Copyright 0 1990 by the Genetics Society of America

Repression of P Element-Mediated Hybrid Dysgenesis in Drosophila melanogaster Michael J. Simmons, John D. Raymond, Korise E. Rasmusson, LorenM. Miller, Christopher F. McLarnon andJoseph R. Zunt Department of Genetics and Cell Biology, University of Minnesota, St. Paul, Minnesota55108-1095 Manuscript received August 3, 1989 Accepted for publication November 14, 1989 ABSTRACT Inbred lines derived from a strain called Sexi were analyzed for their abilities to repress P elementmediated gonadal dysgenesis. One line had high repression ability, four had intermediate ability and two had very low ability. The four intermediate lines alsoexhibited considerable within-line variation for this trait; furthermore, in at least two cases, this variation could not be attributed to recurringP element movement. Repression of gonadal dysgenesis in the hybrid offspring of all seven lines was due primarily to a maternaleffect; there was no evidence for repression arising de novo in the hybrids themselves. In one of the lines, repression ability was inherited maternally, indicating the involvement of cytoplasmic factors. In three other lines, repression ability appeared to be determined by partially dominant or additive chromosomal factors; however, there was also evidence for a maternal effect that reduced the expression of these factors in at least two of the lines. In another line, repression ability seemed to be due to recessive chromosomal factors. All seven lines possessed numerous copies of a particular P element, called K P , which has been hypothesized to produce apolypeptide repressor of gonadal dysgenesis. This hypothesis, however, does not explain why the inbred Sexi lines varied so much in their repression abilities. It is suggested that some of this variation may be due todifferences in the chromosomal position of the K P elements, or that other nonautonomous P elements are involved in the repression of hybrid dysgenesis in these lines.

T

H E many families of transposable elements that have been identified in Drosophila melanogaster exhibit considerable variety in size, structure and behavior. One of these, the P family, consists of elements less than 3 kb long, and is responsible for a condition known as P-M hybrid dysgenesis (KIDWELL,KIDWELL and SVED 1977; BINCHAM,KIDWELL and RUBIN 1982).Thiscondition is normally confined tothe germ line, where it is manifested by elevated mutation rates, chromosome breakage, segregation distortion and sterility (ENCELS1989). As its name implies, hybrid dysgenesis occurs in the progeny of crosses between strains. However, not all inter-strain crosses produce dysgenic offspring. Genetic and molecular analyses have demonstrated that dysgenesis requires a specific protein, the P transposase, aswell as a cellular environment that allows P element movement. The transposase is encoded by the four open reading frames of structurally complete (2.9 kb) P elements (KARESSand RUBIN1984; ENGELS 1984; LASKI,RIO and RUBIN 1986; RIO, LASKIand RUBIN 1986). Incomplete elements, which are deletion derivatives of these 2.9-kb elements, cannotmake the transposase, but, as longastheirterminal and T h e publication costs of this article were partly defrayed by the payment of page charges. This article nust therefore be hereby marked “advertisement” in accordance with 18 U.S.C. $1734 solely to indicate this fact. Genetics 1 2 4 669-676 (March, 1990)

subterminal sequences are intact, they can be acted upon by it (ENCELS 1984,1989; SPRADLINCand RUBIN 1982). Such elements are functionally nonautonomous since their movement requiresthe presence of complete, or autonomous, P elements. Several laboratories are currently investigating the cellular conditions that control P element activity. Early studies had identified two alternative states, or cytotypes, that permit or repress movement (ENCELS 1979a, b, 198b). 1 The M cytotype is permissive, while the P cytotype is repressive. These two states are characteristic of certain types of Drosophila strains, which are designated M and P, respectively. M cytotype strains completely lack P elements, whereas P cytotype strains possess many different kinds. Detailed studies have shown that cytotype is determined jointly by a combination of chromosomal and cytoplasmic factors, and that the influence of the latter can persist forat least two generations (ENGELS 1979a). These cytoplasmic factorsaccountfor the different levels of dysgenesis that are seen in genetically identical hybrids from reciprocal crosses between P and M strains (ENGELS 1979a,b; SIMMONSet al. 1980). In such crosses, the cytotype of ahybrid is usually the same as that of its mother. Thus, hybrids fromthe cross P female X M male inherit the P cytotype, which represses P element activity, whereas

