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On the origin of new lethal chromosomes and its rate in laboratory populations of Drosophila willistoni
SHORT
I c h r o m o s o m e s and its rate Ilistoni*
tory
pulation genetics, lethal chromoson e eliminated b y selection. The pict nthetic lethals, lethal chromosom he generality of the occurrence of species of Drosophila1,9, 4 5, includ e new concept into models of popu :e scarcity of data ". The d a t a repo: roblem of the relative importance of m u t a t i o n and of recon Lof lethal second chromosomes in laboratory populatio relations of Dro 5 populations were started with 4 types of lethal second sec, chro~ aaintained in population cages, A, B, C, D, and E. The Th flies in he second chromosome genotypes nz/rl, nl/rg, n~/rl and a n n.2/r 2 al /ra, ha~r4, n4/ra and n4/r4. The symbols nl, rig, na and n4 refer t aosomes isolated from a natural population, while r~, r~ ra and r "1, r2, is induced b y radiation. Therefore all the original second se chr ations were lethal, and each one had an initial freque tuency of o. The populations were periodically sampled, and th the frequei d chromosomes studied using the methods described b y Maga
dered more [ and ~thals
stoni. :ics is ar on n the
sloni. each, md C ) and ethal erent f the ethal The
E I IEQUENCY
OF LETHALS
IN THE
EXPERIMENTAL
POPULATIONS
Population A Generation Chromosomes studied F r e q u e n c i e s of o r i g i n a l l e t h a l s O b s e r v e d f r e q u e n c i e s of new l e t h a l s E s t i m a t e d frequencies of new l e t h a l s
3-4 Io3 0.679 0.058 o.154
9 io 127 0.37 ° 0.082 o. I15
Population B 25 I35 0.222 0.066 0.078
3 4 85 o.576 0.047 O, I O 0
9-Io i 16 0.244 o,o47 0.057
lethal chromosomes (Ix) obtained from the samples were identified b y allelism t h r o u g h crosses with balancedd strains having the original lethals and the laboratory lethal marked chromosome 20)7. W h e n all the crosses of Ix /2o 7 with every one of the 7, n~/2°7, n9/2o7 for lethals from cages A, B and C and balanced strains rl/2o7, r2/2o7, ad n4/2o7, for lethals from cages D and E, gave normal with ra/2o7, r4/2o7, na/2o7 and non-allelic to the original lethals, and therefore to be a flies, the lx was considered nc ae. The technique permits the detection of new lethals new lethal second chromosome. only in second chromosomes; t h a t were made free of the original lethals through :he frequencies of the original lethals and the frequencies recombination. Table I gives the of the detected new lethals in the whole sample. It is reasonable to assume t h a t new :ate in chromosomes t h a t are lethal-free and in chromolethals originate at the same rate somes t h a t have the original lethals. The data permit an estimate to be made of the cals, in all the chromosomes tested, including those with rate of occurrence of new lethals ee of them. This was done b y generalizing for the whole the original ones and those free in p r o g r e s s w i t h financi a l a i d from t h e U S -A EC ( c o n t r a c t AT * This w o r k is p a r t of p r o j e c t s in (3o-~)-2733), T h e R o c k e f e l l e r F o umn d a t i o n , F A P E S P , CNPq, C A P E S a n d C N E N .
Mutation Res., 3 0966) 458-46o
(
ew lethals obtained among the chl d at Table I as estimated frequencJ samples were taken are shown in g 3r second-chromosome lethals an )OBZHANSKY et al? as 0.0098 or o.o te the frequencies of new lethals p ons the elimination of lethals by h ethals produced by mutation alon, being an overestimate as the elimin was not considered. Inspection of chroI after 3-4 generations the frequencies of new lethal second sec o.154, o.Ioo, o.212, o.314 in populations A, B, D and E, and o tions in population C. The rate of origin of new lethal second chromosomes chromo, is than that expected only by mutation. The only ex tfianation lethal chromosomes observed is due to recombination Ltion. Moreovc in the first few generations, under the experimental conditions co chromosomes were due to recombination than to mutation. mut~ Th~ conclusions of DOBZHANSKY'Sgroup 2,5,9. Another point in favour of the above interpretal ~retation is gi~ of the populations according to their origins. The populations po I intervals and of similar origins, A, B, on the one hand, ham and D, similar among themselves as to the frequencies of new n letha xl~vv
pulation C '06 87 31
6-7 44 0.273 o.2ol o.219
Population D 14-15 80 0.363 o.125 0.227
3-4 59 o.559 o. 119 o.212
9- 1 ° 14o o.3o 7 O.lO5 o.132
x~Lxtaxo
!tee of the thals. The n Table I. als in D. Ltion. This mutation. is negligiimated as heterozyshow that re already -2 generalch higher excess of show that lore lethal infirm the
~
--
',/.±xv
)mparison the same ther, were [6 for the .tu.t
Population E 25 146 0.287 0.090 O.ll 3
3-4 75 o.573 o.196 o.314
9-1o 124 o.2o9 o.o87 0.096
25 213 0.342 o.133 o.I52
first first AB sample, 1.156 for DE). However, A and B differ diJ significantly from C and D, the x 9 being lO.O83 for the estimated frequencies of new lethals of the first sam ple. 1 While the occurrence off new lethals through mutation should be similar in th the 4 populations, the production tion of synthetic lethals through recombination must dept 9end itution 8. upon the chromosomal constitu =onstitution A final point to be consid sidered is that a high frequency of new lethals is rapi considered pidly attained, and it is main1Lained with oscillations and decreases due to recombinatio tional synthesis and dissociation on and to selection. This seems to indicate that equilibri uilibrium for recombinational lethals lals is rapidly reached whereas that for mutational1lethals lethal is only slowly obtained.
