James C. Fogleman

Extinction of Montane Populations of the Northern Leopard Frog (Rana pipiens) in Colorado

Between 1973 and 1982 nine populations of the northern leopard frog in the Red Feather Lakes region of Larimer County, Colorado, failed to reproduce. These failures all resulted in extinction of the populations. One area formerly supporting a population was recolonized in 1980, but no frogs were observed at any of the nine sites in 1981 or 1982. Six of the populations went extinct because the breeding ponds dried up. The remaining populations were small enough to be susceptible to random events, but the nature of these events is unknown.

Coadaptation ofDrosophila and yeasts in their natural habitat

The mutualistic interactions of cactophilicDrosophila and their associated yeasts in the Sonoran Desert are studied as a system which has evolved within the framework of their host cactus stem chemistry. Because theDrosophila-yeast system is saphrophytic, their responses are not thought to directly influence the evolution of the host. Host cactus stem chemistry appears to play an important role in determining where cactophilicDrosophila breed and feed. Several chemicals have been identified as being important. These include sterols and alkaloids of senita as well as fatty acids and sterol diols of agria and organpipe cactus. Cactus chemistry appears to have a limited role in directly determining the distribution of cactus-specific yeasts. Those effects which are known are due to unusual lipids of organpipe cactus and triterpene glycosides of agria and organpipe cactus.Drosophilayeast interactions are viewed as mutualistic and can take the form of (1) benefits to theDrosophila by either direct nutritional gains or by detoxification of harmful chemicals produced during decay of the host stem tissue and (2) benefits to the yeast in the form of increased likelihood of transmission to new habitats. Experiments on yeast-yeast interactions in decaying agria cactus provide evidence that the yeast community is coadapted. This coadaptation among yeasts occurs in two manners: (1) mutualistic increases in growth rates (which are independent of the presence ofDrosophila larvae) and (2) stabilizing competitive interactions when growth reaches carrying capacity. This latter form is dependent on larval activity and results in benefits to the larvae present. In this sense, the coadapted yeast community is probably also coadapted with respect to itsDrosophila vector.

Ecological and Evolutionary Importance of Host Plant Chemistry

In the last two decades, there has been an increasing interest in the inter-disciplinary subject of the chemical ecology of insects. A large part of this subject concerns the chemistry of the interactions between insects and their host plants. The cactus-microorganism-Drosophila model system of the Sonoran Desert provides an excellent opportunity to pursue the subject of chemical ecology in a system which is also amenable to the study of evolutionary and ecological genetics. By examining the chemical interactions between the desert Drosophila and their cactus host plants, insights into aspects of the habitat that impact on the fitness of the flies can be gained. The cactophilic Drosophila in this model system feed and breed in necrotic stems of columnar cacti. In order to do this, the flies must be able to locate suitable rot pockets, assimilate required nutrients, and be able to tolerate whatever toxic compounds might be present in the cactus tissue. Microorganisms which grow in the developing rot serve as a food source for the Drosophila as well as modify the cactus tissue both physically and chemically.

Induction by alkaloids and phenobarbital of Family 4 Cytochrome P450s in Drosophila : evidence for involvement in host plant utilization

Abstract In vertebrates, cytochrome P450s of the CYP2 and CYP3 families play a dominant role in drug metabolism, while in insects members of the CYP6 and CYP28 families have been implicated in metabolism of insecticides and toxic natural plant compounds. A degenerate 3′ RACE strategy resulted in the identification of fifteen novel P450s from an alkaloid-resistant species of Drosophila. The strong (17.4-fold) and highly specific induction of a single gene (CYP4D10) by the toxic isoquinoline alkaloids of a commonly utilized host-plant (saguaro cactus) provides the first indication that members of the CYP4 family in insects may play an important role in the maintenance of specific insect-host plant relationships. Strong barbiturate inducibility of CYP4D10 and two other D. mettleri P450 sequences of the CYP4 family was also observed, suggesting a pattern of xenobiotic responsiveness more similar to those of several vertebrate drug-metabolizing enzymes than to putative vertebrate CYP4 homologs.

REPRODUCTIVE ISOLATION IN SONORAN DESERT DROSOPHILA

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The role of phytosterols in host plant utilization by cactophilicDrosophila.

The Cactus-Drosophila Model System of the Sonoran Desert consists of four endemic species ofDrosophila (D. mojavensis, D. nigrospiracula, D. mettleri andD. pachea) and five species of columnar cacti (agria, organpipe, saguaro, cardón and senita). Extensive collection records indicate that each cactus species has only one species ofDrosophila as the primary resident. The elimination of six of the twenty possible random combinations ofDrosophila species and cactus species can be attributed directly to phytosterols.Drosophila pachea has a strict requirement for Δ7-sterols such as 7-cholestenol and 7-campestenol. Since Δ7-sterols are found only in senita cactus,D. pachea cannot use agria, organpipe, saguaro or cardón as host plants. The lipid fractions of agria and organpipe are chemically similar and contain high concentrations of several 3β,6α-dihydroxysterols. Larval viability tests using chemical constitutents of organpipe cactus demonstrate that the sterol diols are toxic toD. nigrospiracula but not to the resident, species,D. mojavensis. Agria and organpipe are therefore unsuitable as host plants forD. nigrospiracula. These results suggest that phytosterols play a major role in determining host plant utilization by cactophilicDrosophila in the Sonoran Desert.

BEHAVIORAL DIFFERENTIATION BETWEEN TWO SPECIES OF CACTOPHILIC DROSOPHILA III. OVIPOSITION SITE PREFERENCE

Oviposition site preferences of Drosophila nigrospiracula and D. mettleri were determined for cactus versus soaked-soil substrates, light versus dark areas, and upper versus lower position in an attempt to explain the behavioral basis for the separation of their larval niches. The results show that D. nigrospiracula females discriminate oviposition sites mainly on the basis of substrate type, while D. mettleri females use substrate type and position as the primary basis of their site preference. These conclusions are supported by field data. Viability studies showed that D. nigrospiracula larvae cannot survive in the soaked-soil substrate while D. mettleri larvae survive equally well in either substrate. The evolution of soil-breeding is discussed, and the distinction is made between primary and secondary preferences in the case of D. mettleri with the soaked-soil/saguaro ecosystem.

Genotype-specific habitat selection for oviposition sites in the cactophilic species Drosophila buzzatii

Isofemale lines of the cactophilic species, Drosophila buzzatii, exhibit genetic variation for their oviposition response to cactus yeast species in the laboratory. In general, interactions between yeast species preclude the use of pairwise preferences as predictors of preferences in three-way choice experiments. Two isofemale lines with relatively high laboratory preference for ovipositing on the yeast Pichia cactophila (as opposed to Cryptococcus cereanus) and two isofemale lines with relatively low preference for P. cactophila were used in a series of field release experiments to determine if laboratory preferences were also realized under field conditions. The influence of yeast species on both settling behaviour (long-distance response) and oviposition preference (short-distance response) were tested. The four lines were identical in their settling behaviour, preferring P. cactophila. The analysis of the oviposition preference tests showed significant line effects which correlated with the laboratory results. Thus a genetic component for oviposition preference under laboratory and field conditions was demonstrated and this strengthens the evidence for genotype-specific habitat selection in D. buzzatii. One low line, however, did not differ significantly from the two high lines under field conditions. A laboratory retest of this low line showed that the laboratory preference had not changed. The reason for the difference in the two situations is unknown but undoubtedly is attributable to uncontrolled variables under the field situation. Settling behaviour and oviposition response, in general, appear to be proximately linked to differences in the volatiles produced by the different yeast species.

Comparisons of yeast florae from natural substrates and larval guts of southwestern Drosophila

SummaryThe yeast florae in the natural substrates of four desert and three non-desert Drosophila species were compared both qualitatively and quantatively to the yeast present in the guts of Drosophila larvae living in those substrates. The desert species breed in rotting cacti and the other Drosophila were found breeding in necrotic oranges. Larvae of one cactophilic species, D. mojavensis, and larvae of all of the species utilizing oranges (D. melanogaster, D. pseudoobscura, and D. arizonensis) were found to contain non-random samples of the yeasts available in their respective substrates. Larval preference behavior is most likely responsible for these differences. The other cactophilic Drosophila (D. nigrospiracula, D. mettleri, and D. pachea) did not exhibit significant differences when the yeast florae of their larvae and substrates were compared. Selective feeding by larvae appears to be related to the degree of polyphagy in that only larvae of polyphagous species are selective. Trade-off between generalism and specialism at two biological levels is discussed.

The Molecular Basis of Adaptation in Drosophila

In the field of population genetics, genetic variation is of primary importance because it serves as the basis for evolutionary adaptation. Adaptation, another primary concern in population genetics, and one where Bruce Wallace has made numerous scientific contributions, refers to the integration of the phenotype into its environment (Hartl and Clark, 1989). With the advent of the techniques of molecular genetics, we are now able to look at genetic variation at the DNA level. Although this higher resolution provides a wealth of information and, in some cases, can shed light on previously intractable questions, it is certainly one step more removed from the phenotype. It also can quickly become overwhelming: regulatory variation versus variation in structural genes, and the complexity of mapping genotypes into phenotypes especially for quantitative traits, all in order to get at the genetic basis of an adaptation, seems impossibly intricate. The question then is, how can we best make progress in this scientific arena? One reasonable solution is to very carefully select an amiable system in which to work. Such a system should involve an organism that is genetically tractable and genes whose function in a natural environment can be investigated.