THE PARTHENOGENETIC CAPACITIES AND GENETIC STRUCTURES OF SYMPATRIC POPULATIONS OF MERCATORUM AND

ALAN R. TEMPLETON Department of Biology, Washington University, St. Louis, Missouri 63130 Manuscript received August 4, 1978 Revised copy received December 5, 19178

ABSTRACT Drosophila mercatorum is a sexual species that can reproduce partheno- genetically in the laboratory. A previous study showed that a natural popula- tion of D. mercatorum inhabiting the Kamuela garbage dump on the Island of Hawaii could produce both viable parthenogenetic adults and self-sustaining parthenogenetic lines. The present study deals with a second screen for parthenogenesis and an isozyme survey performed on natural populations of D. mercatorum and D. hydei caught in patches of Opuntia tuna about 10 kilometers from Kamuela. Both cactus-patch species produced viable partheno- genetic adults, but only D. mercatorum produced parthenogenetic females themselves capable of parthenogenesis. Moreover, D. mercatorum produced several “hot” lines characterized by high parthenogenetic rates, while all lines of D. hydei had a homogenous low rate. The parthenogenetic capacity of the cactus-patch D. mercaiorum was lower than that of the garbage-dump D. mercatorum. Moreover, both the cactus-patch D. mercatorum and D. hydei had lower levels of polymorphism (26% and 22%, respectively) then the garbage- dump D. mercatorum (44%), and both cactus-patch populations had hetero- zygote deficiencies with respect to Hardy-Weinberg equilibrium, unlike the garbage-dump population. Consequently, these data do not support the idea that decreased levels of heterozygosity in a sexual population increase the chance that sexual females will produce totally homozygous, parthenogenetic progeny.

TALKER (1954) surveyed 28 species of for their ability to reproduce parthenogenetically and discovered at least some capacity for par- thenogenetic development in 23 of them. However, only three actually produced adult parthenogenetic progeny (D. parthonogenetica, D. polymorpha and D. affinis) . Since STALKER’Spioneering work, parthenogenetic adults have been discovered in five other normally sexual species of Drosophila (D. robusta; CARSON1961; D. mematorum; CARSON1967; D. anamssae and D. pallidosa; FUTCH1973; D. paulistorum, EHRMAN,personal communication). Of these species, D.mercatorum is of particular interest because its parthenogenetic stocks are easily reared in the laboratory (CARSON1967). Moreover, TEMPLETON, CARSONand SING (1976) showed that new Parthenogenetic strains of D. mer-

Genetics 92: 128?-1293 August, 1979. 1284 A. R. TEMPLETON catorum can be established with relative ease from wild-caught females from natural sexual populations. In order to insure that the high parthenogenetic capacity observed in the natural population of D. mercatorum reported by ‘rEMPLEToN, CARSONand SING (1976) is not unique or nonrepeatable, a second collection of D. mercatorum was made in November of 1976. This paper deals with the results of that collec- tion. Moreover, since STALKER’S(1954) work showed that laboratory stocks of many Drosophila species can initiate parthenogenetic development, it is possible that the high parthenogenetic capacity displayed by natural populations of D. mercatorum is not unique to that species, but may be generalized to natural populations of other species. Hence, a population of Drosophila hydei that is sympatric with the D. mercatorum population was also sampled and screened for parthenogenetic Capacity. D. hydei was included in STALKER’S original sur- vey, and did show some early parthenogenetic development. However, no par- thenogenetic adults or even pupae have been described in this species. An isozyme survey accompanied the Parthenogenetic screen for both species. In this paper, I will describe each species’ parthenogenetic capacity, levels of protein polymorphism and genetic structure, as well as contrast the two sympatric species for these attributes and calculate a genetic distance between them.

