M morphic for gene arrangements in their chromosomes. In D. pseudoobscura,

Transcription

1 EXPERIMETS O CHROMOSOML VRIBILITY I.DROSOPHIL ROBUST MX LEVIT Deprtnzent of Zoology, Columbi University, hte?e, York 2 Received ugust 29, 1950 OST species of Drosophil so fr studied in this respect re poly- M morphic for gene rrngements in their chromosomes. In D. pseudoobscur, D. persimilis, nd D. funebris, this polymorphism is dptive ( DOB- ZHSKY 1943, 1947, 194S, 194Sb, 1948c; DOBZHSKY nd EPLIG 1944; DUBII nd TIIKOV 1945, 1946, 1946b; SPIESS 1950). The dptive nture of the chromosoml polymorphism is shown by three types of evidence. (1) The frequencies of the gene rrngements often show cler geogrphic nd ltitudinl grdients. (2) Cyclic sesonl chnges in reltive frequencies of gene rrngements re observed in some loclities. (3) Experiments on the behvior of chromosomes with different gene rrngements in rtificil popultions in the lbortory show tht inversion heterozygotes possess, s rule, much higher fitness thn do the corresponding inversion homozygotes. CRSO nd STLKER (1947, 1949) STLKER nd CRSO ( 1948) hve discovered chromosoml polymorphism lso in D. robust Sturtevnt, species common in forested regions of the estern United Sttes. In ddition to the usul prcentric inversions, D. robust popultions contin lso t lest two pericentric nd two cytologiclly pprently terminl ones. Geogrphic nd ltitudinl clines hve been demonstrted for certin gene rrngements, but sesonl cyclic vritions re smll or bsent in the popultions living ner St. Louis, Missouri. The work reported in the present pper is study of nturl nd experimentl popultioiis of D. robust in the ew York City re. The evidence shows tht the chromc~soml polymorphism in this species is lso dptive, inversion heterozygotes possessing higher dptive vlues thn do the homozygotes. The work ws strted during the summer of Preliminry collections showed tht the gene rrngements 2L, 2L-1 nd 2L-3 in the left rm of the second chromosome would be especilly suitble for experimentl study. Experimentl popultions with different combintions of the gene rrngements were creted, nd the ensuing chnges in the reltive frequencies of the chromosoml types in these popultions were recorded by mens of periodic smpling. In the spring nd fll of 1948 nd in the spring of 1949 systemtic collecting ws mde in single loclity in ew Jersey to detect possible sesonl vritions in the incidence of the gene rrngements nd to obtin lrger number of strins for further experimentl popultions. 1 Submitted in prtil fulfillment of the requirements for the degree of Doctor of Philosophy in the Fculty of Pure Science, Columbi University. 2 Present ddress : Deprtment of Biology, Virgini Polytechnic Institute, Blcksburg, Virgini. GEETICS 36: 285 My 1951.

2 286 MX LEVIT MTERILS D METHODS Collections of Drosophil for this study were mde in 1947 in deciduous woods on the outskirts of Bronx County, ew York (3) ; in Tibbetts Brook Prk, Westchester County, ew York (10); ner Fort Lee, Bergen County, ew Jersey (6); in the Mriners' Hrbor section of Richmond County, ew York (11) ; t Cold Spring Hrbor, Suffolk County, ew York (2) ; nd t Tenfly, Bergen County, ew Jersey (1). In the prentheses re the number of strins developed from ech plce. Mss cultures were developed from ech collected femle, nd the slivry glnds of twenty to thirty lrve from ech culture were smered in ceto-orcein for qulittive nlysis. The quntittive collections were mde t Englewood Cliffs, Bergen County, ew Jersey, in n re bout 30 x 120 feet. The trps (Mson jrs) were mintined throughout the study by fstening them to brnches with wire. Collected femles were plced individully in culture bottles ; fter five dys they were trnsferred to fresh food. If lrve ppered in either bottle slivry smer ws mde from single femle lrv, nd the recorded rrngements contributed to n " egg smple " (following the terminology of DOBZHSKY nd LEVEE 1948). If no lrve ppered, the femle, presumbly uninseminted, ws mted to mles homozygous for ll chromosoml rms. Exmintion of seven to nine lrve from this cross determined the chromosoml constitution of the collected fly; the sum of these results my be clled n " dult femle smple." The corresponding " dult mle smple " ws obtined from mtings of collected mles nd virgin stock femles, cre being tken in ech instnce to exmine the slivry glnds of t lest one femle lrv. The homozygous stock used for most of the determintions, derived from Westchester County strin, ws homozygous for rrngements XL, XR, 2L, 2R, 3L nd 3R. The popultion cges used for D. pseudoobscur by WRIGHT nd DOB- ZHSKY (1946) re pplicble lso for D. robust. The pproximte durtion of the egg to egg cycle in the cges ws 35 dys t 25"C, 50 dys t 21"C, nd 68 dys t 15$"C. The erliest popultions were tken from pir mtings out of the mss cultures s described by WRIGHT nd DOBZHSKY (1946). The offspring of similr homozygotes from the collections, s well s from these mtings, were retined s homozygous stocks for use in subsequent cges. In ddition, the F1 or Fz from collected flies were used in the mkeup of lter popultions studying 2L vs. 2L-1 or 2L vs. 2L-3 when this would introduce no other rrngements of this rm. In ew York experiments t 15$"C were kept in constnt temperture room. The reltive humidity vried considerbly throughout the yer, the rnge of monthly mens being from t 0.8 percent (Mrch, 1949) to 83.5 zk 0.9 percent (July, 1948). Some experiments t 25 C were lso run in constnt temperture room. The monthly mens of reltive humidity were between 65.8 zk 0.4 percent (Jnury, 1949) nd 76.5 f 0.7 percent (July, 1949). For the other experiments incubtors were used. In one set t 25 C

