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.     

Phylogenetic Relationships among DROSOPHILA LONGICORNIS, DROSOPHILA PROPACHUCA and DROSOPHILA PACHUCA, a Triad of Sibling Species

Drosophila longicornis, D. propachuca and D. pachuca comprise a triad of sibling species. They are morphologically indistinguishable, sympatric forms that, under laboratory conditions, are capable of exchanging genes through the production of fertile F₁ females. However, we have no evidence for introgressive hybridization in nature. The chromosomal constitution of our strains indicates that the ancestral species had the Primitive E gene sequence, and therefore differed from the standard repleta sequence by being Xabc; 2abcg; 3abc. This Primitive E sequence is found in both D. propachuca and D. longicornis. Each of these two species has its own unique rearrangements. D. pachuca is a derived species, which evolved from propachuca. It is cytologically more advanced and has, as its most primitive gene arrangement, one of the more advanced arrangements found in propachuca.     

Microgeographic Differentiation of Chromosomal and Enzyme Polymorphisms in DROSOPHILA PERSIMILIS

We studied microgeographic and temporal genetic differentiation in natural populations of Drosophila persimilis with respect to chromosome inversion and enzyme polymorphisms. Both inversion frequencies and allozyme frequencies varied significantly over short distances. Neither differed significantly between morning and evening collections. Because several studies of the dispersal behavior of this species have been performed, we attempt to fit the observed data to mathematical models which relate dispersion to random genetic drift and to spatially varying selection coefficients. We conclude that the observations are due at least partly to behavioral differences among genotypes. i.e., habitat preferences. These results have implications for genetic load theory and models of selection in heterogeneous environments.     

Genic Variation in Abundant Soluble Proteins of DROSOPHILA MELANOGASTER and DROSOPHILA PSEUDOOBSCURA

Genic variation was surveyed for 20 proteins of Drosophila melanogaster and 18 proteins of D. pseudoobscura. Analysis was by extraction and one-dimensional polyacrylamide gel electrophoresis under nondenaturing conditions, followed by staining with Coomassie Brilliant Blue to detect soluble proteins present in relatively large amounts ("abundant soluble proteins"). D. melanogaster was polymorphic for 65% of its protein loci and an individual was heterozygous for 10% of its loci. The respective figures for D. pseudoobscura were 61% and 11%. These estimates of genic variation fall between previously published estimates obtained for these species by one-dimensional electrophoresis of soluble enzymes and those obtained by two-dimensional electrophoresis of solubilized abundant proteins. However, variation for both species could be strongly partitioned between loci, on the basis of tissue and stage expression of the proteins. The results are discussed with respect to their bearing on the possibility that abundant proteins constitute a distinct class of proteins less polymorphic than soluble enzymes.     

Variation in Biochemical Properties of Allozymes of Xanthine Dehydrogenase in DROSOPHILA PSEUDOOBSCURA

Twenty-six D. pseudoobscura strains isogenic for xanthine dehydrogenase alleles from Mesa Verde, Colorado, were tested for differences in the biochemical properties of different allelic forms of xanthine dehydrogenase. No significant differences in binding affinity (K_m) or substrate specificity of the enzyme were found. Significant variation among strains, in activity (V_max) and among electromorphs, as well as among strains, in thermolability was found. For the few strains tested, the activity and thermolability differences were shown to co-segregate with the electrophoretic mobility of the variant allele.     

GENERAL-PURPOSE GENOTYPES FOR HOST SPECIES UTILIZATION IN A NEMATODE PARASITE OF DROSOPHILA

The nematode Howardula aoronymphium parasitizes several species of mushroom-feeding Drosophila. A survey of isofemale strains of H. aoronymphium and a 25-generation selection experiment revealed that this species does not comprise host races, and that it harbors little heritable variation for adaptation to specific hosts No tradeoffs in performance on the different host species were evident. General-purpose genotypes, which can utilize all host species, characterize H. aoronymphium. An important feature of the natural history of these nematodes—correlated epidemiology across host species—is postulated to be both a cause and a consequence of the evolution of general-purpose genotypes in this species.     