664

M. J. Simmons et al.

hybrids from the reciprocal cross inherit the M cytotype, whichallows this activity. The latter type of hybrid is therefore much more likely to manifest dysgenic traits. The offspring of first generation backcrosses to the male parent also show the influence of these cytoplasmic factors, since they tend to have the same cytotype as their grandmothers; the persistence of this influence through two generations establishes that it is not simply a maternal effect, but rather, a bonafide case of maternal inheritance. In subsequent backcrosses, the cytotype is apt to switch to that of the backcrossing strain, indicating that it is ultimately determined by factors onthe chromosomes. Such changes appear to be sudden and complete, for intermediate or transitional states are rarely observed (ENGELS 1979a). More recent studies have revealed considerable variation in the inheritance of these regulatory states (KIDWELL1983, 1985; SIMMONS et al. 1987; BLACKet al. 1987). This variation is seen in many strains that carry P elements, especially those derived from populations in Europe, Africa and Asia (ANXOLABEHERE et al. 1984, 1985). Such strains are referred to as M' or pseudo M because they permit P element movement in some of their hybrid progeny.Detailed studies with one of these strains, called Sexi, have suggested that it possesses a system for regulating P elements that is different from cytotype (KIDWELL1985). We have investigated P element regulation in seven inbred lines derived from the Sexi strain. Such lines should be fixed for many of the P elements they carry, especially if no autonomous P elements are present. The regulatory properties of these inbred Sexi lines have been investigated by studying the repression of gonadal dysgenesis, an abnormality that occurs when the embryonic germ line fails to develop. In the adult, this condition is manifested by rudimentary gonads that are incapable of producing gametes (ENGELS and PRESTON1979; KIDWELL and NOVY1979). The resulting sterility is often referred to as dysgenic or GD sterility. The precise etiology of this condition is not known, but it probably stems fromP-element mediatedchromosomebreakage anddominant lethal mutations (ENGELSet al. 1987; SIMMONS et al. 1987; RASMUSSON et al. 1990). Gonadal dysgenesis occurs in the hybrid offspring of certain crosses. Typically, one of the parents in these crosses induces P element activity in the offspring by contributing transposase-producing P elements. The other parent may contribute factors that regulate the level of this activity, thereby influencing the likelihood that gonadal dysgenesis will occur. If, in a series of crosses, the inducer contribution is held more or less constant, it is possible to estimate the relative effects of the regulatory factors that are contributed by the other parent. This is done simply by

noting the proportion of dysgenic offspring that are obtained from each mating. A high proportion would indicate thattheparentcontributed weakly tothe regulation of P element activity, whereas a low proportion would indicate that it contributed strongly. We shall refer to this measure of regulatory ability as the "repression potential" of the parent. If the parent is a male, this potential can only involve chromosomally transmitted factors, whereas, if it is a female, both chromosomal and cytoplasmic factors can be involved. In this study we have sought todeterminethe strength and variability of the repression potential of flies from each of theinbred Sexi lines. We have investigated whether this potential varies within and among the lines, and whether it can exist stably as a state in between the M and P cytotypes. In addition, we have investigated whether or not the repression of gonadal dysgenesis involves amaternaleffect;previous analyses of M' strains have not considered this issue (KIDWELL1985;BLACKet al. 1987). We have also studied the inheritance of repression potential, utilizing hybrids from reciprocal crosses between the Sexi lines and a trueM strain. Experimental tests with these hybrids have allowed us to separate the genetic and cytoplasmic factors that contribute to repression potential. Finally, we have analyzed DNA from each of the Sexi lines, focusing on one type of P element that has been proposed to account for the repression of gonadal dysgenesis. This putative repressor, called the K P element, has been found in many M' strains, including Sexi, whereit is especially abundant (BLACK et al. 1987).Our combined genetic and molecular data have been used toevaluatethe K P repressor hypothesis. MATERIALS A N D METHODS Stocks: The stocks used in these experiments are listed below. Information about the genetic markers and rearranged chromosomes can be found in LINDSLEYand GRELL(1968) and in SIMMONS et al. (1987). True M stocks (with the M cytotype and devoid of P elements): 1. bw; st, a stock homozygous for recessive autosomal

markers. 2 . ,445; bw; st, a stock homozygous for the X chromosome balancer Muller-5, as well as the autosomal markers bw and st.

