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.     

Endangered Cactus Restoration: Mitigating the Non-Target Effects of a Biological Control Agent (Cactoblastis cactorum) in Florida

The moth Cactoblastis cactorum (Berg), the poster child of weed biological control in Australia, has recently invaded the United States and threatens native cacti. Concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two known populations exist in the wild. We made three separate outplantings of O. corallicola, designed to bolster the number of extant cacti and to test the effectiveness of three different treatments to protect the cacti from Cactoblastis. In one outplanting, we tested the associational susceptibility hypothesis and found that cacti planted more than 20 m away from the common prickly pear cactus, Opuntia stricta, which act as a reservoir of Cactoblastis, were just as frequently attacked and killed by Cactoblastis as cacti planted within 5 m. In addition, Cactoblastis attack was greater in the shade than in the sun. In the second outplanting, we minimized the attack from Cactoblastis by using protective cages planted at least 500 m from O. stricta in areas not inhabited by cacti. Cages attracted the attention of local animals, which destroyed the cages and trampled the cacti inside to death. Crown rot caused high mortality in this outplanting. In the third outplanting, again conducted at least 500 m away from O. stricta, fertilization did not reduce crown rot mortality. We suggest that increasing populations of O. corallicola in Florida, by means of outplantings, will remain a challenge because of death from Cactoblastis when planted in areas where cacti normally grow and because of death from crown rot in areas where they do not. Because Cactoblastis is moving rapidly northward and westward and has already reached Charleston, South Carolina, rare cacti in the rest of the U.S. Southeast may be in danger. Eventually, many cactus species in the U.S. South, Southwest, and Mexico will likely be threatened by this moth.     

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.     

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.

     

The ecology of the yeast flora in necroticOpuntia cacti and of associatedDrosophila in Australia

A survey was made of the yeast communities isolated from necrotic tissue of 4 species of prickly-pear cacti (Opuntia stricta, O. tomentosa, O. monacantha, andO. streptacantha) which have colonized in Australia. Yeast communities were sampled from a number of localities and at different times. Cactus specific yeasts accounted for 80% of the total isolates, and the 3 most common species contributed 63% of the total. Comparisons of the species compositions of the yeast communities indicated that the differences among communities were greater betweenOpuntia species than between different localities within a single cactus species, and also that differences between years were greater than average differences between localities within years. Multivariate statistical tests of association between yeast community and physical features of rots indicated that temperature, pH, and age of rot all exerted some influence on the structure of the yeast community. Similar analyses involvingDrosophila species inhabiting these cactus rots suggested the existence of complex associations betweenDrosophila community, yeast community, and physical and chemical attributes of the cactus necroses.     

Comparing the effects of the exotic cactus-feeding moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) and the native cactus-feeding moth, Melitara prodenialis (Walker) (Lepidoptera: Pyralidae) on two species of Florida Opuntia

This study examined the effects of the native cactus moth borer, Melitara prodenialis, and the invasive cactus moth borer, Cactoblastis cactorum, on two common cactus species, Opuntia stricta and O. humifusa at coastal and inland locations in central Florida. Opuntia stricta were present only at coastal sites and O. humifusa were present at coastal and inland sites. Throughout the duration of the study, coastal plants were subject to damage solely by C. cactorum and inland plants solely by M. prodenialis. Results showed marginally significantly higher numbers of eggsticks on O. stricta than O. humifusa and significantly higher numbers at coastal sites than at inland sites. There was also significantly higher moth damage on O. stricta than O. humifusa and at coastal sites than inland sites, but not significantly so. However, there was a higher level of plant mortality for O. humifusa than for O. stricta and a significantly higher level of cactus mortality at inland sites when compared to coastal sites. This increased mortality may be due to increased attack by true bugs, Chelinidea vittiger, and by Dactylopius sp., combined with attack by M. prodenialis. Inland plants also tended to be smaller than coastal plants and could be more susceptible to the combined effects of all insects. Further long-term research on coastal cactus survival when attacked and unattacked by Cactoblastis is necessary to fully determine the effects of this moth on Opuntia survival.     