3 VRIBILITY I DROSOPHIL 287 the monthly mens of reltive humidity rnged from 47.2 F 0.3 percent (pril, 1949) to 59.2 % 0.7 percent (June, 1949). In nother 25" incubtor the monthly mens vried from percent in Mrch, 1949, to 92.0 k 0.2 percent in ugust, One experiment ws kept in 21" incubtor t generlly close to 70 percent humidity. On September 8, 1949, number of the experiments were moved to Virgini Polytechnic Institute, Blcksburg, Virgini, nd continued in incubtors there. Becuse of the long life cycle of D. robust considerble problem in these experiments ws the prevention of mite infections nd contmintions with foreign Drosophil species. Benzyl benzote described by WLLCE ( 1948) nd 5 percent dust of the miticide " DMC " nd pyrophyllite (DHM nd BUER 1949) in the cups with growing lrve were used to control the mites. (Technicl compound " DMC " obtined through the courtesy of the Sherwin-Willims Compny, Clevelnd, Ohio, nd pyrophyllite from the R. T. Vnderbilt Compny, ew York, ew York.) CHROMOSOML POLYMORPHISM I TURL POPULTIOS Dt concerning the frequencies of the gene rrngements t Englewood Cliffs, ew Jersey, re summrized in tble 1. The fifteen gene rrngements found in more thn one loclity by CRSO nd STLKER (1947) re ll represented. The reltive frequencies of the gene rrngements in this popultion re generlly similr to their northern popultions. rrngements tht show most obvious increses from south to north, XL-1, 2R, nd 3R, show t Englewood Cliffs the highest frequencies yet recorded for these rrngements; correspondingly, XL, 2R-1, nd 3R-1 show their lowest recorded frequencies. nother inversion more common in northern thn in southern popultions, the pericentric inversion, 2L-3, hs here lmost s high frequency s in Iow. rrngements common in southern United Sttes, such s XL-2 nd 2L-2, re comprtively rre in Englewood Cliffs. Comprison of the rrngement frequencies in spring nd fll smples discloses no significnt differences. The differences within the 1948 collections re probbly ttributble to smpling errors ; even were they rel, the spring, 1949, dt show tht the chnges re not cyclic. n utumnl virginl dipuse similr to tht described by CRSO nd STLKER (1948) ws found t Englewood Cliffs; ll except the first two out of 84 mture-ppering femles collected in the fll of 1948 were uninseminted in nture. Zygotic frequencies in the egg smple, dult mle smple, dult femle smple, nd combined dult smple (dt kept on file to minimize publiction costs) were studied by mens of LEVEE'S t-test (DOBZHSKY nd LEVEE 1948). In no instnce is there significnt difference, t the 5 percent level using,both tils of the curve, between the number of heterozygotes nd homozygotes observed nd the number expected in ech smple by HRDY'S formul. The dult femle smple does show lrge excess of heterozygotes for the X-left rrngements but this is probbly not significnt (P = 0.088). In

4 288 MX LEVIT

5 VRIBILITY I DROSOPHIL 289 most other cses " t '' is positive, indicting n excess of homozygotes; the devition of this type is gretest for the combined dult smple of second-left rrngements (P = 0,102). In unpublished dt CRSO nd STLKER found more significnt excesses of homozygotes. turl popultions of D. pseudoobscur hve n excess of inversion heterozygotes in dult mle smples ( DOBZHSKY nd LEVEE 1948). Tble 2 shows gmetic frequencies in dult smples, by seson. Surprising differences re disclosed between the gene rrngements crried by mles nd femles in the fll, 1948, collection. The femles seem to hve significntly higher frequency of XL nd 2L nd lower frequency of XL-1 nd 2L-3 thn do the mles; the excess of 2L is highly significnt (P less thn 0.01). The dt suggest tht selection for the crriers of these rrngements is not equl in the sexes. The dt suggest lso tht these sex differences show sesonl vrition; for exmple, mles seem to crry less 2L thn femles in the fll but more 2L in the spring. The dtt were tested for sesonl nd sexul differences simultneously by compring the frequencies of the r- TBLE 2 Sesonl comprison of gmetic frequencies (in percent) of XL nd 2L rrngements in dult smples. nx XL XL-1 XL-2 I n 2L 2L-1 2L-2 2L-3 Fll mles Fll femles Spring mles Spring femles Totl mles Totl femles EXPERIMETL POPULTIOS Tble 3 contins dt describing the initil compositions of the popultion cges in this study. Experiments of DOBZHSKY, HOLZ, nd SPSSKY (1942) demonstrted the presence of mny recessive deleterious mutnts in chromosomes of D. pseudoobscur derived from nturl popultions. Presumbly this is no less true for D. robust; hence, n ttempt ws mde to ensure heterogeneity of gene contents of the chromosomes used by including minimum of five different strins of ech gene rrngement. The number of strins used, listed in the tble s " number of kinds of chromosomes," repre-

