and satya - geneticsr. a. norman* and satya prakash department of biology, university of rochester,...

DISTINCTIONS AMONG ALLELIC VARIANTS ASSOCIATED WITH CHROMOSOME 3 INVERSIONS IN DROSOPHILA PSEUDOOBSCURA

AND DROSOPHILA PERSIMILIS

R. A. NORMAN* AND SATYA PRAKASH

Department of Biology, University of Rochester, Rochester, New York 14627

Manuscript received June 27, 1980 Revised copy received September 18, 1980

ABSTRACT

Efforts were made to discriminate new genetic variants among electrophoretic alleles that are associated with chromosome 3 inversions of Drosophila pseudoobscura and D. persimilis. Apparent genetic similarities for electxopho- retic alleles between these two species and among the common inversions they carry were reexamined by altering gel concentration and buffer pH. At the amylase locus, the 1.09 electrophoretic allele could be further separated into two allelic classes that differentiated the WT and KL arrangements. Similarly, the 0.84 electrophoretic allele was divided into two allelic classes, one characteristic of the Santa Cruz phylad arrangements, TL and SC, and the other found in strains of the Standard phylad arrangements and CH. Uncommon amylase alleles proved to be different alleles in the two species. No new allelic variants, however, could be found among strains with the amylase 1.00 allele, the commonest allele in the Standard phylad of both species. No major new allelic variation was detected for acid phosphatase3 and larval protein-IO that revealed any further differentiation among species or inversions. Variation at all three loci in strains of the Bogota population remained genetically similar to variation in strains of mainland D. pseudoobscura.

MEASURES of genetic differentiation among natural populations, chromosome inversions and species are commonly based on the electrophoretic vari-

ation of proteins. If differences are found and their genetic basis confirmed through Mendelian analysis, then genetic divergence is substantiated. However, if electrophoretic similarities result from such comparisons, one cannot be certain whether the absence of differentiation is due to the inability of electrophoresis to discriminate allelic variants. In Drosophila pseudoobscura and D. persimilis, attempts have been made to uncover further structural gene variation by using altered gel concentrations and pH of the buffer. The results of such studies have depended upon the enzyme examined. For the almost monomorphic loci, hexokinase-1 (BECKENBACH and PRAKASH 1977), octanol dehydogenase-I ( COYNE and FELTON 1977) and malic dehydrogenase (COYNE and FELTON 1977), there has been little or no change in the profile of variation. However, for the previously polymorphic loci, esterase-5 (MCDOWELL and PRAKASH 1976; COBBS and PRAKASH 1977; COYNE, FELTON and LEWONTIN 1978), xanthine dehydrogenase ' Present address: Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138.

Genetics 96: 727-741 November, 1980.

728 R. A. N O R M A N AND S. PRAKASH

(COYNE 1976; SINGH, LEWONTIN and FELTON 1976; PRAKASH 1977a) and alcoho€ dehydrogenase-6 (COYNE and FELTON 1977), which is the acetaldehyde oxidase of PRAKASH, LEWONTIN and HUBBY (1969), many new allelic variants have been detected.

Populations of Drosophila are also polymorphic for chromosome inversions. A well known example is chromosome 3 of D. pseudoobscura and D. persimilis (DOBZHANSKY and EPLING 1944). Each species has its own set of overlapping inversions related through the phylogeny in Figure 1. The ST arrangement is shared and thus relates the chromosome phylogenies of the two species. Because recombination is drastically reduced in inversion hetemkaryotypes (DOBZHAN- SKY and EPLJNG 1948; LEVINE 1956), the genetic contents of the arrangements may be sufficiently isolated to allow divergence by natural selection, random drift and new mutations. For polymorphic protein and enzyme loci, the associations of specific electrophoretic alleles with groups of inversions have revealed differences among the genic contents of arrangements ( PRAKASH and LEWONTIN

WT

KL 1

RD \13?" D. persimilis ST

D. pseudoobscura ST

"HYPOTHETICAL"

sc

/- \ CH TL

FIGURE 1 .-Phylogeny of chromosome 3 inversions in Drosophila pseudoobscura and D. persimilis. WT ( Whitney) , KL (Klamath), RD (Redwoods), ST (Standard), PP (Pikes Peak), AR (Arrowhead), SC (Santa Cruz), TL (Treeline) and CH (Chiricahua) from DOBZHANSEY and EPLING 1944).

CHROMOSOME 3 INVERSIONS 729

1968, 1971; PRAKASH 1976a). Because of these associations, the frequencies of certain allozymes closely parallel the geographical variation of inversions.

