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.     

Oviposition preference and life history traits in cactophilic Drosophila koepferae and D. buzzatii in association with their natural hosts

Drosophila koepferae and D. buzzatii are two closely related cactophilic species inhabiting the arid lands of southern South America. Previous studies have shown that D. buzzatii breeds primarily on the necrotic cladodes of several Opuntia cacti and D. koepferae on the rotting stems of columnar cacti of the genera Trichocereus and Cereus. In this paper, we analyze the patterns of host plant utilization in a locality where both Drosophila species are sympatric. Field studies showed an absence of differential attraction of adult flies to the rots of two major host cacti: O. sulphurea and T. terschekii. However, the proportion of D. buzzatii flies emerged from the rotting cladodes of O. sulphurea was significantly higher than in T. terschekii. In laboratory experiments, egg to adult viability in single species cultures varied when both Drosophila species were reared in media prepared with O. sulphurea or T. terschekii. In addition, between-species comparisons of flies emerged from single species cultures showed that D. buzzatii adults were smaller and developed faster than D. koepferae. Furthermore, analysis of flies emerged in mixed species cultures showed differences in oviposition preference and oviposition behavior. We discuss the observed between-species differences and suggest that these traits are the result of adaptation to specific patterns of spatial and temporal predictability of their respective preferred host plants: columnar are less dense and less ephemeral resources, whereas the opuntias are more abundant, and fast rotting cacti. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Population genetics and geographic variation of alcohol dehydrogenase (Adh) paralogs and glucose-6-phosphate dehydrogenase (G6pd) in Drosophila mojavensis

Populations of Drosophila mojavensis from the deserts of the Baja California peninsula and mainland Mexico utilize different cactus hosts with different alcohol contents. The enzyme alcohol dehydrogenase (ADH) has been proposed to play an important role in the adaptation of Drosophila species to their environment. This study investigates the role of ADH in the adaptation of the cactophilic D. mojavensis to its cactus host. In D. mojavensis and its sibling species, D. arizonae, the Adh gene has duplicated, giving rise to a larval/ovarian form (Adh-1) and an adult form (Adh-2). Studies of sequence variation presented here indicate that the Adh paralogs have followed different evolutionary trajectories. Adh-1 exhibits an excess of fixed amino acid replacements, suggesting adaptive evolution, which could have been a result of several host shifts that occurred during the divergence of D. mojavensis. A 17-bp intron haplotype polymorphism segregates in Adh-2 and has markedly different frequencies in the Baja and mainland populations. The presence of the intron polymorphism suggests possible selection for the maintenance of pre-mRNA structure. Finally, this study supports the proposed Baja California origination of D. mojavensis and subsequent colonization of the mainland accompanied by a host shift.     

Temporal and Spatial Allozyme Variation in the South American Cactophilic Drosophila antonietae (Diptera; Drosophilidae)

Drosophila antonietae is an endemic South American cactophilic species that uses Cereus hildmaniannus rotting cladodes as breeding sites. We assessed temporal and spatial intrapopulational allozyme variation of two natural populations. Our results suggest that environmental variation (rain precipitation) is probably influencing allozyme temporal variation. Moreover, it seems that D. antonietae does not have intrapopulation structure and has N _ev (variance effective size) 83 and N _ec (number of adult flies that colonize each rotting cladode) = 21. The deficiency of heterozygotes found must be due to null alleles, a temporal Wahlund effect, or selection against heterozygotes. Assortative mating and inbreeding are discarded. This is the first report on allozyme variation in D. antonietae. It gives some insight on intrapopulational genetics through space and time for this species. This is important to understand its general genetic variability and will be essential to future works on the natural history and evolution of this species.     

Estimation of migration from a perturbation experiment in natural populations of Drosophila buzzatii Patterson & Wheeler

In order to estimate migration and gene flow, allele frequencies in populations at two sites separated by 120 m were differentially perturbed by the continuous release over 413 days of flies homozygous at particular allozyme loci. The effects of perturbation were determined by genotype assay at two collections prior to, thirteen during and nine after the perturbation period. Maximum likelihood methods were developed to estimate migration into the two populations from the homozygous releases, and migration between the two populations. The successful perturbation of allele frequencies in a natural population is demonstrated. A plateau in allele frequencies during perturbation and a return to original frequencies following cessation of perturbation was most likely due to selection during development against recessive alleles concurrently introduced into the populations by the released flies. There is unequivocal evidence for short distance gene flow between the two populations. The migration rates estimated at ten times over a nine month period were extremely variable, but with higher population density at one site positively related with migration from that site to the other.     

