Genetic instability in Drosophila Melanogaster: Mutable miniature (mμ)

Mutation Research, 29 (1975) 77-84 re) Elsevier Scientific Publishing Company, A m s t e r d a m - - P r i n t e d in The Netherlands

G E N E T I C I N S T A B I L I T Y IN D R O S O P H I L A

77

MELANOGASTER:

MUTABLE M I N I A T U R E (ml,)*

M. M. G R E E N

Department of Genetics, University of California, Davis, Calif. (U.S.A.) (Received December I8th, 1974)

SUMMARY

A new mutable gene, mutable miniature wing (m"), is described. This mutable gene mutates spontaneously at an inordinate rate both germinally and somatically. Two classes of reversions of rn~ have been found in approximately equal frequency: those to an allele with an intermediate phenotype (rn i) and those to a subliminal allele (rn~) equivalent to wild type in phenotype. Reversions appear to be mutationally stable. The chronology of genetic events leading to the discovery of rn, implicates, but does not prove, the insertion of a "foreign" DNA segment as the basis of mutability.

INTRODUCTION

Prior to the in depth comprehensive genetic analysis of mutable genes in maize initiated by McCLINTOCKL the most detailed study of the genetics of a mutable gene was that of DEMEREC1 on the mutable miniature wing gene (mr-3 o:) in Drosophila virilis. Over a period of 15 years until the stock was inadvertently lost DEMEREC, in a series of investigations on rnt-3 oc described a number of genetic properties of this mutable gene. These included the conditions both genetic and environmental which affect the mutability of rnt-2 o: and the mutational properties of derivatives of rot-3 oc. A summary of the principal results (DEMEREC~) indicates that in spite of painstaking and careful research, no clearcut explanation of the underlying basis of mutability could be delineated from the studies of rot-3 oc. This is not altogether surprising because the research was ahead of its time. The loss of the mt-s oc stock brought the study of mutable genes in Drosophila to a temporary halt. However, the discovery of a series of independent mutable genes of the white locus in D. melanogaster (@ GREEN* for a summary) has rekindled the interest in mutable gene research. Herein will be reported the occurrence for the first time a mutable miniature wing mutant (m.) in D. meli~nogaster. For several reasons m~ is more than a genetic curiosity. It represents the first example of a mutable gene in D. melanogaster outside the white locus. In its somatic and germinal mutability m" * Supported by National Science Foundation grant G.B. 27599.

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M. M. GREEN

mimics the m u t a t i o n a l b e h a v i o u r of mr- 3 oc of D. virilis. The unique origin of rn~ p e r m i t s some speculation as to w h y m u t a b l e genes are m u t a b l e . MATERIALS AND METHODS

The several m u t a n t genes, their p h e n o t y p e s , s y m b o l s a n d m a p location, used in this s t u d y are listed in Table I. All crosses were carried out at a controlled room t e m p e r a t u r e of 22 °. A s t a n d a r d cornmeal Brewer's y e a s t m e d i u m was used for all crosses. Details of crosses will be given in the t e x t where a p p r o p r i a t e . TABI.E I SYNOPSIS OF MUTANTS WITH SYMBOLS USED IN TEXT (AFTER LINDSLEY AND GRELL 6)

Symbol

Phenotype

M a p location

y y~k

yellow b o d y color allele of y scute, b r i s t l e s a b s e n t zeste eye color w h i t e eye color w h i t e - s p o t t e d , allele of w m a l e v i a b l e d e f i c i e n c y of w split bristles echinius, r o u g h eyes singed- 3, b r i s t l e s g n a r l e d r a s p b e r r y eye color v e r m i l i o n eye color miniature wings dusky, small wings furrowed, eyes f u r r o w e d a n d b r i s t l e s gnarled fork ed b r i s t l e s c o m p o u n d d o u b l e - X , e q u i v a l e n t to attached-X. deficiency for b o t h m a n d f w balancer X chromosome marked with d o m i n a n t B a r eye

