Cactophilic Drosophila Research Articles

Premating Isolation is Determined by Larval Substrates in Cactophilic Drosophila mojavensis

Premating Isolation is Determined by Larval Substrates in Cactophilic Drosophila mojavensis

Genetic Variation in Cactophilic Drosophila for Oviposition on Natural Yeast Substrates

Genetic Variation in Cactophilic Drosophila for Oviposition on Natural Yeast Substrates

Remating effects on the genetic structure of female life histories in populations of Drosophila mojavensis.

A quantitative genetic analysis of nine adult fitness components was performed in two populations of cactophilic Drosophila mojavensis under natural conditions of fermenting cactus and ethanol vapour. Female progeny from 18 sires and 36 dams were treated to a range of six exposure periods to males to assess effects of remating frequency on female fitness. Lifetime fecundity increased with increasing male exposure, but longevity showed an intermediate optimum with temporary exposure to males of 2-4 days. Narrow-sense heritabilities were significant for egg production traits while broad-sense heritabilities were significant for longevity-related traits. Positive genetic correlations between components of fitness were expressed among functionally related traits, e.g. longevity was positively correlated with lifetime fecundity, the number of clutches laid, clutch size, and the number of eggs laid per day. Negative genetic correlations were detected between early and late life fecundity suggesting genetic tradeoffs among components of adult fitness.

Breeding structure of natural populations of Drosophila buzzatii: effects of the distribution of larval substrates

The population structure of several Australian populations of the cactophilic Drosophila buzzatti was investigated, with seasonal samples from two populations analyzed for six polymorphic allozymes. Significant inbreeding throughout the year was detected in both populations, and significant but relatively slight differences in allele frequencies between collections were found in one population. Little significant 2-locus linkage disequilibrium was detected and the variances of linkage disequilibrium coefficients were generally consistent with genetic drift. Individual breeding substrates (rotting cactus cladodes) were collected and the adult flies emerging from them were scored for their allozyme genotypes. The data suggest that approximately ten individuals contribute to the progeny emerging from a rot. The influence of the sizes and distributions of breeding substrates on the maintenance of genetic variation is discussed.

Evolution of the Mojavensis Cluster of Cactophilic Drosophila with Descriptions of Two New Species

The mojavensis cluster of the repleta species group of Drosophila (Drosophilidae: Diptera) consists of three species. One is newly described as D. navojoa. A second species, described here as D. arizonae, replaces D. arizonensis, which has become a junior subjective synonym for D. mojavensis, the third species in the cluster. A phylogeny of the three species is presented, based on chromosomal inversions, morphology, and the ability to produce hybrids. Breakage points are assigned for all inversions, and male genitalia are figured; 186 crosses were made from 225 possible combinations among 15 geographic strains from the southwestern United States, Mexico, and Guatemala. It is confirmed that D. mojavensis and D. arizonae are very closely related and shown that D. navojoa is more distantly related in regard to all criteria. This relationship is supported by the geographical positions of the ancestral gene sequences in each species, which show a sequential northwest movement (D. navojoa----D. arizonae----D. mojavensis) from southern Mexico to southern California and northern Arizona. The relationship is also supported by the fact that D. navojoa breeds in Opuntia cactus, an ancestral behavior, whereas the other two species breed chiefly in Stenocereus cacti, a derived behavior. The possible role of this host plant shift in speciation is discussed.

Ecological aspects of the heritability of body size in Drosophila buzzatii.

