Genetic variation and plasticity of thorax length and wing length in Drosophila aldrichi and D. buzzatii

Reaction norms across three temperatures of development were measured for thorax length, wing length and wing length/thorax length ratio for ten isofemale lines from each of two populations of Drosophila aldrichi and D. buzzatii. Means for thorax and wing length in both species were larger at 24 °C than at either 18 °C or 31 °C, with the reduction in size at 18 °C most likely due to a nutritional constraint. Although females were larger than males, the sexes were not different for wing length/thorax length ratio. The plasticity of the traits differed between species and between populations of each species, with genetic variation in plasticity similar for the two species from one locality, but much higher for D. aldrichi from the other. Estimates of heritabilities for D. aldrichi generally were higher at 18 °C and 24 °C than at 31 °C, but for D. buzzatii they were highest at 31 °C, although heritabilities were not significantly different between species at any temperature. Additive genetic variances for D. aldrichi showed trends similar to that for heritability, being highest at 18 °C and decreasing as temperature increased. For D. buzzatii, however, additive genetic variances were lowest at 24 °C. These results are suggestive that genetic variation for body size characters is increased in more stressful environments. Thorax and wing lengths showed significant genetic correlations that were not different between the species, but the genetic correlations between each of these traits and their ratio were significantly different. For D. aldrichi, genetic variation in the wing length/thorax length ratio was due primarily to variation in thorax length, while for D. buzzatii, it was due primarily to variation in wing length. The wing length/thorax length ratio, which is the inverse of wing loading, decreased linearly as temperature increased, and it is suggested that this ratio may be of greater adaptive significance than either of its components.     

Genetic and phenotypic variation for flight ability in the cactophilicDrosophila species,D. aldrichi andD. buzzatii

The flight ability ofDrosophila aldrichi (Patterson & Crow) andD. buzzatii (Patterson & Wheeler) using tethered flights, was measured with respect to age-related changes, genetic variation and adult body size variation induced by rearing at different larval densities.Drosophila buzzatii flew for much longer thanD. aldrichi, especially females, but age-related changes in flight duration were significant only forD. aldrichi. Effects of body size on flight ability were significant inD. buzzatii, but not inD. aldrichi. InD. buzzatii, there was a significant genotype-environment interaction (larval density × line) for flight duration, with short and average flight duration isofemale lines showing longer flights, but a long flight duration line shorter flights as body size decreased (i.e., as larval density increased). Heritability estimates for flight duration were similar in the two species, but flight duration showed no significant genetic correlations with developmental time, body size or wing dimensions (except for one wing dimension inD. buzzatii). Although not significantly different between the species, heritabilities for life-history traits (adult size and developmental time) showed contrasting patterns — with higher heritability for body size (body weight and thorax length) inD. buzzatii, and higher for developmental time inD. aldrichi. In agreement with limited previous field evidence,D. buzzatii is better adapted for colonization than isD. aldrichi.     

Reaction norms across and genetic parameters at different temperatures for thorax and wing size traits in Drosophila aldrichi and D. buzzatii

Reaction norms across seven constant and one fluctuating temperature of development were measured for thorax length and several wing size traits for up to 10 isofemale lines of each of the cactophilic Drosophila species, D. aldrichi and D. buzzatii, originating from the same locality. Maximum thorax length was reached at different low to intermediate temperatures for the two species, whereas wing length was highest at the lowest temperature in both species. Various ratio parameters showed pronounced species differences. The reaction norm for the wing loading index (wing length/thorax length) decreased monotonically with temperature in both species, but was much steeper and spanned a wider range in D. aldrichi than in D. buzzatii, suggesting either that wing loading is not a good characterization of flight capacity or, more likely, that flight optimization does not occur in the same manner in both species. The vein ratio (distal length/proximal length of the third vein) increased with temperature in D. buzzatii but decreased in D. aldrichi. Wing development in the two species thus is very different, with the proximal part of the wing in D. buzzatii more closely allied to the thorax than to the distal part. Among line variation was significant for all traits in both species, and most pronounced for thorax length and the ratio parameters. Coefficients of variation were significantly different between the species for all traits, with those in D. aldrichi higher than in D. buzzatii. Genetic variance in plasticity was significant for all traits in D. buzzatii, but only for seven out of 12 in D. aldrichi. Additive genetic variances for all traits in both species were significantly larger than zero. Genetic correlations between thorax length and several wing length parameters, and between these and wing area, were positive and generally significant in both species. The genetic correlation between the distal and the proximal length of the third vein was not significantly different from zero in D. aldrichi, but negative and significant in D. buzzatii. Heritabilites varied significantly among temperatures for almost all traits in both species. Phenotypic variances were generally higher in D. aldrichi than in D. buzzatii, and commonly highest at the extreme temperatures in the former species. At the high temperature the genetic variances also were usually highest in D. aldrichi. The data clearly suggest that the process of thermal adaptation is species specific and caution against generalizations based on the study of single species.     