I-O.O

sc z w wSp w

spl ec sn 3 ras v m

dy fw f

C(~)DX 1)f(±)m-fw FM 7

i o.o i i .o i-i.5

1-3.o 1-5. 5 i 21.o I 32.8 i 33 I 36.I 1-36.2 i 38.3 I 56.7

RESULTS

T h e origin o f m r

Elsewhere% 1° a t a n d e m d u p l i c a t i o n associated with the w locus has been described in some detail because it a p p e a r s to be genetically unstable. One criterion of this i n s t a b i l i t y is t h a t in females, in the absence of crossing over, deficiencies of p a r t or all of the d u p l i c a t i o n are g e n e r a t e d at an u n u s u a l l y high rate. In the course of c o n d u c t i n g a genetic analysis of one such p u t a t i v e deficiency, females h e t e r o z y g o u s for the deficiency X chromosome a n d a wild t y p e X c h r o m o s o m e were o b t a i n e d . The deficiency X chromosome was m a r k e d with the m u t a n t s S ~k, sc, z, w s~ a n d ec; the wild t y p e X c h r o m o s o m e d e r i v e d from the s t a n d a r d Canton-S s t r a i n was u n m a r k e d . Several i d e n t i c a l crosses were m a d e s i m u l t a n e o u s l y in which three heterozygous females were crossed to y~ w - spl s n 3 males. I n one such cross, in a d d i t i o n to the e x p e c t e d progeny, two males were found of identical p h e n o t y p e : the wings were r e d u c e d in size comp a r a b l e to the p h e n o t y p e associated w i t h the m u t a n t d y or with a m o d e r a t e m m u t a n t (compare Figs. I a n d 2). Since two identical m u t a n t males o c c u r r e d - - a n u n c o m m o n

NEW

MUTABLE

GENE

IN

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DROSOPHILA

mutational e v e n t - - t h e y were crossed to C(±)DX, y f females and a stock established. The males bred true: all male progeny were phenotypic copies of their fathers, all females were C(z)DX, y f l i k e their mothers. Thus the new wing mutant is sex-linked. Since subsequently this wing mutant proved to be recessive it is not inconceivable that among the F1 females sibs of the two mutant males other instances of the mutant occurred. No search was made. The new wing mutant was designated dS a because it was noncomplementary in compound with a standard dy mutant and complementary with tile mutant m (cf. Fig. I). It was maintained as an unselected culture in four vials with d S 3 males crossed to C(±)DX, y f females. Prior to storing this stock among the routinely maintained laboratory stocks, each vial was checked for purity. In one vial in addition to the dy TM males, several males were found with an extreme wing phenotype mimicking that of the mutant rn (cf. Fig. 3). These males were separated out and crossed to C(2r)DX, y f females. By and large the progeny were as expected: the wing phenotype of the sons was inseparable from that of their fathers and the daughters were all C(z)DX, yr. However, included among the sons were several unexpected individuals. These included males with a mosaic wing phenotype (cf. Figs. 4 and 5) and males with a wing phenotype essentially that of wild type. These exceptional males motivated a systematic investigation of the new wing mutant derived from dS 3. Since the new wing mutant mimicked m, males were crossed to females homozygous for a standard m mutant. All F1 females were miniature wing in phenotype establishing allelism to m. Since in subsequent generations within the stock mosaic and "wildtype" males were found, the mutant was designated miniature-mutable or m~. In other ensuing crosses ml~ behaved as a m allele. Females dy/rnl, and dS3/rnt ' (cf. Fig. I) were wild type in wing phenotype just as are dy/m females. Females Df(~)m-fw/ml'

