Larval selectivity for yeast species by Drosophila mojavensis in natural substrates

The yeast flora found in the major substrates of Drosophila mojavensis and in larval guts was studied both qualitatively and quantitatively. Quantitative studies show that, in four of the five substrates tested, the larvae did not contain a random sample of the yeasts present in the substrates. One widely distributed cactus yeast, Pichia cactophila, was typically in greater frequency in the larvae than in the substrates. Another cactus yeast, Candida sonorensis, typically exhibited the opposite relationship. Laboratory tests support larval preference behavior rather than differential digestion as being primarily responsible. Larvae are capable of distinguishing between patches of different yeast species and spend more time in patches of preferred yeasts. D. mojavensis appear to be ecological (host plants) generalists and physiological (yeasts) specialists in contrast to the other cactophilic Drosophila. Selective feeding by D. mojavensis larvae in natural substrates may represent optimal foraging behavior.

Hierarchical structure in the Drosophila mojavensis cluster (Diptera: Drosophilidae)

Sequences from three gene regions from the nuclear and mitochondrial genomes were used to examine the extent and nature of hierarchical structure in the Drosophilamojavensis cluster (Drosophila arizonae, D. mojavensis and D. navojoa) of the D. repleta species group. To determine the genetic divergence of these three species, sequence data were analyzed using maximum parsimony and population aggregation analysis. Individual and combined gene genealogies indicate that D. arizonae and D. mojavensis are neither diagnosable nor monophyletic with respect to one another. Although D. navojoa has differentiated from D. arizonae and D. mojavensis, as diagnosed by nuclear gene sequences, it may have undergone a reticulation event with D. arizonae. Our results suggest that either these taxa are still undergoing differentiation at the molecular level or have experienced gene flow in the recent past.

Genetic divergence within the Drosophila mayaguana subcluster, a closely related triad of Caribbean species in the repleta species group

The mayaguana triad of the Drosophila repleta species group includes D. mayaguana, D. straubae, and D. parisiena, the latter two of which are very similar when examined morphologically. Many morphological characters used to define these taxa are quantitative and overlap substantially among some forms--it is only through suites of such characters that species can be identified. We apply Population Aggregation Analysis and tree building methods to five rapidly evolving gene regions--the mitochondrial AT rich region and the nuclear acetylcholinesterase, hunchback, mastermind, and vestigial loci to test the morphological species delineations within the morphocryptic mayaguana triad. We find that D. mayaguana is diagnosable using DNA sequences, but the other two species form a non-diagnosable paraphyletic assemblage. A single ecological factor, oviposition substrate, is an important diagnostic character distinguishing D. straubae from D. parisiena, highlighting the importance of examining a diverse array of data (morphological, molecular, ecological, and behavioral) when defining species limits.

Phylogenetic analysis of the repleta species group of the genus Drosophila using multiple sources of characters

The species in the repleta group of the genus Drosophila have been placed into five subgroups-the mulleri, hydei, mercatorum, repleta, and fasciola subgroups. Each subgroup has been further subdivided into complexes and clusters. Extensive morphological and cytological analyses of the members of this species group have formed the foundation for the proposed relationships among the members of the repleta species group. Fifty-four taxa, including 46 taxa belonging to the repleta species group, were sequenced for fragments of four genes-16S ribosomal DNA (16S), cytochrome oxidase II (COII), and nitrogen dehydrogenase 1 (ND1) of the mitochondrial genome and a region of the hunchback (hb) nuclear gene. We also generated a partial data set of elongation factor 1-alpha (Ef1alpha) sequences for a subset of taxa. Our analysis used both DNA characters and chromosomal inversion data. The phylogenetic hypothesis we obtained supports many of the traditionally accepted clades within the mulleri subgroup, but the monophyly of taxonomic groups outside of this subgroup appears not to be supported. Phylogenetic analysis revealed one well-supported, highly resolved clade that consists of closely related members of the mulleri and buzzatii complexes. The remaining taxa, a wide assortment of taxonomic groups, ranging from members of other species groups to members of several subgroups and members of three species complexes from the mulleri subgroup are found in poorly supported arrangements at the base of the tree.

Ultrastructural examination of the insemination reaction in Drosophila

The insemination reaction is a swelling of the female vagina caused by the male ejaculate. This postmating phenomenon is common among species in the genus Drosophila. It could act as a plug securing male paternity. It is not clear, however, what benefits it provides to the female. The structure formed in the female vagina is expelled in some species and disappears gradually in others suggesting different phenomena. Based on ultrastructural examination of the vaginal contents of five Drosophila species (D. mettleri, D. nigrospiracula, D. melanogaster, D. mojavensis, and D. hexastigma), we propose three terms to describe these vaginal structures: the sperm sac, the mating plug, and the true insemination reaction. Each term describes a distinct structure associated with a specific female postmating behavior. This study questions the concept of the insemination reaction as a single phenomenon and discusses its possible functions from an evolutionary perspective.

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.

Four decades of inversion polymorphism in Drosophila pseudoobscura

We report data that continue the studies of Dobzhansky and others on the frequencies of third-chromosome inversions in natural populations of Drosophila pseudoobscura in North America. The common gene arrangements continue to be present in frequencies similar to those described four decades ago, and the broad geographic patterns also remain unchanged. There is only one pronounced trend over time: the increase in frequency of the Tree Line inversion in Pacific coast populations.