MATERIALS AND METHODS

All were collected shortly after sunrise or before sunset on November 20 and 21, 1976, near Kamuela, Hawaii. Kamuela was also the site of the 1974 collection of D. mercatorum reported by TEMPLETON,CARSON and SING (1976), but the primary collecting site for the 1974 collection was the Kamuela garbage dump. Since 1974, the dump has been modernized and no Drosophila of any species were found there. All flies from the 1976 collection were caught on two large patches of Opuntia tuna cactus only 2.25 m apart. The cactus patches were located a little past the mile 5 marker on Highway 27 just north of Kamuela and about ten km from the garbage dump. The elevaticn was approximately 946 m above sea level. Some of the flies were aspirated directly off the cactus pads, but most were attracted to baits. Two types of baits were used: a standard banana bait and rotten guava bait. D. mercatorum were found only in the guava baits and never in the banana baits, even when the two baits were placed side by side. D. hydei also preferred the guava baits, but a iew were captured in the banana baits. Best results were obtained for both species when the baits were placed inside the cactus patches. Only two species of Drosophila were collected in the cactus patches: D. hydei and D. mer- catorum (38 female and 49 male D. hydei were collected, us. 21 female and 66 male D. mercatorum). Some other collections were made in the open nearby and consisted almost exclusively of D. mehogaster. The D. hydei and D. mercatorum females were placed singly into shell vials with a standard cornmeal-molasses-agar food medium sprinkled with live yeast. The males were used for an isozyme survey, as were the females after they had produced progeny. Of the 21 D. mercatorum females, 20 produced progeny. These progeny were approximately equal numbers of males and females, indicating that the F, progeny thus obtained were the result of sexual matings in nature. Several of the emerging F, daughters were isolated as virgins from each of the 20 iso-female lines. Each of these virgin F, female lines were designated by “K-z-F,”, where K refers to Kamuela and z is a number assigned to the wild-caught female parent of the line (z = 35, . . . ,54; numbers 1 through 34 refer to lines established from the 1974 collection of D. mercatorum). Twenty virgin F, daughters from a single wild-caught THE CAPACITY FOR PARTHENOGENESIS 1285 female were placed in a shell vial to lay eggs. The daughters were transferred to a fresh vial every three days for a total of 33 days of egg laying. The number of eggs laid in each vial was counted. This counting was facilitated by the habit of D. mercatorum of laying its eggs around the edge of the vial and the food medium. Hence, a starting point was marked on the vial, which was then slowly rotated through 360" while the eggs were counted with the aid of a hand counter. The counting of the eggs not on the edge was facilitated by cutting a grid onto the food before the virgins were placed in the vial. The number of offspring a single wild-caught female produced was usually quite large, so that two to three replicas of each of the 20 K-z-F, virgin lines were usually made (i.e., a total of 40 to 60 virgin daughters from each wild-caught female were used to obtain unfertilized eggs). However, the eggs were counted only for the first replica, and the total number of unfertilized eggs obtained from each iso-female line was estimated by the number of eggs counted in the first replica times the number of replicas. The remaining F, daughters and sons of each wild-caught female were crossed to establish an iso-female sexual lines were established and were designated as Kz-0-Bi where Bi indicates the line is bisexual, z = 35, . . . ,54, and 0 refers to the fact that no bridge crosses have occurred (see CARSON1967). Twenty-four of the D. hydei females produced progeny, with all sex ratios being nearly 50:50. The design of the parthenogenetic screen was the same as that for the D. mercatorum virgins, except that D. hydei females tended to lay fewer eggs and thus produced fewer progeny in the laboratory. Therefore, only 20 to 40 (one to two replicas) of D.hydei virgins were used to obtain unfertilized eggs us. the 40 to 60 for D.mercatorum. To compensate for the smaller num- ber of virgins and the fact that D. hydei females laid fewer eggs than D. mercatorum females, the total egg-laying period was extended to 48 days. The D. hydei virgin female lines were designated by KH-r-F,, where H indicates hydei and z = 1, . . . ,38 (some of the z's were missing because only 24 of the 38 wild-caught females produced progeny). The D.hydei females tended to lay most eggs on the interior food surface and not around the edges of the vial. Thus, although they laid fewer eggs, they were much more time-consuming to count. To save time, all vials were counted up to sometime between the fourth and seventh change; thereafter, only every second or third change was counted. The eggs in the uncounted vials were estimated by linear extrapolation between the two counted vials straddling the uncounted vials in time. Finally, 24 iso-female bisexual lines were established for D. hydei and designated KHz-O-Bi. unfertilized egg developing into a viable adult) was estimated to be 1.1 X

RESULTS The results with respect to the parthenogenetic capacity of D. mercatorum are given in Table 1. The viable parthenogenetic rate (the probability of an