i

i ~

ii

ii¸ !i

ii!ii ~ ~ ~i~ii~ ~i i~

~i~i~~ ~!~il!~i~i~i~!i~ !!~i

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M.M. GREEN

Figs. I 5. Wings of dy va and ,~f' individuals (all X36 ). ~, wild type wing of a (lyTa/m~' female; 2, wing of a d3,Ta female; 3, wing of a ml~ female; 4, wings of a mosaic m, male; 5, a mosaic m*' male. were also m in phenotype. Mapping experiments localized m, between v and fw, and comparatively tightly linked to the latter. Thus the genetic and phenotypic results are consistent with the conclusion that m, is a m allele. In the course of these genetic experiments it became clear t h a t m ' mutates at an inordinately high rate. Within a genetically marked m ' stock, maintained en masse as ras v m , males crossed to C(_r)DX, y f females, each generation numerous phenotypic exception were observed among the males. In tile main these exceptions fall into the following three classes: (I) mosaic males whose wing phenotype paralleled that given in Figs. 4 and 5; (2) males whose wing phenotype mimicked t h a t of the dy m u t a n t and (3) males whose wing phenotype appeared indistinguishable from that of wildtype. In an unselected culture males of classes (2) and (2) soon took over. Thus an estimate of the m u t a t i o n rate of m" was deemed desirable. A simple experiment was set up to estimate the mutability of my. Individual m" males o-12 h old were m a t e d to C ( I ) D X , y f females in half-pint milk bottles containing medium. Females were allowed to oviposit for six days after which all parents were discarded. To be included in the m u t a t i o n estimate, arbitrarily males were required to produce a m i n i m u m of IOO sons. A m o n g 46 m~ males tested, 42 produced the m i n i m u m n u m b e r of male progeny and were included in the m u t a t i o n estimate. The average n u m b e r of male progeny scored was 247.2 (range 134-394). Exceptional males were found among the progeny of each of the 42 males tested. The exceptional males fell into the three phenotypie classes spelled out above, viz. mosaic males, dy-like males, and wild-type-like (m+-like) males. The number of exceptional male progeny per parental m , male varied from 2-124. Thus all males produced at least two mosaic males ; 18 males produced, mosaic males plus dy-like males ; 14 males produced mosaic males plus m+-like males; and two mv males produced all three classes of exceptional male progeny. The dy-like exceptions per m~ male ranged from I - I 2 O ; tile m+-like exceptions ranged from 1-78 and the mosaic males ranged from

NEW

MUTABLE

TABLE

GENE

IN DROSOPHILA

8I

II

A R E P R E S E N T A T I V E SAMPLE OF T Y P I C A L R E S U L T S D E R I V E D FROM T H E CROSS m " ~ ×

Male No.

Phenotype and number o f F 1 c~c~ mv dy-like m+-like

Mosaic

Number of ~c~ scored

I 3

274 148

3 12o

---

IO 4

287 272

7 9 12 15 22 27 36

23 ° 281 223 196 294 228

--22 -I ---

-3 I II -78

7 7 9 8 II 9 6

237 291 255 215 306 281 294

288

C(z)DX, yf~

2 19. Rather than reproduce the extensive data, a selected sample of typical results is presented in Table II. Exceptional males derived from each cross of m, males by C(z)DX, y f females were tested further according to the following scheme. Where a large cluster of dylike males was recovered, a sample of exceptional males was crossed separately to females FM7/Df(I)rn:[w, to females homozygous rnu and to females homozygous dy. Where only one dy-like or m+-like exceptional male occurred, he was crossed to C(z)DXyffemales and his sons were tested to Df(±)rn-fw, to dy and to rn,. A sample of FI mosaic males was tested by crossing individual males to C(z)DX, y f females. Mosaic males selected for testing included those sibs of clusters of dy-like males (e.g. those derived from male 3, Table II), those sibs of clusters of m+-like males (e.g. those derived from male 2 9, Table II), and those in which the mosaic males represented the only exceptions (e.g. those derived from male 7, Table II). Progeny tests were obtained on all dy-like and m+-like exceptions. The results were identical for all exceptions and are summarized as follows. Each @-like or m+-like exceptional X chromosome in compound with Df(z)rn-fw resulted in females unmistakably mutant in wing phenotype. Thus the wings of females dy-like/Df(z)rmfw approximated rn in phenotype and the wings of females rn+-like/Df(z)m-fw approximated dy in phenotype. Identical results were obtained when the dy-like and m+-like X chromosomes were compounded to rn,. However, in compound with dy, the dy-like and m+-like X chromosomes resulted in females inseparable from wild type in wing phenotype. Thus it is fair to conclude that the dy-like exceptions are functional m mutants of intermediate wing phenotype (designated rni) and the m+-like exceptions represent subliminal m mutants (designated ms). So far as could be determined from these phenotypic tests, all the members of a cluster are identical. Thus the occurrence of clusters is most likely due to a premeiotic mutational event, with the cluster size predicated on the time before meiosis the event occurred. Further tests of the mosaics males were also unremarkable. All told, 18 males were tested. Overall the mutational properties of the mosaic males were equivalent to those of any rnv males selected at random from the ma stock. Thus the fact that a mosaic male was sib to a cluster of dy-like or m+-like males had no bearing on its mutability. In mutability such males behaved no differently from mosaic males which occurred as the only exceptions. Thus, for example, a mosaic male, the son of male 3, Table II, on progeny testing to C(z)DX, y f females produced the following male