Department of General Biology, Faculty of Philosophy, Sciences ciences and Lettres, University of S~o Paulo
(Brasil)
A . B R I T O DA CUN CUNHA
L . E . DE MAGALHJ AGALH~ES TOLEDO J. S. DE TOLE S. A . T O L E D O J R .
H. M. L. DE SOUZA Sol Mutation Res., 3 (1966) 458-46o 458-
SHOR atural populations, X I [ I . R e c o m b i n a t
bility in
9bscura, Genetics, 31 (1946) 269-29o. B. SPASSKY AND N. SPASSKY, Release
triability
ophila prosaltans, Genetics, 44 (I959) 75XrD N. SPASSKY, A c o m p a r a t i v e s t u d y of osophila, Genetics, 37 (I952) 65o-664. Y, Effects of disruptive selection, VI. ,
~?S in t w o )IEOEOIII(?
) ~26. c variability t h r o u g h recombination, VI
,,illisloni,
variability t h r o u g h recombination, l
theory,
NHA, J. S. DE TOLEDO, S. A. TOLm~O
\RGA, g .
and their s u p p r e s s o r s in experimental hila willistoni, Mutation Res., 2 (1965) 45 54[AGALH,~ES, L. E., J. S. DE TOLEDO AND A. B. DA CUNHA, Th The n a t u r e o illistoni, Genetics, 52 (1965) 599 6o8. NSKY, Release PASSKY, B., N. ~PASSKY, H. LEVENE AND TH. [)OBZHA [)OBZHANSK~ arough recombination, 1. Drosophila pseudoobscura, Genetics, 43 (1958 )
~f l)roso-
:eived June 7th, 1966 ration Res., 3 (1966) 458 46o
vosophila triabilitv
I c h r o m o s o m e s and its rate Ilistoni*
tory
pulation genetics, lethal chromoson e eliminated b y selection. The pict nthetic lethals, lethal chromosom he generality of the occurrence of species of Drosophila1,9, 4 5, includ e new concept into models of popu :e scarcity of data ". The d a t a repo: roblem of the relative importance of m u t a t i o n and of recon Lof lethal second chromosomes in laboratory populatio relations of Dro 5 populations were started with 4 types of lethal second sec, chro~ aaintained in population cages, A, B, C, D, and E. The Th flies in he second chromosome genotypes nz/rl, nl/rg, n~/rl and a n n.2/r 2 al /ra, ha~r4, n4/ra and n4/r4. The symbols nl, rig, na and n4 refer t aosomes isolated from a natural population, while r~, r~ ra and r "1, r2, is induced b y radiation. Therefore all the original second se chr ations were lethal, and each one had an initial freque tuency of o. The populations were periodically sampled, and th the frequei d chromosomes studied using the methods described b y Maga
dered more [ and ~thals
stoni. :ics is ar on n the
sloni. each, md C ) and ethal erent f the ethal The
E I IEQUENCY
OF LETHALS
IN THE
EXPERIMENTAL
POPULATIONS
Population A Generation Chromosomes studied F r e q u e n c i e s of o r i g i n a l l e t h a l s O b s e r v e d f r e q u e n c i e s of new l e t h a l s E s t i m a t e d frequencies of new l e t h a l s
3-4 Io3 0.679 0.058 o.154
9 io 127 0.37 ° 0.082 o. I15
Population B 25 I35 0.222 0.066 0.078
3 4 85 o.576 0.047 O, I O 0
9-Io i 16 0.244 o,o47 0.057
lethal chromosomes (Ix) obtained from the samples were identified b y allelism t h r o u g h crosses with balancedd strains having the original lethals and the laboratory lethal marked chromosome 20)7. W h e n all the crosses of Ix /2o 7 with every one of the 7, n~/2°7, n9/2o7 for lethals from cages A, B and C and balanced strains rl/2o7, r2/2o7, ad n4/2o7, for lethals from cages D and E, gave normal with ra/2o7, r4/2o7, na/2o7 and non-allelic to the original lethals, and therefore to be a flies, the lx was considered nc ae. The technique permits the detection of new lethals new lethal second chromosome. only in second chromosomes; t h a t were made free of the original lethals through :he frequencies of the original lethals and the frequencies recombination. Table I gives the of the detected new lethals in the whole sample. It is reasonable to assume t h a t new :ate in chromosomes t h a t are lethal-free and in chromolethals originate at the same rate somes t h a t have the original lethals. The data permit an estimate to be made of the cals, in all the chromosomes tested, including those with rate of occurrence of new lethals ee of them. This was done b y generalizing for the whole the original ones and those free in p r o g r e s s w i t h financi a l a i d from t h e U S -A EC ( c o n t r a c t AT * This w o r k is p a r t of p r o j e c t s in (3o-~)-2733), T h e R o c k e f e l l e r F o umn d a t i o n , F A P E S P , CNPq, C A P E S a n d C N E N .
