A SECONDARY MUTATION FROM WHITE TO A DARKER ALLELOMORPH IN DROSO- PHILA MELANOGASTER

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1 A SECONDARY MUTATION FROM WHITE TO A DARKER ALLELOMORPH IN DROSO- PHILA MELANOGASTER BY L. C. DUNN ANATOMICAL INSTITUTE, OSLO, NORWAY M ANY spontaneous mutations have occurred at the white-eye locus in Drosophila, producing the well known series of multiple allelomorphs, ranging in color from the red of the wild type eye through coral (woo), blood (wbl), eosin (we), cherry (wch), apricot (wa), buff (wbf), tinged (wf), ivory (w'), and ecru (we.) to white (w). Nearly all of these have arisen by mutation of a wild type gene to a recessive allelomorph. Rarely one of these mutant allelomorphs has again mutated, for example eosin has on several occasions mutated to white. Such secondary mutations which result in a further departure from wild type have been called by MOHR (922) progressive mutations. Secondary mutations in the contrary direction from mutant allelomorphs to or toward wild type are much rarer. The majority of these are complete reverse mutations from mutant to wild type at one step. The rarest mutation of all is from a mutant to another allelomorph which represents only a partial return to the wild type characters. This has been called regressive mutation and is only known to have occurred once spontaneously at the white locus, viz., the original mutation of eosin from white as reported by MORGAN and BRIDGES (96). It is of interest therefore to record the origin of a new allelomorph of white which can be shown to have arisen not from wild type but by regressive mutation from white, and which provides a useful new member of this series of allelomorphs. The new mutant was found as a single male among the progeny of the culture described below: TABLE. CuZture C 25 (lo/27/34). 9 w/w; L'lh/Cy X d b pr vg"' L ~ Q L/Cy Q L cf w; L/Cy cf w; L d WX; L/Cy! The exceptional male had yellowish-brown eyes of about the color of honey, darker and yellower than buff, a little lighter hnd less Hereditaa XSI. 8

2 4 L. C. DUNN orange than apricot. From its appearance and manner of origin it was judged to be an allelomorph of white and was therefore crossed to white, wild type and cherry (wch) females with the following results: TABLE 2. Culture C88b. Q white (w/w; Cy) X d.honey))(w"; L/Cy) CY 9 LKY Q LQ d d d light honey light honey light honey w; Cy w; L/Cy w; L All daughters were intermediate in color between the new mutant color and white. All sons were white. TABLE 3. Q cherry ych X d wx; L/Cy ))honey)) WCh 9 9 d d light cherry light cherry I cherry cherry C88c CY L cy I, All daughters were intermediate in color between cherry and the new mutant color. All sons were cherry. A mating of the mutant male to wild type (C 88 d) gave only wild type eye color in sons and daughters: 65 Cy ( d), 68Lobe ( d). These results show clearly that the mutant male contained a recessive gene allelomorphic with white. This has been called honey (wh). His composition was thus wh; L/Cy and the mutation must have occurred in the X-chromosome of his white mother, i. e. w -+ wh. The presence of other genes excludes the possibility of origin from any other culture. The mother of the mutant was from an inbred stock of white containing also a Lobe allelomorph (L", possibly the same as L2 of other workers), a TI chromosome lethal P, balanced over Curly. This stock had been under continuous observation for two years and had produced no other mutants. The original source of the white gene of this stock is unknown, A homozygous stock of the new mutant was subsequently extracted from the cross to wild type and freed from other mutant genes. The colors of its compounds with white, apricot, cherry and Notch 8

3 A SECONDARY MUTATION 5 (a deficiency for the white locus) were compared with the homozygous types in the same cultures. Observations of the pure honey stock showed that the eye color of the male is clear light brownish yellow, with a slightly lighter,halo* at the edge of the eye. It most nearly resembles the apricot mutant (w") but differs from it in being slightly lighter and in having no orange or pinkish tinge. The eyes of the female are slightly but distinctly darker than those of the inale with less halo and a slight orange tinge, whereas in all three of the apricot allelomorphs, the males have lighter eyes than the females (BRIDGES, personal communication). In the same culture newly hatched honey flies can be easily separated from apricot and from cherry. The eyes of the honey flies darken slightly and very slowly with age, whereas apricot and cherry darken more quickly, so that the differences between honey and both of Ihe other mutants increase with age. The following combinations have been observed under comparable conditions of age and culture and are arranged in decreasing order of darkness Q Q wch/wch))( WchlWh? WCh/W dd WCh))W") w") w wa/wa)wa/wh)wh/wh)wa/w)wa/n8)wh/w)wh/nb)w/w The order of the allelomorphs studied is very clearly in order of decreasing darkness cherry, apricot, honey, white. The greatest differences are those between cherry and apricot and between honey and white. Honey female is only slightly lighter than tipricot female. The same order is found in the compounds of honey: wh/wch)wh/wa)wh/wh) wh/w. Here in addition it is clear that the compound with cherry is pinkish, while that with apricot has more orange tinge than honey. Among the other compounds the relative order of w"/w" and wch/wh is not clear since these have not been coinpared in the same culture. wch/wh is probably the darker. The most difficult classifications are those involving separation of w8/wa, wa/wh, wh/wh and wa/w, although our ratios show that after practice these classes can be separated. One unexpected fact was observed in the inability to separate wch/wh from wc/w. These classes were nearly alike in color indicating a similar diluting effect of w and w" on cherry, although the wa/wh and wa/w compounds were separable. In view of MOHR'S (923) discovery that a deficiency for the white