TABLE 1 Results of the screen for parthenogenetic capacity in the virgin daughters of wild-caught Drosophila mercatorum females

Parthenogenetic progeny No. of eggs No. of counted in No.,of Estimated total 3rd instar No. of No. of Stock replica 1 replicas no. of eggs larvae pupae adults K-F, 1 1,265 3 33,795 2 0 0 K-41-F1 12,435 3 37,305 5 5 5 K-43-F, 12,498 3 37,494 1 1 1 K-5&Fl 14,221 3 42,663 1 1 I 16 remaining 1 to3 isofemale 183,500 replicas 459,829 0 0 0 lines per line Average: 11,696 Sum: 61 1,086 9 7 7 1286 A. R. TEMPLETON However, the fact that five of the seven parthenogenetic adults were produced by one line (K41-F1) indicates that this rate is not homogeneous over all lines. The x2 value of homogeneity (this and all subsequent tests involving partheno- genetic rates are based on the likelihood ratio procedures given in TEMPLETON, CARSONand SING1976) with respect to viable rate over all lines is 27.72, with 19 d.f. (0.1 > P > 0.05), and the x2 value of homogeneity over all lines excluding K-41-F, is 7.88 with 18 d.f. The difference between these two xz values is a test of the null hypothesis that the viable rate of K-41-F, is no different from that of the remaining lines. This difference is 19.84 with one d.f. Therefore, KAI-F, has a significantly higher viable rate (1.3 x than the remaining lines (3.5 X which are quite homogeneous among themselves. This was the same pattern obtained from the 1974 sample in which three lines had a homo- geneous viable rate of 2.6 x while the others had a homogeneous viable rate of 1.2 x 1t5. To see if the 1974 and 1976 samples were homogeneous in viable rates, the x2 value of homogeneity was calculated over all lines and both years to be 82.73, with 50 d.f. The probability of homogeneity is less than 0.005, but this is not surprising since the viable rates are not even homogeneous within a year. Another test of homogeneity across years is to assume that all the “hot” lines (i.e., those lines producing three or more parthenogenetic progeny) have a homogeneous rate irrespective of year. The homogeneity x2 value of the hot lines irrespective of year (K-23, 25, 31 and 41) is 2.22, with three d.f. The homogeneity x2 value of all other lines irrespective of year is 24.61, with 46 d.f. Consequently, there is homogeneity across years when the data are partitioned into “hot” us. remaining lines. Moreover, there is no significant difference in the frequency of “hot” lines in the two samples (the frequency of hot lines in 1974 was 0.097 f 0.053, where 0.053 is the standard deviation, us. 0.050 -+ 0.049 for 1976), but this is not surprising given the small sample sizes. In summary, the 1974 and 1976 samples of D.mercatorum appear homogeneous with respect to the viable parthenogenetic rate. Another measure of parthenogenetic capacity is the “true rate.” i.e., the probability of an unfertilized egg developing into a viable adult that is itself capable of reproducing parthenogenetically. Only one of the seven partheno- genetic D.mercatorum females was capable of reproducing parthenogenetically (K-43), so that the true rate is 1.6 x for the 1976 data. Since only one female could reproduce parthenogenetically, there is no power to test homo- geneity among the 1976 lines. However, the 1976 rates can be tested us. the homogeneous 1974 true rate of 1.6 x This test yields a x2 value of 10.91, with 1 d.f. Therefore, there is a statistically significant difference between the 1974 and the 1976 true rate;, with the 1976 flies having a parthenogenetic capac- ity an entire order of magnitude lower than that of the 1974 flies. The results with respect to the parthenogenetic capacity of the Drosophila hydei lines are given in Table 2. As can be seen, the D. hydei lines did produce two viable parthenogenetic adults, the first ever recorded in this species. The estimated viable rate is 5.2 x and is homogeneous across lines (XI = 9.98 with 23 d.f.). Moreover, this viable rate is homogeneous with the 1976 D.mer- THE CAPACITY FOR PARTHENOGENESIS 1287 TABLE 2

Results of the screen for parthenogenetic capacity in the virgin daughters of wild-caught Drosophila hydei femaks

