Phylogeography of the Cactophilic Drosophila and Other Arthropods Associated with Cactus Necroses in the Sonoran Desert

Risk of predation increases susceptibility to parasitism via trait-mediated indirect effects

The presence of natural enemies can cause organisms to change habitat use, foraging behavior, and/or resource allocation in response to a perceived risk, responses that may come at the cost of other fitness-related traits. Since most species encounter multiple natural enemies in nature, defensive behaviors against one attacker may make the focal organism more vulnerable to attack by a different natural enemy. Anti-predator behaviors can lead to trait-mediated indirect effects, such as an increased risk of parasitism and vice versa. Few empirical studies have examined the response of a single focal species to the risk of attack by multiple species. Our experiments provided the cactiphilic fly Drosophila nigrospiracula with opportunities to prioritize either anti-predator (e.g., reduced activity) or anti-parasite behavior (e.g., increased activity) at the cost of increased infection or predation, respectively. We experimentally show that when flies were exposed to ectoparasitic mites, in the presence of predator (jumping spider) cues, flies incurred increased levels of infection compared to flies without predator cues. The mean infection prevalence increased by 80% and the infection intensity increased by 180%. However, the presence of parasite cues had no analogous effect on predation rates, which suggests that flies prioritized predation risk over parasite defense at the cost of increased infection. We provide empirical evidence that the presence of multiple threats can lead to trait-mediated indirect effects, with important consequences for host-parasite and food web dynamics, and the ecology of fear.

Navigating the landscape of fear: Fruit flies exhibit distinct antipredator and antiparasite defensive behaviors

Most organisms are at risk of being consumed by a predator or getting infected by a parasite at some point in their life. Theoretical constructs such as the landscape of fear (perception of risk) and nonconsumptive effects (NCEs, costly responses sans predation or infection) have been proposed to describe and quantify antipredator and antiparasite responses. How prey/host species identify and respond to these risks determines their survival, reproductive success and, ultimately, fitness. Most studies to date have focused on either predator-prey or parasite-host interactions, yet habitats and ecosystems contain both parasitic and/or predatory species that represent a complex and heterogenous mosaic of risk factors. Here, we experimentally investigated the behavioral responses of a cactophilic fruit fly, Drosophila nigrospiracula, exposed to a range of species that include parasites (ectoparasitic mite), predators (jumping spiders), as well as harmless heterospecifics (nonparasitic mites, ants, and weevils). We demonstrate that D. nigrospiracula can differentiate between threat and non-threat species, increase erratic movements and decrease velocity in the presence of parasites, but decrease erratic movements and time spent grooming in the presence of predators. Of particular importance, flies could distinguish between parasitic female mites and nonparasitic male mites of the same species, and respond accordingly. We also show that the direction of these NCEs differs when exposed to parasitic mites (i.e., risk of infection) versus spiders (i.e., risk of predation). Given the opposing effects of predation versus infection risk on fly behavior, we discuss potential trade-offs between parasite and predator avoidance behaviors. Our findings illustrate the complexity of risk assessment in a landscape of fear and the fine-tuned NCEs that arise in response. Moreover, this study is the first to examine these behavioral NCEs in a terrestrial system.

Genetic variation in disease resistance in Drosophila spp. is mitigated in Drosophila sechellia by specialization to a toxic host

We found that Drosophila species vary in their susceptibility to the broad-spectrum entomopathogen, Metarhizium anisopliae (strain Ma549). Generalist species were generally more resistant than dietary specialists, with the cactophilic Drosophila buzzatii and Drosophila sechellia, a specialist of the Morinda citrifolia (Morinda) fruit, being most susceptible. Morinda fruit is reported to be toxic to most herbivores because it contains Octanoic Acid (OA). We confirmed that OA is toxic to Drosophila spp., other than D. sechellia, and we also found that OA is highly toxic to entomopathogenic fungi including Ma549 and Beauveria bassiana. Drosophila sechellia fed a diet containing OA, even at levels much less than found in Morinda fruit, had greatly reduced susceptibility to Ma549. This suggests that specializing to Morinda may have provided an enemy-free space, reducing adaptive prioritization on a strong immune response. Our results demonstrate that M. anisopliae and Drosophila species with divergent lifestyles provide a versatile model system for understanding the mechanisms of host-pathogen interactions at different scales and in environmental context.