6 290 MX LEVIT sents the miniml number of kinds of chromosomes, since the prentl flies of strin my contribute more thn single chromosome with the sme gene rrngements. Popultions 1 nd 2 were derived from the sme pir mtings, hence from identicl strins ; with two exceptions the sme ws true of 3 nd 4. In cges 1-16, initil popultions contined equl numbers of mles nd femles. The experimentl results re given s gmetic frequencies, the bulkier zygotic dt being kept on file. Unless otherwise noted, the chromosome number per smple is 300. The smple of the prentl popultion, tken bout two weeks fter the experiment begn, ws clled control smple. The TBLE 3 Composition of the initrl popultions in 21 popultion cges. Experiment number Dte begun Mrch 8, 1948 Mrch 18, 1948 ugust 30, 1948 ovember 12, 1948 ovember 12, 1948 ovember 17, 1948 ovember 19, 1948 December 1, 1948 December 22, 1948 Jnury 5, 1949 Jnury 18, 1949 Jnury 25, 1949 September 23, 1948 Jnury 10, 1949 My 13, 1949 Jnury 8, 1949 pril 1, 1949 My 27, 1949 June 1, 1949 ovember 14, 1949 ovember 30, 1949 Gmetic umber of kinds Temp. frequencies of chromosomes umber O c. of flies 2L 2L-1 2L-3 2L 2L-1 2L % % % 76 15%.e.* *-* % 20 15% 20 -e * e < ??? smple tken fter the first genertion htched ws clled FI. Since the intervls between subsequent smples were equl to the genertion time, they were nlogously lbelled ; no impliction tht the smpled popultion consisted wholly of tht genertion is intended.. Popultions with 2L nd 2L-1 Tble 4 shows the results of two experiments with the rrngements 2L nd 2L-1 mintined t bout 156OC. The first of these, cge 2, pprently reched n equilibrium t bout 30 percent 2L nd 70 percent 2L-1. For clcultion of the dptive vlues of the gene rrngements in this environment, cge 5, where chnges continued for severl genertions, is more suitble.

7 Smple VRIBILITY I DROSOPHIL 291 TBLE 4 Smples from experimentl popultions of-2l us. 2L-1 t 15%' nd 21 ' c. (2 nd 5 t 151/2', 2 t 21 "). Cge 2 Cge 5 Cge 2 2L 2L-1 2L 2L-1 2L 2 L-1 Initil Control F, F* F, F, F, 47.7 F, F, F, F *n = 200. tfirst smple in Virgini t Using the method of itertion nd the underlying ssumptions described in WRIGHT nd DOBZHSKY (1946) gives the following results : Genotype W 2L/2L s = L/2L $ = L- 1/2L t = W represents the reltive dptive vlue, tking tht of the heterozygote to be 1.00, s, the selective disdvntge of the 2L homozygotes, nd t, the selective disdvntge of the 2L-1 homozygotes; " " is the equilibrium frequency of 2L expected from these selection coefficients. Here both homozygotes pper to be nerly lethl reltive to the heterozygotes. The indicted equilibrium point is pprently the one obtined. With the exception of the smll F3 smple, the dt fit closely the curve determined by these coefficients. When cge 2 hd pprently cesed chnging, it ws trnsferred to 21 C nd 70 percent humidity, nd renmed cge 2. Tble 4 shows tht chnge occurred t the new temperture in the reverse direction from the chnge in cge 2. Unfortuntely, the experiment hd to be discontinued before the point could be determined. The smll chnge observed in the third genertion mkes it likely tht n equilibrium ws being pproched, if not lredy reched. t 25" these gene rrngements were studied in three groups of experiments (tble 5). Cges 1 nd 12X were kept in constnt temperture room where the verge humidity vried from 65 to 76 percent. In o. 1 the fll of 2L frequency from the control of 28 percent during the first three genertions certinly seemed to be produced by selection, but beyond this the experiment gives no cler results. Humidity in the room decresed t this time, nd the errtic chnges observed my be due to selection, the sign of which