Parthenogenetic progeny No. of eggs No. of counted in No. of Estimated total 3rd instar No. of No. of Stock replica 1 replicas no. of eggs larvae pupae adults KH-3-F, 1,720* 1 1,720 1 1 0 KH-7-F, 16,226 1 16,226 1 1 1 KH-36F1 15,281 1 15,281 1 1 1 21 remaining 1 to2 isofemale 309,524 replicas 348,488 0 0 0 lines per line Average: 14,827 Sum: 381,715 3 3 2

* Only nine daughters were obtained from the KH-3 wild-caught female. catorum rate with K-41-F, excluded (x2= 1.08 with 1 d.f.). Consequently, the only noticeable difference between D.hydei and D. mercatorum in viable rate is the absence of “hot” lines in the hydei. However, since only 24 lines were examined, such “hot” lines may well exist at low frequency. Neither of the two parthenogenetic D. hydei produced any parthenogenetic progeny. The KH7-0-Im (Im stands for “impaternate”) female laid no eggs and died a few days after eclosion. The KH36-0-Im female was maintained as a virgin for two weeks after eclosion and laid 435 unfertilized eggs during this period, none of which hatched. At that point, the KH36-0-Im female was mated to a male from the KH36-0-Bi stock to produce the stock KH36-1-Bi, which is to be used in future studies on parthenogenesis in D. hydei. Therefore, the estimated true rate in D.hydei is 0. However, if D.hydei had a true rate equal to that of the 1976 D. mercatorum population (1.6 x lo-”, the probability of observing no parthenogenetic flies capable of parthenogenesis in the D. hydei would be 0.53. Consequently, D. hydei could have a parthenogenetic capacity comparable to that of D.mercatorum, but more extensive collections are needed to determine if it has a non-zero true rate of the order of A summary of the isozyme survey is provided in Table 3. The results of the 1974 survey (only part of which was published in TEMPLETON,CARSON and SING1976) are also provided for comparison. In the 1976 D.mercatorum collec- tion, ten of 23 systems were segregating, as opposed to seven of 16 for the 1974 collection. Defining a polymorphic locus to be one at which the frequency of the most common allele is less than 0.95, 26% of the loci are polymorphic in the 1976 collections us. 44% in 1974. This difference in level of polymorphism is not due to the fact that more loci were scored in 1976 since, using only the 16 loci scored for both samples, 25% of the loci are polymorphic in 1976 us. &% in 1974. However, both D. mercatorum collections are polymorphic at levels commonly encountered for Drosophila, with the 1974 sample being very close to the mean for all Drosophila (NEVO1978). The D.hydei collection was segre- gating at ten of 23 loci, and was polymorphic at 22% of the loci. This is some- 1288 A. R. TEMPLETON what below the D. mercatorum samples, but both the 1976 D. mercatorum and D. hydei populations are close to the average for the Drosophila repleta group (NEVO1978). As can also be seen from Table 3, D. mercatorum and D. hydei share very few isozyme bands. This can be quantified by using NEI’S (1972) genetic dis- tance and identity measures. The two D. mercatorum samples have an average identity of 0.99, and a genetic distance of 0.01. The average genetic identity between the 1976 sympatric populations of D.mercatorum and D. hydei is 0.11, and the genetic distance is 2.24. Genotype frequency data were obtained directly from the wild-caught flies and tested for deviations from Hardy-Weinberg equilibrium. This analysis is presented in Table 4. Only those. systems in which the most common allele frequency was less than 0.85 were used in order to insure adequate numbers in the genotypic categories. Furthermore, in the case of LAP in D. hydei, the rare

TABLE 3 Results of Kamuela isozyme surveys performed on wild-caught flies of Drosophila mercatorum caught in 1974 at the garbage dump and in 1976 on cactus patches, and of Drosophila hydei caught in 1976 on cnctus patches