Mutation Res., 3 0966) 458-46o
(
ew lethals obtained among the chl d at Table I as estimated frequencJ samples were taken are shown in g 3r second-chromosome lethals an )OBZHANSKY et al? as 0.0098 or o.o te the frequencies of new lethals p ons the elimination of lethals by h ethals produced by mutation alon, being an overestimate as the elimin was not considered. Inspection of chroI after 3-4 generations the frequencies of new lethal second sec o.154, o.Ioo, o.212, o.314 in populations A, B, D and E, and o tions in population C. The rate of origin of new lethal second chromosomes chromo, is than that expected only by mutation. The only ex tfianation lethal chromosomes observed is due to recombination Ltion. Moreovc in the first few generations, under the experimental conditions co chromosomes were due to recombination than to mutation. mut~ Th~ conclusions of DOBZHANSKY'Sgroup 2,5,9. Another point in favour of the above interpretal ~retation is gi~ of the populations according to their origins. The populations po I intervals and of similar origins, A, B, on the one hand, ham and D, similar among themselves as to the frequencies of new n letha xl~vv
pulation C '06 87 31
6-7 44 0.273 o.2ol o.219
Population D 14-15 80 0.363 o.125 0.227
3-4 59 o.559 o. 119 o.212
9- 1 ° 14o o.3o 7 O.lO5 o.132
x~Lxtaxo
!tee of the thals. The n Table I. als in D. Ltion. This mutation. is negligiimated as heterozyshow that re already -2 generalch higher excess of show that lore lethal infirm the
~
--
',/.±xv
)mparison the same ther, were [6 for the .tu.t
Population E 25 146 0.287 0.090 O.ll 3
3-4 75 o.573 o.196 o.314
9-1o 124 o.2o9 o.o87 0.096
25 213 0.342 o.133 o.I52
first first AB sample, 1.156 for DE). However, A and B differ diJ significantly from C and D, the x 9 being lO.O83 for the estimated frequencies of new lethals of the first sam ple. 1 While the occurrence off new lethals through mutation should be similar in th the 4 populations, the production tion of synthetic lethals through recombination must dept 9end itution 8. upon the chromosomal constitu =onstitution A final point to be consid sidered is that a high frequency of new lethals is rapi considered pidly attained, and it is main1Lained with oscillations and decreases due to recombinatio tional synthesis and dissociation on and to selection. This seems to indicate that equilibri uilibrium for recombinational lethals lals is rapidly reached whereas that for mutational1lethals lethal is only slowly obtained.
Department of General Biology, Faculty of Philosophy, Sciences ciences and Lettres, University of S~o Paulo
(Brasil)
A . B R I T O DA CUN CUNHA
L . E . DE MAGALHJ AGALH~ES TOLEDO J. S. DE TOLE S. A . T O L E D O J R .
H. M. L. DE SOUZA Sol Mutation Res., 3 (1966) 458-46o 458-
SHOR atural populations, X I [ I . R e c o m b i n a t
bility in
9bscura, Genetics, 31 (1946) 269-29o. B. SPASSKY AND N. SPASSKY, Release
triability
ophila prosaltans, Genetics, 44 (I959) 75XrD N. SPASSKY, A c o m p a r a t i v e s t u d y of osophila, Genetics, 37 (I952) 65o-664. Y, Effects of disruptive selection, VI. ,
~?S in t w o )IEOEOIII(?
) ~26. c variability t h r o u g h recombination, VI
,,illisloni,
variability t h r o u g h recombination, l
theory,
NHA, J. S. DE TOLEDO, S. A. TOLm~O
\RGA, g .
and their s u p p r e s s o r s in experimental hila willistoni, Mutation Res., 2 (1965) 45 54[AGALH,~ES, L. E., J. S. DE TOLEDO AND A. B. DA CUNHA, Th The n a t u r e o illistoni, Genetics, 52 (1965) 599 6o8. NSKY, Release PASSKY, B., N. ~PASSKY, H. LEVENE AND TH. [)OBZHA [)OBZHANSK~ arough recombination, 1. Drosophila pseudoobscura, Genetics, 43 (1958 )
~f l)roso-
:eived June 7th, 1966 ration Res., 3 (1966) 458 46o
vosophila triabilitv