Genome-wide analysis of ruminant Staphylococcus aureus reveals diversification of the core genome

Staphylococcus aureus causes disease in humans and a wide array of animals. Of note, S. aureus mastitis of ruminants, including cows, sheep, and goats, results in major economic losses worldwide. Extensive variation in genome content exists among S. aureus pathogenic clones. However, the genomic variation among S. aureus strains infecting different animal species has not been well examined. To investigate variation in the genome content of human and ruminant S. aureus, we carried out whole-genome PCR scanning (WGPS), comparative genomic hybridizations (CGH), and the directed DNA sequence analysis of strains of human, bovine, ovine, and caprine origin. Extensive variation in genome content was discovered, including host- and ruminant-specific genetic loci. Ovine and caprine strains were genetically allied, whereas bovine strains were heterogeneous in gene content. As expected, mobile genetic elements such as pathogenicity islands and bacteriophages contributed to the variation in genome content between strains. However, differences specific for ruminant strains were restricted to regions of the conserved core genome, which contained allelic variation in genes encoding proteins of known and unknown function. Many of these proteins are predicted to be exported and could play a role in host-pathogen interactions. The genomic regions of difference identified by the whole-genome approaches adopted in the current study represent excellent targets for studies of the molecular basis of S. aureus host adaptation.     

Genetic Analysis of Phototactic Behavior in DROSOPHILA SIMULANS

Phototaxis mazes have been employed to select photopositive and photonegative strains of Drosophila simulans. The results suggest that phototactic behavior in D. simulans, as in other Drosophila species, is a polygenic trait. Hybridization using divergent strains revealed that the genes controlling negative phototactic behavior in D. simulans are autosomal, as opposed to D. melanogaster in which negative phototactic behavior is known to be very strongly sex-linked.     

GENETIC AND ENVIRONMENTAL DETERMINANTS OF FOOD PREFERENCE IN DROSOPHILA TRIPUNCTATA

Mark-release-recapture field experiments involving two isofemale strains of Drosophila tripunctata revealed that strain identity strongly and consistently affected the preferences of both males and females for mushrooms versus tomatoes. Females, but not males, showed an augmented preference for the type of food on which they had been kept prior to release. The behavior of F2 flies from reciprocal crosses between the two strains demonstrated that genetic variation for food preference is autosomal and largely additive. Because mating often occurs in the vicinity of food in the wild, positive assortative mating with respect to genes for food preference may lead to greater phenotypic variance in preference, which could increase the variety of food resources used by a population.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

Nonrandom Association between Structural Amy and Regulatory map Variants in DROSOPHILA MELANOGASTER

Populations of Drosophila melanogaster were investigated for variation in structural Amy genes, coding for different electrophoretic variants, and regulatory genes that determine the tissue-specific production patterns of α-amylase in the midguts of adults and larvae. Analysis of strains homozygous for second chromosomes extracted from three cage populations of different geographical origin revealed a consistent nonrandom association between Amy and midgut activity pattern (map) variants of α-amylase in adults and third-instar larvae. The origin and maintenance of the linkage disequilibrium between Amy and map genes are discussed.     

Genetic Control of Adh Expression in DROSOPHILA MELANOGASTER

Natural variants displaying different levels of expression of the gene for alcohol dehydrogenase (Adh) were subjected to genetic mapping experiments. The strains studied carry one of the two common electrophoretic forms of the enzyme. The difference among Adh-fast strains appears to be due to multiple loci with trans-acting effects. Differences among Adh-slow strains are due to modifiers or quantitative sites located very close to the structural gene (less than 0.05 map unit) or part of it. The modifiers detected in the Adh^s strains seem to operate only on the structural allele in the cis-position.—A modifier that affects the ratio of ADH levels in larvae and adults was also detected in the Adh^s strains. This modifier is also closely linked to Adh and is cis-acting.     

Spatial variation in host preference in the endangered epiphytic bromeliad Tillandsia carlos-hankii

Although most vascular epiphytes are generalist, some exhibit host preferences. The latter may vary spatiotemporally driven by variation in environmental conditions. However, to our knowledge no previous studies have evaluated this variation. The aim of this study was to analyze variation in host preferences and population structures in the endangered bromeliad Tillandsia carlos-hankii in two pine-oak forests which differ in environmental conditions, tree composition and community structure. Our questions were: i) does Tillandsia carlos-hankii prefer certain host species over others?; ii) does this preference vary between sites?; iii) how does population structure vary between hosts species and sites? The two sites chosen were an east-facing (I_NE) and a west-facing slope (C_NW). The tree community was sampled within an area of 0.1 ha at each site. Preferred host species were identified using two criteria: a) the degree of colonization; and b) the probability of colonization. In addition, population size-structure on each colonized tree was evaluated to compare between sites and host species. The results showed that almost every tree in the community was colonized by this bromeliad. At both sites Quercus rugosa was a preferred host species, while Q. laurina and Mirsyne juergensenii were preferred at I_NE but limiting at C_NW. In turn, Pinus teocote was limiting at both sites. The percentage of small trees that were colonized depended on species identity. Population structures differed between sites and host species. Thus, both host preferences and local population behavior vary spatially given the differential conditions associated with the two sites.     

