The data on which these calculations are based are given in table 48.
Table 48.—Data on the linkage of white, lethal sb, and miniature, from Stark, 1915.
The locus of this lethal is at 16.7; the locus of lethalsawas found to be at 23.7, so that the lethal at 16.7 is evidently the second lethal or lethalsbwhose advent gave rise to the high sex-ratio. This interpretation is in accord with the curve which Miss Stark published, for although the mode which corresponds to lethalsais weak, the mode at 15-16 is well marked.
The two other lethals, lethalsscandsd, which came up in the course of these experiments by Miss Stark, are treated in other sections of this paper.
BAR.
(Plate II, figures 12 and 13.)
The dominant sex-linked mutant called bar-eye (formerly called barred) appeared in February 1913 in an experiment involving rudimentary and long-winged flies (Tice, 1914). A female that is heterozygous for bar has an eye that is intermediate between the rounded eye of the wild fly and the narrow band of the bar stock. This heterozygous bar female is always readily distinguishable from the normal, but can not always be separated from the pure bar. Bar is therefore nearly always used as a dominant and back-crosses are made with normal males.
Bar is the most useful sex-linked character so far discovered, on account of its dominance, the certainty of its classification, and its position near the right end of the X chromosome. The locus of bar at 57 was determined on the basis of the data of table 65.
NOTCH.
A sex-linked dominant factor that brings about a notch at the ends of the wings appeared in March 1913, and has been described and figured by Dexter (1914, p. 753, and fig. 13, p. 730). The factor acts as a lethal for the male. Consequently a female heterozygous for notch bred to a wild male gives a 2:1 sex-ratio; half of her daughters are notch and half normal; the sons are only normal. The actual figures obtained by Dexter were 235 notch females, 270 normal females, and 235 normal males.
The location of notch in the X chromosome was not determined by Dexter, but the mutant has appeared anew three or four times and the position has been found by Bridges to be approximately at 2.6.
DEPRESSED.
Several mutations have appeared in which the wings are not flat. Of these the first that appeared was curved (second chromosome), in which the wings are curved downward throughout their length, but are elevated and held out sidewise from the body; the texture is thinner than normal. The second of these wing mutants to appear was jaunty (second chromosome), in which the wings turn up sharply at the tip; they lie in the normal position. The third mutant, arc (second chromosome), has, as its name implies, its wings curved like the arc of a circle. The fourth mutant, bow (first chromosome, fig.c), is like arc, but the amount of curvature is slightly less. The fifth mutant, depressed (first chromosome, fig.g), has the tip of its wings turned down instead of up, as in jaunty, but, as in jaunty, the wing is straight, except near the tip, where it bends suddenly. These stocks have been kept separate since their origin, and flies from them have seldom been crossed to each other, because in the succeeding generations it would be almost impossible to make a satisfactory classification of the various types. But that they are genetically different mutations is at once shown on crossing any two, when wild-type offspring are produced. For instance, bow and arc are the two most nearly alike. Mated together (bow ♂ by arc ♀), they give in F1straight-winged flies which inbred give in F29 straight to 7 not-straight (i.e., bow, arc, and bow arc together).
Depressed wings first appeared (April 1913) among the males of a culture of black flies. They were mated to their sisters and from subsequent generations both males and females with depressed wings were obtained which gave a pure stock. This new character proved to be another sex-linked recessive.
LINKAGE OF DEPRESSED AND BAR.
Depressed (not-bar) males mated to (not-depressed) bar females gave bar daughters. Two of these were back-crossed singly to depressed males and gave the results shown in table 49. Males and females were not separated, since they should give the same result.
Table 49.—P1depressed♀ ♀ ×bar♀ ♀.B.C. F1bar♀ ×depressed♂ ♂.