3. C ( I ) D X , y f / Y / y cin w f ' s ~ ( f ) ' " " ~a gstock , in which the females have attached-X chromosomes and the males have an X chromosome with a temperature-sensitivelethal mutation (su(f)""g) that aids in the collection of virgin females. 4. C( I ) D X , y f / Y / y shi'"; bw; s t , another attached-X stock in which the males have an X chromosome with a different temperature-sensitive lethal mutation (shi"). This stock is also homozygous for bw and st. 5. y sn3 u car, a stock homozygous for four recessive Xlinked markers, including an extreme allele of the singed (sn)locus. Stocks containing P elements:

Repression of Hybrid Dysgenesis

6. a2,an inbred, wild type strain with P cytotype that is capable of inducingahighfrequency of GD sterility in dysgenic crosses (ENCELSand PRESTON1979). 7. C(I)DX, y f/y/sn"; r2,an attached-X strain with the genetic background of the r2stock. T h e males in this strain carrythe P element-insertion mutation singed-weak (sn"') (ENGELS1979b, 1984; ROIHA, RUBINand O'HARE 1988), which becomes unstable in the presence ofthe P transposase. However, in this stock the sn"' mutation is stabilized by the P cytotype. 8. y snw/y+Y; bw; s t , an M cytotype stock homozygous for snw. T h e only P elements in this stock are nonautonomous elements located in the vicinity of the singed locus. 9. M5-B#1, an inbred M' strain derived from a Muller-5 balancer stock from the University of Birmingham, England KIDWELL and RUBIN1982). All the P elements (BINCHAM, in this strain are nonautonomous (SIMMONS et al. 1987). 10. v6 an inbred P cytotype strain capable of repressing, b u t incapable of inducing, GD sterility (ENCELSand PRESTON 198 1; SIMMONS et al. 1984). P[ry' Sal11 (89D), a stock homozygous for a 11. single P element in which a frameshift mutation has been and RUBIN1984). created in the Sal1 restriction site (KARESS 12. C ( I ) D X ,y fIYIT-5 (p.3'),y snw; bw; st, a subline ($3') of the T-5 stock described by SIMMONS et al. (1987). T h e T5 X chromosome in the males carries autonomous P elements capable of destabilizing sn". To maintain this stock, T-5,y sn"; bw; st males that are phenotypically weak singed are crossed to C(I)DX, y f ; bw; st females (from stock 4 above) at 2 1 . In previousexperiments, theT-5 X chromosome was unable to induce a significant frequency of GD sterility in a pure M genetic background (SIMMONS et al. 1987); however, by the time of the present experiments, this chromosome had acquired a marked ability to doso. 13. Sexi, a wild type stock derived from a single inseminated female caught in Spain in the mid-1970s (KIDWELL 1985). Genetic analysis has indicated that this stock possesses and some autonomous P elements UONCEWARD,SIMMONS HEATH1987). Inbred lines were produced from this stock by 2 1generations of full-sib mating at 25'. Thereafter, the lines were maintained by small mass matings at 21 T h e inbreeding commenced with Go in May, 1985and was completed by Gn4 in March 1986. In G I I ,G L 2and GIs, the inbreeding scheme was relaxed in order to obtain enough flies for genetic tests. Experimental methods:Stocks and experimental cultures were raised in vials and half-pint milk bottles on a standard cornmeal-molasses medium. Typically, the flies that were used to initiate experiments came from cultures that had been reared at 25 T h e basic methods fortesting the germ line instability of sn"' and for determining the frequency of gonadal dysgenesis (GD sterility) in hybrid flies were given by KOCUR, DRIERand SIMMONS (1986) and by SIMMONS et al. (1 987),respectively. Slight differences are noted in the text. DNA forSouthern analysis was extractedfromadult females. Samples (3-6 pg) of DNA were digested with restriction enzymes according to the supplier's instructions (Bethesda Research Laboratories). T h e DNA fragments were separated in agarose gels by electrophoresis and then transferred to GeneScreenPlus nylon membranes (Dupont) by capillary blotting. T h e membranes were hybridized with radiolabeled DNA probes that had been synthesized with ['"PIdCTP (3000 Ci/mM, Amersham) from fragments of the plasmid ~ ~ 2 5 (O'HARE . 1 and RUBIN 1983) using a random primer DNA labeling system (Bethesda Research Laboratories). Plasmid fragments were isolated from restrictionenzyme digestions that were electrophoresed in low O

O .