The influence of herbivory and weather on the vital rates of two closely related cactus species

Herbivory has long been recognized as a significant driver of plant population dynamics, yet its effects along environmental gradients are unclear. Understanding how weather modulates plant–insect interactions can be particularly important for predicting the consequences of exotic insect invasions, and an explicit consideration of weather may help explain why the impact can vary greatly across space and time. We surveyed two native prickly pear cactus species (genus Opuntia) in the Florida panhandle, USA, and their specialist insect herbivores (the invasive South American cactus moth, Cactoblastis cactorum, and three native insect species) for five years across six sites. We used generalized linear mixed models to assess the impact of herbivory and weather on plant relative growth rate (RGR) and sexual reproduction, and we used Fisher's exact test to estimate the impact of herbivory on survival. Weather variables (precipitation and temperature) were consistently significant predictors of vital rate variation for both cactus species, in contrast to the limited and varied impacts of insect herbivory. Weather only significantly influenced the impact of herbivory on Opuntia humifusa fruit production. The relationships of RGR and fruit production with precipitation suggest that precipitation serves as a cue in determining the trade‐off in the allocation of resources to growth or fruit production. The presence of the native bug explained vital rate variation for both cactus species, whereas the invasive moth explained variation only for O. stricta. Despite the inconsistent effect of herbivory across vital rates and cactus species, almost half of O. stricta plants declined in size, and the invasive insect negatively affected RGR and fruit production. Given that fruit production was strongly size‐dependent, this suggests that O. stricta populations at the locations surveyed are transitioning to a size distribution of predominantly smaller sizes and with reduced sexual reproduction potential.     

Early events in speciation: Cryptic species of Drosophila aldrichi

Understanding the earliest events in speciation remains a major challenge in evolutionary biology. Thus identifying species whose populations are beginning to diverge can provide useful systems to study the process of speciation. Drosophila aldrichi, a cactophilic fruit fly species with a broad distribution in North America, has long been assumed to be a single species owing to its morphological uniformity. While previous reports either of genetic divergence or reproductive isolation among different D. aldrichi strains have hinted at the existence of cryptic species, the evolutionary relationships of this species across its range have not been thoroughly investigated. Here we show that D. aldrichi actually is paraphyletic with respect to its closest relative, Drosophila wheeleri, and that divergent D. aldrichi lineages show complete hybrid male sterility when crossed. Our data support the interpretation that there are at least two species of D. aldrichi, making these flies particularly attractive for studies of speciation in an ecological and geographical context.     

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.     

The Diet and Reproductive Schedules of Leptonycteris curasoae curasoae and Glossophaga longirostris elongata (Chiroptera: Glossophaginae) on Curaçao

Two bat species, Leptonyrteris curasoae and Glossophaga longirostris, are the principal pollinators of at least two of the three species of columnar cacti that grow on the semiarid island of Curaçao, Netherlands Antilles. I examined the importance of the cacti in the diets of the bats and found that 85–91 percent of their diet samples contained cactus pollen and seeds. At least 43 percent of the samples from each species contained cactus pollen andlor seeds exclusively. Leptonycteris curasoae consumes nectar and pollen of Ceiba pentandra and Agave spp. at the beginning of the dry season and G. longirostris also consumes a few other plant products in the wet season, but both bat species depend nutritionally on cacti. Female bats give birth to one pup per year, and the periods of parturition and lactation in each species correspond to peaks in the reproductive phenology of the two most abundant columnar cactus species. From personal observations and a review of the literature, I determined that bats were unlikely to fly to the mainland to feed, although L. curasoae may do so. I conclude that the interdependence of bats and cacti is suggestive of coevolution, and that columnar cacti are critical for the survival and persistence of nectar-feeding bats on Curaçao.     

