Non-random localization of lethals in Drosophila

Mutation Research

411

Elsevier Publishing Company, A m s t e r d a m Printed in The Netherlands

NON-RANDOM LOCALIZATION OF L E T H A L S IN D R O S O P H I L A

R A P H A E L I"ALI( AND E L I E Z E R LIlrSCHYTZ

Labo~'atory of (;enelics, The Hebrew U~iversil3', Jem~salem (Israel) (Received March iSth, 19(~8)

SUMMARY

Inconsistencies in allelism tests were observed by SALCEDA among the lethals, obtained from SANKARANARAYANAN'S populations with radiation histories. These were interpreted as being due to a series of partially overlapping "deletions" clustered within short segments of the chromosome. Lethal mutants in these segments would have been protected from effective elimination by natural selection if they were completely, or nearly completely recessive. If also inversions with breakage points in these segments persisted longer in populations, the non-random distribution of breakage points of inversions from natural populations could be understood.

In a study of the fate of populations of Drosophila rnelanogaster which had a history of extensive irradiations, SANKARANARAYANAN14 observed an increase inthe allelism frequency of lethals, despite the considerable decline in their frequencies in the populations since irradiation was discontinued. His conclusion was that the elimination of lethals did not occur entirely at random and that some lethals were more likely to be eliminated than others. SALCEDA la, who studied the allelism between lethals from SA~
412

RAPHAEL

FALK, ELIEZER

LIFSCHYTZ

which a p p e a r e d only once, was allelic both with let(2)63a-I8, which a p p e a r e d 3 times, a n d with let(2)63a-23, which a p p e a r e d I I times. L e t h a l s -18 a n d -23 were not allelic with each other. PROUT tOO f o u n d t h a t a c o m m o n lethal c h r o m o s o m e in WAH~ACE'S old p o p u l a t i o n @5 was allelic to m a n y o t h e r lethals t h a t were i n d i v i d u a l l y non-allelic (WaLLaCE, personal c o m n m n i c a t i o n ) . Our recent studies of i n d u c e d lethals in short segments of the X - c h r o m o s o m e of Drosophila melanogaster 7 9 r e v e a l e d t h a t p r a c t i c a l l y all whole b o d y recessive lethal n m t a n t s i n d u c e d in p o s t - m e i o t i c cells b y X - r a y s were small chromosomal a b e r r a t i o n s : t h e y could be a r r a n g e d into a consistent linear a r r a y of " d e l e t i o n s " - - a f f e c t i n g one or more a d j a c e n t g e n e s - - t h r o u g h allelism tests. Such a p a t t e r n is u s u a l l y not o b s e r v e d when allelism of lethals d i s t r i b u t e d over the length of the chromosome is examined, s i m p l y because the p r o b a b i l i t y of o v e r l a p p i n g a m o n g short " d e l e t i o n s " is low. However, when TANO ~8 l o c a t e d lethals in the second chromosome of D. melanogaster b y using a series of o v e r l a p p i n g deficiencies, lie found t h a t 9 out of the 132 lethals obt a i n e d from various sources were l o c a t e d in an e x t r e m e l y short segment of the chromosome. A t least some of these lethals m u s t have been small " d e l e t i o n s " . SALCEI)A'S o b s e r v a t i o n s in a selected sample of lethals resemble the p a t t e r n of the " c o m p l e m e n t a t i o n m a p " t h a t we o b s e r v e d in our preselected c h r o m o s o m a l segment. Therefore, the simplest e x p l a n a t i o n for the inconsistencies in the allelisnl tests o b s e r v e d b y SALCEDA is t h a t m a n y lethals were "deletions" of various extent. Some lethals which were o b s e r v e d only once, or those which were found more t h a n once b u t h a d no inconsistent overlaps w i t h o t h e r lethals, m a y be considered to be "nonclustered l e t h a l s " or lethals in " s m a l l clusters". T h e y to() m a y be " d e l e t i o n s " a n d insofar as allelism was observed, it might have been due to p a r t i a l overlaps only; b u t this c a n n o t be d e t e r m i n e d from the a v a i l a b l e d a t a . There were a c t u a l l y 4 cases of " s m a l l clusters" where not more t h a n two genes per cluster could be detected. A b o u t half of the lethals could, however, be a r r a n g e d in one or at most two extensive clusters s p a n n i n g at least 5 15 genes in which lethal m u t a t i o n s m i g h t occur (Tables I a n d II). iI'A 1"I1.1~ i DISTRIBUTION