This paper is concerned with three of the loci-amylase ( A m y ) , larval protein- 10 (Pt-IO) and adult acid phosphatase-3 (Ap-3)-whose alleles are associated with inversions of chromosome 3 in D. pseudoobscura and D. persimilis. The main allelic differentiation for these loci occurs between the Standard phylad (ST, AR and PP of D. pseudoobscura and WT, MD, KL, RD and ST of D. persimilis) and the Santa Cruz phylad (TL, SC and CH) . There are three amylase allozymes in these species that are strongly associated with inversions. The Standard phylad is largely whereas the Santa Cruz phylad has a high frequency of Am~".~4. In D. persimilis, the WT arrangement has predominantly

which is found much less frequently in the other arrangements. At the Pt-IO locus, the two phylads are again differentiated; Pt-lU1.O4 is associated with the Standard phylad, and Pt-I01.06 is associated with the Santa Cruz phylad. At this locus, the sibling species, D. miranda, which is much more distantly related to D. pseudoobscura and D. persimilis than these latter two are to each other, is monomorphic for Pt-IU1.04. At the Ap-3 locus, the Santa Cruz phylad is mainly

whereas, the ST and AR arrangements are monomorphic for A P - ~ ~ . O O .

The PP arrangement which is similar to Santa Cruz phylad arrangements, however, is seemingly monomorphic for A P - ~ O . ~ ~ . It should be noted that these associations are not complete; that is, low frequencies of the alternative alleles are found in many of the arrangements, especially at the A m y locus.

The purpose of this paper is to examine the apparent genetic similarities among species and inversions for alleles at these loci, using altered gel concentration and buffer pH. We wish to know if further structural gene variation can give any information on the extent of differentiation among these species and the common inversions they carry. We have also attempted to detect genetic differentiation for a sample of strains from a Bogota, Colombia, population that shows geographica1 isolation and partial hybrid sterility with other populations of D. pseudoobscura, the so-called mainland D. pseudoobscura.

'MATERIALS AND METHODS

Strains: Strains of D. pseudoobscura from both Mather, California, and Bogota, Colombia, were made isogenic for chromosome 3 by the use of balanced lethal stocks. WYATT ANDERSON provided several of the Bogota strains and a strain contai-ning the Amy1.05 allele from Ame- cameca, Mexico. D. persimiEis strains were made homokaryotypic for chromosome 3 and were further inbred for 8 or more generations by sib matings. ELIOT SPIESS provided several lines of D. persimilis. The D. persimilis strains were from California populations: Mather, Humboldt Co., and McDonald ranch. D. miranda strains were from Mather CA, and Sisters, Oregon, and were inbred for 4 to 5 generations by sib matings.

Electrophoresis: Electrophoresis was performed in acrylamide gels of varying concentrations with the ratio of bis-acrylamide to acrylamide kept constant. Two buffers of different p H s were used: (1) Tris-borate, pH 8.9, consisted of 0.075 M Tris and 0.012 M borate. (2) Borate Tris-HC1, pH 7.1, consisted of 0.19 M borate and 0.04 M Tris with the addition of HC1 to bring the p H to 7.1. The duration of electrophoretic runs was extended so that migration distances were much greater than those used to score alleles in the original studies of (PRAKASH and LEWONTIN (1968) and PRAKASH (1976a). Each condition was selected so as to give good resolution of enzyme or

730 R. A. N O R M A N A N D S. PRAKASH

protein bands. No variant was considered genetic unless it was repeatable from gel to gel and could be distinguished in heterozygotes with a reference allele.

For amylase, 5 separate gel conditions were used: 5%, 7% and 9% acrylamide gels at p H 8.9, and 5% and 7% gels a t pH 7.1. The gels contained 0.025 M CaC1,. Staining was done as described in NORMAN and PRAKASH (198Oa). Environmentally induced alterations of amylase mobilities, probably resulting from variation in the condition of stocks, occurred commonly in early attempts to uncover variants. Maintaining the strains in bottles under uncrowded conditions, as well as keeping adults on fresh food, was necessary to avoid this effect. For acid phosphatase-3, gel conditions of 4% and 6% at pH 8.9 and 5% at p H 7.1 were used. Higher concentration of gels gave unfavorable banding for this enzyme. Due to variation in staining intensity, as well as nonrepeatable mobility variation between homozygous strains, all comparisons of AP-3 mobilities were done in heterozygotes derived from crosses to the same reference strain CH-2 with a 0.87 mobility allele. Staining was done as described in NORMAN and PRAKASH (1980~). For larval protein-IO, only 1.04/1.06 heterozygotes were used to screen for new alleles. Gel concentrations of 3.8% and 5.8% were used with pH 8.9 buffer with the addition of 1.33 mM EDTA. In the pH 7.1 buffer, Pt-IO migrates very slowly, and only a 4.2% gel concentration was used. The protein was detected with a 0.05% solution of Coomassie blue in a 7:7:1 mixture of water: methanol: acetic acid.

Relative amylase activity measures of the Bogota strains in Table 2 were assayed by the procedure described in NORMAN and PRAKASH (1980a).