Biofilm development and enhanced stress resistance of a model,mixed-species community biofilm

Most studies of biofilm biology have taken a reductionist approach, where single-species biofilms have been extensively investigated. However, biofilms in nature mostly comprise multiple species, where interspecies interactions can shape the development, structure and function of these communities differently from biofilm populations. Hence, a reproducible mixed-species biofilm comprising Pseudomonas aeruginosa, Pseudomonas protegens and Klebsiella pneumoniae was adapted to study how interspecies interactions affect biofilm development, structure and stress responses. Each species was fluorescently tagged to determine its abundance and spatial localization within the biofilm. The mixed-species biofilm exhibited distinct structures that were not observed in comparable single-species biofilms. In addition, development of the mixed-species biofilm was delayed 1–2 days compared with the single-species biofilms. Composition and spatial organization of the mixed-species biofilm also changed along the flow cell channel, where nutrient conditions and growth rate of each species could have a part in community assembly. Intriguingly, the mixed-species biofilm was more resistant to the antimicrobials sodium dodecyl sulfate and tobramycin than the single-species biofilms. Crucially, such community level resilience was found to be a protection offered by the resistant species to the whole community rather than selection for the resistant species. In contrast, community-level resilience was not observed for mixed-species planktonic cultures. These findings suggest that community-level interactions, such as sharing of public goods, are unique to the structured biofilm community, where the members are closely associated with each other.     

Experimental test of biotic resistance to an invasive herbivore provided by potential plant mutualists

Understanding the influence of resident species on the success of invaders is a core objective in the study and management of biological invasions. We asked whether facultative food-for-protection mutualism between resident, nectar-feeding ants and extrafloral nectar-bearing plants confers biotic resistance to invasion by a specialist herbivore. Our research focused on the South American cactus-feeding moth Cactoblastis cactorum Berg (Lepidopetra: Pyralidae) in the panhandle region of Florida. This species has been widely and intentionally redistributed as a biological control agent against weedy cacti (Opuntia spp.) but arrived unintentionally in the southeast US, where it attacks native, non-target cacti and is considered a noxious invader. The acquired host-plants of C. cactorum in Florida secrete extrafloral nectar, especially on young, vegetative structures, and this attracts ants. We conducted ant-exclusion experiments over 2 years (2008 and 2009) at two sites using potted plants of two vulnerable host species (O. stricta and O. ficus-indica) to evaluate the influence of cactus-visiting ants (total of eight species) at multiple points in the moth life cycle (oviposition, egg survival, and larval survival). We found that the presence of ants often increased the mortality of lab-reared C. cactorum eggsticks (stacks of cohered eggs) and larvae that we introduced onto plants in the field, although these effects were variable across sites, years, host-plant species, ant species, and/or between old and young plant structures. In contrast to these “staged” encounters, we found that ants had little influence on the survival of cactus moths that occurred naturally at our field sites, or on moth damage and plant growth. In total, our experimental results suggest that the influence of cactus-visiting ants on C. cactorum invasion dynamics is weak and highly variable.     

Association between Opuntia species invasion and changes in land-cover in the Mediterranean region

In Mediterranean regions, biological invasions pose a major threat to the conservation of native species and the integrity of ecosystems. In addition, changes in land‐cover are a widespread phenomenon in Mediterranean regions, where an increase in urban areas and major changes from agricultural abandonment to shrub encroachment and afforestation are occurring. However, the link between biological invasions and changes in land‐cover has scarcely been analyzed. We conducted a regional survey of the distribution of the two alien prickly‐pear cacti Opuntia maxima and O. stricta in Cap de Creus (Catalonia, Spain) and related patterns of invasion to spatially explicit data on land‐cover/change from 1973 to 1993 to test the hypotheses that the two Opuntia species invade areas that have experienced large land‐cover transformations. We found that Opuntia invasion is particularly high in shrublands and woodlands located near urban areas. O. maxima are over‐represented in the shrublands and O. stricta in the woodlands that were former crops. Crop coverage has dropped by 71% in this 20‐year period. This study highlights the role of past land‐cover in understanding the present distribution of plant invasions.     