Phylogeography of Falagonia mexicana Sharp, 1883 (Coleoptera, Staphylinidae, Aleocharinae)

Falagonia mexicana is an aleocharine distributed from northern Mexico to Guatemala and El Salvador. It is associated with Atta mexicana ants and lives within their piles of waste or external debris. The phylogeography and historical demography of 18 populations from Mexico, Guatemala, and El Salvador were studied. The data set encompasses a 472 bp fragment of the COI. Results suggest that F. mexicana was originated during Middle Pliocene (ca. 0.5 Mya), starting its diversification at the Upper Pleistocene and Holocene. Populations were recovered forming at least four main lineages, with a significant phylogeographic structure. Evidence of contemporary restricted gene flow was found among populations. The historical demography suggests that the geographic structure is due to recent physical barriers (e.g., Isthmus of Tehuantepec) rather than ancient geological events. Also, recent geological and volcanic events in the east of the Trans-Mexican Volcanic Belt and the Sierra Madre Oriental might be responsible for the restricted gene flow among populations. Skyline-plot analyses suggested that a demographic expansion event took place at the end of the Late Quaternary glacial-interglacial cycles.

Plastic Variation in the Phyletic Lineages of Cactophilic Drosophila meridionalis and Relation to Hosts as Potential for Diversification

The insect/plant interaction is known to be a trigger for diversification and even speciation. Experimental analyses on fitness traits and phenotypic variation using alternative host sites have been performed to understand the process of diversification relative to insect/plant interactions. For cactophilic species of Drosophila, the speciation process is considered an adaptive radiation in response to the exploration of species of the Cactaceae as breeding and feeding sites. In this work, we analyzed life history and morphological traits in individuals from two phyletic lineages (Evolutionarily Significant Units ESU) of the cactophilic species Drosophila meridionalis (Wasserman 1962) (Diptera: Drosophilidae) raised from media prepare. The characters analyzed corresponded to viability, developmental time, and four morphological measurements. The experiments were performed in a semi-natural medium prepared with fermenting tissues of the natural hosts, Cereus hildmaniannus and Opuntia monacantha. Viability, development time, and three morphological measurements were influenced by lineage, suggesting differentiation between the lineages. However, in O. monacantha, the mean viability was greater (~15%) and development time was longer (~336 h) than in C. hildmaniannus (~11% and ~301 h, respectively). Only the developmental time was significantly affected by the host cactus. In general, ESU group A had better values than ESU group BC for the evaluated traits. This finding suggested differentiation between the two lineages and different plastic responsiveness to the contrasting environments of the hosts, and that C. hildmaniannus may be a relatively stressful environment for the larvae, as for other Drosophila species.

Her Majesty's Desert Throne: The Ecology of Queen Butterfly Oviposition on Mojave Milkweed Host Plants

Butterfly-host plant relationships can inform our understanding of ecological and trophic interactions that contribute to ecosystem function, resiliency, and services. The ecology of danaid-milkweed (Apocynaceae) host plant interactions has been studied in several biomes but is neglected in deserts. Our objective was to determine effects of plant traits, seasonality, and landscape-level host plant availability on selection of Mojave milkweed (Asclepias nyctaginifolia A. Gray) by ovipositing monarch butterflies (Danaus plexippus plexippus) and queen butterflies (Danaus gilippus thersippus) in the Californian Mojave Desert. We surveyed all known Mojave milkweed locations in the Ivanpah Valley, California (n = 419) during early, mid-, and late spring in 2017. For each survey, we counted monarch and queen butterfly eggs on each Mojave milkweed plant. We also measured canopy cover, height, volume, and reproductive stage of each Mojave milkweed plant. We counted a total of 276 queen butterfly eggs and zero monarch butterfly eggs on Mojave milkweed host plants. We determined that count of queen butterfly eggs significantly increased with increasing Mojave milkweed canopy cover. Additionally, count of queen butterfly eggs was: (1) greater on adult Mojave milkweed plants than on juvenile and seedling plants and greater on juvenile Mojave milkweed plants than on seedling plants; and (2) greater during early spring than mid-spring-we recorded no eggs during late spring. Based on aggregation indices, queen butterfly eggs occurred on Mojave milkweed plants in a nonrandom, clustered pattern throughout the Ivanpah Valley. We provide the first evidence of trophic interactions between queen butterflies and Mojave milkweed at multiple spatial scales in the Mojave Desert, suggesting that conservation and management practices for both species should be implemented concurrently. Given its role as an herbivore, pollinator and prey, the queen butterfly may serve as a model organism for understanding effects of anthropogenic disturbance (e.g., solar energy development) on "bottom-up" and trophic interactions among soils, plants and animals in desert ecosystems.

Genetic Diversity and Demographic History in the Cactophilic Drosophila repleta Species Group (Diptera: Drosophilidae) in North America Inferred from Mitochondrial DNA Barcodes

Genetic diversity in mitochondrial DNA barcodes, comprising a segment of the cytochrome c oxidase subunit I (COI) gene, was used to infer demographic histories in selected taxa of the cactophilic Drosophila repleta species group in North America. Haplotype and nucleotide diversities were determined in 16 taxa based on both previously published and new sequences. Haplotype diversity (h) differed dramatically in different taxa, varying from h = 0 in Drosophila eremophila, Drosophila hexastigma, and Drosophila bifurca to h = 0.99 in Drosophila hamatofila. Genetic diversity indices and sample sizes were sufficient to infer demographic histories from mismatch distribution analysis and Bayesian skyline plots for 9 taxa: Drosophila mojavensis baja, Drosophila mojavensis sonorensis, Drosophila arizonae, Drosophila aldrichi, D. hamatofila, Drosophila spenceri, Drosophila mainlandi, Drosophila mettleri, and Drosophila nigrospiracula. Evidence was found for both population expansions and relatively stable populations in these species. Demographic history varied dramatically in subspecies of D. mojavensis, showing a relatively stable population size over time in D. m. sonorensis from the mainland Sonoran Desert whereas a large population expansion was evident in D. m. baja from the Baja California Peninsula, providing support for the hypothesis that the split of sister species D. mojavensis and D. arizonae from a common ancestor occurred on the mainland rather than the peninsula as proposed by others. No evidence was found for a causal relationship between a stable or expanding population and host plant shifts from prickly-pear cactus to columnar cacti, which has occurred independently in many taxa of the repleta group.

Genetics and Distribution Modeling: The Demographic History of the Cactophilic Drosophila buzzatii Species Cluster in Open Areas of South America

Biodiversity is the result of historical and recurrent events acting on populations and species. The Drosophila buzzatii species cluster is distributed along a diagonal of open areas in South America. Combining genetic analyses with species distribution models we evaluated the influence of climatic changes in the demography history of this cluster. We performed a Bayesian Skyline analysis and reconstructed the ancestral areas based on mitochondrial cytochrome oxidase I (mtCOI) gene. We modeled the current distribution and projected it to past (mid-Holocene and Last Glacial Maximum) and future. Our results demonstrate that climate change plays a critical role in historical demography and in defining the current and future geographic ranges of these species. The inter-Andean dry valleys and the Chiquitano Seasonally Dry Tropical Forests (SDTF) in Bolivia are considered the ancestral area for the D. buzzatii cluster. From this area, the migration route was through a west-east corridor from central Andes, throughout Bolivia, Paraguay and Argentina toward eastern and northeastern Brazil, along the edges of rain forest. The responses of the species to the climatic changes differ from the dominant assumptions of expansion during dry/cold weather and contraction during wet/warm weather that characterized the glacial cycles. We suggest that the influence of ecology and the potential responsiveness of each taxon to the environmental dynamics should be considered as well. Predictions for the future suggest a decline of suitable areas for the cluster, threatening biodiversity of these habitats. This work showed the importance of an integrative analysis of genetics and geography information to improve the inferences about demographic history hypotheses for the cluster.

Contrasting Plasticity in Ovariole Number Induced by A Dietary Effect of the Host Plants between Cactophilic Drosophila Species

Under the preference-performance hypothesis, natural selection will favor females that choose oviposition sites that optimize the fitness of their offspring. Such a preference-performance relationship may entail important consequences mainly on fitness-related traits. We used the well-characterized cactus-Drosophila system to investigate the reproductive capacity in the pair of sibling species D. buzzatii and D. koepferae reared in two alternative host plants. According to our hypothesis, ovariole number (as a proxy of reproductive capacity) depends on host plant selection. Our results indicate that the capacity of D. buzzatii showed to be mild, only increasing the number of ovarioles by as much as 10% when reared in its preferred host. In contrast, D. koepferae exhibited a similar reproductive capacity across host cacti, even though it showed a preference for its primary host cactus. Our study also revealed that D. buzzatii has a larger genetic variation for phenotypic plasticity than its sibling, although ovariole number did not show clear-cut differences between species. We will discuss the weak preference-performance pattern observed in these cactophilic species in the light of nutritional and toxicological differences found between the natural host plants.

Phylogeographic patterns of Merodon hoverflies in the Eastern Mediterranean region: revealing connections and barriers

We investigated the phylogeographic patterns of Merodon species (Diptera, Syrphidae) in the Eastern Mediterranean. Ten species were sampled on five different islands and mainland sites as a minimum. All samples were screened for their mtDNA COI barcode haplotype diversity, and for some samples, we additionally generated genomic fingerprints. The recently established zoogeographic distribution categories classify these species as having (1) Balkan distribution; (2) Anatolian distribution; (3) continental areas and large islands distribution; and (4) with wide distribution. The ancestral haplotypes and their geographical localities were estimated with statistical parsimony (TCS). TCS networks identified as the ancestral haplotype samples that originated from localities situated within the distributional category of the species in question. Strong geographical haplotype structuring was detected for many Merodon species. We were particularly interested to test the relative importance of current (Aegean Sea) and past Mid-Aegean Trench) barriers to dispersal for Merodon flies in the Aegean. We employed phylogenetic β-diversity (Pβ total) and its partition in replacement (Pβ repl) and richness difference (Pβ rich) to test the importance of each explanatory variable (interisland distance, MAT, and island area) in interisland differences using partial Mantel tests and hierarchical partitioning of variation. β-Analyses confirmed the importance of both current and past barriers to dispersal on the evolution of group. Current interisland distance was particularly important to explain the replacement of haplotypes, while the MAT was driving differences in richness of haplotypes, revealing the MAT as a strong past barrier whose effects are still visible today in the phylogenetic history of the clade in the Aegean. These results support the hypothesis of a highly restricted dispersal and gene flow among Merodon populations between islands since late Pleistocene. Additionally, patterns of phylogeographic structure deduced from haplotype connections and ISSR genome fingerprinting data revealed a few putative cases of human-mediated transfers of Merodon spp.

Genetic diversity and population genetics of mosquitoes (Diptera: Culicidae: Culex spp.) from the Sonoran Desert of North America

The population genetics and phylogenetic relationships of Culex mosquitoes inhabiting the Sonoran Desert region of North America were studied using mitochondrial DNA and microsatellite molecular markers. Phylogenetic analyses of mitochondrial cytochrome c oxidase subunit I (COI) from mosquitoes collected over a wide geographic area, including the Baja California peninsula, and mainland localities in southern Arizona, USA and Sonora, Mexico, showed several well-supported partitions corresponding to Cx. quinquefasciatus, Cx. tarsalis, and two unidentified species, Culex sp. 1 and sp. 2. Culex quinquefasciatus was found at all localities and was the most abundant species collected. Culex tarsalis was collected only at Tucson, Arizona and Guaymas, Sonora. The two unidentified species of Culex were most abundant at Navojoa in southern Sonora. Haplotype and nucleotide diversities in the COI gene segment were substantially lower in Cx. quinquefasciatus compared with the other three species. Analysis of molecular variance revealed little structure among seven populations of Cx. quinquefasciatus, whereas significant structure was found between the two populations of Cx. tarsalis. Evidence for an historical population expansion beginning in the Pleistocene was found for Cx. tarsalis. Possible explanations for the large differences in genetic diversity between Cx. quinquefasciatus and the other species of Culex are presented.

Model-based comparisons of phylogeographic scenarios resolve the intraspecific divergence of cactophilic Drosophila mojavensis

The cactophilic fly Drosophila mojavensis exhibits considerable intraspecific genetic structure across allopatric geographic regions and shows associations with different host cactus species across its range. The divergence between these populations has been studied for more than 60years, yet their exact historical relationships have not been resolved. We analysed sequence data from 15 intronic X-linked loci across populations from Baja California, mainland Sonora-Arizona and Mojave Desert regions under an isolation-with-migration model to assess multiple scenarios of divergence. We also compared the results with a pre-existing sequence data set of eight autosomal loci. We derived a population tree with Baja California placed at its base and link their isolation to Pleistocene climatic oscillations. Our estimates suggest the Baja California population diverged from an ancestral Mojave Desert/mainland Sonora-Arizona group around 230,000-270,000years ago, while the split between the Mojave Desert and mainland Sonora-Arizona populations occurred one glacial cycle later, 117,000-135,000years ago. Although we found these three populations to be effectively allopatric, model ranking could not rule out the possibility of a low level of gene flow between two of them. Finally, the Mojave Desert population showed a small effective population size, consistent with a historical population bottleneck. We show that model-based inference from multiple loci can provide accurate information on the historical relationships of closely related groups allowing us to set into historical context a classic system of incipient ecological speciation.

Phylogeography of Saproxylic and Forest Floor Invertebrates from Tallaganda, South-eastern Australia

The interaction between physiogeographic landscape context and certain life history characteristics, particularly dispersal ability, can generate predictable outcomes for how species responded to Pleistocene (and earlier) climatic changes. Furthermore, the extent to which impacts of past landscape-level changes ‘scale-up’ to whole communities has begun to be addressed via comparative phylogeographic analyses of co-distributed species. Here we present an overview of a body of research on flightless low-mobility forest invertebrates, focusing on two springtails and two terrestrial flatworms, from Tallaganda on the Great Dividing Range of south-eastern Australia. These species are distantly-related, and represent contrasting trophic levels (i.e., slime-mold-grazers vs. higher-level predators). However, they share an association with the dead wood (saproxylic) habitat. Spatial patterns of intraspecific genetic diversity partly conform to topography-based divisions that circumscribe five ‘microgeographic regions’ at Tallaganda. In synthesizing population processes and past events that generated contemporary spatial patterns of genetic diversity in these forest floor invertebrates, we highlight cases of phylogeographic congruence, pseudo-congruence, and incongruence. Finally, we propose conservation-oriented recommendations for the prioritisation of areas for protection.