population genetics of drosophila amylase. v. genetic ... - Europe PMC

Copyright 0 1984 by the Genetics Society of America

POPULATION GENETICS O F DROSOPHILA AMYLASE. V. GENETIC BACKGROUND AND SELECTION O N DIFFERENT CARBOHYDRATES JEFFREY R. POWELL AND GEORGE D. AMATO Department ofBiology, Yale University, N m Haven, Connecticut 0651 1 Manuscript received September 20, 1983 Accepted December 16, 1983 ABSTRACT Frequency changes in amylase allozymes and patterns of tissue-specific expression of amylase have been monitored in laboratory populations of Drosophila pseudoobscura maintained on media in which the only carbohydrate source was maltose or starch. Nonrandom changes occurred in patterns of expression, whereas no patterns in allozyme frequency changes were discernible. The nature of the pattern changes was similar to an identical study done on populations derived from a natural population several hundred miles from the population used in the present experiments. However, in the previous study nonrandom changes in allozyme frequencies were also noted. Evidently, selection on the Drosophila amylase system differs depending upon the genetic background of the population. Furthermore, the evolutionary dynamics of structural gene variants and those regions controlling its expression may be independent, a result consistent with DNA sequence data.

HAT natural selection molds adaptations in populations is nearly universally T accepted. However, the specific nature of the genetic variants contributing to adaptations is less well understood. Presently, there is controversy over the relative roles of changes in genes (i.e., amino acid substitutions) compared with changes in their regulation of expression in adaptive evolution ($ WILLS 1973; WILSON1976). In an attempt to shed some light on the problem, we embarked on a research program designed to compare the evolutionary dynamics of naturally occurring polymorphisms of both types. The stimulus for this program and DOANE(1978), who clearly documented was the seminal work of ABRAHAM genetically determined variants in the regulation of a-amylase expression in the midguts of Drosophila. Electrophoretic variants (allozymes) of the structural gene, Amy, are also widespread, thus allowing such comparative studies. Previous studies in this series include confirmation of the genetic control of and tissue-specific midgut patterns of expression in D. pseudoobscura (POWELL LICHTENFELS 1979), a survey of geographic variation in frequencies of midgut patterns and allozymes (POWELL 1979) and interspecific comparisons (POWELL, RICO and ANDJELKOVIC 1980). Most relevant to the present report is a study of the effects of maintaining laboratory populations on medium containing different and ANDJELKOVIC 1983). Replicas of the same founding carbohydrates (POWELL Genetics 106: 625-629 April, 1984.

626

J. R. POWELL AND G. D. AMATO

population were maintained for more than 2 yr on medium in which the only carbohydrate was starch (the substrate of amylase) or maltose (the product of amylase). Nonrandom changes in frequencies of both Amy alleles and midgut patterns of expression were observed. In particular, on starch medium the Amy F allele was favored, and a pattern of expression of amylase confined to the very anterior of the midgut was favored. On maltose medium, no consistent changes in either polymorphism were detected. A total of 14 populations, seven on each medium, was studied; all were derived from the same natural population, Bryce Canyon, Utah. T o test the robustness of the conclusion that these polymorphisms respond to selection in certain environments, we have repeated the experiments using a different base population. The results are both similar and different. MATERIALS AND METHODS

The base population for the present experiments was a set of 11 isofemale lines begun by flies captured in the Chiricahua Mountains in southern Arizona during the summer of 1980; WYATT ANDERSON kindly sent us these strains. This sight is some 425 miles south of Bryce Canyon. D. pseudoobscura in this area, as in Bryce Canyon, are nearly monomorphic for the AR third chromosome. All 11 strains were checked at least twice to assure that they were homozygous AR; during the course of the experiments, samples were taken from the cages and chromosomes were reexamined. N o third chromosome gene arrangements other than AR were ever found. Details of the experimental design can be found in POWELL and ANDJELKOVIC (1983), and the description here will be brief. The 11 isofemale lines were mixed about equally and used to begin laboratory populations in cages which support a population of more than 3000 adults; at least 500 flies from the mixed population per cage were the founders. Two cages, designated I"' and I P , had medium in which the only carbohydrate sourcs was starch. Two cages, designated I"" and 11", had medium in which the only carbohydrate source was maltose. Periodically, egg samples were taken on standard cornmeal-molassesmedium, and flies were developed on this medium at low densities and optimal temperature. Adults emerging were assayed for Amy allele frequencies by polyacrylamide electrophoresis, and the pattern of expression along the midgut was determined as described by ABRAHAM and DOANE(1978). Designation of midgut patterns follows DOANE(1980) and POWELL and LICHTENFELS (1979). The anterior midgut (AMG) is divided into three regions designated 1, 2 and 3; the posterior midgut (PMG) is divided into two regions, 1 and 2. Presence of activity is indicated by the number and absence by a zero. Thus, pattern 120-00 would have activity in the two most anterior regions but not elsewhere. RESULTS