OF INDEPENDENT

LETHAL MUTATIONS

Poptdation

N o ! i n cluster

Clustered

Total

<~[~chtslered

A l/ () 1)

21 22 12 J8

14 15 8 [ I[ 25

35 37 .31 -13

4 °.0 .40.5 ] 0I. 3 j 58.2

TABI.E

5[3.I

lI

GROUPING

L E T H A L S IN C L U S T E R S

Population

Lelhals Jbzold (rely once

L e l h a l s in s m a l l chtslers and~or alleles

Lelhals i n extensive clusters

Total

A B

18 14

7 I2

6

l2

20 24 32 29

45 5° 5° 5°

C 1)

Non-linear

io

lethals

l I

were considered

M u l a l i o n t¢.es., 5 ( 1 9 0 8 ) 411 4 [ 6

twice as half a lethal,

N O N - R A N D O M L O C A L I Z A T I O N O F L E T H A L S IN

Drosophila

413

44 4~

A

42

.43~

29~

j..Z5~

2 6 ~

~20 '-& 9 t.j_

z4 t6

B

6

14 (461 42--

i

""47 3..E-~

{

~36

50~ 49

29 i

'

,

I

I

~ i

I

~

't,,_~5m '__2.O

55 C

44 -38 4 5 40

i!;r

!

~t 4_.& ',5 5O

~ 0 ~..~.. ~

2o 24 _t

{51 9 25j2_ (t21 (~.4124 29 -D

46

2o

2.~.2

tO

3 40 28 ~ g ~

I 1I 1II ]g ~ 71~Zll~lglX X XI ]tllXlnxlgXg Fig. i. "Complementation m a p s " of l e t h a l s f r o m p o p u l a t i o n s A t o D . T h e n u m b e r s of t h e l e t h a l s a r e t h o s e g i v e n b y SALCEDA. N u m b e r s i n p a r e n t h e s i s : n o n - l i n e a r l e t h a l s .

The "complementation maps" constructed from SALCEDA'Sallelism data are given in Fig. I. Population A was unirradiated. The total dose given to each of the other three populations was 12o oooR, but while it was given at a rate of 2oooR/ generation to population B, the rate was 4oooR/generation to population C and 6oooR/generation to population D. It should be noted that in each of the first three populations there was one inconsistency in the linear map (lethal Nc4o in population A, lethal @46 in population B, and lethal N:II in population C), while there were 8 such inconsistencies in the linear map in population D (lethals ~ 2 , 4, 5, 12, 14, 33, 47, 50). Such non-linear lethals could be due to the presence of two independent 2VIutation Res., 5 (1968) 4 1 1 - 4 1 6

414

RAPHAEL

FALK,

ELIEZER

LIFSCHYT2

lethals in some of the chromosomes. Alternatively, these non-linear lethals could be due to several types of induced aberrations, such as inversions, both breaks of which caused lethal mutations, without deleting the whole segment between the lethals. The fact that such non-linear lethals were found mainly in population D, which received 6oooR/generation, supports the interpretation that complex aberrations were the main cause of these non-linearities. From these "complementation maps" it nmst be concluded that the selected lethals were not randomly distributed over the chromosoum. Rather, groups of Io 15 genes, concentrated within short segments of the chromosome, out of the odd 4 o o 5oo genes which may mutate to lethal alleles% ~a are those that persist in the populations (Table III). It nmst be noted, however, that the clustering given in the "complelnentation maps" is a minimal estimate: it is probable that some "missing links" were not included in the samples of lethals chosen bv SALCEI)A. "I'ABLt*" NUMBER

111 OF CLUSTFRS

OF LFTHAL

GENES

O F G I V E N S I Z E S I N T H E VY, R I O U S P O P U L A T I O N S

Popzfla-

3 1 z n i m a ! n~*mber ol,qem~s in c h f s t e r

limz

z

~X B (."

2I 17 12

2

l)

12

2

2

3

4

6

.5

7

8

9

zo

I2

zz

z3

OF SALCEDA

I4

±5

I !