RESULTS

This paper is particularly concerned with re-examining the apparent electrophoretic similarities among species, inversions and between the isolated population of Bogota, Colombia, and mainland D. pseudoobscura populations. For this reason, several strains of each common gene arrangement in D. pseudoobscura and D. persimilis were selected that carried the associated alleles. Strains carrying rare or “wrong” alleles were examined when appropriate comparisons could be made between species or inversions. The results of this attempt to discriminate new structural alleles of these three loci are presented in Table 1. The new allelic designation, using only the realtive mobilities in pH 8.9 and pH 7.1 buffers, pro- vides a complete description for the variants that were found. For example, the amylase allozyme 0.86(0.76) means that these strains have an allozyme that at pH 8.9 migrates 86%, and at pH 7.1 migrates 76% of the distance of the I.OO(i.00) amylase allozyme.

Amylase (Amy) : Five amylase electromorphs that could be compared between species and/or inversions were available in either strains isogenic for chromosome 3 or inbred lines. We have detected four additional alleles among these strains with some important implications for genetic differentiation of the A m y locus.

The 0.84 allozyme is common in the Santa Cruz phylad, which includes the CH, TL and SC arrangements, all of which are represented in our sample. In addition, one 0.84 strain of AR and one of ST were examined. No new variants within each of the TL, SC or CH arrangements could be detected under any condition. A comparison of TL strains with CH-2 in 5% gels at pH 7.1, however, disclosed a consistently slower variant in the TL strains (Figure 2). While the small mobility difference between the two strains could not be resolved as a two-banded pattern in 0.86 (0.76) /0.86 (0.74) heterozygotes, the mobility difference is consistently evident when both 0.86(0.76) and 0.86(0.74) are compared in 0.86


TABLE 1

Comparisons of electrophoreiic allele variation among species, inversions and the Bogota population of Drosophila pseudoobscura*

D. pseudoobscura

Bogota Mather D. persimilis Original allelic - D. New

LOCUS allele) designation$ TL SC TL CH AFi PP ST ST KL RD MD WT miranda

A m y 0.84 0.86(0.76) 2 1 1

1.00 1.09

0.92

1.05

Ap-3 0.98

I .oo Pt-IO 1.04

1.06

0.86(0.74) 1.00(1.00) 1.09(1.12) 1.09(1.08) O.SO(0.99) 0.90(0.96) 1.02(1.00) 1.04(1.10) 0.98 (0.98) 0.97(0.98) 1.00(1.00) 1.04(1.00) 1.045 1.06(1.08j 1.055

4 5 9 1 3 8 9 9 1 8 2 4 1

8 4

1

1 1 2

1 3 4 6 3 1

6 6 5 2 4 1 2 4 4 4 6 1 6 2 3 4 4

I 4 3 6 4

I

* Each table entry refers to the number of strains examined that were found to have the same

+Allele of PRAILASH, LEWONTIN and HUBBY (1969) for A m y and Pt-10 and of PRAKASH

$ The first number is the relative mobility of the allozyme in 5% gel, ph 8.9 buffer. The num-

electrophoretic allele under all conditions.

(1976a) for Ap-3.

ber in parenthesis is the mobility in 5% gel, pH 7.1 buffer.

(0.76)/1.00 and 0.86(0.74)/1.00 heterozygotes (Figure 3). Further comparisons of the AR-4, ST-3 and CH-2 strains did not resolve any other Amy alleles. The same is true of the TL and SC strains from Bogota when compared to the TL strains of Mather, California. Thus, 0.84 strains can be divided into two allelic classes, each characteristic of a different set of gene arrangements.

The 1.00 electrophoretic allele is one of the predominant Amy alleles in both D. pseudoobscura and D. persimilis. Mobility variation was compared among gene arrangements that are associated with the 1 .OO allozyme as well as one 1 .OO strain each of TL and WT and three strains of CH. Five separate gel conditions failed to reveal any new allelic forms (Table 1 ) . Crosses of 1.00 strains with different gene arrangements to the same 0.86 (0.76) CH-2 strain and the subsequent comparison of 1.00 mobilities in these 0.84 (0.76) /1 .OO heterozygotes failed to detect any consistent differences in mobility. Thus, for the 1.00 mobility class, the electrophoretic identity between the two species and among the many inversions is not altered.

The 1.09 electromorph is common only in the WT arrangement of D. persimilis. Examination of several WT strains revealed no new Amy alleles under any condition. A comparison of 1.09 in WT-10 with the 1.09 in KL at 5 % gel pH 7.1 buffer, however, disclosed a slower variant in the KL arrangement (Figure 4).


FIGURE 2.-Gel showing mobility difference at pH 7.1 between Amy O.SS(0.76) and Amy 0.86(0.74). A: TL-1 0.86(0.74). B: AR-4 O.SS(0.76). C: TL-4 0.86(0.74). D: ST-3 O.SS(0.76). E: TL-6 0.86(0.74). F: CH-2 0.86(0.76).