O.

665

melting point agarose gels. After hybridization, the membranes were washed at 65" in 0.1 X SSC, 0.1 % SDS, sealed in plastic bags and exposed to X-ray film with intensifying screens (Dupont) at -80". Membranes that were used for rehybridization were stripped of previously bound probeby shaking at 42" in 0.4 N NaOH and then in 0.1 X SSC, 0.5% SDS, 0.2 M Tris (pH 7.5). RESULTS

Potential for inducing hybrid dysgenesis:T o provide background information for the study of repression potential, each of the inbred lines derived from the M' strain called Sexi was tested for its ability to induce hybrid dysgenesis. Two traits, sn" mutability (ENGELS1979b, 1984) and GD sterility (ENGELSand PRESTON1979; KIDWELL and NOVY1979), were studied. In the sn" tests, males from each Sexi line were mass-mated to y sn"; bw; st females at 25". The resulting hybrids were then crossed to allow the detection of mutations of sn" occurring in their germ lines. Both sne and sn(+)mutations could be detected in the crosses with thehybrid males, but only $ne mutations could be detected in the crosses with the hybrid females; the reason is that these females already carried a wild type singed allele (KOCUR, DRIER and SIMMONS1986). In both sets of crosses, some of theapparentmutations of sn" were subsequently tested for complementation with sn3 to determine if they were genuine. Because the mutability of sn" requires the P transposase, the occurrenceof any bona f i d e mutations would indicate that an inbred line carried at least one transposase-producing P element on its chromosomes. The sn" mutability tests were initiated four timesin generations 11, 18, 38 and 45-during the propagation of the inbred lines (Table 1). In all these tests, only one of theinbred lines, Sexi.3,exhibiteda marked ability to destabilize sn" in hybrid flies. The persistence of this ability indicated that Sexi.3 maintained at least one autonomous P element in its genome.Autonomouselements were also present in Sexi. 1 , Sexi.2 and Sexi.4, since each of these induced a few bonafide mutations of sn"; however, Sexi.4 seems to have lost this ability (and, presumably, its autonomous P elements) sometime during the experiments. The other inbred lines showed little or no ability to destabilize snw. No mutations were induced by Sexi.5 at anytime, and only a few apparent mutations (which were not tested for authenticity) were induced by Sexi.6 and Sexi.7-al1in the experiment initiated in GI]. The absence of any bona fide mutations from these last three lines suggests that they did not have autonomous P elements, or if they did, that these elements were lost early in the propagation of the lines. For the GD sterility tests, males from each of the inbred lines were individually mated to bw; st females

M. J. Simmons et al.

666

TABLE 1 Mutability of sn" among the F, progeny of crosses between y sn"; bw; st females and inbred Sexi males monitored over 44 generations ~

______~

~~

GI I Stor!,

Sex of Fl

N"

Sexi.1

Male 48 1320 Female 42 1376 Sexi.2 Male 49 1084 Female 44 1233 Sexi.3 Male 48 1002 Female 1270 45 Sexi.4 Male 941 45 Fenlale 47 1463 Sexi.5 Male 117548 Female 047 1522 1423 Sexi.6 Male 56 049 (8)1122 0 1087 Female 42 38 0 1 100 Sexi.7 Male 48 0 1113 Female 0 1439 47 1636 50

49

+'

wb

_____~~ ~~~~

GI,

0

ed

1V

w

0 0 0 (2y

Gs*

+

e

N

w

101 0 0 57 1303 2335 89 2492 0 50 1458 0 49 980 58 01382 0 0 46 1204 1 54 1586 (53) (54) 1362 60 (62) (45) 58 1376 (22) 49 (14)1345 1614 56 3+(3) 142458 (1) 0 0140457 (1) 51 1657 1742 (1) 57 1704 0 0 53' 977 0 0 0 59 0 1378 51 0 52 1612 1070 0 0 58 1301 1205 0 56 1309 0 0 58 0 1539 0 1730 (1) 55