Coexistence of ecologically similar colonising species. II. Population differentiation in Drosophila aldrichi and D. Buzzatii for competitive effects and responses at different temperatures and allozyme variation in D. aldrichi

Drosophila aldrichi and D. buzzatii are cactophilic species that colonised Australia about 55–60 years ago. They are sympatric only in Australia. Thus they may be in the process of adapting to new environments and to each other, and diversifying among local, possibly isolated, populations. Larval competitive effects for three populations of each species (Roma, Planet Downs, and Binjour) were measured on semi-natural cactus rots at three temperatures, with preadult viability, developmental time and adult body weight scored for each sex and species. Populations of both species varied in their responses to the other species as competitor, and one D. buzzatii population (Roma) reduced larval performance of D. aldrichi significantly more than did other D. buzzatii populations. Geographic divergence for the three traits was similar in both species, with a relative performance index derived from these traits highest for Roma, second for Binjour, and least for the Planet Downs population of each species. The Roma D. aldrichi population was the most different from the other populations for the performance index and in terms of genetic distances derived from allozyme frequencies. Additionally, comparisons of climatic variables among the population localities showed that the Roma environment was most different from the others. Differential natural selection in different areas of the cactus distribution may be a major cause of population divergence in both species. Drosophila aldrichi is superior for some fitness components at the highest temperature. Thus temperature variation throughout the cactus distribution may contribute to the different ranges of these two species, with competitive exclusion of D. aldrichi in the southern, cooler region of the cactus distribution, but coexistence in the northern, warmer region.     

Intraspecific variation ofDrosophila buzzatii larval breeding success onOpuntia: A yeast-plant-insect relationship

The cactophilic species,Drosophila buzzatii, normally breeds in decaying pockets ofOpuntia cladodes, in which there is a complex interaction with the microbial flora, especially yeast species. Isofemale lines were used to estimate genetic variation among larvae reared on their natural feeding substrate. Four naturally occurring cactophilic yeast species isolated from the same Tunisian oasis as theDrosophila population were used. Two fitness components were studied for each line, viability and developmental time. Genetic variations amongD. buzzatii lines were observed for both traits. A significant yeast species x isofemale line interaction for viability was also evidenced, suggesting the occurrence of specialized genotypes for the utilization of breeding substrates. This genetic heterogeneity in the natural population may favor a better adaptation to the patchily distribution of yeasts.     

Geographical distributions,relative abundance and coexistence of Drosophila aldrichi and Drosophila buzzatii in Australia

Abstract Climatic data and collection records for the cactophilic Drosophila aldrichi and Drosophila buzzatii for 97 localities were used to examine the effects of geographical location, season, host plant species and climatic factors on their range and relative abundance. Temporal variation in relative abundance was assessed from monthly collections over 4 years at one locality. Effects of weather variables over the 28 days before each collection were examined. A generalized linear model of the spatial data showed significant geographical variation in relative abundance, and significant climatic effects, with the proportion of D. aldrichi higher in the warm season, and increasing as temperature variation decreased and moisture indices increased. The temporal data gave generally concordant results, as D. aldrichi proportion was higher in summer and autumn, and increased as maximum and minimum temperatures increased, and as variation in maximum temperature decreased. In a laboratory competition experiment, D. aldrichi eliminated D. buzzatii at 31°C, but was itself eliminated at 18°C and 25°C. The range of D. buzzatii is constrained only by availability of its host plant, Opuntia species, although its relative abundance is reduced in the northern part of its distribution. The range of D. aldrichi, from central Queensland to northern NSW, Australia, is entirely within that of D. buzzatii, and its relative abundance decreases from north to south. Both climate and weather, particularly temperature variability, have direct effects on the relative abundances of the two species, and both likely act indirectly by influencing the outcome of interspecific competition.     

Larval age differences and competition between Drosophila aldrichi and D.buzzatii

Abstract.

• 1 Competitive interactions among larvae of Drosophila species using the same habitat will usually involve individuals varying in age and size. Older, larger larvae might be expected to outcompete younger, smaller larvae.
• 2 Newly hatched laryae of Drosophila aldrichi and D.buzzatii were placed on induced cactus necroses (simulating the natural habitat), either simultaneously, or with a 1 or 2 day difference in the time at which individuals of the other species were added.
• 3 Evidence for competitive differences with larval age were found. In both species, body weight was significantly higher for those larvae added first than for later addition larvae, and a longer time to complete development was required for those larvae added 2 days later. However, pre-adult viability of D.aldrichi was significantly higher for larvae started 1 or 2 days after larvae of D.buzzatii. For D.buzzatii, viability was lowest for larvae started simultaneously with D.aldrichi, but not different between those started either before or after D.aldrichi.
• 4 Considering all three traits, no clear advantage for the early addition larvae was observed.

     

EVOLUTIONARY GENETICS OF OVIPOSITION PREFERENCE IN SWALLOWTAIL BUTTERFLIES

Models of the evolution of host shifts and speciation in phytophagous insects are often built upon the assumption that host selection is under simple genetic control, perhaps even a single locus. The genetic basis for differences in host-plant preference by ovipositing insects was investigated using two closely related species of swallowtail butterfly, Papilio oregonius and P. zelicaon, which differ in the plant families on which females oviposit. Both species had been shown previously to vary within populations in host selection. A combination of analyses using reciprocal interspecific crosses and isofemale strains within populations indicated that oviposition preference in these species is determined significantly by one or more loci on the X chromosome, which female Lepidoptera inherit only paternally. Hence, preferences in hybrid females tended toward the paternal species. This is the first insect group for which partial control of oviposition preference has been localized onto a particular chromosome. In addition, one or more loci on another chromosome(s) appear to contribute to preference, as indicated by the partially intermediate preferences of some hybrid crosses. The overall differences in preference in the reciprocal interspecific hybrids were restricted to differences in the distribution of eggs laid among the local host plants of these two Papilio species; the reciprocal crosses did not differ in the small percentage of eggs laid on a novel potential host species. The variation in host selection found among the isofemale strains reinforces earlier results for these strains, indicating that there is genetic variation in host selection within these populations. Overall, the results indicate that the evolution of oviposition preference in these species involves genetic changes at two or more chromosomes with the X chromosome playing an important role in determining preference.     

Genetic history of a colonizing population: Drosophila buzzatii (Diptera: Drosophilidae) in Australia

Drosophila buzzatii Patterson & Wheeler, a cactophilic species that feeds and breeds in the rotting tissues of various Opuntia cactus species, was inadvertently introduced to Australia from Argentina sometime during the period 1931–1936. After a bottleneck at introduction, its spread through the cactus distribution was probably very rapid as a result of natural dispersal from the site of introduction and from three other foci colonized from the introduction site by human intervention. By 1940, the Opuntia distribution and consequently that of D. buzzatii was reduced to spatially isolated populations, with probable further bottlenecking of at least some of the D. buzzatii populations. Allozyme data (primarily six polymorphic loci) from flies collected during April 1972 to February 1996 at 67 localities were used to examine current population differentiation and relationships, as well as to infer aspects of their demographic history. Although there is significant isolation‐by‐distance, genetic relationships among the populations are not simply related to geographical distance, implying that genetic drift has contributed to population differentiation. However, the biotic and, to an extent, the physical environment are not the same in Australia as in Argentina. Consequently, exposure to novel environments has led to local adaptation and further population differentiation. Genetic variation and the structure of Australian populations apparently are determined by founder effects (drift) at the level of individual breeding sites (cactus rots), by diversifying selection among rots within a locality, as well as by drift and geographically varying selection among localities. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 682–698.     

VARIATION IN PREFERENCE AND SPECIFICITY IN MONOPHAGOUS AND OLIGOPHAGOUS SWALLOWTAIL BUTTERFLIES

Although variation in oviposition preference and specificity for host plants has been demonstrated within populations of a variety of oligophagous insect species, it is unknown whether genetic variation in host choice is lost within populations of monophagous species. Analysis of a locally monophagous butterfly species, Papilio oregonius, and a locally oligophagous species, P. zelicaon, showed significant variation in oviposition preference within populations of both species. Females of both species chose primarily their native hosts. Nonetheless, the percentages of eggs laid by individual females among the plant species and the number of plant species on which individual females laid eggs differed significantly among isofemale strains within populations. Moreover, some females within all isofemale strains of both species laid a few eggs on Foeniculum vulgare, an umbelliferous species that does not occur in the native habitats of these populations but is a host for Papilio species in other geographic areas. The results suggest that local monophagy and oligophagy in these species reflect the relative ranking among potential plant species. Both populations harbor variation in oviposition choice that could allow for host shifts if these populations invaded new habitats.     

Differential responses to artificial selection on oviposition site preferences in Drosophila melanogaster and D. simulans

The preference–performance relationship in plant–insect interactions is a central theme in evolutionary ecology. Among many insects, eggs are vulnerable and larvae have limited mobility, making the choice of an appropriate oviposition site one of the most important decisions for a female. We investigated the evolution of oviposition preferences in Drosophila melanogaster Meigen and Drosophila simulans Sturtevant by artificially selecting for the preference for 2 natural resources, grape and quince. The main finding of our study is the differential responses of D. melanogaster and D. simulans. Although preferences evolved in the experimental populations of D. melanogaster, responses were not consistent with the selection regimes applied. In contrast, responses in D. simulans were consistent with expectations, demonstrating that this species has selectable genetic variation for the trait. Furthermore, crosses between D. simulans divergent lines showed that the genetic factors involved in grape preference appear to be largely recessive. In summary, our artificial selection study suggests that D. melanogaster and D. simulans possess different genetic architectures for this trait.     

Variation of life-history and morphometrical traits in Drosophila buzzatii and Drosophila simulans collected along an altitudinal gradient from a Canary island

Variation in three life‐history traits (developmental time, preadult viability and daily female productivity) and five morphometrical traits (thorax length, wing length, wing width, wing/thorax ratio and wing‐aspect ratio) was studied at three developmental temperatures (20, 25 and 30 °C) in Drosophila buzzatii and Drosophila simulans collected on the island of La Gomera (Canary Archipelago). The flies originated from five closely situated localities, representing different altitudes (from 20 to 886 m above sea level) and a range of climatic conditions. We found statistically significant population effects for all traits in D. buzzatii and for most of the traits in D. simulans. Although no correlations of trait values with altitude were detected, geographical patterns for three life‐history traits and body size in D. buzzatii indicated that short‐range geographical variation in this species could be maintained by local climatic selection. Five of eight traits showed population‐by‐temperature interactions either in D. buzzatii or in D. simulans, but in all cases except wing width in D. buzzatii this could not be interpreted as adaptive responses to thermal conditions in the localities. The range of plastic changes across temperatures for particular traits differed between species, indicating a possibility for different levels of environmental stress experienced by the natural populations. The reaction norm curves and the response of within‐population variability to thermal treatments suggested better adaptations to higher and lower temperatures for D. buzzatii and D. simulans, respectively. The levels of among‐population differentiation depended on developmental temperature, implying environmental effects on the expression of the genetic variance. At 20 and 25 °C, interpopulation variability in D. buzzatii was higher than in D. simulans, while at 30 °C the opposite trend was observed. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 119–136.     

Adaptive diversity in genetic control of egg-laying behavior in Drosophila melanogaster

Summary

In the absence of insemination or of a suitable oviposition site, Drosophila melanogaster females react with egg retention patterns which differ as to the position in the genital tract at which the mechanism operates (ovarioles for initial retention; uterus for oviposition blocking) and to physiological basis. A study of the first laboratory generation of isofemale lines, founded by using three different samples of wild flies collected the same year from a natural population, showed a significant genetic variability in initial egg retention by virgin females and in the capacity for oviposition blocking by mated flies reacting to an unfavorable oviposition site. Comparison of isofemale lines showed that these two adaptive behavior patterns also differ in their genetic determination. They are controlled, and therefore can react, independently to selection pressures resulting from seasonal variation in the natural environment.     

Behavioral genetics of Drosophila ananassae

Drosophila ananassae has a unique status among Drosophila species because of certain peculiarities in its genetic behavior. The most unusual feature of this species is its relatively high frequency of spontaneous male recombination. The results of studies on non-sexual behavior, such as phototactic responses, eclosion rhythm, and preferences for oviposition and pupation sites, lead us to suggest that this behavior is under polygenic control, with a substantial amount of additive genetic variation. Sexual isolation has been reported in D. ananassae with the degree of such isolation being stronger in isofemale lines than in natural populations. The significant variations seen in the mating propensity of several isofemale strains, inversion karyotypes and wild type strains, the diminishing effects of certain mutations on the sexual activity of males, and the positive responses to selection for high and low mating propensity point to a genetic control of sexual behavior in D. ananassae. Males contribute more to variation and thus are more subject to intrasexual selection than females. There is a positive correlation between sternopleural bristle number, mating propensity and fertility in D. ananassae. This correlation between morphometric traits and mating success suggests that larger flies are more successful in mating than smaller ones. There is also evidence for adaptive plasticity and a trade-off between longevity and productivity in D. ananassae. Rare, specific courtship song parameters that provide males with a mating advantage have also been reported in different geographic strains of D. ananassae. The remating behavior of males and females, sperm displacement, and the bi-directional selection for female remating speed indicate that post-mating behavior in this species may also be under genetic control. The occurrence of size assortative mating further indicates that there is size-dependent sexual selection in D. ananassae.     

GROWTH TEMPERATURE AND ADULT PIGMENTATION IN TWO DROSOPHILA SIBLING SPECIES: AN ADAPTIVE CONVERGENCE OF REACTION NORMS IN SYMPATRIC POPULATIONS?

Phenotypic plasticity of abdomen pigmentation was investigated in populations of the sibling species Drosophila melanogaster and D. simulans, living in sympatry in two French localities. Ten isofemale lines of each population and species were grown at different constant temperatures spanning their complete thermal range from 12 to 31°C. Genetic variability between isofemale lines was not affected by growth temperature, but was consistently less in D. simulans. For all traits, the dark pigmentation of the abdominal segments decreased according to growth temperature, in agreement with the thermal budget adaptive hypothesis. The shapes of the response curves were different between the abdominal segments, but for a given segment, quite similar in the two species. On average D. simulans was lighter than D. melanogaster, but the difference was mainly expressed at higher temperatures. An interesting result was the difference observed between the two localities: flies from the colder locality (Villeurbanne) were found to be darker than flies from the warmer locality (Bordeaux). Interestingly, this difference was expressed only at low temperatures, 21°C and below, that is, at temperatures encountered in natural conditions. This suggests an adaptive response resulting in a change of the shape of reaction norm and involving genotype-environment interactions. When comparing the genetic structure of geographic populations for quantitative traits, several laboratory environments should be preferred to a single one.     

Molecular Population Genetics of the α<Emphasis Type="Italic">-Esterase5</Emphasis> Gene Locus in Original and Colonized Populations of <Emphasis Type="Italic">Drosophila buzzatii</Emphasis> and Its Sibling <Emphasis Type="Italic">Drosophila koepferae</Emphasis>

Several studies have suggested that esterase-2 (EST-2) may be the target of natural selection in the cactophilic fly Drosophila buzzatii. In this work, we analyzed nucleotide variation in a fragment of α-esterase5 (αE5), the gene encoding EST-2, in original (Argentinian) and colonized (Australian) populations of D. buzzatii and in its sibling D. koepferae. Estimates of nucleotide heterozygosity in D. buzzatii were similar in Australia and Argentina, although we detected a loss of singletons in colonized populations, suggesting a moderate founder effect. Interspecific comparisons revealed that D. buzzatii was more polymorphic for nonsynonymous variation, whereas D. koepferae was more variable for synonymous and noncoding sites. The two major chromosomal arrangements (2st and 2j) in D. buzzatii displayed similar levels of nucleotide variation, whereas 2jz ³ was monomorphic. The sequenced region allowed the discrimination of a greater number of EST-2 protein variants in the Australian sample than in the Argentinean sample. In D. koepferae, nucleotide variation in αE5 does not depart from neutral expectations, although tests of population structure were significant for silent variation. In contrast, D. buzzatii has probably undergone a recent population expansion in its South American range. In addition, the McDonald and Kreitman test revealed an excess of nonsynonymous polymorphism in both original and colonized populations of this species. Electronic Supplementary Material Electronic Supplementary material is available for this article at and accessible for authorised users. [Reviewing Editor: Dr. Richard Kliman]     

Micro‐climate determines oviposition site selection and abundance in the butterfly Pyrgus armoricanus at its northern range margin

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LIMITS TO THE SOUTHERN BORDER OF DROSOPHILA SERRATA: COLD RESISTANCE,HERITABLE VARIATION,AND TRADE-OFFS

There are a number of evolutionary hypotheses about why species distributions are limited, but very little empirical information to test them. We present data examining whether the southern distribution of Drosophila serrata is limited by cold responses. Species comparisons were undertaken for cold resistance, development time, and viability at 15°C and 25°C for D. serrata and other species with a more southerly distribution (D. melanogaster, D. simulans, and D. immigrans). Relative to the other species, D. serrata had a long development time at both temperatures and a low level of cold resistance. Using isofemale lines collected in different seasons, central and marginal populations were compared for cold resistance, as well as development time and viability at 14°C. The border population had a relatively higher resistance to cold shock in postwinter collections, but there was no population differentiation for prewinter collections or for the other traits. The presence of variation among isofemale lines within the border populations suggests that genetic variation as measured in the laboratory is unlikely to limit range expansion. Population cages were used in the field to determine if D. serrata persisted over winter at borders. Although all cages yielded adult offspring at northern sites, only a few produced offspring at or just south of the border. In contrast, all cages with D. simulans produced adult offspring, suggesting that climatic factors limited D. serrata numbers. Offspring from surviving adults showed a phenotypic trade-off between fecundity and cold resistance. Comparisons of the cold resistance of field males and females with their laboratory-reared offspring provided evidence for heritable variation in field-reared flies. Overall, the results suggest that cold stress is important in limiting the southern distribution of D. serrata, but it seems unlikely that a lack of genetic variation restricts range expansion.     

Ancestral population reconstitution from isofemale lines as a tool for experimental evolution

Experimental evolution is a powerful tool to study adaptation under controlled conditions. Laboratory natural selection experiments mimic adaptation in the wild with better‐adapted genotypes having more offspring. Because the selected traits are frequently not known, adaptation is typically measured as fitness increase by comparing evolved populations against an unselected reference population maintained in a laboratory environment. With adaptation to the laboratory conditions and genetic drift, however, it is not clear to what extent such comparisons provide unbiased estimates of adaptation. Alternatively, ancestral variation could be preserved in isofemale lines that can be combined to reconstitute the ancestral population. Here, we assess the impact of selection on alleles segregating in newly established Drosophila isofemale lines. We reconstituted two populations from isofemale lines and compared them to two original ancestral populations (AP) founded from the same lines shortly after collection. No significant allele frequency changes could be detected between both AP and simulations showed that drift had a low impact compared to Pool‐Seq‐associated sampling effects. We conclude that laboratory selection on segregating variation in isofemale lines is too weak to have detectable effects, which validates ancestral population reconstitution from isofemale lines as an unbiased approach for measuring adaptation in evolved populations.     

Coexistence of ecologically similar colonising species III. Drosophila aldrichi and D. buzzatii: larval performance on,and adult preference for,three Opuntia cactus species

Two Drosophila species, D. buzzatti and D. aldrichi, coexist on several species of Opuntia cacti in Australia, primarily on O. tomentosa and O. streptacantha in the northern part of the cactus distribution, and on O. stricta in the south. Thorax length of field-collected adults was less, and the variance in length greater, than that for flies reared on simulated rots in the laboratory, indicating that these species are affected by crowding in nature. A larval performance index, measured on simulated cactus rots at low, moderate and high densities in single-species cultures, and at moderate and high densities in mixed-species cultures, was used to compare the relative intensity of intra- and interspecific competition at the same total larval density per 5 g necrotic cactus. Larval performance of both fly species was greatest on O. streptacantha, intermediate on O. tomentosa, and least on O. stricta in both single-species and mixed-species cultures. On O. stricta, the performances of D. aldrichi and D. buzzatii were not different when in single-species cultures, but that of D. aldrichi decreased significantly in mixed-species cultures. On the other two cactus species, the performances of D. aldrichi and D. buzzattii were not different in mixed-species cultures. The order of preferences by adult females for the cacti differed from that for larval performance, with females of both species prefering O. stricta. Analysis of microbial numbers growing on the cacti showed little difference among cacti at the rot age used for testing adult preference, but later growth was greater on O. tomentosa and O. streptacantha, the cacti that best supported larvae. Differential larval performance on O. stricta may contribute to the rare presence of D. aldrichi in the southern part of the cactus distribution, while the superior quality of O. tomentosa and O. streptacantha (larger rot size and higher microbial concentration) may reduce competition and facilitate cocxistence of the fly species in the north.