GENETICS OF HOST-CACTUS RESPONSE AND LIFE-HISTORY EVOLUTION AMONG ANCESTRAL AND DERIVED POPULATIONS OF CACTOPHILIC DROSOPHILA MOJAVENSIS

The extent of host-specific genetic variation for two life-history traits, egg to adult developmental time and viability, and one morphological trait closely tied to fitness, adult thorax size, was exposed by employing a nested half-sib/full-sib breeding design with Baja and mainland populations of Drosophila mojavensis recently extracted from nature. This study was motivated by the presence of substantial variation in life histories among populations of D. mojavensis that use the fermenting tissues of particular species of columnar cacti for feeding and breeding in the Sonoran Desert. Full-sib progeny from all sire-dam crosses were split into cultures of agria cactus, Stenocereus gummosus, and organ pipe cactus, S. thurberi, to examine patterns of genotype-by-environment interaction for these fitness components. Baja flies expressed shorter egg-to-adult developmental times, higher viabilities, and smaller body sizes than mainland flies consistent with previous studies. Significant sire and dam components of variance were exposed for developmental time and thorax size. Genotype-by-environment interactions were significant at the level of dams for developmental time and nearly significant for viability (P = 0.09). Narrow- and broad-sense heritabilities were influenced by host cactus, sex, and population. No strong pattern of genetic correlation emerged among fitness components suggesting that host-range expansion has not been accompanied by formation of coadapted life histories, yet the ability to estimate genetic correlations and their standard errors was compromised by the unbalanced nature of the data set. Genetic correlations in performance across cacti were slightly positive, evidence for ecological generalism among populations explaining the observed pattern of multiple host cactus use within the species range of D. mojavensis.     

Opposing forces of seed dispersal and seed predation by mammals for an invasive cactus in central Kenya

The invasive erect prickly pear cactus (Opuntia stricta) has reduced rangeland quality and altered plant communities throughout much of the globe. In central Kenya's Laikipia County, olive baboons (Papio anubis) frequently consume O. stricta fruits and subsequently disperse the seeds via defecation. Animal‐mediated seed dispersal can increase germination and subsequent survival of plants. However, consumption of seeds (seed predation) by rodents may offset the potential benefits of seed dispersal for cactus establishment by reducing the number of viable seeds. We investigated foraging preferences of a common and widely distributed small mammal—the fringe‐tailed gerbil (Gerbilliscus robustus), between O. stricta seeds deposited in baboon faeces versus control O. stricta seeds. In addition to providing evidence of seed predation on O. stricta by G. robustus, our data show that seed removal was higher (shorter time to use) for seeds within faeces than for control seeds. G. robustus clearly prefers seeds within faeces compared to control seeds. These results suggest that high abundances of rodents may limit successful establishment of O. stricta seeds, possibly disrupting seed dispersal via endozoochory by baboons.     

GENETIC VARIATION IN CACTOPHILIC DROSOPHILA FOR OVIPOSITION ON NATURAL YEAST SUBSTRATES

Theory predicts that environmental heterogeneity in space or in time can maintain genetic polymorphism. Stable polymorphisms are expected to be more readily maintained if there are genotype specific habitat preferences. Genotype specific preferences for oviposition sites in Drosophila could be a major factor promoting habitat selection, and thus the maintenance of genetic variation. This hypothesis is being tested using the cactophilic species, D. buzzatii and D. aldrichi, where available evidence indicates a potential for such habitat selection, the habitats (oviposition sites) being yeast species found in the natural environment of these flies (cactus rots). Genetic variation for oviposition preferences was tested using isofemale lines—for D. buzzatii, a total of 60 lines from seven localities widely distributed through the species range in Australia, and for D. aldrichi, 21 lines from three of these localities. Females were given a choice of five yeast species as oviposition sites. Genetic variation for oviposition preferences on these natural substrates was demonstrated. There was significant variation among isofemale lines within populations in their patterns of preferences for oviposition on the five yeast species. However, analyses of preferences for each yeast species separately showed that the genetic variation for preferences relates to only three of the five species. Heritabilities of individual female preferences for these three species were low, ranging up to 9%. Little geographic differentiation was apparent among populations, most likely due to similar selection regimes within each population. Within populations, this kind of habitat selection could act to maintain polymorphisms, both at loci determining the habitat preferences and at other loci in linkage disequilibrium with them.     