GENETIC POPULATION STRUCTURE OF DROSOPHILA TRIPUNCTATA: PATTERNS OF VARIATION AND COVARIATION OF TRAITS AFFECTING RESOURCE USE

Drosophila tripunctata is an ecological generalist, using both fruits and mushrooms as breeding sites. Isofemale strains of this species were established from seven populations over a wide part of its range and assayed for electrophoretic variability, oviposition-site preference, and larval performance on several types of substrates. Significant variation among strains within populations was found for oviposition-site preference, larval development time on tomatoes versus mushrooms, and tolerance (as measured by development time) of the mushroom toxin α-amanitin. Even populations at the periphery of the range, which electrophoretic data suggest have been through bottlenecks, harbored levels of variation for oviposition-site preference approximately equal to that found in central populations. All correlations between preference and various measures of larval performance were close to zero. Thus, there is no evidence for sympatric divergence of host races or for coadapted complexes of genes related to host specificity. Strains with higher-than-average amanitin tolerance tended to develop more rapidly on tomatoes than on nontoxic mushrooms, whereas the less-tolerant strains had slower development on tomatoes. This suggests that there may be genetically based correlations and trade-offs in larval performance on different breeding sites. No geographic differentiation among populations was found for either oviposition-site preference or any measure of larval performance. There is also very little electrophoretic variation among populations. Thus, the species as a whole, rather than local populations, appears to be the unit of evolution with respect to resource use in D. tripunctata.     

Distinctions among Allelic Variants Associated with Chromosome 3 Inversions in DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS

Efforts were made to discriminate new genetic variants among electrophoretic alleles that are associated with chromosome 3 inversions of Drosophila pseudoobscura and D. persimilis. Apparent genetic similarities for electrophoretic alleles between these two species and among the common inversions they carry were reexamined by altering gel concentration and buffer pH. At the amylase locus, the 1.09 electrophoretic allele could be further separated into two allelic classes that differentiated the WT and KL arrangements. Similarly, the 0.84 electrophoretic allele was divided into two allelic classes, one characteristic of the Santa Cruz phylad arrangements, TL and SC, and the other found in strains of the Standard phylad arrangements and CH. Uncommon amylase alleles proved to be different alleles in the two species. No new allelic variants, however, could be found among strains with the amylase 1.00 allele, the commonest allele in the Standard phylad of both species. No major new allelic variation was detected for acid phosphatase-3 and larval protein-10 that revealed any further differentiation among species or inversions. Variation at all three loci in strains of the Bogota population remained genetically similar to variation in strains of mainland D. pseudoobscura.     

Estimation of Genetic Variability in Natural Populations of DROSOPHILA SIMULANS by Two-Dimensional and Starch Gel Electrophoresis

Genic variation in natural populations of Drosophila simulans was surveyed using allozymic and two-dimensional electrophoretic techniques. Consistent with some previous reports, allozymic heterozygosity appeared lower than in the sibling species D. melanogaster (0.07 vs. 0.16). No variation was detected by two-dimensional electrophoresis of 19 lines scored for 70 abundant proteins. This is consistent with reported reductions in estimates of genic heterozygosity by two-dimensional electrophoresis in D. melanogaster, Mus musculus, and man. Although the amount of intraspecific variation detected in abundant proteins was lower than that detected for allozymes in D. simulans and D. melanogaster, the genetic distances between the sibling species calculated from the two data sets are not significantly different (0.35 and 0.20). The allozyme and two-dimensional electrophoresis data confirmed the impression from other measures of genetic variation (mitochondrial DNA restriction maps and inversion polymorphisms) that D. simulans is substantially less variable than D. melanogaster.     

Genic Heterogeneity at Two Alcohol Dehydrogenase Loci in DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS

A sequential electrophoretic survey of the second chromosome loci, alcohol dehydrogenase-6 (Adh-6) and octanol dehydrogenase ( Odh), was performed on 147 isochromosomal lines of Drosophila pseudoobscura and 60 lines of its sibling species, D. persimilis. Gels run with a variety of acrylamide concentrations and buffer pH's revealed the presence of 18 alleles of Adh-6 in the two species, where only eight had been previously detected by conventional electrophoretic methods. Only two alleles were added with our techniques to the previous total of nine in both species at the largely monomorphic Odh locus. Both enzymes show a predominance of one allele, with the other variants being fairly rare. There was no evidence of increased genetic divergence between the two species, but we found a striking increase in differentiation of Adh-6 alleles between the main body of D. pseudoobscura populations and the conspecific isolate from Bogotá, Colombia. These results are compared with our previous surveys of xanthine dehydrogenase in these species and discussed in reference to theories of genic polymorphism.     