A H FIGURE 3 . 4 e l illustrating mobility difference at pH 7.1 between Amy, 0.86(0.74) and

0.86(0.76) in heterozygotes obtained from crosses to the ST-6 strain with an Amy l.OO(l.00) allele. A: TL-1, Mather 0.86(0.74) x ST-6. B: B-9(TL), Bogota 0.86(0.74) x ST-6. C: AR-4, Mather, 0.86(0.76) x ST-6.


The new allele 1.09( 1.08) in KL, was easily resolved as a two-banded pattern in heterozygotes with the WT 1.09(1.12) allozyme. All four KL 1.09 strains, from two different populations, had identical mobilities in every gel condition.

Two rarer Amy alleles that were represented in the different species and in different inversions were also compared. The 0.92 allozyme was clearly resolved into two variants by extended electrophoretic runs in 5% gels a t pH 7.1 (Figure 4). The 0.92 allozyme from the ST-5 strain of D. pseudoobscura was faster in mobility than the 0.92 allozyme of ST and MD gene arrangements of D. persimilis. The 0.92 allozyme of the ST and MD gene arrangements of D. persimilis did not differ in mobility under any electrophoretic condition. Two strains with 1.05 allozymes were also distinguished as having different alleles between species in extended runs in 5% gels a t pH 8.9 and 7.1 (Figure 4).

Adult acid phosphatase-3: The Ap-3 polymorphism consists of two common alleles. D. pseudoobscura has both a 0.98 and 1.00 electrophoretic allele, whereas D. pcrsimilis is monomorphic for the 2.00 allele. The search for variation among gene arrangements and species was confined to comparisons among 0.87/1.00 and 0.87/0.98 heterozygotes derived from independent crosses to the same reference strain, CH-2, which carries the rare 0.87 allele. This procedure effectively reduced the nonallelic mobility variation as mentioned in MATERIALS AND METH- ODS. All strains listed in Table 1 were examined using this procedure at three different gel conditions. Only one new variant could be reliably detected by these methods. This strain, B-9 from Bogota, displayed a slower mobility 0.98 a l l o z p e 0.97(0.98) in gels of buffer pH 8.9. The extensive similarity between D. pseudoobscura and D. persimilis and among inversions of the Standard phylad as well as

FIGURE 4 . 4 e l illustrating mobility differences in Amy alleles at pH 7.1. A: KG1.09(1.08) X

084(0.76). E: ST (D. persimilis) O.gO(0.96) x 0.84(0.76). F: ST (D. pseudoobscuru) 0.90 (0.99) x 0.84(0.76).

l~oO(l.00). B: WT-1.09(1.12) x l.OO(1.00). C: CU-1.04(1.10) x 0.84(0.76). D: KL1.02(1.00) X


FIGURE 5 . 4 e l illustrating mobility difference at pH 8.9 between Pf-10 2.06 and 1.055 alleles in heterozygotes obtained from crosses to the PP-1 strain with a PI-20 1.04 allele. A: B-1,1.055 X 1.04 B: CH, 1.06 x 1.01. C: TL, 1.06 x 1.04.

the 0.98 similarity among inversions of the Santa Cruz phylad (including Bogota) and PP remains unchanged.

Larval protein-I0 (Pt-IO): Comparisons of Pt-10 mobilities among isogenic strains proved unsuitable since slight mobility differenccs between strains neither Mendelized nor proved to be consistent. High gel concentrations (> 6%) gave unfavorable resolution for Pt-10 protein. At pH 7.1, Pt-10 mobility is radically altered in acrylamide gels. The mobility is drastically reduced, and in heterozygotes Pt-10 migrates as one band intermediate to the 1.04 and 1.06 bands in homo- zygotes. It is likely that the Pt-10 protein band at pH 7.1 represents a complex of Pt-10 with some other protein. At pH 8.9, comparisons were done with 1.04/1.06 heterozygotes, while at pH 7.1, 1 .04 and 1.06 homozygous lines were compared. D. miranda strains were also included since they have the 1.04 allozyme. Electro- phoresis uncovered two new variants each present only once. At pH 8.9, the ST- H2 strain of D. persimilis has a slightly faster 1.04 allozyme, whereas the B-1 strain of Bogota displays a slightly slower 1.06 allozyme (Figure 5 ) . No distinctions could be made among species, inversions or the Bogota population in general that are not evident in the previously described associations of 1.04 and 1.06.