8 292 MX LEVIT 3 I d J r( r : U M U M

9 VRIBILITY I DROSOPHIL 293 chnged with the chnges in humidity. Evidence for this comes from the equilibrium of bout percent which 2L pprently ttined when the environment becme stbilized. n lterntive hypothesis would ssume n equilibrium between 15 nd 20 percent 2L which ws obscured by smpling errors nd genetic drift. Responsibility of smpling errors is contrdicted by the zygotic dt for F3-Fs which show greter differences thn might be expected from smpling vrition (P bout 0.01 for twelve degrees of freedom). In cge o. 12 no significnt chnges were detected during two nd twothirds genertions. The seeming inbility of the 2L frequency to increse beyond 13.7 percent my, considering smple errors, support the hypothesis of n equilibrium frequency of percent 2L in this environment. Cges 3 nd 1 were kept t 25' but t lower humidity. Cge 1 ws continution of experiment 1. Once in the new conditions (see tble 5) the frequency of 2L fell gin, pprently reching n equilibrium round 8 percent 2L. This result seems very unlikely on bsis of genetic drift lone. Even better evidence for nturl selection in n experimentl popultion comes from cge 3. Strting with high frequency of 2L nd low one of 2L-1, this popultion showed clerly directionl chnges for five genertions, the fll in 2L nd rise in 2L-1 pprently cesing fter frequency of 33 percent 2L nd 67 percent 2L-1 hd been reched. fter tht, few irregulr chnges occurred, nd the cge ws discontinued fter the eleventh genertion. The popultion ws lwys smll; thus, it contined 152 flies in Fl1 genertion. Clcultion of dptive vlues using ll the dt for o. 3 gives the following results : Genotype W 2L/2L 0.26 s = L/2L LL I: L1/2L t = 0.36 The expected equilibrium proportion of 2L grees with the one pprently ttined. However, the inclusion of severl genertions of no pprent chnge hs considerble effect on the clcultions. Thus, if the dt used re restricted to the period until equilibrium seemed to be ttined (Fb) the following picture of the selection coefficients emerges : Genotype W 2L/2L 0.39 s = L/2L GZL = L- 1/ 2 L t z 0.08 The dptive vlues of the heterozygotes nd the 2L-1 homozygotes seem to hve been more nerly similr during the period of rpid chnge thn ws indicted by the previous clcultion. n equilibrium is gin predicted. However, this equilibrium is very different from one pprently ttined in this cge; in fct, it closely resembles the one ttined in cge 1 s well s severl results t this temperture t higher humidities (cges 12, 12, nd the

10 294 MX LEVIT first prt of cge 1). Below re the clculted expected frequencies of 2L on the bsis of ech set of dptive vlues for comprison with the observed results : Smple Control ctul e Expected (First set) Expected (Second set) S s expected, the dptive vlues indicting n equilibrium of 11.6 percent 2L (second set) fit the gmetic dt of the first five genertions more closely thn do the ones in the first set. If, then, this is the correct set for these genertions, the discrepncies between observed nd expected in the lter dt would men tht the dptive vlues of the genotypes re different when the frequencies re bout 33 percent 2L nd 67 percent 2L-1 in this environment thn they re t higher frequencies of 2L or lower ones of 2L-1. However, the clcultions bsed on ll the dt (first set) lso generlly gree, within the commonly ccepted limits of smpling error, with the observed frequencies during the period of rpid chnge. The only exception is the FS smple, which hs t = 2.20, nd this lso is not highly significnt (P = 0.028). Since it is lso simpler, becuse no vrition of the dptive vlues need be postulted, the hypothesis of dptive vlues leding to n equilibrium of 33 percent 2L is probbly the preferred one. Experiments 4, 12, nd 3 were kept in the 25" incubtor t bout 90 percent humidity. These cges consistently contined lrge popultions ; for exmple, F3 of cge 12 numbered 4562 individuls. Tble 5 shows tht the chnges in o. 4 were of the sme generl nture s those in its sister cge, o. 3, but they were slower rriving t the equilibrium point. The clculted selection coefficients re : Genotype 2L/2L 2L/2L-l 2L-1/2L-1 W oo 0.65 s = 0.65 t = 0.35 e,, = Cge 12, intended s check on popultion 4, showed no significnt chnges. Experiment 3 determined whether the equilibrium ttined in o. 3, of which it ws continution, would chnge in higher humidity. The vilble dt indicte no chnge, thus possibly duplicting the result in cge 4. Becuse of the reltive rrity of 2L-3 in the 1947 collections, experimentl

11 VRIBILITY I DROSOPHIL 295 popultions involving this pericentric inversion were not begun until ovember, It ws, therefore, not possible to crry out s mny tests in different environments s ws possible for 2L nd 2L-1. It is hoped to study the dptive properties of 2L-3 more fully lter. B. Experiments with 2L nd 2L-3 The studies with 2L nd 2L-3 re shown in tble 6. t the lower temperture these consist of cges 6 nd 11. Cge 11, which could be crried on for only two genertions, showed no significnt chnges. o. 6 did show significnt chnges for number of genertions. These chnges mke it likely tht these rrngements re responsive to nturl selection in this environment nd indicte tht the dt of cge 11 resulted from beginning the experiment t or ner the equilibrium point. The reltive dptive vlues of the genotypes in experiment 6, bsed on the vilble dt, re: Genotype W 2L/2L 0.53 s = L/2L-3 1.oo G, L-3/2L t = 0.36 The gmetic frequencies expected from these dptive vlues gree closely with those found in the smples. This pplies to the period of rpid chnge s well s the lter dt, indicting tht nturl selection ws governed by the sme dptive vlues throughout the rnge of frequencies covered by this experiment. ot explined, however, is the discrepncy between the equilibri for experiments o. 11 nd o. 6. t the higher temperture lso, the cge begun with high frequency of 2L-3, o. 13, showed no significnt directionl chnge. n increse in frequency of 2L nd decrese in 2L-3 did occur during the second nd third genertions, but the popultion reverted to its initil frequencies therefter. This suggests these re equilimbrium frequencies for 2L or 2L-3 t this temperture. The popultion begun with low frequency of 2L-3, in this cse o. 8, showed stedy, lbeit slow, decreses in the frequency of 2L nd increses in 2L-3, pprently due to nturl selection. This experiment hd to be discontinued before the finl outcome could be determined. To see if the results in o. 13 could be duplicted, o. 16 ws begun with 50: 50 proportions of 2L nd 2L-3. The control smple showed few 2L-1 chromosomes hd lso been introduced. The min chnge observed is the rise in 2L-1, obscuring the reltions of 2L nd 2L-3. These results indicte tht the crriers of 2L nd 2L-3 hve different dptive vlues, probbly resulting in equilibri, t both tempertures, but the dt do not show conclusively whether the equilibri differ t the different tempertures. C. Experiments involving 2L-1 nd 2L-3 The severl experiments involving these overlpping inversions re shown in tble 7. Cges 7 nd 10 show no significnt chnges nd pprently dem-

