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Drosophila Mercatorum and Drosophila Hydei

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THE PARTHENOGENETIC CAPACITIES AND GENETIC STRUCTURES OF SYMPATRIC POPULATIONS OF DROSOPHILA MERCATORUM AND DROSOPHILA HYDEI 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 Drosophilidae 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 flies 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.
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