DISCUSSION

Additional variants at Pt-10, AP3, and Amy loci detected electrophoresis: For the loci Pi-IO and Ap-3, the near absence of any further structural gene variation leaves the pattern of allozyme variation among inversions and species un-

CHROMOSOWE 3 INVERSIONS 735

altered. All three sibling species still appear to share the 1.04 electrophoretic allele at the Pt-10 locus, whereas D. pseudoobscura and D. persimilis have the same 1.00 allele at the Ap-3 locus. The results for these loci are similar to those obtained for the Odh-1, M d h (COYNE and FELTON 1977) and Hez-I (BECKEN- BACH and PRAKASH 1977) loci in that few new variants were detected in altered conditions of electrophoresis. Based upon the increase in the number of alleles found for Est-5 (MCDOWELL and PRAKASH 1976; COBBS and PRAKASH 1977; COYNE, FELTON and LEWONTIN 1978) and X d h (COYNE 1976; SINGH, LEWONTIN and FELTON 1976; PRAKASH 1977a), a significant increase in the number of Amy alleles was expected because A m y ranks just below Est-5 and X d h in the number of alleles detected in these three species under standard conditions of electrophoresis (PRAKASH 1977b). Nine allelic classes are now discernable from the five classes examined. This result is similar to the increase in alleles for Adh-6 (COYNE and FELTON 1977), the aldehyde oxidase-I of PRAKASH, LEWONTIN and HUBBY (1969), which showed about a two-fold increase in the number of mobility classes. The discovery that the l .05 and 0.92 allozymes of amylase in both D. pseudoobscura and D. persimilis are different in the two species suggests that other uncommon amylase allozymes may not be identical between species. It seems likely that these alleles have arisen independently by mutation subsequent to speciation. This result is in accord with COYNE (1976) and COBBS and PRAKASH (1977), who could distinguish shared electrophoretic alleles at the X d h and Est-5 loci between D. pseudoobscura and D. persimilis. X d h and Est-5 were quite divergent for alleles under one condition of electrophoresis, with each species having a different most frequent mobility allele. Sequential electrophoresis has shown that many variants in the shared classes are truly species specific. These results, however, may not be true in general since Est-5 similarity between D. miranda and D. pseudoobscura has not been altered by varying conditions of gel electrophoresis (COBBS and PRAKASH 1977).

The other “new” allozymes found for amylase have particular relevance to the genetic differentiation of inversions. If the detection of a new structural allele 0.86(0.76) in the 0.84 class proves to distinguish gene arrangements, as our finding suggests, then the degree of association of amylase allozymes with inversions is stronger than first thought. The distinction between the 0.84(0.76) allele of the Standard phylad strains and the 0.84(0.74) allele of the Santa Cruz phylad strains means that the reported sharing of the 0.84 allele in gene arrangements of these two phylads (PRAKASH and LEWONTIN 1968) is overestimated. Likewise, the frequency of occurrence (- 17%) of the 1.09 allele in KL is likely an overestimate since the 1.09 allozyme 1.09(1.12) found in WT is not equivalent to the 1.09, 1.09 (1 .OS) , of KL. These new variants are also distinguished on the basis of relative activities (NORMAN and PRAKASH 1980a). The WT-1 strain with a 1.00 allele cannot be distinguished in mobility from the 1.00 allozyme in other inversions; yet, its relative activity (NORMAN and PRAKASH 1980a) and developmental variation in expression (NORMAN and PRAKASH 1980b) clearly distinguish this amylase from other 1.00 strains. The apparent genetic similarity between WT and other arrangements, due to a substantial fre-

736 R. A. NORMAN A N D S. PRAKASH

quency of 1 .OO in WT ( PRAKASH and LEWONTIN 1968), is also likely to be smaller than electrophoretic data suggest.

electrophoretic allele class is in accord with the results of POWELL (1979) and POWELL, RICO and ANDJELKOVIC (1980), our findings of additional variants in the 0.84 and 1.09 classes are clearly not. These authors reported no new amylase variants under a variety of electrophoretic conditions. The following might account for this dis- crepancy. The 1.09(1.08) variant of the KL arrangement may not have been present in samples studied by POWELL, RICO and ANDJELHOVIC (1980) since the 1.09 electromorph is uncommon in the KL arrangement. Populations studied by POWELL (1979), however, should have included both CH and TL inversion types. I n this case, the use of isofemale lines may not have allowed the discrimination between 0.84(0.76) and 0.84(0.74) that we report in this paper using isogenic lines for various chromosome 3 inversions since these variants cannot be reliably distinguished in 0.84(0.76) / 0.84(0./’4) heterozygotes.

Genetic differentiation of Bogota: Table 2 contains data on the relative amylase activities of strains derived from the Bogota population of D. pseudoobscura. The mean relative activity among these strains is 54.2% (standard deviation, 1.20%). The mean activity among TL strains of Mather, relative to the same reference strain, is 54.8 % (standard deviation, 1.5 1 % ) (NORMAN and PRAKASH 1980a). Amylase relative activities, amylase structural alleles, AP-3 relative activities (NORMAN and PRAKASH 1980c), Ap-3 alleles, Pt-IO alleles, Pt-12 alleles, as well as Hex-1 alleles and Hex-1 relative activities (BECKEKBACK and PRAKASH (1977), show that little differentiation for chromosome 3 loci exists between strains of Bogota and strains containing Santa Cruz phylad arrangements from Mather, CA. The differentiation among inversions contrasts rather dramatically with this failure to find any substantial genetic differences between Bogota and mainland D. pseudoobscura.