The heritability of thorax length in the cactophilic Drosophila buzzatii was determined for flies from each of 10 rotting cactus cladodes. For each rot, emerging flies were used as parents of progeny reared in the laboratory. The methods used were full sib analysis with the parents mated assortatively and also offspring-parent regression. From this, heritabilities were measured for the laboratory environment and for the natural environment of the rotting cladode. For the laboratory environment, h2 = 0.3770 +/- 0.0203 and for the natural environment h2 = 0.0936 +/- 0.0087 within rots and h2 = 0.0595 +/- 0.0123 for a population drawn randomly from different rots. Because of the possibility of genotype-environment interaction between the laboratory and rot environments, the methods of B. Riska, T. Prout and M. Turelli were used to show it is possible that there is no such interaction, but if there is, the above natural heritabilities are approximate lower bounds. These results are related to the general problem of determining heritabilities in nature where it is impractical to measure both parents and progeny in nature. Determining heritability not only in nature but in relation to subdivision into ephemeral patches (cladodes in this case) has an important bearing on natural selection response and to general theories of stabilizing selection proposed to explain the existence of genetic variation. Attempts were made to detect selection by using the size of emerging adults as an indicator of various levels of larval stress. No selection was detected, but the power to do so was very weak. Differences between progeny means from different rots indicated some genetic differences between rots which can be adequately explained by small numbers of founders. This suggests a random fine scale subdivision amounting to FST = 0.1483 +/- 0.0462.

Adult Life Span of Cactophilic Drosophila: Interactions among Volatiles and Yeasts

-Volatile compounds found in rotting cactus tissue were investigated for their effect on the adult life span of cactophilic and generalist Drosophila under axenic conditions and in the presence of cactophilic yeasts. A low concentration of acetone vapors extended the longevity of axenic D. mojavensis females with or without the presence of yeast, but acetone extended the life span of males only in the presence of an acetone-utilizing yeast (Candida sonorensis). Methanol extended the adult life span of both sexes of D. mojavensis only in the presence of a yeast (C. sonorensis) that could use methanol as its sole carbon source. No increase in longevity was observed for flies exposed to low concentrations of propionic acid vapor. The interaction between methanol and C. sonorensis affected only the longevity of D. mojavensis (a cactophilic fly from a host with low levels of free sugars in fresh tissue) and not the longevity of D. buzzatii (a cactophilic fly from a host with higher levels of free sugars) or D. melanogaster (a strain of a generalist fly that was collected from sugary cactus fruit).

Microbial colonization of injured cactus tissue (Stenocereus gummosus) and its relationship to the ecology of cactophilic Drosophila mojavensis.

Necrotic tissue of agria cactus (Stenocereus gummosus) serves as a feeding and breeding substrate for Drosophila mojavensis. This fly species is one of the four endemic Drosophila species in the Sonoran Desert. Freeze injuries were created in arms of agria cactus in Mexico to study the events of microbial colonization. Facultative anaerobic bacteria were the first microbes to be detected, and the exclusion of large arthropods by covering the injuries with netting did not affect bacterial colonization. Yeast growth lagged behind bacterial growth by 2 days, and excluding arthropods delayed the detection of yeasts by an additional 2 days. Thus, insects (such as Drosophila species) and other arthropods do play a role in the colonization of agria rots by yeasts. All injuries were attractive to D. mojavensis within 5 days, and these flies were shown to be carrying significant densities of both bacteria and yeasts. Analysis of the volatile compounds present in the developing rots over time indicated that the volatile pattern is dynamic. Ethanol and acetic acid were the two volatile substances most likely responsible for the initial attraction of the injuries for Drosophila species.

The vectoring of cactophilic yeasts by Drosophila.

At two locations in the Sonoran Desert, yeasts were sampled from species of Drosophila, the flies' cactus hosts, and other neighboring sources of cactophilic yeasts to determine the relation between the yeasts vectored by the fly and the yeasts found in their breeding sites. D. mojavensis, D. nigrospiracula, and D. mettleri vectored yeast assemblages significantly more similar to the yeast species found on the rot from which the flies were collected than to the yeasts found on other rots from the flies host cactus or other rotting cactus at the same site. Rots with Drosophila had fewer yeast species than those without flies, suggesting that flies were associated with younger rots. Rots with flies and the Drosophila also had more yeast species with the capability to produce ethyl acetate than rots without flies. The results support the contention that cactophilic Drosophila feed on a subset of the yeasts available in an area, and may act to maintain differences among the yeast communities found on different species of cactus.