The Drosophila Serido Speciation Puzzle: Putting New Pieces Together

The D. seridosuperspecies is a complex mosaic of populations distributed over a vast part of South America and showing various degrees of genetical divergence. We have analyzed its chromosomal constitution in 16 new localities of southeastern and southern Brazil. Both the metaphase and salivary gland chromosomes show a sharp split of these populations in two groups. Four populations, fixed for inversion 2e ⁸and showing the type I karyotype, represent the southwestern limit of D. seridotype B, which inhabits the Cerrado in central-western Brazil. The remaining populations are homozygous for 2x ⁷, an inversion also fixed in the Caatinga populations of northeastern Brazil. However, their karyotype, in those populations analyzed, belong to a different type (V) from that of the Caatinga populations. Populations in this second group are polymorphic for five inversions on chromosome 2 plus another on chromosome 5 and show considerable interpopulation differentiation. The breakpoints of chromosome 2 inversions are described and the inversion loops of several heterokaryotypes are presented. Biogeographical information suggests that there are clear ecological differences between the two groups of populations as well as among the populations within the second group. The possible role of host plants in promoting the genetic divergence among the D. seridopopulations is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microsatellite loci in the cactophilic species Drosophila antonietae (Diptera; Drosophilidae)

Morphometric and isoenzymatic data have showed little variation among Drosophila antonietae populations. This suggests historic gene flow, due to the distribution of these populations, which are associated with cacti along rivers, and natural selection on these markers. Microsatellite loci have high variability and are ideal for detecting gene flow and analysing population structure. Here we describe the isolation of seven polymorphic microsatellite loci in D. antonietae.     

Developmental Time and Thorax Length Differences Between the Cactophilic Species Drosophila Buzzatii and D. Koepferae Reared in Different Natural Hosts

Drosophila buzzatii and Drosophila koepferae are two cactophilic sibling species whose ranges partially overlap in Northwestern and Western Argentina. Both species can utilize the decaying tissues of both Opuntia and columnar cacti as breeding sites. Though D. buzzatii and D. koepferae are not differentially attracted to Opuntia and columnar hosts, the composition of the communities of flies emerging from natural substrates of both cacti differed significantly in a natural population. The objective of this paper is to analyze whether intra and/or interspecific competition affects development time and thorax length in D. buzzatii and D. koepferae when both species are reared in single and mixed species culture and fed with semi-natural media prepared with fermenting materials of Opuntia sulphurea(tuna) and Trichocereus terschekii(cardón). Our results showed that both traits differ significantly between flies raised in different hosts and that differences between D. koepferae and D. buzzatii species for both thorax length and development time depend on the type of culture (mixed vs. single species). In addition, the host by type of culture interaction was significant. We also observed thorax length differences between Drosophila species and type of culture. Our present data suggest that the effect of intra and interspecific competition varied between the two traits investigated and between species. However, competition alone cannot explain the differential pattern of resource utilization shown by D. buzzatii and D. koepferae in the natural population studied.     

Genetic differentiation of island populations: geographical barrier or a host switch?

In the Sonoran desert, there exists a diverse community of cactophilic drosophilids that exploit toxic, rotting cactus tissue as a food resource. The chemistry of the necrotic cactus tissue varies among species, and several drosphilid species have evolved specialized detoxification mechanisms and a preference for certain cactus types. In the present study, we compared the genetic structure of two columnar cactus species, Drosophila mettleri and Drosophila mojavensis, and two prickly pear species, Drosophila mainlandi and Drosophila hamatofila, which have all recently colonized Catalina Island off the coast of southern California. Because there are no columnar cactus species on Catalina Island, the two columnar specialists underwent a host switch to prickly pear cactus, the only cactus present on the island. Previous genetic studies of D. mettleri and D. mojavensis showed significant genetic differentiation between mainland and island populations, which could result from restricted gene flow as a result of the San Pedro Channel, or because of a host switch to prickly pear. To distinguish between these possibilities, we analyzed the genetic structure of the prickly pear species aiming to isolate the effects of geography versus host switching. The results obtained show little to no genetic differentiation for the prickly pear species, supporting the hypothesis that the genetic differentiation of the two columnar species is a result of a host switch from columnar cacti to prickly pear. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.