Whereas our finding of no new alleles in the

TABLE 2

Mean relative activities of amylase in ytrains from the Bogota pcpulation carrying the 0.86(0.74) Amy allele*

Strain Mean relative activity & 1 S.D.: Nt B-14(0) 53.3 f 4.99 4 B-7 54.2 i- 2.06 4 B-9 54.6 f 2.07 5 B-18 (0) 53.6 & 2.55 5 B-1 54.0 k 2.55 5 B-2 56.6 4.88 4 B-I0 53.0 t 2.00 6

* All activities are expressed as percentage of total amylase activity in heterozygotes resulting

+ S.D. is the standard deviation of the samples. 2 N is the sample size.

froin crosses to the PP-1 strain of Mather. CA, which carries the AmyI.00 allele.


SINGH (1979) reported virtually complete genetic differentiation at the Pt-lU locus of the Bogota population when compared to five mainland D. pseudoobscura populations. These populations, however, consist mainly of Standard phylad inversions, which are not found at all in Bogota population (DOBZHANSKY eP al. 1963). Because Standard phylad inversions are associated with an allele rarely found in Santa Cmz phylad inversions and not at all in. Bogota (PRAKASH and LEWONTIN 1968), the differentiation SINGH described is related to inversion differentiation and not to geographical differentiation of the Bogota population. We have mentioned (see RESULTS) that the electrophoretic mobility of Pt-IO is altered in different buffers, and Pt-10 may be complexed with other proteins. In our work, the mobility at pH 7.1 of the Pt-10 protein complex is analogous to the mobility of what SINGH describes as a Pt-6 protein that appears only in the TGA buffer system (pH 8.0) Also in SINGH’S study, Pt-6 displays a pattern of diffep- entiation between Bogota and mainland populations that is very similar to the Pt-IO polymorphism he described; the strong correlation between the mobilities of variants at Pt-lU and Pt-6 indicates that the variation may be due to the same locus. We feel that this Pt-6 differentiation is due to variation at the Pt-IO locus and thus related to inversion differentiation and not to the isolation of Bogota.

These results for chromosome 3 loci are unlike those reported for Est-5 (COYNE, FELTON and LEWONTIN 1978), X d h (SINGH, LEWONTIN and FELTON 1976) and Adh-6 (COYNE and FELTON 1977). For these loci, Bogota strains are borh polymorphic and appear to have alleles that are not found in strains from mainland D. pseudoobscura. The discovery of polymorphism at these loci in Bogota is not wholly unexpected since these genes are segregating for so very many alleles in D. pseudoobscura populations. It is not clear, however, how much of the genetic change observed at some loci in Bogota is the result of divergence subsequent to geographic isolation solely due to mutation, random drift and natural selection and how much may be associated with founder effects. I t should also be noted that loci other than X d h and Est-5 show substantially different frequencies between mainland and Bogota populations (PRAKASH, LEWONTIN and HUBBY 1969; PRAKASH 1976b). Pgm-1 and Est-6 have an allele fixed in Bogota that occurs in frequencies of only 10% to 40% in mainland populations. At Hez-2, mainland populations appear monomorphic for an allele that has a frequency of only 30% in Bogota; whereas, an allele unique to Bogota is found there at a frequency of 70%. At Pt-8, an allele that has a frequency of -87% in Bogota is found in frequencies of only 1 % to 3% in mainland populations. At the acph-l locus (AYALA and DOBZHANSKY 1974), the most common allele in Bogota has a frequency of 83%, but occurs in only 1% of strains from the mainland. Patterns of polymorphism at these loci suggest that random drift or natural selection, coupled with a founder effect, has substantially altered allele frequencies in Bogota. Because of this possibility, and in the absence of any through sampling of Mexican and Guatemalan populations, the likely ancestors of the Bogota founder population (DOBZHANSKY et al. 1963) , caution must be exercised in terming alleles in Bogota unique. The origin of partial reproductive isolation rapidly and due to random drift in the Bogota population, as suggested previously (PRAKASH 1972), may be

738 R. A. NORMAN AND S. PRAKASH

supported by cases of “hybrid dygenesis” found in D. melanogaster (KIDWELL, KIDWELL and SVED 1977). With the available evidence, it cannot be concluded that the partial reproductive isolation of Bogota from mainland D. pseudoobscura has been accompanied by major genetic differentiation in the form of new alleles.