+ 0

G45

e

N

w

+

e

0 0

79 67 78 76

1609 1358 1521 1679

0

0 3 0 0

0 (52) 0 77

0

2 (69) 1+ (15) 0 152978 0 76 1493 0 69 0 72 0 76 0 0 76 74

1

0

1618

0

1385 2139 1558 1994

0 0

0 0 0 0 0

0 0

FI flies were mated at 25" and the F:! progeny were scored until the 15th day after mating. Number of FI cultures. Number of weak singed F2 progeny. ' Number of wild type F2 progeny. Number of extreme singed F2 progeny. Numbers in parentheses indicate Fs progeny that were not tested to confirm the apparent mutation of sn"'. 'One culture in which a suppressor of sn" was segregating was not tallied in these data. a

'

TABLE 2 GD sterility among the F, daughters of crosses between bw; st females and inbred Sexi males

Stock

62 54 55

Sexi. 1 Sexi.2 Sexi.3 Sexi.4 Sexi.5 Sexi.6 Sexi.7 bw; stb

52 72 46 56 a

No. males No. FI females tested" sterile examined

6 5 5 6 4 7 4 5

76

Percent F I females

0 0

0

0

The inbred Sexi males came from GsH. Control.

at 29" to produce hybrid daughters. These were then examinedfor the presence or absence of eggs as described in SIMMONS et al. (1 987).As Table 2 shows, not even Sexi.3 induced a significant percentage of sterility in hybrid females. Additional data from experiments discussed belowcorroborate thesefindings. Potential for repressing gonadal dysgenesis: Females from the seven inbred Sexi lines exhibited different abilities to repress GD sterility in their hybrid offspring. Table 3 presents theresults of experiments thatmonitored this ability over29generations.In each experiment, a sample of females from an inbred line were crossed individually to males from the P strain ~2 at 29". As many as 12 of the daughters from each cross were thenexamined for GD sterility. Throughout the course of these experiments, Sexi.4 and Sexi.7 behaved like the control strain,bw; st, since

nearly all their hybrid offspring were sterile. In contrast, Sexi.3 behaved like a P cytotype strain; its hybrids showed almost no sterility in any of the experiments. The other inbred lines, Sexi.l, Sexi.2, Sexi.5 and Sexi.6, produced a mixture of fertile and sterile offspring, indicating that they possessed intermediate repression potential. The distributions of sterility amongthe hybrids from each of theintermediate lines are shown in Figure 1. Each distribution was constructed by plotting the frequency of sterility among the daughtersof individually tested females. It is clear from these plots that the factors affecting hybrid sterility varied continuously within the lines. It is also clear that some of the lines changed gradually during the course of the experiments. For instance, the distribution of Sexi.l was initially concentratedattheupperend of the scale; however, by G47it was concentrated at thelower end, having passed through a highly dispersed state. The distributions of Sexi.2 and Sexi.6 were also initially concentrated at the upper end of the scale, but by G47,both were dispersed over essentially the entire range. Inall three cases, a leftward shift had occurred, indicating the emergenceof greater repression potential. In contrast to these, the distribution of Sexi.5 remainedmore or less unchanged throughoutthe period of study. Repression of sterility involves a maternal effect: The above experiment.s have shown that GD sterility is repressed in the hybrid offspring of females from some of the inbred Sexi lines. However, they have not shown whether this repression arises in the hybrids

Repression of Hybrid Dysgenesis

667

TABLE 3 GD sterility among the F1daughters of crosses between ?TZ males and inbred Sexi females monitored over 29 generations GW

G,n

G47

G40

G32

Stock

R;"

nb

GD+sE

.I;