Columnar cacti as sources of energy and protein for frugivorous bats in a semi‐arid ecosystem

Columnar cacti constitute the dominant elements in the vegetation structure of arid and semi‐arid New World ecosystems representing a plethora of food resources for vertebrate consumers. Previous stable isotope analysis in Central Mexico showed that columnar cacti are of low importance to build tissue for frugivorous bats. We used carbon stable isotope analysis of whole blood and breath samples collected from four species of frugivorous bats (Sturnira parvidens, Sturnira ludovici, Artibeus jamaicensis, and Artibeus intermedius) to reconstruct the importance of cactus plants in their diet. Breath samples were collected within 10 min (B₁₀) of bat capture and ~12 h after capture (B₇₂₀), representing the oxidation of recently ingested food and of body reserves, respectively. We expected that bats relied primarily on non‐cactus food to construct tissues and fuel oxidative metabolism. Non‐cactus food strongly predominated for tissue building, whereas oxidative metabolism was supported by a moderate preponderance of non‐cactus food for B₁₀ samples, and a moderate preponderance of cactus food or an equal contribution of both sources for B₇₂₀ samples. Artibeus and Surnira species appear to cover a narrow part of the diet with cactus food, confirming that the incorporation of nutrients derived from these plants is not generalized among vertebrate consumers.     

The importance of shrub cover for four cactus species differing in growth form in an Andean semi‐desert

Question: Factors influencing seedling establishment are known to vary between open sites and those protected by plant cover. In many desert regions, protected microhabitats below shrubs are essential for establishment of many cactus species. Very little is known about these factors for Andean cacti and how the importance of vegetation cover varies with cactus species. Are Andean cacti associated more frequently to vegetation cover than to open ground? Are they associated to certain shrub species? Is the distributional pattern in relation to cover similar for different cactus species? In what microhabitat (below or away from shrubs) are cactus seeds more abundant? These questions are addressed for the case of an Andean semi‐desert. Location: Semi‐arid tropical Andes, La Paz department, Bolivia. Methods: We examined 132 isolated shrubs = 50 cm along a line across two microhabitats: areas below and away from shrubs/trees. Shrub crown size was measured. The among‐shrub samples were taken from open spaces contiguous to each of the sampled shrubs. In both microhabitats, all cactus species were recorded. The cardinal direction of the cacti was also registered. Correlation between canopy diameter and number of beneficiaries was evaluated for Prosopis flexuosa. The cactus seed bank in each microhabitat was also studied. Results and Conclusions: The four cactus species found behaved differently in relation to shrub canopies. These distributional differences could be due to differences in growth form. Columnar cacti apparently need the shade of shrubs. Only the columnar species is able to grow near the base of the tallest nurse species. The opuntioid cacti studied seem more facultative: although apparently preferring shrub un‐der‐canopies, they are able to establish in open ground. The globose cactus is the most indifferent to the presence of plant cover. These patterns parallel others found in North America. The capacity of different cacti to appear in open spaces could be related to vegetative propagation, and not necessarily to seedling tolerance of heat.     

The influence of cactus size and previous infection on bird deposition of mistletoe seeds

Abstract Tristerix aphyllus (Loranthaceae) is a bird-dispersed mistletoe that infects several cactus species in central and north-central Chile. At Las Chinchillas National Reserve it parasitizes two species of columnar cacti (Echinopsis chilensis and Eulychnia acida) and its seeds are dispersed exclusively by the Chilean mockingbird Mimus thenca. The prevalence of T. aphyllus on cacti depended on host species and size. Echinopsis chilensis was parasitized at a higher, and E. acida at a lower, frequency than expected from their relative abundances. In both species, the incidence of T. aphyllus parasitism increased with cactus height. Birds perched and deposited seeds primarily on parasitized cacti, which resulted in an aggregated distribution of seeds. By preventing bird perching and thus reducing seed deposition, the long spines of E. chilensis and E. acida may help protect cacti against T. aphyllus infestation. However, the seeds of T. aphyllus produced a slow-growing radicle that reached up to 91 mm long in 43 days. Radicles of this length are unique among mistletoe species and may have evolved to overcome the barrier posed by cactus spines.     

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, ?? , ??–??.