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14 298 MX LEVIT onstrte dptive equivlence of the two gene rrngements t the low temperture. The behvior is different t 25". Here significnt chnge ws found in the popultion, o. 14, begun with high frequency of 2L-3 nd low frequency of 2L-1. Clcultion of dptive vlues for the genotypes in this popultion, bsed on the smples of genertions tht developed in ew York ( Control-F5), gives the following results : Genotype 2L-3/2L-3 2 L- 1/2 L-3 2 L- 1 /2 L- 1 W s = 0.83 t = %L Hd no chnge of environment tken plce, it is not inconceivble tht this equilibrium might hve been reched. However, it is pprent tht in Virgini there ws new chnge leding to equilibrium round 50 percent of ech rrngement. The environmentl fctors responsible for the new chnge re presently under investigtion. Popultion o. 9, strted with high frequency of 2L-1 nd low one of 2L-3, showed no significnt chnges in five genertions. Upon trnsfer to lower humidity, however, s o. 9, this popultion chnged in mnner reminiscent of the trnsfer of popultion 1 to lower humidity (cge I, tble 7). The decrese in 2L-3 nd increse in 2L-1 in 9 is similr to wht is expected from the ew York equilibrium indicted for this environment in cge 14. n opposite chnge seemed to occur in Virgini in the sme direction, but not to the sme extent, s the new chnge noted in cge 14. Since significnt chnges were noted only t the low humidity, the possibility exists tht t higher humidity t this temperture (s in cge 9), the crriers of these rrngements re dptively neutrl. Cge 15, run t the TBLE 8 Popultion counts of experimentl popultions. pproximte Temp. Humidity Cge Totl Femles Mles Cge genertion O C. Style flies % % % 15% 15% ew* Old* 0 Id ew Old Old ew Old Old Old Old ew ew ew * explntion in text.

15 VRIBILITY I DROSOPHIL 299 sme humidity s o. 9, is n ttempted test of this hypothesis. Its results re inconclusive. Size of the popultions Tble 8 shows the counts mde of vrious experimentl popultions long with the pproximte men humidity during the previous genertion period. Femles re consistently more common thn mles. t the high temperture the order of mgnitude of the totl count seems directly relted to the reltive humidity. t similr humidities, it is greter t high thn t low temperture. t similr tempertures nd humidities, the newer cges, those with netting insted of glss on two windows, nd, therefore, with more evportion surfce, tend to hve the smller popultions. DISCUSSIO The Drosophil robust popultion of Englewood Cliffs, ew Jersey, repetedly smpled for the purposes of the present investigtion (see tble l), shows no cyclic sesonl chnges in chromosoml composition, being in this respect like the St. Louis popultion studied by CRSO nd STLKER (1949). It my be noted tht no sesonl chnges re observed for the two gene rrngements, XL-1 nd 2L-3, which show cler geogrphic nd ltitudinl grdients (CRSO nd STLKER 1947; STLKER nd CRSO 1948) but whose sesonl vrition could not be determined in Missouri (CRSO nd STLKER 1949). The vilble dt confirm the conclusion of CRSO nd STLKER tht the dptive function of the chromosoml polymorphism in D. robust is not relted to sesonl chnges in the environment. In this respect the species resembles D. persimilis rther thn D. pseudoobscur or D. funebris ( DOBZHSKY 1943, 1948 ; DUBII nd TIIKOV 1945). Experiments on rtificil popultions nevertheless show tht the gene rrngements in D. robust re not dptively neutrl. The reltive frequencies of the gene rrngements in experimentl popultions often show rpid nd directionl chnges, clerly cused by opertion of nturl selection. In no cse, however, do these chnges led to elimintion of one nd estblishment of the competing gene rrngement. Insted, the reltive frequencies of the gene rrngements rech equilibri, nd the composition of the popultions becomes more or less constnt. Equilibri my be produced by nturl selection if heterozygotes re t ll frequencies dptively superior to the homozygotes. In the text the reltive dptive vlues of the genotypes were clculted on the bsis of this hypothesis, nd the results expected from the clculted dptive vlues generlly corresponded well with the results ctully obtined. Implicit in these clcultions were two further ssumptions : tht the dptive vlues were constnt throughout the rnge of rrngement frequencies nd tht they were identicl in the sexes. There is some evidence, however, tht these ssumptions my represent n oversimplifiction. Thus, in severl experiments (see especilly o. 3, tble 5) the equilibrium pprently ttined my not be the