n

GD+sE

N

n

GD+sE

N

n

GD+sE

N

n

Sexi.1 Sexi.2 Sexi.3 Sexi.4 Sexi.5 Sexi.6 Sexi.7

32 35 34 27 26 17 35 11

383 420 408 318 309 160 416 132

96.8 f 1.8 86.7 f 2.3 5.9 f 1.6 99.4 f 0.4 60.5 f 4.1 86.3 & 3.2 97.5 f 0.9 100

25 25 25 25 23 15 25 14

300 294 299 299 273 160 300 131

99.3 f 0.5 87.3 f 2.7 0 99.7 f 0.3 46.5 f 4.5 52.1 f 6.0 98.7 f 0.9 99.1 rt 0.9

25 25 29 25 29 23 25 25

300 300 345 292 334 194 293 273

70.7 f 6.7 64.0 f 5.0 1.4 f 0.6 94.3 f 1.7 59.7 f 4.0 43.7 f 5.9 99.2 f 0.6 100

28 41 25 29 25 25 25 22

330 477 295 308 267 271 294 230

32.7 f 3.9 71.7 f 3.5 0 95.9 f 3.2 57.2 f 4.4 62.3 f 3.5 99.7 f 0.3 98.8 f 0 . 6

19 34 27 19 36 22 38 33

207 351 323 223 406 212 433 296

bw;st

GD

+SE

11.4 f 3.0 48.0 f 4.9 0.3 f 0.3 86.4 rt 5.7 41.3 f 3.1 58.6 f 3.9 97.0 f 1.4 98.5 f 0 . 7

Number of females crossed individually to P:!males. Total number of F, daughters examined. Unweighted average percentage of daughters with GD sterility f standard error. Sexi.1

0

'.O

Sexi. 5

Sexi.2

Sexi.6

t J tc M uutci I

00

I

U

G

4

0

1 1 m

100 0

100

0

100

0L

J

G1004

7

Percent GD Sterility FIGURE1 .-Distributions of GD sterility among the daughtersof inbred lines that had intermediate repression potential. T h e frequency indicates the proportion of daughters from a given female that were sterile. Results from five different generations areshown.

themselves, or is due to a maternal effect. T o investigate this issue, each inbred line was crossed reciprocally to flies that carried a T-5 X chromosome. This chromosome has sn" plus at least one transposaseproducing P element and is able to induce high levels of GD sterility. By comparing the frequencies of sterility in the hybrid females from these crosses, it was possible to determineif a maternaleffect was involved. The T-5-bearing flies came from crosses between T-5, y sn"; bw; st males and C( I ) D X , y f; bw ; st and M5; bw; st females; each T-5 male was mated at 21 to both types of females. Then, from each pair of O

crosses, T-5 sons from the first mating and M5/T-5 daughters from thesecond were crossed, respectively, to females and males from each of the inbred Sexi lines; the same crosses were also carried out with two control stocks, bw; st and ~ p instead , of the Sexi lines. In all these crosses, single pair matings were used and the cultures were incubated at 29". In the next generation, samples of T-5/+ (and, where appropriate, M5/+) females from each culture were examined for GD sterility. As a check on the frequency of sterility induced by the Sexi and control stocks themselves, a parallel experiment was performed in which the T-5 X chromosome was replaced by the y snw X chromosome from stock 8. Before examining the results, it should be noted that thesnwallele on theT-5 X chromosome was highly mutable. The average mutation rate, 0.423 +: 0.019, was estimated by scoring the sons of the matings between the T-5 males and the C( I ) D X , y f; bw; st females at the beginning of the experiment (26 matings, each transferred twice, for a total of 1174 progeny scored). Thisrate is comparable tothe rates induced by the strongest P strains and indicates that the autonomousP elements on theT-5 X chromosome were able to generate much transposase activity. In contrast, they sn" X chromosome that was used in the parallel experiment was unable to generate any transposase activity. Table 4 presents the main results. First, consider the data from crosses 1 and 2, which tested the ability of the Sexi and control strains to induce GD sterility without the T-5 X chromosome. In these crosses, y sn"/M5 females (cross 1) and y snW males (cross 2) were mated to flies from each of the Sexi and control stocks. Since these males and females did not carry any transposase-producing P elements, any sterility that occurred in their offspring must have been induced by elements contributed by their mates. It is clear from the data that none of the Sexi lines was able to induce much sterility in either cross 1 or cross