4 6 L. C. DUNN locus (Notch 8) has a greater diluting effect on the allelomorphs blood, coral, eosin, apricot and buff than the white gene itself, tests were made to determine whether Notch 8 has a similar effect on honey and whether this would serve further to distinguish wh and w'. Honey and apricot males were crossed with Notch 8 females and with white females. From the Notch crosses the Notch females showed the mutant eye colors and these were compared with the respective compounds with white. w"/n8 was much darker than wh/n8 and had distinctly more orange tinge. The N8 compound was in each rase lighter than the corresponding white compound, as follows: w"/w) w"/n~)w"/w) w"/n8. w"/n8 was only slightly darker than Wh/w and separation was not certain. wh/n8 was pale yellow, slightly but distinctly lighter than wh/w. Thus the diluting effect of Notch 8 serves further to confirm the distinctness of the new mutant from apricot which it most nearly resembles. I have had no opportunity to compare honey with buff, but from the descriptions of SAFIR (96) and MORGAN, BRIDGES and STURTEVANT (925) buff is apparently much lighter in color, shows little or no sex-dimorphism, and has a pinkish tinge which honey lacks entirely '. DISCUSSION. From the above facts it can be concluded that honey is a new and different allelomorph of white which arose by spontaneous regressive mutation of a white gene. The reversibility of mutation has already been proved by the occurrence of spontaneous reverse mutations at this locus (eosin) and at others, although it remains a very rare occurrence except in the cases of very mutable genes such as those studied by DEMEHEC (928). Reverse mutations have been induced by irradiation in the case of the forked and scute loci by PATTERSON and MULLER (930) and at the white locus by TIMOFI~EFF-RESSOVSKY (928-33). Lo~asov and SMIRNOV (934) report a mutation of eosin to a darker allelomorph following treatment with ammonia, while JOLLOS (935) has reported two cases of complete reversion of eosin and of white to wild type, although he gives no supporting evidence. The extensive experiments of JOHNSTON and WINCHESTER (934) have From a cross of Ns/lz 49 X whone exceptional honey son was found. This was tested and found to be sterile and so was probably XO from non-disjunction. The eye color (was that of a normal honey male so that the sex dimorphism in honey eye color is probably not due to the Y-chromosome.

5 A SECONDARY MUTATION 7 shown that irradiation induces reverse mutations with much lower frequency than mutations from wild type allelomorphs, and they found no reverse mutations at all at the white locus (w" gene treated). Regressive mutation at the white locus is thus very rare indeed, although the evidence from the present case leaves no doubt that it does occur spontaneously. In addition to providing a new useful allelomorph for marking the white locus, honey adds a new step in the remarkable series of eye color changes already reported at this locus. The order of effect of the genes of this series as summarized by MORGAN, BRIDGES and SCHULTZ (93) and with the new allelomorph inserted'is as follows: COlOrS : coriil blood eosiii cherry apricot honey buff tinged ivory white Allels : wco / wtll ) w80) we ) wc ) wa ) wh )wj>f) wt ) wi ) w Sex diff.: d)q d)q Q))d Q))d Q))d d)q Q)d Qjd? Gid 9- Syinbols: ) darker, )) much darker, ) slightly darker. I am indebted to Prof. 0. L. MOHR for hospitality and advice, and to Miss JEANNE COYNE for technical assistance, LITERATURE CITED.. DEMEREC, M The behavior of mutable genes. - Zschr. f. ind. Abst.- u. Vererb.-lehre, Supp.-bd. I: JOHNSTON, 0. and WINCHESTER, A. M Studies on reverse mutations in Drosophila melanogaster. - Anier. Naturalist, 68: JOLLOS, V Inherited changes produced by heat treatment in Drosophila melanogaster. - Genetica, 6: LOBASOV, M. and SMIRNOV, F C. R. de 'Acad. des Sciences de l'u.r.s.s., p. 77. (Russian.) 5. MOHR, 0. L Cases of mimic mutations and secondary mutations in the X-chromosome of Drosophila melanogaster. - Zschr. P. ind. Abst.- u. Vererb.-lehre, 28: A genetic and cytological analysis of a section deficiency involving four units of the X-chromosome in Drosophila melanogaster. - Zschr. f. ind. Abst.- u. Vererb.-lchre, 32: MORGAN, T. H. and BRIDGES, C. B. 96. Sex linked inheritance in Drosophila. - Carnegie Publication MORGAN, T. H., BRIDGES, C. B. and SCHUL'rZ, J. 93. The constitution of the germinal material in relation to heredity. - Carnegie Year Book, 30: MORGAX, T. I., BRIDGES, C. B. and STURTEVANT, A. H The genetics of Drosopliila. - Bibl. Genetica,.

6 8 L. C. DUNN 0. MORGAN, T. H., STURTEVANT, A. H. and BRIDGES, C. B The constitution of the germinal material in relation to hcredity. - Carnegie Year Book, 27: PATTERSON, J. T. and MULLER, H. J. 930; Are progressive mutations produced by x-rays? - Genetics, 5: SAFIR, S. R. 96. Buff, a new allelomorph of white eye color in Drosophila ampelophila. - Genetics, : STERN, C Multiple Allelie. - Handbuch der Vererbungswissenschaft, Lief. 4: TIMOFEEFF-RESSOVSKY, N. W Reverse genovariations and the genovariability in different directions.. The production of reverse genovariations in Drosophila by x-ray treatment. - Journ. of Hered., 2: