First Record of Drosophila buzzatii (Patterson & Wheeler) (Diptera: Drosophilidae) Emerging from a Non-Cactus Host

Drosophila buzzatii (Patterson & Wheeler), a typical cactophilic species of the repleta group, is registered for the first time emerging from Melon (Cucumis melo) in western Argentina. The analysis of inversion polymorphism and genetic diversity of mitochondrial cytochrome oxidase subunit I gene (mtCOI) provided additional evidence that corroborated the presence of a high proportion of D. buzzatii among the flies emerged from melon. This finding set the scenario for a broader range of possible hosts and host-related distribution and dispersion for this widespread species.

Developmental thermal plasticity among Drosophila melanogaster populations

Many biotic and abiotic variables influence the dispersal and distribution of organisms. Temperature has a major role in determining these patterns because it changes daily, seasonally and spatially, and these fluctuations have a significant impact on an organism's behaviour and fitness. Most ecologically relevant phenotypes that are adaptive are also complex and thus they are influenced by many underlying loci that interact with the environment. In this study, we quantified the degree of thermal phenotypic plasticity within and among populations by measuring chill-coma recovery times of lines reared from egg to adult at two different environmental temperatures. We used sixty genotypes from six natural populations of Drosophila melanogaster sampled along a latitudinal gradient in South America. We found significant variation in thermal plasticity both within and among populations. All populations exhibit a cold acclimation response, with flies reared at lower temperatures having increased resistance to cold. We tested a series of environmental parameters against the variation in population mean thermal plasticity and discovered the mean thermal plasticity was significantly correlated with altitude of origin of the population. Pairing our data with previous experiments on viability fitness assays in the same populations in fixed and variable environments suggests an adaptive role of this thermal plasticity in variable laboratory environments. Altogether, these data demonstrate abundant variation in adaptive thermal plasticity within and among populations.

Genetic architecture of olfactory behavior in Drosophila melanogaster: differences and similarities across development

In the holometabolous insect Drosophila melanogaster, genetic, physiological and anatomical aspects of olfaction are well known in the adult stage, while larval stages olfactory behavior has received some attention it has been less studied than its adult counterpart. Most of these studies focus on olfactory receptor (Or) genes that produce peripheral odor recognition. In this paper, through a loss-of-function screen using P-element inserted lines and also by means of expression analyses of larval olfaction candidate genes, we extended the uncovering of the genetic underpinnings of D. melanogaster larval olfactory behavior by demonstrating that larval olfactory behavior is, in addition to Or genes, orchestrated by numerous genes with diverse functions. Also, our results point out that the genetic architecture of olfactory behavior in D. melanogaster presents a dynamic and changing organization across environments and ontogeny.

Patterns of variation in wing morphology in the cactophilic Drosophila buzzatii and its sibling D. koepferae

Drosophila buzzatii and D. koepferae are two sibling species that breed on the necrotic tissues of several cactus species and show a certain degree of niche overlap. Also, they show differences in several life history traits, such as body size and developmental time, which probably evolved as a consequence of adaptation to different host plants. In this work we investigate the ecological and genetic factors affecting wing morphology variation both within and between species. Three wing traits were scored, distal and proximal wing length and width in isofemale lines reared in two of the most important host cacti: Opuntia sulphurea and Trichocereus terschekii. Our results revealed that differences between species and sexes in wing size and shape were significant, whereas the cactus factor was only significant for wing size. Intraspecific analyses showed that differences among isofemale lines were highly significant for both size and shape in both species, suggesting that an important fraction of variation in wing morphology has a genetic basis. Moreover, the line by cactus interaction, which can be interpreted as a genotype by environment interaction, also accounted for a significant proportion of variation. In summary, our study shows that wing size is phenotypically plastic and that populations of D. buzzatii and D. koepferae harbour substantial amounts of genetic variation for wing size and shape. Interspecific differences in wing size and shape are interpreted in terms of spatial predictability of the different host plants in nature.

Genotype by environment interactions in viability and developmental time in populations of cactophilic Drosophila

The genetic and ecological basis of viability and developmental time differences between Drosophila buzzatii and D. koepferae were analysed using the isofemale line technique. Several isofemale lines were sampled from pairs of allopatric/sympatric populations of each species. Flies were reared in media prepared with decaying tissues of two of the main natural cactus hosts of each species. This experimental design enabled us to evaluate the relative contribution of phenotypic plasticity, genetic variation and genotype by environment interaction (G x E) to total phenotypic variation for two fitness traits, viability and developmental time. Our results revealed significant G x E in both traits, suggesting that the maintenance of genetic variation can be explained, at least in part, by diversifying selection in different patches of a heterogeneous environment in both species. However, the relative importance of the factors involved in the G x E varied between traits and populations within species. For viability, the G x E can be mainly attributed to changes in the rank order of lines across cacti. However, the pattern was different for developmental time. In D. buzzatii the G x E can be mainly accounted for by changes in among line variance across cacti, whereas changes in the rank order of lines across cacti was the main component in D. koepferae. These dissimilar patterns of variation between traits and species suggest that the evolutionary forces shaping genetic variation for developmental time and viability vary between populations within species and between species.

Oviposition acceptance and fecundity schedule in the cactophilic sibling species Drosophila buzzatii and D. koepferae on their natural hosts

We tested for the occurrence of oviposition acceptance for different media prepared with cactus tissues of three alternative cactus hosts: Opuntia sulphurea, O. quimilo and Trichocereus terschekii for 4 consecutive days in lines of two Drosophila buzzatii populations and one population of D. koepferae. Our results showed that the former laid significantly more eggs on both Opuntia cacti than on T. terschekii, whereas D. koepferae preferred T. terschekii. In addition, fecundity schedules differed between species: D. buzzatii laid similar numbers of eggs along the four-day sampling period, whereas D. koepferae showed an oviposition peak on the second day of egg collection on T. terschekii. We suggest that the between-species disparities observed in oviposition acceptance and fecundity schedule may be related to the temporal and spatial predictability of Opuntia versus T. terschekii (cardón) as part of the different adaptive strategies that have evolved after the split of D. koepferae and D. buzzatii from their recent common ancestor. Therefore, the willingness to accept hosts would be an important factor in the habitat selection and in the maintenance of species diversity.

Functional genomics of odor-guided behavior in Drosophila melanogaster

The avoidance response to repellent odorants in Drosophila melanogaster, a response essential for survival, provides an advantageous model for studies on the genetic architecture of olfactory behavior. Transposon tagging in a highly inbred strain of flies in combination with a rapid and simple statistical behavioral assay enables the identification of not only large phenotypic effects, but also small aberrations from wild-type avoidance behavior. The recent completion of the sequence of the Drosophila genome facilitates the molecular characterization of transposon-tagged genes and correlation between gene expression and behavior in smell-impaired (smi) mutant lines. Quantitative genetic analyses of a collection of smi lines in a co-isogenic background revealed an extensive network of epistatic interactions among genes that shape the olfactory avoidance response. Candidate genes for several of these transposon-tagged smi loci implicate genes that mediate odorant recognition, including a novel odorant binding protein; signal propagation, including a voltage-gated sodium channel; and a protein containing multiple leucine rich repeats and PDZ domains likely to be involved in postsynaptic organization in the olfactory pathway. Several novel genes of unknown function have also been implicated, including a novel tyrosine-regulated protein kinase. The discovery and characterization of novel gene products that have major, hitherto unappreciated effects on olfactory behavior will provide new insights in the generation and regulation of odor-guided behavior. The identification and functional characterization of proteins encoded by smi genes that form part of the olfactory subgenome and correlation of polymorphisms in these genes with variation in odor-guided behavior in natural populations will advance our understanding of the genetic architecture of chemosensory behavior.

Temporal and spatial variation of inversion polymorphism in two natural populations of Drosophila buzzatii

The inversion polymorphism of the cactophilic fly Drosophila buzzatii was studied in two natural populations. We assessed the temporal changes and microspatial population structure. We observed a significant increase in the frequency of arrangement 2J at the expense of 2ST in both populations. These gene arrangements appear to affect the life-history of flies differently. Environmental heterogeneity explains the karyotype coexistence in nature. The analysis of population structure showed that differentiation of inversion frequencies among individual breeding sites, the rotting clacodes of Opuntia vulgaris, was highly significant. The karyotypic frequencies did not depart significantly from Hardy-Weinberg expectations, neither in individual rots nor in the total population. These results suggest that the observed population structure can be easily accounted by random genetic drift.

The effect of polymorphic inversions on body size in two natural populations of Drosophila buzzatii from Argentina

Previous works in a colonized and an original population of Drosophila buzzatii have shown a consistent relationship between the inversion polymorphism and thorax length, a measure of body size. However, the populations studied in those reports share a close genealogical relationship as suggested by several lines of evidence. In the present paper, we revisit this issue by analysing the correlation between second chromosome arrangements and thorax length in two Argentinian natural populations (Termas de Río Hondo and Arroyo Escobar) from different biogeographic areas with different host plants. Our findings are: (1) inversion frequencies were significantly different between populations; (2) the mean thorax length of flies collected in both populations was not significantly different; and (3) we obtain confirming evidence that flies carrying 2st, the ancestral gene order, have on average a smaller body size than those carrying the derived arrangements (2j and 2jz3). These results suggest that the biometrical effect of inversions on body size previously described are due to genetic differences between arrangements and not to the close historical relationship between the populations studied in previous reports.

The evolutionary history of Drosophila buzzatii. XXXIII. Are Opuntia hosts a selective factor for the inversion polymorphism?

Previous work has shown fitness differences among chromosomal arrangements by means of selection component analysis in two Drosophila buzzatii natural populations, one of which is native to Argentina and the other a colonized population from Carboneras, Spain. Founder effects or niche shifts were proposed to explain the differences observed in the pattern of pleiotropic effects of inversions on fitness components. In this paper, we address the possible role of niche shifts by determining whether differential attraction to, oviposition on, or utilization of the rotting cladodes of two different Opuntia species (O. quimilo and O. ficus-indica) occurred among individuals carrying different second chromosome karyotypes in a natural Argentinian population. Through the analysis of more than 2500 individuals comprising five different life cycle stages associated with the necroses of these two cactus species, we found that the distributions of inversion frequencies in samples of adult flies, third instar larvae and emerging adults collected on both Opuntia species were not significantly different. Likewise, no evidence of differential oviposition was observed. These findings suggest that niche shifts cannot, solely, account for the changes observed in the Carboneras population. In addition, the selection component analysis did not reveal any significant relationship between chromosomal arrangements and the fitness components tested. These results suggest either that fitness differences might be too small to be detected or that the assumptions of the model concerning the mode of selection may not be tenable in the studied population.

An adaptive chromosomal polymorphism affecting size-related traits, and longevity selection in a natural population of Drosophila buzzatii

Size-related phenotypic variation among second-chromosome karyotypes in Drosophila buzzatii was examined in an Argentinian natural population. For all measured traits (thorax and wing length; wing, head and face width), this inversion polymorphism exhibited a significant and (additive) linear contribution to the phenotypic variance in newly emerged wild flies. The results suggest that only overall body size, and not body shape, is affected, as no karyotypic variation was found for any trait when the effects of differences in within-karyotype size were removed with Burnaby's method. Likewise, in an experiment of longevity selection in the wild, variation in chromosomal frequencies was verified in the direction predicted on the basis of: (i) previous studies on longevity selection for body size in the wild and (ii) the pattern of chromosomal effects we observed on size. The direction of such selection is consistent with a pattern of antagonistic selection detected in previous studies on the inversion polymorphism.

Genetic structure is determined by stochastic factors in a natural population of Drosophila buzzatii in Argentina

D. buzzatii is a cactophilic species associated with several cactaceae in Argentina. This particular ecological niche implies that this species is faced with a non-uniform environment constituted by discrete and ephemeral breeding sites, which are colonized by a finite number of inseminated females. The genetic consequences of this population structure upon the second chromosome polymorphism were investigated by means of F-statistics in a natural endemic population of Argentina. The present study suggests that differentiation of inversion frequencies in third instar larvae among breeding sites has taken place mainly at random and selection is not operating to determine the structure of this population. The average number of parents breeding on a single pad seems to be similar to the number colonizing Opuntia ficus indica rotting cladodes in Carboneras, a derived population from Spain. There is no significant excess of heterokaryotypes within pads or in the population as a whole. The results obtained in the present study suggest that the potential role of selective versus stochastic factors relative to the among pad heterogeneity in the population here studied is different from that of the Spanish population previously reported. Potential mechanisms responsible for these differences are discussed.

The evolutionary history of Drosophila buzzatii. XXVII. Thorax length is positively correlated with longevity in a natural population from Argentina

The correlation between body size and longevity was tested in an Argentinian natural population of Drosophila buzzatii. Mean thorax length of flies newly emerging from rotting cladodes of Opuntia vulgaris was significantly smaller than that of two samples of flies caught at baits. The present results which might be interpreted as directional selection for longevity favoring larger flies are in agreement with previous results achieved in a Spanish natural population of D. buzzatii. Flies emerging from different substrates showed significant differences in thorax length, suggesting that an important fraction of phenotypic variance can be attributed to environmental variability. However, laboratory and field work in different populations of D. buzzatii showed a significant genetic component for thorax length variation.

The evolutionary history of Drosophila buzzatii. XXIV. Second chromosome inversions have different average effects on thorax length

We demonstrate a genetic correlation between rearrangements of the second chromosome of D. buzzatii and thorax length, as a measure of body size. The results indicate that 2j and 2jz3 arrangements are correlated with large size, whereas 2st arrangement is correlated with small size. Some inversions (2st and 2jz3) show dominant effects and others (2j/jz3) exhibit overdominance. These results show that at least 25 per cent of body size variation may be accounted for by the studied karyotypes. The possible integration of the genotypic, phenotypic and fitness levels, and also the possible implications to life-history evolution theories, are discussed. These results suggest that, under moderate to high heritability values, some kinds of chromosomal endocyclic and/or balancing selection may be valuable mechanisms for maintenance of body size variation.