Population D.mercatorum D. hydei Sample size: 72 84 80 System Allele 1974 1976 Allele - 1976 HEX 1.0000 1.oo bands unclear - G-6-PD 0.6000 0.5437 15 1 .oooo 0.4OOo 0.4563 6-PGD 0.8000 1.0000 8 1 .om0 0.2000 O.oo00 GOT-1 n.s.* 0.9w -3 1 .WO 0.0536 GOT-2 n.s. 1.oOOo 13 0.5727 20 0.4273 PGM 10 0.9938 1.moo 1.0000 14 0.0063 PGI 1.0000 1.om0 14 I .moo a-GPDH 1.WO 1.moo 30 I .00m ALD-I 1.woo 1.Oooo 2 I .OOoo ALD-2 1.mo 1.moo -17 1.0000 MDH o.oo00 0.0357 12 0.9688 1.Oooo 0.9643 17 0.0313 IDH 1.OoOo 1.0000 15 1.oom FUM n.s. 1.oOOo -2 I .moo ME 1.oOOo I .oooo 8 0.9813 10 0.0188 XDH 0.2083 0.1845 I5 0.4563 0.7917 0.8155 17.5 0.5438 APH ns. 0.041 7 13 0.9938 0.9583 20 0.0063 APH-SL n.s. 1.oooo no staining 25 0.0203 THE CAPACITY FOR PARTHENOGENESIS 1289 TABLE 3-Continued

(sex limited to females) ACPH -1 (S) 0.2750 0.0119 15 0.9932 -7 (F) 0.7241 0.9881 20 0.0068 LAP 25 n.s. 0.4881 22 0.4932 28 0.5119 23 0.4865 A0 no clear staining I3 0.7250 14 0.2750 ADH -20 (S) 0.0603 0.0060 -20 1.om0 -25 (F) 0.9397 0.9940 ODH 29 n.s. 1.ooao 34 1.0000 EST-A 39 (SI 0.4701 0.4107 41.5 1.woo 45 (F) 0.5299 0.5893 EST-B 38.5 (S) 0.6769+ 0.0595 42.5 1.oOOo 39.5 (M) 0.6667 42 (F) 0.3231 0.2738 EST-SL no strain 21.5 0.5114 (sex limited to males)

* n.s. = not scored. +The 38.5 and 39.5 alleles were not resolved in the 1974 data. All alleles are designated by a number giving their electrophoretic mobility in mm. Some of the D. mercatorum alleles had previously been designated by letters, and these are indicated in parenthesis. The frequency of each allele is then given. allele 25 was pooled with allele 23; similarly, for EST-B in D.mercatorum, the rare allele 38.5 was pooled with 42. All seven parthenogenetic D.mercatorum adults were also screened by electro- phoresis. All proved to be totally homozygous, which is consistent with partheno- genetic reproduction via pronuclear duplication, a mechanism that results in total homozygosity and is the primary mode of parthenogenesis in all previously

TABLE 4 Chi-square goodness-of-fit duesfor Hardy-Weinberg proportions at some polymorphic loci in [he 1974 and 1976 D. mercatorum collections and the 2976 D. hydei collection

D. mercatorum D.hydei System 1974 1976 1976 EST-A 1.8380 0.6809 - EST-B 0.4751 0.1071 - EST-SL __ - 4.4447* LAP - 0.7537 0.2148 XDH 0.3909 5.1835* 5.8004* ACPH 0.1485 - - 6-PGD ( 9 9 only) 0.0521 -- - GOT-2 __ - 2.6586 A0 - - 2.7360

* Significant at the 0.05 level. All significant deviations were associated with o heterozygote deficiency. The 38.5 and 42 alleles were pooled for EST-B in D.mercatorum, and the 23 and 25 alleles were pooled for LAP in D.hydei. All degrees of freedom are one. 1290 A. R. TEMPLETON studied parthenogenetic strains of D. mercatorum (CARSON,WEI and NEIDER- HORN 1969; CARSON1973).