I

i [

Since it is plausible that lethals with different "complementation" patterns are independently induced lethals, the allelism due to multiple recovery of the same original nmtants will be less than that given by SALCEOA (compare our Table IV with SALCEDA'STable 3)- It is also probable that many of those lethals which appeared repeatedly were only' seemingly identical. They may be independent "deletions" of varying extent, all spanning a section only some loci of which may nmtate to lethals. TABLV. 1V ALLELISM

Popzflatim~

FREQUENCY"

O F L I ¢ T H A L S I N THIS I ~ O P U L A T I O N S

),la.vim2¢m tinzes lethal r e p r e s e n l e d ~n s a m p l e •

2

3

2 ,'; 27 21

5 ~ 7

i

l

B C

t

T

l)

.~5

4

2

A

4

1

5

6

7

8

i i

i

Non-random localization of lethals in chromosonles derived from populations was observed repeatedly in the past. Thus SPIESS et al. 17 found a non-random distribution of lethals in the chromosomes obtained from WALLACE'S population 3 3 , among them a cluster of non-allelic lethals in the same segment where TANOis located his cluster of independent lethals. Although PAIKp2 located the lethals he found in tile second chromosome of D. mdanogaster in Korean populations only to the chromosome arm, he too noted a non-random distribution of lethals. One prediction suggested by 2~Jztlalio;~ J~cs., 5 ( [ 0 0 8 )

411

410

NON-RANDOM LOCALIZATION OF LETHALS IN

Drosophila

415

results such as those of SALCEDAis that many of the lethals from the extensive cluster in one population would be allelic to lethals of the other populations. This prediction was fulfilled by TANO'Sts finding a cluster of lethals in the same segment of the chromosome as that of the cluster in the study of SPIESS et al? 7. Also PAIK'S'~ observation that there was more than the expected allelism between populations indicates a non-random distribution of clusters of lethals in natural populations. These patterns of clustering suggest that there is some common functional design for the genes in those segments of the chromosome. It is possible for instance that the characteristic of lethal mutants in such segments is just that they are completely, or nearly completely, recessive--and not heterotic as suggested by SANKARANARAYANANand ~ALCEDA.

This localization of lethals could also explain the well-known phenomenon of non-random distribution of breakage points of inversions in natural populations of Drosophila. NOVITSKI~° proposed that because of mechanical tension at meiosis breaks occurred preferentially near preexisting breakage points in inversion heterozygotes. Later the "splinter" hypothesis of preferential non-restitution of broken ends in unpaired segments of the chromosomes was proposed to explain this phenomenona,H,v~, 2°. In another series of studies~,~, 4 no accumulation of induced breakage points near the sites of the original breaks in inversion heterozygotes could be demonstrated. On the other hand, it was found by SPERLICH16 that while new inversions, induced by X-rays in the chromosomes of a highly polymorphic population of D. subobscura were, as a rule, eliminated within a short time, one inversion which had one of its breakage points practically at the same site as that of five other natural inversions, was retained at a considerable frequency for a long time. He suggested that rearrangements at certain regions of the genome endowed their carriers with a "heterotic position effect" which gave the genes in the inverted region a "start" for the establishment of a "coadapted" system. Recently VANN2~ demonstrated how inversions which were in themselves deleterious were kept in the population if the increased fitness endowed by their contents was greater than the disadvantage of the inversions themselves. It is therefore not essential to assume "position effect" heterosis, as long as it is assumed that breaks at some sites are causing less damage to the genotype than at other sites, so that some aberrations take longer to be eliminated from the populations than others. The presence of chromosome segments in which lethal mutants might be maintained longer in populations thus provides the sites where inversion breaks have better chances to be established. This study was supported by a research grant Aio-AH- 9 from the Research and Technical Program Division, U.S. Department of Agriculture. We wish to thank Dr. G. SIMCH~N for stinmlating discussions. t(l~l;]~; R1,;NC I,;S 1 BARAK, 1L., The frequency of c h r o m o s o m e breakage induced in s t r u c t u r a l heterozygotes, Israel J. Zool., I2 (I903) 224 . "2 BARZILAV, N., Radiation sensitivity of s t r u c t u r a l heterozygotes, Ph.D. thesis, H e b r e w University, Jerusalem, A u g u s t 1967. 3 BERNSTEIN, N., AND E. GOLDSCHMIDT, Chromosonae breakage in s t r u c t u r a l heterozygotes, Amer. Naturalist, 95 (I90I) 53-56. 4 (~OLDSCHMIDT,E., E. BARAK, N. BERNSTEIX AND R. FALK, The restitution of radiation induced breaks in s t r u c t u r a l heterozygotes of Drosophila, Proc. zzth Interm Co~tgr. Genel., i (I9()3) 72. 3lutation Res., 5 (I968) 4 r t - 4 1 6