Chromosome inversions, genetic differentiation and adaptive evolution: The first report of inversion-allozyme associations in D. pseudoobscura and D. persimilis showed that inversions of the Standard phylad are largely distinct from those of the Santa Cruz phylad for alleles of Pt-10 and A m y and that the associations transcend the two species. Other polymorphic loci, Pt-12 (PRAKASH and LE- WONTIN 1971) and Ap-3 (PRAKASH 1976a), showed somewhat different patterns of inversion-allozyme associations, thus increasing the genetic distinctions among arrangements. Through the measure of relative activities for AP-3 (NORMAN and PRAKASH 1980~) and Amy allozymes (NORMAN and PRAKASH 1980a) and the comparison of relative activities of Amy allozymes between larvae and adults (NORMAN and FRAKASH 1980b), the amount of genetic polymorphism now discernable at these loci has altered our perception of inversion differentiation in two ways. First, many inversions that are associated with the same allozyme at a locus have proven to be actually genetically differentiated for relative activities. Second, the so-called “wrong” alleles are, in many cases, different structurally and/or in activity from the same electrophoretic allele in arrangements in which this allele occurs at high frequencies. This new pattern of variation, as it contrasts with the original data, is summarized in Table 3. There is obviously much more genic divergence among inversions, even among those within the same phylad, than had been realized.

Originally, evidence for genetic differences among inversions was based on the frequencies of lethals, relative fitnesses in laboratory experiments, and persistent altitudinal and geographical clines ( DOBZHANSKY 1970). Presumably, these types of differences reflect variation in physiological and adaptive abilities of inversion types, which are realized in natural populations and allow the main- tenance of inversion polymorphisms. The geographical variation in chromosome 3 inversions and associated allozymes of D. pseudoobscura and D. persimilis con- trast dramatically with the genetic similarity of geographical populations for loci on other chromosomes lacking inversion polymorphisms ( PRAKASH, LEWONTIN and HUBBY 1969). The same type of pattern among populations due to associations of allozymes with inversions is found in D. robusta (PRAKASH 1973; PRA- KASH and LEVITAN 1973; 1974) and D. subobscura (CHARLESWORTH et crl. 1979; LOUKAS, KRIMBAS and VERGINI 1979). The genetic variation necessary for differential adaptation of populations to different environments may reside largely among inversions.

If evolutionary time is long and the mutation rate is larger than the recombination rate, then both natural selection and random drift may lead to inversion differentiation (NEI and LI 1980). Divergence due to natural selection should be faster. We cannot exclude either of these evolutionary forces as a cause of differentiation at Amy, Ap-3, Pt-10 or Pt-12, but if the variation found for these loci is

m w il

3

C H R O M O S O ~ E 3 INVERSIONS 739


effectively neutral with respect to the evolution of fitness differences among inversions, then clearly for loci that are part of these adaptive gene complexes the genetic divergence among inversions must be even greater. The differential adaptation of these chromosome inversions may result from the accumulation of very small fitness differences resulting from genetic changes at many loci - a process greatly enhanced by the large reduction in recombination that inversions provide.

We are grateful 10 WYATT ANDERSON and ELIOT SPIESS for providing strains. This research was supported by Public Health Service grant GM-19217 and by a predoctoral genetics training grant.

LITERATURE CITED

AYALA, F. and TH. DOBZHANSKY, 1974 A new subspecies of Drosophila pseudoobscura. Pan. Pac.

BECKENBACH, A. T. and S. PRAKASH, 1977 Examination of allelic variation at the hexokinase

CHARLESWORTH B., D. CHARLESWORTH, M. LOUKAS and K. MORGAN, 1979 A study of linkage

COBSS, G. and S. PRAKASH, 1977 A comparative study of the esterase-5 locus in Drosophila

Ento. 50: 211-219.

loci of Drosophila pseudoobscura and D. persimilis. Genetics 87 : 743-761.

disequilibrium in British populations of Drosophila sub~bscura. Genetics 92 : 983-994.

pseudoobscura, D. persimilis and D. miranda. Genetics 85: 697-711.

vealed by varied techniques. Genetics 84: 593-607. Genic heterogeneity at two alcohol dehydrogenase loci in

Drosophila pseudoobscura and Drosophila persimilis. Genetics 87 : 285-304. Extent of genetic variation at a highly

polymorphic esterase locus in Drosophila psaudoobscura. Proc. Natl. Acad. Sci. U.S. 75:

Genetics of the Evolutionary Proseers. Columbia Univ. Press, New

Contributions to the Genetics, Taxonomy, and Ecology of Drosophila pseudoobscura and its Relatives. Carnegie Inst. Washington Publ. 554. -, 1948 The suppression of crossing over in inversion heterozygotes of Drosoph:'la pseudoobscura. Proc. Natl. Acad. Sci. U.S. 34: 137:141.

Genetics of natural populations. XXXI. Genetics of an isolated marginal population of Drosophila pseudoobscura. Genetics 48: 91-103.

Hybrid dysgenesis in Drosophila melanogaster: a syndrome of aberrant traits including mutation, sterility and male recombination. Genetics 86: 813-833.