M. J. Simmons et al.

668

2, confirming earlier results (Table 2). In each case, the frequency of sterility was comparable to that obI served in the crosseswith the bw; st controls.In +I +I +I+I +I +I 8: +I ~+I+Im +I m * - m m - m contrast, the other control strain, 7r2, induced a high ?v\ S -Wim&& W&m;& m e " level of sterility in the offspring of cross 1, but almost L v none in the offspringof cross 2. These opposite results w09wam*pP-mm -*mm*P-P-om $ C m - * m m - w were expected from the known properties of the 7r2 6 strain (ENGELS1979a; ENGELSand PRESTON1979). m m * o m m m m m z m - e m m m * Now consider the data from cross 3, which demonstrate that the T-5 X chromosome could induce a v?s~v?z~~~ high frequency of GD sterility, both by itself and in I +I+I+I+I+I $1 +I+I+I combination with chromosomes from the inbred Sexi 094?4?c???c? r-09wQi*mOiQ, ?.o w lines. In this cross, T-5/M5 females were mated to mmP-mmwmP-oI males from each of the Sexi and control stocks. Both n P - m m * m o Q i m T-5/+ and M 5 / + females emerged in the progeny. In P-Q,ammP-mm 3 s d m m m m m m m m all cases, the T-5/+ females suffered appreciable ste6 rility, whereas the M5/+ females were usually fertile. *mm*'C.Iaem z m m m m m m m m m m (One exception is the cross with 7r2, where a high ~P-Q,P-Q,oQ,-Q, frequency of sterility was observed-and expected-in w ccj4;oj4;oj4L6;ojo vi B both classesof progeny.) The large and consistent +I+I+I+I+I +I +I +I +I c??9419"09? differences between these two classes of females indi? . 0 *- m m" m * o - m P - m Q, "m cate thatthe sterility-inducing factors were linked e 2 -! ;o mainly to the T-5 X chromosome. This is most plainly r - - - * ~ m m m m a s 6 1 * m a m m P - - a 9 seen in the results with the bw; st controls; 79.7% of m m m m m m m m e c; 0 the T-5/+ females were sterile, comparedto only 7.8% e m o o * m a * m 2h z m m m m m m m m m m of the M5/+ females. The low frequency of sterility w n in the M5/+ class probably reflects the action of P w elements that had transposed from the T-5 X chro4 d a m - a $1 +I +I 0 +I 0 +I+I +I mosome to other chromosomes in the genome in a 4 s b -2 $ 0 1?? 3 2e.o -09 previous generation. This might also explain the low .-.* E i frequency of sterility amongtheM5/+ females from 3 2 a - m - m t - m w m $ e -mp~--mwcnoa 0 the crosses involving the Sexi lines. P e - * - m m m -E In cross 3 it is interesting tonotethattheT-5/+ 0 6 b 0 2 offspring of the Sexi lines tendedto show more sterilI D P m m w m m m z m-*-h m m m 5 1 ity thanthe T-5/+ offspring of the bw; st controls. This suggests thatthe paternally derived Sexi chroc? m 0 A 0 I 2 mosomes actually enhancedthe sterility that was inLr +I 0 0 0 0 +I 0 +I t l 0 duced by the T-5 X chromosome (see also thedata of 0, $ 0 L RASMUSSONet al. 1990).In only one case (Sexi.6) was E .5 there noticeably less sterility than in the bw; st cont.Q,m'C.IP--mm2 s mm-wP-P-mm0 0" trols, but this was not significant (0.05 < P < 0.10 by m m m m m m m m m e .-c 0 2 a one-tailed Mann-Whitney rank sum test). The inc 2 m m m P - a Q , - m z w y creased sterility that was observed with the Sexi lines m m m m m m m m m .Lc 4 is similar to that seen with another M' strain, Muller.-c ? E 5 Birmingham (SIMMONSet al. 1987), and might be b EE +I 0 0 0 0 +I 0 +I +I +I $ g due to interactions between the T-Zgenerated transg: .2 :$ k + g ? 5E posase and the largenumber of P elementsthat were 09 2 g inherited from these lines (see RASMUSSON et al. 1990). r u m - - o m * m P - w To see that the repression of GD sterility in the 6 A 2 $ z c wmDmo mI mmw mm Pm- mm m- m0 9 EZd o A g & A g o ; % % 5 i k > offspring of the Sexi lines involves a maternal effect, 5 5 Z A 1 A ~ ~ ~ consider the data from cross 4. In this cross, T-5 males - a 0 9 o Q , u x g x gc"z12 5 ammmmmmmr m m m m m Z& S o + 1 0 0 2 were mated to females from each of the Sexi and 4 ' e x 7 x .? control stocks. Although the daughters from this cross - m m * m w P " . 2 o $ u