Table 1 shows the frequencies of both midgut patterns and Amy alleles in the four populations over a period of 800 days. The sample size for the midguts is between 60 and 66 for each cage at each sample; 96 genes were sampled for Amy for each cage. These populations have only two Amy alleles which we designate F and S and almost certainly correspond to PRAKASH and LEWONTIN'S( 1 968) 1.00 and 0.84 alleles, respectively. There is no pattern of Amy allele changes evident. Even after more than 2 yr all four cages have very similar gene frequencies, which are not significantly different from the original frequencies. Thus, there is no evidence for selection on these alleles. We might also note that these results give no evidence for drift being sufficient. This adds to our confidence that such experimental populations have rather large effective population sizes, and bottlenecks are not common. The changes in midgut patterns are significant and somewhat complex. Be-

627

DROSOPHILA AMYLASE GENETICS

m m

m m a m ~ ~ m am + m

m

9

9999999

919

19

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

mmmm(D

m a m m a - o m

19999

99199119

CO

c?

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

w a m a w m m + m a m v - m m a

9990999199911999

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

* m m m c o m a * m m o m - ~ m m m

1991999199191999 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

a m m m m m - a t - m m a m m + m

999999?9999?99?"

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

m - m a - m m a a * t . o - m m -

1"19"?19??11""11

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

m ~ + m o - c o - + - m m m o m o

? 9 9 4 4 4 ? 4 ? 9 ? " " 9 1 ?

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0100

- m m

I

o m 0 - m m

I 3

o m 0 - m m

c

o m 0 - m m

c

I

628

J. R. POWELL AND G.

D.

AMATO

cause the sample size is small for several patterns, statistical tests on the raw data are suspect. Thus, we have combined categories for testing. In essence, we have ignored the PMG and combined categories with the same AMG pattern. Thus, we have four categories, A E, B F, C G and D H (lettered designations as in Table 1). A FUNCAT analysis (SAS Institute, Cary, North Carolina) was performed on the combined data; this is similar to an analysis of variance based on chi-squares. Table 2 presents the results. There are significant cage effects indicating that the frequencies are different in different cages; the cage X time effect indicates that the cage differences were not static but changed over the course of the experiment. When the data in Table 1 are examined, one can see some patterns in these significant changes. In the starch cages the frequency of AMG pattern 100 (regardless of PMG) has increased, whereas in the maltose cages it has decreased. In maltose cages, AMG 123 has increased to 27% by day 800, whereas in the starch cages, it has remained 8 and 14%.

+

+

+

+

DISCUSSION

T h e general conclusion is that there is a tendency for limiting amylase activity to the most anterior region of the AMG in starch cages, whereas amylase activity spread more evenly along the midgut is favored in maltose cages, regardless of what is happening in the PMG. This result is similar to that obtained previously when a different base population (Bryce) was used (POWELLand ANDJELKOVIC 1983). In that study, the pattern 100-00 increased rather dramatically in the starch cages, whereas little or no change occurred in maltose cages. The main difference is that, in the present study, the PMG pattern did not seem to respond as it did with the Bryce populations. Based on the four populations studied in the present report, we would not be overly confident of the conclusions stated. However, because they are similar to those obtained with 14 other cages, we feel that they are quite significant. Thus, a total of 18 cage populations have been studied for changes in midgut activity patterns, using two different base populations. The robustness of the conclusion (that selection affects this polymorphism differently in starch and maltose environments) is established. In contrast, the results on the frequency of Amy alleles are not so clear. Previously (POWELL and ANDJELKOVIC 1983), we did obtain evidence of selection on this polymorphism as well. However, there is no evidence of selection in the present data. We can only conclude that the “genetic background” can affect the outcome of selection at Amy. An alternative explanation is that the F and S alleles are not really the same in the two populations. However, neither POWELL (1979) nor NORMAN (1978) found “hidden”alleles at Amy of D.pseudoobscuru by varying TABLE 2 FUNCAT analysis of data in Table I combining categories as discussed in text Source

Intercept

Day Cage Day X cage

d.f.

XP

P

3 9 9 27

236.5 49.2 21.9 48.7