Crossing over and inversions in coadapted systems. Am. Naturalist 90: 4 1 4 5 .

The genetics of DrosophJla subobscura populations. IX. Studies on linkage disequilibrium in four natural populations. Genetics 93: 49 7-523.

Allelic heterogeneity within allozymes separated by electrophoresis in Drosophila pseudoobscura. Prm. Natl. Acad. Sci. U.S. 73 : 4150-4153.

Nonrandom association between electromorphs and inversion chromosomes in finite populations. Genet. Res. (Camb.) 35: 65-83.

COYNE, J. A., 1976

COYNE, J. A. and A. A. FELTON, 1977

COYNE. J. A., A. A. FELTON and R. C. LEWONTIN, 1978

Lack of genic similarity between two sibling species of Drosophila as re-

5090-5093. DOBZHANSKY, TH., 1970

DOBZHANSKY, TH. and C. EPLING, 1944 York.

DOBZHANSKY, TH., A. S. HUNTER, 0. PAVLOVSKY, B. SPASSKY and B. WALLACE, 1963

KIDWELL, M. G., J. F. KIDWELL and J. A. Svm, 1977

LEVINE, R. P., 1956

LOUKAS, M., C. B. KRIMBAS and Y. VERGINI, 1979

MCDOWELL, R. E. and S. PRAKASH, 1976

NEI, M. and W.-H. LI 1980

CHROMOSOME 3 INVERSIONS 74 1

NORMAN, R. A. and S. PRAKASH, 1980a Variation in activities of amylase allozymes associated with chromosome inversions in Drosophila pseudwbscura, D . persimilis, and D. miranda, Genetics 95: 187-209. -, 1980b Developmental variation in amylase allozyme activity associated with chromosome inversions in Drosophila persimilis. Genetics 95 : 1001- 1011. -, 1980c Activity variants of acid phosphatase-3 among chromosome 3 inversions of Drosophila pseudoobscura. Genetics 96: 743-755.

Population genetics of Drosophila amylase. 11. Geographic patterns in D . pseudoobscura. Genetics 92: 613-622.

Population genetics of Drosophila amylase. 111. Interspecific variation. Evolution 34: 209-213.

Origin of reproductive isolation in the absence of apparent genic differentiation in a geographic isolate of Drosophila pseudoobscura. Genetics 72: 143-155. -, 1973 Patterns of gene variation in central and marginal populations of Drosophila robusta. Genetics 75: 347-369. -, 1976a Genetic differences between third-chrmosome inversions of Drosophila pseudoobscura. Genetics 84: 787-790. --, 1976b Further studies on gene polymorphism in the mainbody and geographically isolated populations of Drosophila pseudoobscura. Genetics 85: 713-719. - , 1977a Allelic variants a t the xanthine dehydrogenase locus affecting enzyme activity in Drosophila pseudwbscura. Ge- netics 87: 159-168. -, 1977b Genetic divergence in closely related sibling species Drosophila pseudoobscura. Drosophila persimilis, and Drosophila miranda. Evolution 31 :

Association of alleles of the Esterase-I locus with gene arrangements of the left arm of the second chromosome in Drosophila robusta. Genetics 75: 371-379. -, 1974 Association of alleles of the malic dehydrogenase locus with a peri- centric inversion in Drosophila robusta. Genetics 77 : 565-568.

A molecular approach to the study of genic heterozygosity in natural populations. 111. Direct evidence of coadaptation in gene arrangements of Drosophila. Proc. Natl. Acad. Sci. U.S. 59: 398-405. - , 1971 A molecular approach to the study of genic heterozygosity in natural populations. V. Further direct evidence of coadaptation in inversions of Drosophila. Genetics 69: 4115-408.

A molecular approach to the study of genic heterozygosity in natural populations. IV. Patterns of genic variation in central, marginal and isolated populations of Drosophila pseudoobscura. Genetics 61 : 841-858.

Genic heterogeneity within electrophoretic “alleles” and the pattern of variation among loci in Drosophila pseudoobscura. Genetics 93 : 997-101 8.

Genetic heterogeneity within electrophoretic “alleles” of xanthine dehydrogenase in Drosophila pseudoobscuru. Genetics 84:

Corresponding editor: M. NEI

POWELL, J. R., 1979

POWELL, J., M. RICO and M. ANDJELKOVIC, 1980

PRAKASH, S., 1972

14-23. PRAKASH, S. and M. LEVITAN, 1973

PRAKASH, S. and R. C. LEWONTIN, 1968

PRAKASH, S., R. C. LEWONTIN and 5. L. HUBBY, 1969

SINGH, R. S., 1979

SINGH, R. S., R. C. LEWONTIN and A. A. FELTON, 1976

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and satya - geneticsr. a. norman* and satya prakash department of biology, university of rochester,...

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