DISCUSSION The findings reported in this paper indicate that the discovery of a non-zero parthenogenetic capability in the Kamuela garbage dump sexual population of Drosophila mercatorum (TEMPLETON,CARSON and SING 1976) is not unique to that particular population of D. mercatorum, nor even to the D. mercatorum species. The discovery of parthenogenetic adults in D.hydei in conjunction with STALKER’Sprevious work (1954) suggests that many sexual species of Drosophila may have a latent parthenogenetic capacity hidden in their natural populations. The existence of such a capacity is a necessary prerequisite for any theory of parthenogenetic speciation in . None of the parthenogenetic adults in D. hydei proved capable of partheno- genetic reproduction themselves (Le., the estimated true rate = 0). However, the number of eggs screened for the D. hydei was sufficiently small that the hypothesis of a true rate comparable to that of the sympatric population of D. xercatorum (10-O) cannot be rejected. Nevertheless, both the D.hydei and the D.mercatorum collected in 1976 had a true rate significantly lower than that of the D. mercatorum collected in 1974 us. le5);moreover, the D.hydei lacked the “hot” lines found in D. mercatorum. That a D. hydei population might differ significantly from a D.mercatorum population is not at all sur- prising. Although both species belong to the repleta species group, they are in different subgroups. Chromosomally, the two species are very distinct ( WASSER- MAN 1962), and with respect to isozyme loci, their average NEI (1972) identity is only 0.11. Moreover. the two species differ considerably in many important life-history attributes (e.g., egg laying capacity, generation time, time to sexual maturation, etc.). The two species thus have very distinct gene pools. Hence, if the latent Parthenogenetic capacity has any genetic component, it would be expected that the two species differ in this capacity as part of their overall genetic distinctiveness. The differences between the 1974 and 1976 D. mercatorum collections are more difficult to explain. First of all, the differences are significant only with respect to the true rate, not to the viable rate. This homogeneity for the viable rate coupled with the lack of homogeneity for the true rate indicates that the set of genes responsible for surviving the genetic consequences of parthenogenesis (in this case, total homozygosity) may not be completely identical to the set of genes that allows the female to reproduce parthenogenetically. This conclusion is further supported by some laboratory data (ANNESTand TEMPLETON1978) and by the lack of association between viable and true rates. Thus, in the pooled 1974 and 1976 samples, a total o€ 22 viable parthenogenetic females was pro- duced, six of which were themselves capable of parthenogenetic reproduction. As previously shown, the viable rates are nonhomcgeneous because four “hot” lines are present. These lines accounted for 16 of the 22 viable adults, but only THE CAPACITY FOR PARTHENOGENESIS 1291 €our (25%) of these viable adults from the hot lines could reproduce partheno- genetically. On the other hand, two (33%) of the viable adults produced by the nonhot lines were capable of reproducing parthenogenetically. These num- bers are not significantly different, and indicate that the lines “hot” in terms of viable rate are no more likely to produce females capable of parthenogenesis than the nonhot lines. The significant differences in true rates between the 1974 and 1976 D.mer- catorum samples amounts to an entire order of magnitude and cannot be ignored. Perhaps this difference reflects an underlying genetic dissimilarity be- tween those two D.mercatorum populations. For example, the 6-GPD F allele had a frequency of 0.2 in the 1974 collection, but was entirely absent in the 1976 collection. A similar, but less extreme, pattern was seen for the S allele at ACPH, which was 0.28 in 1974 but 0.01 in 1976, and for the S allele at ADH, which was 0.060 in 1974 but 0.006 in 1976. All of these differences are significant at the 0.05 level of probability. Thus, these two populations of D.mercatorum had significantly different allele frequencies at three of eight segregating loci scored for both samples. These two populations also differ in the pattern of genotype frequencies. As shown in Table 4, all five tested loci from the 1974 sample were in Hardy- Weinberg equilibrium, while three out of four were from the ’76 sample. More- over, the locus sigiificantly deviating from Hardy-Weinberg equilibrium in the 1976 sample is Xdh and is characterized by a heterozygote deficiency in 1976, but not in 1974 (there is no significant difference in allele frequency at the Xdh locus between the two samples). Moreover, this pattern of heterozygote defici- ency is true of all polymorphic loci from the 1976 sample. If the population were truly in Hardy-Weinberg equilibrium, we would expect about half of the loci to show a heterozygous excess and half a deficiency, due to sampling error. Yet, all six of the polymorphic loci show a deficiency in 1976 (probability = 0.008) us. four out of seven polymorphic loci showing a heterozygous deficiency in 1974 (probability = 0.273). Consequently, the trend towards