4x6

RAPHAEL FALK, ELIEZER LIFSCHYTZ

5 GOLDSCHMIDT, E., J. WAHRMAN, :\. LFDERMANN-I{LEIN AND i{. WEISS, A two years' s u r v e y of population d y n a m i c s in Drosophila melanogaster, Evolution, 9 (I955) 353-3666 lVES, P. T., The genetic s t r u c t u r e of American p o p u l a t i o n s of Drosophila melanogasler, Ge*~elics, 30 (19451 I67-196. 7 LIFSCHYTZ, 12~., AND R. FALK, A systenl for the line s t r u c t u r e analysis of c h r o m o s o m e segments, Drosophila [~fi)rm. 5"err., 43 (19(17). bl LIFSCHYTZ, I~., AND t . lgALK, Analysis of the flue s t r n c t u r e of the chronaosome with induced lethals ill a p r e d e t e r m i n e d segnleut, Proc. I8lh 2Vieeting Israel (;e~2elies Socletp D e c e m b e r 1967. 9 LIFSCHYTZ, E., AND 1¢.. FALK, Fine s t r u c t u r e analysis of a ehronlosolne segment in Drosophila mela~wgasler, in p r e p a r a t i o n . lO NOVlTSKI, E., Chronlosome variation in Drosophila alhabasca, (;elzelics, 31 (~946) 508-524. 11 NovITsE[, F., C h r o m o s o m e breakage in inversion heterozygotes, Amer. Nalztralis/, 95 (I9611 250 25 [. I 2 I)AIK, Y. I~., Genetic wtriability in K o r e a n populations of Drosophila melanogaster, livoh~tion, l 4 (~96o} 293-3o3. 13 ~ALCEDa, g. 5I., Recessive lethals ill second chronlosomcs of l)rosopldla melanogaster with radiation histories, Genetics, 57 (1007) 691-699. 14 SANKARANARAYANAN,t(., Sonic c o m p o n e n t s of genetic loads iu irradiated e x p e r i m e n t a l populatious of Drosophila melat~ogaster, Genetics, 54 (T966) t 2 i - 13o. 15 SANKARANARAYANAN, 1(., hlfluence of selection on the viability of irradiated experimental p o p u l a t i o n s of Drosophila melanogasler, Genetics, 57 (1967) 687-690. I6 SPERLICH, D., Experinlentelle Beitrage zum P r o b l e m des positiven Heterosiseiiektes bei der s t r u k t u r p o l y m o r p h e n Art Drosophila subobscm'a, Z. Vererbm*gsl., 9 ° (19591 273 287. 17 SI'IESS, E. B., R. I3. HELLING AND ~I. R. CAPENOS, Linkage of a u t o s o m a l lethals from a laborat o r y p o p u l a t i o n of Drosophila melanogaster, Genetics, 48 ( t 963) r 377-1388. 18 TANO, S., Studies on the clustered recessive lethal genes on the second chroluosome of Drosophila mela~wgaster, Japan../. Genet., 41 (I966} 299 3o8. 19 THOMPSON, P. E., Effects of sonic autosomal inversions on lethal m u t a t i o n s in Drosophila mela~ogasler, Genetics, 45 (I96°) t567-158o. 2o THOMPSON, P. E., Asynapsis and m u t a b i l i t y in Drosophila melanogaster, Gem'ties, 47 (1962) 337-349. 2 ~ TOB:XRL I., Effects of t e n l p e r a t u r c on the viability of heterozygotes of lethal c h r o m o s o m e s in Drosophila melanogaster, Evolutio*z, 53 (I966) 249-259. 22 VANN, E., The fate of X - r a y induced c h r o m o s o m a l r e a r r a n g e m e n t s introduced into l a b o r a t o r y p o p u l a t i o n s of Drosophila melanogaster, Amer. Naturalist, ioo (19601 425-449. 23 WALLACE, B., Allelism of second chronlosome lethals ii1 Drosophila melanogas/er, Proc. Xatl. Acad. Sei. (U.S.), 36 (I95 o) 654-657.

Mutation Res., 5 (19681 411-416