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M.M. GREEN

progeny: z 2 o m~', 5 mosaic, i rn~. Similarly a mosaic male, son of male 29, Table II, on crossing to C ( z ) D X , y f females produced the following male progeny: I2Z mr,, 5 mosaic. E q u i v a l e n t results were o b t a i n e d on testing mosaic males which were the only exceptions. These results suggest t h a t the mosaic males arise b y more or less strictly somatic events i n d e p e n d e n t of the m u t a t i o n a l events which are incorporated in the germ line. Nonetheless, progeny tests of other mosaic males which were r a n d o m ly selected from the mr' stock d e m o n s t r a t e d t h a t some were both somatically and germinally mosaic. I n one case such a mosaic male crossed to C ( z ) D X , 3' f female:~ produced the following male offspring: 6o m~', 45 mi, a n d 3 mosaic. In a n o t h e r case a mosaic male crossed to C ( I ) D X , y f females produced these male progeny: 24 m~', I I 3 m ~ a n d 6 mosaic. Thus some m~' males m a y be only somatically mosaic, others m a y be b o t h germinally a n d somatically mosaic. M u t a b i l i t y of mr' is not confined to males. Male progeny of single homozygous rnt, females include clusters of both rni a n d rn~ males as well as mosaic males. The results o b t a i n e d are, in principle, not different from those o b t a i n e d b y testing m~ males a n d therefore need not be elaborated here. I t should be noted t h a t homozygous m , females which are somatically mosaic are rare indeed in comparison to such m~' males. This is not u n e x p e c t e d since both m i / m , a n d rn~/rn~' are m i n i a t u r e in wing phenotype. The inordinate m u t a b i l i t y of rn~ posed three relevant questions: what are the mutabilities of dy 73 from which rn~' appears to have arisen, of the rn~ derivatives of rm' a n d of the m s derivatives of m , ? J u d g e d b y its behavior in an en masse m a i n t a i n e d stock, dy TM appeared to be m u t a t i o n a l l y stable. Nonetheless, an a t t e m p t was made to estimate the m u t a b i l i t y of dy 7a b y the following experiment. Newly emerged d v TM males were i n d i v i d u a l l y crossed to 6 females D f ( z ) m - f ~ , / F M 7. Females were allowed to oviposit for s i x d a y s a n d then discarded. Since d y / D f ( z ) r n - f w females are d i m i n u t i v e wing in p h e n o t y p e a n d can be easily distinguished from m/Df(z)m-fa., and d3,+/Df(z) mf w , these F~ females were scored. Reversions of dy 7a to @+ or m u t a t i o n s of @7.~ to m were sought. The results are not unusual. A total of 46 males was tested. Each produced more t h a n ioo scorable FI females (range I 2 I 268 females per male; mean n u m b e r of females per male was 2o8.3). All in all 958I F 1 females were scored and no m u t a t i o n s to d>,+ or to m were found. I n comparison to mr,, ely :a is n m t a t i o n a l l y stable. Stocks of six i n d e p e n d e n t m ~ a n d six i n d e p e n d e n t m~ m u t a n t s were m a i n t a i n e d in mass cultures of m u t a n t males b y C ( z ) D X , y f females for several m o n t h s and were periodically checked. A m o n g the rn ~ r n u t a n t s no n m t a t i o n s of rn~ to m + or to rn were found. Similarly a m o n g the rn~ stocks, no m u t a t i o n s of rn~ to m were uncovered. This superficial testing suggested t h a t m ~a n d m ~ m u t a n t s , as compared to m~', are m u t a t i o n ally stable. A detailed check was made b y m a k i n g an in depth s t u d y of one rn~ a n d one m~ m u t a n t . M u t a n t s selected for s t u d y were a rn t designated here rn i~°7 m a r k e d with the sex-linked eye color m u t a n t s ras v a n d a rn~ designated here as m ~9 also m a r k e d with ras v. Mutations of n¢ ~7 to m or m + were sought in two different experiments. I n one e x p e r i m e n t single ras v n¢ 27 males were crossed to 5 lz~°~ rn females a n d the FI females scored for both m u t a t i o n s to m a n d reversions to m +. A total of 45 fertile males was tested a n d 2I 2z 9 F1 females scored (average n u m b e r of females scored per male was 471.5 ; range 298 559). No m u t a t i o n s to m or m + were found. I n a second e x p e r i m e n t i n d i v i d u a l ras v rn ~7 males were crossed to C ( I ) D X , y f females a n d the F~ males