ADAPTATION AND SPECIALIZATION IN A TWO-RESOURCE ENVIRONMENT IN DROSOPHILA SPECIES

We assayed two components of performance (development time and survivorship), on food medium with and without ethanol, in laboratory populations of Drosophila simulans and D. melanogaster for which ethanol-medium was a novel food resource. These assays were done before and after 12 generations of rearing in either one-(regular medium only) or two-resource (regular medium and ethanol medium) environments. Initially, D. simulans was highly susceptible to ethanol, whereas D. melanogaster was relatively unaffected. After 12 generations in the two-resource environment, D. simulans showed significantly improved mean performance on ethanol medium; mean performance of D. melanogaster did not significantly change. Variation among families for both traits was higher on ethanol medium in D. simulans. Variation in D. melanogaster was not significantly affected by ethanol level, suggesting that resource quality was more important than novelty per se. In both species, the least variation was seen in populations after 12 generations in the two-resource environment. For development time in D. simulans, the decrease in variation was largely due to reduced variation within families, suggesting the evolution of canalization. Development time on the two media was not negatively correlated. In D. simulans, correlations measured before and after the experiment were not heterogeneous, suggesting that trade-offs in performance did not block diet expansion. In D. melanogaster, correlations became significantly less positive after 12 generations in the two-resource environment, supporting the view that correlations between performance on different resources may become less positive over time through selection.     

PUPARIATION SITE PREFERENCE WITHIN AND BETWEEN DROSOPHILA SIBLING SPECIES

Holometabolous insects pass through a sedentary pupal stage and often choose a location for pupation that is different from the site of larval feeding. We have characterized a difference in pupariation site choice within and between sibling species of Drosophila. We found that, in nature, Drosophila sechellia pupariate within their host fruit, Morinda citrifolia, and that they perform this behavior in laboratory assays. In contrast, in the laboratory, geographically diverse strains of Drosophila simulans vary in their pupariation site preference; D. simulans lines from the ancestral range in southeast Africa pupariate on fruit, or a fruit substitute, whereas populations from Europe or the New World select sites off of fruit. We explored the genetic basis for the evolved preference in puariation site preference by performing quantitative trait locus mapping within and between species. We found that the interspecific difference is controlled largely by loci on chromosomes X and II. In contrast, variation between two strains of D. simulans appears to be highly polygenic, with the majority of phenotypic effects due to loci on chromosome III. These data address the genetic basis of how new traits arise as species diverge and populations disperse.     

Tooth Size Variation in Pinniped Dentitions

It is contentious whether size variation among mammalian teeth is heterogeneous or homogeneous, whether the coefficient of variation is reliable, and whether the standard deviation of log-transformed data and the residual of standard deviation on mean variable size are useful replacements for the coefficient of variation. Most studies of tooth size variation have been on mammals with complex-crowned teeth, with relatively little attention paid to taxa with simple-crowned teeth, such as Pinnipedia. To fill this gap in knowledge and to resolve the existing controversies, we explored the variation of linear size variables (length and width) for all teeth from complete permanent dentitions of four pinniped species, two phocids (Histriophoca fasciata, Phoca largha) and two otariids (Callorhinus ursinus, Eumetopias jubatus). Size variation among these teeth was mostly heterogeneous both along the toothrow and among species. The incisors, canines, and mesial and distal postcanines were often relatively highly variable. The levels of overall dental size variation ranged from relatively low as in land carnivorans (Phoca largha and both otariids) to high (Histriophoca fasciata). Sexual size dimorphism varied among teeth and among species, with teeth being, on average, larger in males than in females. This dimorphism was more pronounced, and the canines were larger and more dimorphic relative to other teeth in the otariids than in the phocids. The coefficient of variation quantified variation reliably in most cases. The standard deviation of log-transformed data was redundant with the coefficient of variation. The residual of standard deviation on mean variable size was inaccurate when size variation was considerably heterogeneous among the compared variables, and was incomparable between species and between sexes. The existing hypotheses invoking developmental fields, occlusal complexity, and the relative timing of tooth formation and sexually dimorphic hormonal activity do not adequately explain the differential size variation along the pinniped toothrow.