Differential effects of fatty acid chain length on the viability of two species of cactophilic Drosophila

1. 1. Two columnar cacti in the Sonoran Desert, agria and organpipe, contain medium chain (C 8−C 12) fatty acids. 2. 2. Necrotic tissues of these cacti serve as feeding and breeding substrates for Drosophila mojavensis but not D. nigrospiracula. 3. 3. Results show that capric and lauric acids are the predominant fatty acids of both cacti. 4. 4. Fatty acid chain length exhibits a differential effect on larval viability with caprylic acid (Q) having the greatest and myristic acid (C 14) having the least effect. 5. 5. Drosophila mojavensis is more tolerant of free fatty acids than D. nigrospiracula, and this partly explains the ability of D. mojavensis to utilize agria and organpipe cacti.

Adaptations of Drosophila and Yeasts: their Interactions with the Volatile 2-propanol in the Cactus-Micro organism-Drosophila Model System

The interactions of yeasts growing in decaying cactus tissue with and without 2-propanol were studied with respect to the costs and benefits provided to three cactophilic Drosophila species (D. mojavensis, D. arizonensis and D. buzzatii). Two common cactus yeasts, Candida sonorensis and Cryptococcus cereanus, which can tolerate and metabolize 2-propanol, provide benefits to the three Drosophila species in the presence of the alcohol, as compared with another common cactus yeast, Pichia cactophila, which has less tolerance and cannot metabolize 2-propanol. Because 2-propanol is commonly found in decaying cactus tissue and C. sonorensis and Cr. cereanus are also frequently recovered from the rotting tissue being utilized by the Drosophila species, the interactions described here are viewed as a possible adaptation in which the yeast provides benefits to one of its vectors by metabolism of 2-propanol in the habitat.

Longevity of seven species of cactophilic Drosophila and D. melanogaster on carbohydrates

Adults of 6 species of Drosophila that use decaying prickly pear cactus ( Opuntia sp.) as breeding and feeding sites were compared to each other and to D. nigrospiracula, whose host is saguaro cactus, and to the cosmopolitan D. melanogaster, in their utilization of 21 sugars for longevity (time to 50% mortality). In general, the utilization of sugars by these flies for longevity followed the pattern observed with the other insects. None of the species were able to live very long on solutions of pentoses, uronic acids, inositol, rhamnose, sorbose or the β-linked disaccharides, lactose and cellobiose. Althogh all could use glucose, fructose, sucrose, maltose and melezitose well, their life spans on galactose, mannose, trehalose and raffinose were more variable. Two of the Opuntia feeders were also tested on a number of other carbohydrates. Ribitol, mannitol, sorbitol and xylitol significantly prolonged the life of D. arizonensis but not that of D. wheeleri. Neither species lived long on solutions of arabitol, galactitol, starch, inulin or on arabogalactan.

Heterogeneity of the yeast flora in the breeding sites of cactophilic Drosophila

Yeasts were isolated from 278 rots in cladodes and basal stems of Opuntia stricta sampled over 15 months at one locality. The most common yeast species among 370 nonidentical isolates representing 20 species were Candida sonorensis (27% of isolates) and Pichia cactophila (24%). These species generally were present in all months and throughout the area sampled. Other relatively common species (3–9% of alt isolates) had much more restricted distributions in both time and space. The mean number of yeast species per rot was 1.27 ± 0.082, and no yeasts were isolated from 20% of the rots sampled. Significant differences among months or seasons in average number of yeast species per rot were due primarily to lower numbers in the summer and winter months, while differences among rot types resulted mainly from a higher number in the longer persisting basal stem rots. Both hierarchical diversity analysis and analysis of the distributions of the 10 most common yeast species showed temporal effects (month or season) to be the major component contributing to heterogeneity, while microgeographic effects and rot type also were significant. The seasonal and microgeographic heterogeneity are interpreted in terms of longer persisting rots in part of the locality providing a reservoir for yeast species through the winter and are discussed in relation to the maintenance of genetic variation in cactophilic Drosophila that utilize the rots as feeding and breeding sites.

Behavioral Differentiation between Two Species of Cactophilic Drosophila. II. Pupation Site Preference

Behavioral Differentiation between Two Species of Cactophilic Drosophila. II. Pupation Site Preference

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

The 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.