NEW MUTABLE GENE IN DROSOPHILA

83

scored for m u t a t i o n s . I n this e x p e r i m e n t 44 males p r o d u c e d a m i n i m u m of IOO male progeny. A t o t a l of 13011 males were scored (mean male p r o g e n y per male was 313.8; range 151-363). A g a i n no m u t a t i o n s to m or to m + were found. These results suggest t h a t m l~v as c o m p a r e d to m , is m u t a t i o n a l l y stable. M u t a t i o n s of rn s'9 were also sought in two ways. I n d i v i d u a l ras v m st9 males were crossed to 6-8 D F ( z ) m - f w / F M 7 females a n d the Ft females ras v m s ' 9 / D F ( ~ c ) m f ~ scored for m u t a t i o n s to m or reversions to m +. A t o t a l of 42 ras v m st9 males were fertile a n d 9367 F , females scored (mean females per t e s t e d male was 228.5; range 156-3o5). No m u t a t i o n s were found. In a second e x p e r i m e n t single ras v rn ~9 males were crossed to six C ( z ) D X , y f females a n d the male p r o g e n y scored. Because m ~9 is not r e a d i l y s e p a r a t e d from wild type, only m u t a t i o n s to m were sought in this experiment. Here, too, 42 fertile males p r o d u c e d a t o t a l of 13179 male p r o g e n y (mean male p r o g e n y per t e s t e d male was 313.8; range lO5-4oo ) . Once again no m u t a t i o n s to m were found. Thus m s19 like m 127 a p p e a r s to be m u t a t i o n a l l y stable. DISCUSSION

I t is a m p l y clear from the foregoing d a t a t h a t m~ is a highly m u t a b l e gene, b o t h s o m a t i c a l l y a n d germinally. Several p e r t i n e n t questions are posed b y these d a t a . H o w does the m u t a b i l i t y of mY c o m p a r e with t h a t of mr- 3 o: of D. virilis a n d with other m u t a b l e genes in D. melanogaster ? W h a t is the origin a n d u n d e r l y i n g basis for the m u t a b i l i t y of rn~ ? W h a t is the n a t u r e of the m i a n d m s d e r i v a t i v e s of m , ? In m a n y respects my a n d ml- 3 oc are alike. Their rates of m u t a t i o n b o t h somatically a n d g e r m i n a l l y a p p e a r to be more or less equivalent. There is, however, one d e t a i l wherein m , a n d rot-3 oc a p p e a r to differ : whereas mt-3 oc i n v a r i a b l y m u t a t e d to m t +, m" reverts to m i a n d m s (the l a t t e r m i m i c k i n g m +) a n d t h u s far no reversion to m + has been found. I t is almost certain t h a t h a d mr-3 o~ r e v e r t e d to a t y p e e q u i v a l e n t to m i it would have been detected, since alleles of i n t e r m e d i a t e p h e n o t y p e as the m t locus were known. F u r t h e r m o r e , DEMEREC 1 t e s t e d m + males d e r i v e d from mr-3 oc b y crossing to the s t a n d a r d rnf-z allele of D. virilis. All tests showed t h a t p h e n o t y p i c a l l y m + was i n s e p a r a b l e from wild type. B y comparison with o t h e r m u t a b l e genes in D. melanogaster, m~ a p p e a r s to differ in one n o t e w o r t h y aspect. This is its high somatic m u t a b i l i t y as e v i d e n c e d b y the frequent mosaic males recovered. S o m a t i c m u t a t i o n has been observed a m o n g the several m u t a b l e w m u t a n t s described in D. melanogaster (GREEN4),b u t these are comp a r a t i v e l y infrequent when c o m p a r e d to germinal m u t a t i o n s . F o r the present the absence of a n y clear-cut i n f o r m a t i o n on w h a t m a k e s m u t a b l e genes m u t a b l e precludes a n y discussion of the causes of differential somatic versus germinal m u t a b i l i t y . So far as the origin a n d genetic n a t u r e of m , is concerned, o n l y speculation is possible. As p o i n t e d out at the outset, two fortuitous events a p p e a r to be a s s o c i a t e d with the origin of m~. I t will be recalled t h a t an u n u s u a l X chromosome a p p e a r s to have been involved. Based on genetic evidence, this X chromosome carries a piece of " f o r e i g n " D N A inserted in the environs of the w locus 9. This X chromosome was p r e s e n t in the female from which d y 3 was recovered. P r e s u m a b l y as a first step, dy 73 arose b y t r a n s p o s i t i o n of t h e " f o r e i g n " D N A to the dy locus. In a s u b s e q u e n t second s t e p " f o r e i g n " D N A t r a n s p o s e d from the dy locus to the neighboring m locus t h e r e b y p r o d u c i n g m,. T r a n s p o s i t i o n of " f o r e i g n " D N A a n d of small c h r o m o s o m e segments