16 300 MX LEVIT one expected from the dptive vlues responsible for erlier chnges. This my occur if t certin frequencies the genotypes tke on dptive vlues leding to different equilibrium. Or, the homozygotes, dptively inferior t most frequencies, my be dptively equl to the heterozygotes (nd to one nother) in the vicinity of the frequencies where observed chnges stopped, for the effect of dptive equlity of the genotypes is there indistinguishble from stble equilibrium under nturl selection. In either cse, the postulted stbility of the dptive vlues would be contrdicted. lso, it is probble tht vritions in dptive vlues, possibly relted to smll environmentl vritions, were responsible for temporry chnges noted in severl experiments (especilly in o. 1, tble 5 nd o. 13, tble 6). The observtion mde in the text tht, in nture, crriers of severl rrngements re pprently subject to different selection pressures depending on their sex mkes the ssumption tht given genotype hs the sme dptive vlue in both sexes in the lbortory popultions lso questionble. n lterntive hypothesis for production of dptive equilibri is tht heterozygotes be t lmost ll frequencies intermedite in dptive vlue to the homozygotes but tht the dptive vlue of ech homozygote fll s the frequency of the rrngement increses. There would then be rnge of rrngement frequencies where dptive vlues of the homozygotes were similr or equl, nd here no selective chnges would occur. The mrgins of the rnge would be points of equilibrium. This possibility is more difficult to ttck nd ws not considered in the text. However, instnces described in the text where different equilibri seemed to be ttined by the sme rrngements in the sme environment my well depend on some such explntion. lthough the issue is complicted by strin differences nd other vribles, in some of these cses there ws indeed rnge of frequencies not ttined from either extreme (compre, for exmple, cges 2 nd 5, tble 4; cges 4 nd 12, tble 5; cges 6 nd 11, tble 6). This possibility is supported lso by the excess of homozygotes observed in severl dult smples in nture (discussed in the text). WRIGHT nd DOBZHSKY (1946) point out, however, tht such n explntion my well pply in nture, where genotype my be dptively superior until so numerous tht it must ttempt to use ecologicl niches for which it is not well dpted, but it is more difficult to conceive of such phenomenon in n experimentl popultion cge, where the number of niches is limited. Determintion of zygotic frequencies mong dults in the cges, currently being investigted, should help clrify this problem. In one completed study, Fls dults of cge 4 (tble 5) significnt excess of heterozygotes ws found in femles, the dult mles fitting the HRDY rtio. (Detils of this nd further studies of dults will pper in lter pper.) The observtions on experimentl popultions lend support to the view tht the reltive frequencies of different gene rrngements in nturl popultions re not ccidentl, but re determined by nturl selection. Of course, this pplies to popultions in pprent equilibrium s well s to popultions with chnging gene or inversion frequencies. However, the time of ction of ntu-

17 VRIBILITY I DROSOPHIL 301 rl selection in the life cycle of this fly in nture is not cler. The egg smple, lbeit smll, indictes tht t lest the spring smple is pnmictic nd lcks differentil vibility t the egg stge. The vrious dult smples indicte no differentil survivl of heterozygotes over homozygotes, in either mles or femles, beyond the egg stge; the overll dult smple for left rm second chromosome rrngements even suggest superior survivl vlue of the homozygotes. The remining possibility is greter fecundity or sexul ctivity of the heterozygotes. This hypothesis clls for no devitions from HRDY-WEI- BERG rtios, provided the dptive differences between genotypes re similr in the sexes. Unfortuntely, the hypothesis is exceedingly difficult to test by observtions on nturl popultions ; lbortory experiments re plnned. D CUH (1949) found n even more significnt excess of dult homozygotes for the color types of D. polymorph in nturl popultions, lthough there is evidence the heterozygotes re dptively superior. WLLCE ( 1948) showed, in lbortory experiments, tht superior fecundity of femles heterozygous for the sex-rtio fctor in D. pseudoobscur pprently mintins this fctor in popultions despite the dptive wekness of femles homozygous for it. Femles heterozygous for pericentric inversion my possess inferior fecundity, becuse crossing-over within the inversion would generlly led to invible dupliction-deficiency gmetes. This explins the rrity of pericentric inversions in nturl popultions ( DOBZHSKY 1941). MILLER (1939) found pericentric inversion in nturl popultions of D. lgonquin. Since most individuls with the pericentric inversion lso crry n rrngement differing from it by two overlpping inversions, MILLER suggests this reduces the probbility of crossing-over nd hence erses the theoreticl selective disdvntge. CRSO nd STLKER (1947) point out lso tht single crossovers within the overlp led to bridge nd frgment s in prcentrics. In D. robust the pericentric inversion 2L-3 is sometimes found ssocited with rrngements differing from it by two overlpping inversions, 2L-1 or 2L-2, nd these my " protect " it somewht, but most commonly it is found with 2L, which differs from it by only single inversion. The extent of crossing-over within the inversion is unknown, but the fct tht bout hlf the euchromtic re of the left rm of the second chromosome is involved indictes tht it probbly does occur in 2L/2L-3 femles. Hence, the high frequency of 2L-3 here (tble 1) nd in Mt. Vernon, Iow (CRSO nd STLKER 1947), nd its even higher frequency t elevtions more thn 1400 feet ner Gtlinburg, Tennessee (STLKER nd CRSO 1948), cnnot be explined on the sme bsis s the D. lgonqzrin cse. The equilibri obtined or indicted in the experiments with 2L-3, especilly those involving 2L, show tht more likely explntion for the persistence of this inversion in popultions is the selective dvntge conferred on individuls heterozygous for it by heterotic effects of genes on it with those on 2L nd 2L-1. The dt indicte tht these effects probbly do not depend on differentil mortlity in nture. Under nturl selection the crriers of genotypes best dpted to the en-