heterozygote deficiency is significant in the 1976 sample, while the 1974 population fits Hardy-Weinberg expectations quite well. Thus, the two populations differ not only in allele fre- quency, but in genotype frequency pattern as well. The cactus-patch D. hydei also showed a heterozygote deficiency at all five polymorphic loci (probability under Hardy-Weinberg equilibrium = 0.031 ) . Moreover, the magnitude of this deficiency in D. hydei was greater than seen in the cactus-patch D. mercatorum, as can be seen from Table 4. Thus, both cactus-patch populations share a heterozygote deficiency, while the garbage- dump population fits Hardy-Weinberg expectations quite well. This may indi- cate that the cactus-patch populations of both species tend to be inbred within a patch and/or display n Wahlund effect between patches (since two cactus patches 2.25 meters apart were sampled). The lack of a heterozygote deficiency and higher levels of polymorphism in the garbage-dump population could be explained by the dump (which contained much fruit waste) acting as a bait that attracts individuals from many different cactus patches, which then proceed 1292 A. R. TEMPLETON to mate at random. This speculation also leads to a possible unified explanation for the results with respect to parthenogenetic rates obtained for the two D. mercatorum samples and for the D. hydei sample. As argued by TEMPLETON, CARSONand SING (1976), the transition from sexual to parthenogenetic repro- duction is accompanied by an intense selective bottleneck (direct experimental evidence for the reality of this bottleneck is provided in TEMPLETON,1979). The chance of passing this bottleneck depends upon the level of genetic variability in the base population being selected. In this case, the population consists of the parthenogenetic zygoids produced in unfertilized eggs. Since all parthenogenesis in the Drosophila has so far proven to be automictic (STALKER1954; CARSON1973; MURDYand CARSON1959), the level of genetic variability in the base population of parthenogenetic zygoids is determined primarily by the level of heterozygosity of the female parent (TEMPLETONand ROTHMAN 1973; ASHER 1970). TEMPLETON,CARSON and SING(1976) used this fact to explain the greater ease of establishing parthenogene tic lines from the garbage-dump population of D.mercatorum in contrast to starting from inbred laboratory stocks. This same explanation could also explain the differences observed between the garbage- dump and cactus-patch populations of D. mercatorum, and perhaps between garbage-dump D.mercatorum and cactus-patch D.hydei. The average observed level of heterozygosity from the 1976 cactus-patch D.mercatorum population is 0.08 (based on 23 loci) us. 0.14 for the 1974 garbage-dump population (based on 16 loci). Moreover, the average observed heterozygosity in the cactus-patch D. hydei is 0.08 (based on 23 loci). Consequently, the lower parthenogenetic capacity of the cactus-patch flies may simply be due to the lowered level of genetic variability in the population of their parthenogenetic zygoids as com- pared to the garbage-dump flies. Whatever the cause, this paper and a previous one (TEMPLETON,CARSON and SING1976) show that the chance of producing a totally homozygous partheno- genetic progeny increases with an increase in the level of heterozygosity in the base sexual population (where level of heterozygosity is determined by electro- phoretic surveys or inferred from natural us. laboratory stock contrasts). This observation is in opposition to a prediction made by CUELLAR(1977) that in- breeding in the base sexual population should increase the chance of producing a totally homozygous parthenogenetic progeny, since inbreeding would cause the elimination of recessive lethals. However, this observation is consistent with the hypothesis of an intense selective bottleneck separating the sexual and par- thenogenetic populations in which selection for the creation of a genome co- adapted to total homozygosity and parthenogenesis (TEMPLETON,SING and BROKAW1976; ANNESTand TEMPLETON1978) plays a quantitatively more important role than selection against lethals.

This work was supported by National Science Foundation Grants DEB76-16985 and DEB78-104.55. I thank HARRISOND. STALKERfor his useful criticisms of an earlier draft of this paper, ELLIOTRALIN for his excellent technical assistance, and BONNIEA. TEMPLETONfor aiding in the development of the new collecting techniques. THE CAPACITY FOR PARTHENOGENESIS 1293

NOTE ADDED IN PROOF In August, 1978, SPENCERJOHNSON collected D. mercatorum from the same cactus patches used in the 1976 collection. He kindly sent me some flies from his collection, and 90 isofemale lines were screened for parthenogenesis. Thirty-one parthenogenetic flies were obtained. The viable rate was 2.9 x 10-5, and this can be partitioned into a viable rate of 4.2 x 10-4 for four “hot” lines and of 1.3 x 10-5 for the remaining 86 lines. The frequency of “hot” lines is 4/96 = 0.044. The true rate was 0.93 x 10-6. These results are all homogeneous with the 1976 collection, and confirm the conclusions made concerning the differences between the garbage- dump us. the cactus-patch populations.

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