84

M.M. GREEN

a s s o c i a t e d w i t h m u t a b l e genes h a v e b e e n d e s c r i b e d in maizeT, 8 a n d in Drosophila3, 5. I n m a i z e i n s e r t i o n s of " f o r e i g n " D N A ( - - c o n t r o l l i n g e l e m e n t ) can r e s u l t in m u t a t i o n a l l y s t a b l e m u t a n t s as well as m u t a b l e alleles 8. T h e t r a n s p o s i t i o n of " f o r e i g n " D N A f r o m a s t a b l e m u t a n t to a n o t h e r site to p r o d u c e a n m t a b l e allele is also k n o w n for m a i z e . T h u s t h e t w o steps s u g g e s t e d for t h e mr' are n o t w i t h o u t p r e c e d e n c e . Unf o r t u n a t e l y c o r r e l a t i o n is n o t p r o o f a n d t h e m o d e of origin t o g e t h e r w i t h t h e p r e s e n c e of t h e i n s e r t e d " f o r e i g n " D N A r e m a i n s to be r i g o r o u s l y e s t a b l i s h e d . C o n c e r n i n g t h e n a t u r e of t h e r e v e r s i o n s m ~ a n d m s, t h e r e is at p r e s e n t little t a n g i b l e i n f o r m a t i o n . T h u s m~ seems to m u t a t e e i t h e r to m ~or m s w i t h e q u a l l i k e l i h o o d B o t h a p p e a r to be m u t a t i o n a l l y stable. T h i s s u g g e s t s t h a t if m~' i n v o l v e s an i n s e r t i o n of " f o r e i g n " D N A , t h e m u t a t i o n s to rn~ a n d m~ are a s s o c i a t e d w i t h a " c h a n g e of s t a t e " of t h e " f o r e i g n " D N A . Loss of t h e i n s e r t e d D N A is d e e m e d u n l i k e l y since a r e v e r s i o n to m + w o u l d be t h e e x p e c t e d c o n c o m m i t a n t of such a loss. I n t e r a l l e l i c crossing o v e r e x p e r i m e n t s b e t w e e n m ~ a n d m a p p e d m m u t a n t s m i g h t shed s o m e light on w h e t h e r " f o r e i g n " D N A is i n v o l v e d . T h i s p r o j e c t r e m a i n s to be done. R I~.EE R ENCFS I DEMEREC, M., Miniature-alpha-a second frequently mutating character in Drosophila virilis, Proe. Natl. Acad. Sei. (U.S.A.), I Z (I926) 687-690.

2 I)EMEREC, M., Unstable genes in Drosophila, Cold Spring Harbor Syrup. Quant. Biol., 9 (I94 I) 145 15o. 3 GREEN, M. M., Controlling element mediated transpositions of the white gene in Drosophila melanogaster, Genetics, 61 (1969) 429 441. 4 GREEN, ~I. M., Some observations and comments on nlutable and nlutator genes in Drosophila, Genetics, Suppl. 73 (1973) 187 194. 5 ISlNC, G., AND C. RAMEL, The behavior of a transposing element in Drosophila rnelanogaster, Genetics, June Suppl. 73 (1973) si23. 6 LINDSLEY, D. L., AND E. H. GRELL, Genetic variations in Drosophila melanogaster, Carnegie lnst. ll'ash. Publ., (I968) No. 627. 7 McCLINTOCK, B., Chronlosoule organization and gene expression, Cold Spring Harbor Syrup. Quant. Biol., 16 (I94I) 13-47. 8 McCLINTOCK, B., Induction of instability at selected loci in maize, Genetics, 38 (1953) 579 599. 9 RASMUSON,B., M. M. GREEN AND B.-M. KARLSSON,Genetic instability in Drosophila melanogaster: evidence for insertion mutations, Mol. Gem Genet., 133 (1974) 237-247.