18 302 MX LEVIT vironment leve more surviving progeny thn do crriers of genotypes with lesser fitness. The fitness or " dptive vlue " is, however, composite of reltive superiorities nd inferiorities in number of fctors operting t ll stges of the life cycle. One of the most obvious nd most esily mesured of these is differentil vibility or mortlity, but it is pprent tht fitness depends lso on reltive fecundity, sexul ctivity, longevity nd other fctors. Moreover, ech of these generl clsses is further compounded of prticulr vribles; thus, genotype which produces higher rtio of invible gmetes my yet possess superior fecundity by virtue of lrge totl of gmetes produced. Severl experiments with gene rrngements 2L nd 2L-1 ttined similr equilibri lthough mintined t different tempertures ; nd t ll tempertures used, lss", 21", nd 25"C, the equilibri were t 50 percent 2L or less. Similr results were obtined in the experiments with 2L nd 2L-3, though these re not s extensive nd the results not s definite. This indictes tht heterosis involving these gene complexes is lrgely independent of temperture. Only 2L-1 with 2L-3 shows cler-cut temperture differences in equilibri, these gene rrngements being dptively neutrl or nerly so t lsso but not t 25". However, temperture is but one of mny environmentl vribles to which the rrngements might be dpted. Severl experiments ttempted to study nother vrible, nmely humidity. These showed tht popultions of D. robust re indeed sensitive to this fctor, for the size of the popultion ws consistently greter t higher humidities thn t lower humidities t the sme temperture. Whether ny of the gene rrngements studied re involved in dpttions to this fctor is not so cler, however. When popultion contining 2L nd 2L-1 ws kept t lower humidity (see experiment l, tble 7), the frequency of 2L fell from percent to bout 8 percent, 2L-1 showing corresponding increse. However, severl experiments with these rrngements, notbly 1 nd 12, ttined similr results t very different humidities, while still other experiments, notbly 4 nd 12, ttined very different equilibri t the sme humidity. Repetition of these experiments with better control of humidity nd strin differences should yield interesting results. Controlling humidity in desicctors, HEUTS (1947 nd 1948) ws ble to demonstrte differentil survivl of pupe nd dults of D. pseudoobscur crrying vrious gene rrngements. DOBZHSKY nd ZIMMERIC hve shown tht selective responses by crriers of certin gene rrngements in D. pseudoobscur lso depended on the nture of the yest used (DOBZHSKY 1948d nd unpublished dt). The recent discovery tht the nturl food of D. robust consists of microorgnisms tht occur in slime fluxes (CRSO nd STLKER 1950) my led to experimentl study of the food preference of crriers of different gene rrngements lso in D. robust. In some of the experiments described in this pper the sme rrngements were involved under similr conditions of temperture nd humidity. In none of these duplicte experiments were quite the sme equilibrium proportions

19 VRIBILITY I DROSOPHIL 303 of the rrngements ttined; in some, the differences were quite lrge. Contrstingly, replicte experiments with gene rrngements of Drosophil pseudoobscur usully result in ttinment of uniform equilibri (WRIGHT nd DOBZHSKS 1946; DOBZHSKY 1947 nd 1948~). This difference between D. pseudoobscur nd D. robust my be relted to the sensitivity of D. robzisf to smll environmentl vritions noted on numerous occsions during the course of the experiments. The genetic bsis for this sensitivity, pprently intimtely bound up with the chromosoml polymorphism, should prove n interesting subject for further study. SUMMRY study is mde of chromosoml polymorphism in ntur1 nd experimentl popultions of Drosoplzil robust. The nturl popultion of one loclity in the ew York City re proved to contin the fifteen widespred gene rrngements known in the species. The observed high frequencies of inversions XL-1, 2L-3, 2R, nd 3R re chrcteristic for northern popultions of the species. Cyclic sesonl vrition in the frequencies of inversions hs not been observed, but there is evidence of differentil selection in the two sexes for severl rrngements t different sesons. The dult popultion consists of heterozygotes nd homozygotes in proportions consistent with the HRDY-WEIBERG equilibrium. Twenty-one experimentl popultions contining mixtures in vrious proportions of two prcentric inversions, 21, nd 2L-1, pericentric inversion, 2L-3, in pirs, were plced in ppultion cges nd were kept t three tempertures, 15s0, 21", nd 25 C. t the highest temperture they were lso kept t three different humidities. Most of these popultions showed significnt nd directionl chnges in rrngement frequencies resulting from nturl selection. These popultions usully ttined equilibri between the rrngements t which the competing gene rrngements were present with constnt frequencies. This probbly indictes tht inversion heterozygotes re superior in dptive vlue to homozygotes. In the experiments with 2L nd 2L-1, nd those with 2L nd 2L-3, similr selective responses occurred t ll tempertures used ; experiments with 2L-1 nd 2L-3 showed significnt selective differences t 25" but not t 15$OC. Results of the experimentl popultions indicte tht the frequencies of the gene rrngements in nture re determined by nturl selection, probbly cting through dptive superiority of inversion heterozygotes. Heterosis of this sort lso ccounts for the persistence of pericentric inversion in these popultions. CKOWLEDGMETS The writer is prticulrly indebted to PROFESSOR THEODOSIUS DOBZH- SKY of COLUMBI UIVERSITY for guidnce throughout the course of study; to DRS. HMPTO L. CRSO nd HRRISO D. STLKER of ~VSHIGTOS UIVERSITY, St. Louis, nd DR. L. C. DU of COLUMBI UIVERSITY, for