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.

Relevance of the ecology ofCitrus yeasts to the diet ofDrosophila

A study of the yeast flora of necrotic oranges and associatedDrosophila yielded a total of 221 isolates composed ofKloeckera apiculata (75),Pichia fermentans (75),Pichia kluyveri (50),Torulopsis stellata (17),Hanseniaspora uvarum (2),P. membranaefaciens (1), andCandida vini (1). The yeast species of all samples of oranges and adultDrosophila were very similar; however, the speciesof Drosophila contained a higher proportion ofP. fermentans and a lower proportion ofK. apiculata than was found in the rotting oranges.P. fermentans was subsequently found more frequently on the surface of the necrotic tissue, where the flies feed, than was found internally. SinceP. fermentans characteristically produces a pellicle and pseudomycelium andK. apiculata does not, it is concluded that the growth characteristics of the yeasts are an important factor determining adultDrosophila diets.

Extension of longevity in Drosophila mojavensis by environmental ethanol: differences between subraces

Drosophila mojavensis adults, which breed and feed on necrotic cacti, show an increase in longevity when exposed to atmospheric ethanol. The increase in longevity is accompanied by retention of mature ovarioles and is independent of diet. Differences in longevity among strains from different localities were detected for females. Strains from Arizona and Sonora, Mexico, showed the greatest increase in longevity, while strains from Baja California, Mexico, showed the least increase. These differences may be controlled by the alcohol dehydrogenase locus, the octanol dehydrogenase locus, and modifier genes, because the aduld response is correlated with the frequency of alcohol dehydrogenase alleles, as well as second chromosomal inversions containing the octanol dehydrogenase locus. The longevity response is also consistent with the more uneven distribution and availability of the host plant in Arizona and Sonora, Mexico. Strains from Arizona and Sonora, Mexico, have a high frequency of Adh-S, the allele whose product is heat and pH tolerant. The host plant, organpipe cactus, exhibits extremes in temperature and pH in the same geographic region. Strains from Baja California, Mexico, possess a high frequency of Adh-F, whose product is heat and pH sensitive. The substrate in this region, agria cactus, has moderate temperature and pH extremes and contains relatively high concentrations of isopropanol. Isopropanol is presumable a selective agen favorable to Adh-F. The environmental heterogeneity that is proposed for maintaining the alleles at the alcohol dehydrogenase locus is the interaction of substrate alcohol content with temperature and pH. Substrates that do not contain appreciable amounts of isopropanol and are exposed to high temperatures and exhibit variable pH favor Adh-S, while substrates containing isopropanol and having moderate temperatures and pH favor Adh-F.

The ecology of yeast flora associated with cactiphilic Drosophila and their host plants in the Sonoran desert

Yeasts were isolated from the rotting stems of 7 species of cereoid cacti and 4 species ofDrosophila which utilize them as host plants. The yeast most common among 132 nonidentical isolates from the cacti and 187 nonidentical isolates from the flies, respectively, were:Pichia membranaefaciens (59 and 126),Candida ingens (22 and 8),Torulopsis sonorensis (16 and 20), andCryptococcus cereanus (11 and 14). Isolates capable of utilizingD-xylose were recovered primarily fromD. pachea andL. schotti. Adult flies were present on the substrates whenP. membranaefaciens was at high concentrations. As the pH of the substrates increased, the percent ofC. ingens cells increased relative to other yeast species. Larvae were detected mainly in alkaline substrates, and since adults did not yieldC. ingens to the extent the substrates did,C. ingens may be important in larval nutrition.Torulopsis sonorensis was recovered mainly fromD. mojavensis and its host plants,M. gummosus andL. thurberi. The concentration ofT. sonorensis in the substrates was negatively correlated with the temperature of the substrate.Cryptococcus cereanus was found in high concentrations in suitable tissues for adult flies but most adults did not yield this species to any extent. The yeast habitat diversities from the substrates had the following order:L. thurberi > C. gigantea > C. gigantea soils ≫ M. gummosus > L. schotti > others. Habitat diversity is discussed in relation to the variation of the physical conditions and chemical composition of the substrates. The yeast habitat diversities from the flies had the orderD. pachea > D. mojavensis ≫ D. nigrospiracula > undescribed Species M. The degree of habitat diversity is possibly a function of the surface feeding behavior of the flies.

Allozyme genotype-environment relationships. I. Variation in natural populations of Drosophila pachea

Remarkable geographic and seasonal uniformities in gene and genotype frequencies were found for four enzyme loci. Principal component analysis revealed two patterns of allele frequency changes in three of the allozyme loci (Est-b, Est-c and Acph) and one pattern in the Mdh-d locus. These patterns accounted for 90% to 100% of the variability at these loci. Significant positive correlations were obtained between several of the components and the variability in the non-saponifiable and alkaloid fractions of the senita cactus host plant. The genetic variance within a single locality (Guaymas, Mexico) was as great as the variance between all localities sampled. This is interpreted as a local founder effect. The temporary nature of the rotting cactus breeding site, the delayed maturation and the differential maturation rate of females and males are three factors proposed to cause forced outbreeding and counter the local founder effect. A combination of selection resulting from a direct interaction of the genotype and the environment, in addition to migration, is proposed to maintain the uniform gene frequencies.