20 304 MX LEVIT their dvice nd helpful suggestions; nd to DR. HOWRD LEVEE of COLUM- BI UIVERSITY for his dvice nd sttisticl ssistnce. He is lso indebted to MR. MICHEL SVETI, JH., DR. BRUCE WLLCE, DR. ERST MYR, nd MR. GEORGE STREISIGER for their id in the collections nd to MR. STLEY ZIMMERIG, MR. nd MRS. BORIS SPSSKY, nd MRS. BETTY L. GOERTZE for their id in the lbortory. Completion of the experiments in Blcksburg ws ided by grnt from the ROCKEFELLER FOUDTIO. LITERTURE CITED CRSO, p. L., nd H. D. STLKER, 1947 Gene rrngements in nturl popultions of Drosophil robust Sturtevnt. Evolution 1: Reproduction dipuse in Drosophil robust. Proc. nt. cd. Sci. 34: Sesonl vrition in gene rrngement frequencies over three-yer period in Drosophil robust Sturtevnt. Evolution 3: turl breeding sites for Drosophil robust. Genetics 35: 100. (bstrct) D CUH,. B., 1949 Genetic nlysis of the polymorphism of color pttern in Drosophil polymorph. Evolution 3: DHM, P.., nd C. L. BUER, 1949 The miticidl properties of di (p-chlorophenyl) methyl crbinol in lbortory insect rerings. Science 109: 69. DOBZHSKY, TH., 1941 Genetics nd the origin of species. 2nd ed. XVIII pp. ew York : Columbi University Press Genetics of nturl popultions. IX. Temporl chnges in the composition of popultions of Drosophil pseudoobscur. Genetics 28: Genetics of nturl popultions. XIV. response of certin gene rrngements in the third chromosome of Drosophil pseudoobscur to nturl selection. Genetics 32: Genetics of' nturl popultions. XVI. ltitudinl nd sesonl chnges produced by nturl selection in certin popultions of Drosophil pseudoobscur nd Drosophil persitnilis. Genetics 33: b Chromosoml vrition in popultions of Drosophil pseudoobscur which inhbit orthern Mexico. mer. t. 82: c Genetics of nturl popultions. XVIII. Experiments on chromosomes of Drosophil pseudoobscur from different geogrphic regions. Genetics 33: d Genetic structure of nturl popultions. Crnegie Inst. Wsh. Yb. 47: DOBZHSKY, TH., nd C. EPLIG, 1944 Contributions to the genetics, txonomy, nd ecology of Drosophil pseudoobscure nd its reltives. Crnegie Inst. Wsh. Publ. 554: DOBZHSKY, TH.,. M. HOLZ nd B. SPSSKY, 1942 Genetics of nturl popultions. VIII. Conceled vribility in the second nd fourth chromosomes of Drosophil pseudoobscur nd its bering on the problem of heterosis. Genetics 27: DOBZHSKY, TH., nd H. LEVEE, 1948 Genetics of nturl popultions. XVII. Proof of opertion of nturl selection in wild popultions of Drosophil pseudoobscur. Genetics 33: DUBII,. P., nd G. G. TIIKOV, Sesonl cycles nd the concentrtion of inversions in popultions of Drosophil funebris. mer. t. 79: Structurl chromosome vribility in urbn nd rurl popultions of Drosophil funebris. mer. t. 80: b turl selection nd chromosoml vribility in popultions of Drosophil funebris. J. Hered. 37: HEUTS, M. J., 1947 Influence of humidity on the survivl of different chromosoml types in Drosophil pseudoobscur. Proc. nt. cd. Sci. 33: dptive properties of crriers of certin gene rrngements in Drosophil pseudoobscur. Heredity 2:

21 VRIBILITY I DROSOPHIL 305 MILLER, D. D., 1939 Structure nd vrition of the chromosomes in Drosophil lgoioquilt. Genetics 24: SPIESS, E. B., 1950 Experimentl popultions of Drosophil persinzilis from n ltitudinl trnsect of the Sierr evd. Evolution 2: STLKER, H. D., nd H. L. CRSO, 1948 n ltitudinl trnsect of Drosophil ro6ust Sturtevnt. Evolution 2: WLLCE, B., 1948 Studies on sex-rtio in. Drosophil pset4doo6scur. I. Selection nd sex-rtio. Evolution 2: WRIGHT, S., nd TH. DOBZHSKY, 1946 Genetics of nturl popultions. XII. Experimentl reproduction of some of the chnges cused by nturl selection in certin popultions of Drosophil pseudoobscur. Genetics 31: