THE GENETICS OF AN ISOLATING MECHANISM BETWEEN TWO SIBLING SPECIES OF DROSOPHILA

Matings between Drosophila simulans females and males of the sibling species D. mauritiana are of abnormally short duration. These rapid matings interrupt the transfer of sperm, leading to substantial reproductive isolation in interspecific as compared to intraspecific copulations. Genetic analysis of this behavior shows that it is influenced much more by the male than the female genotype, with genes from D. simulans being dominant. In males, shortened copulation is caused by interspecific divergence at a minimum of three loci, with one gene on each of the major chromosomes. This is an underestimate of the true number of loci affecting the trait, which could be much larger. The two autosomes have the largest effect, whereas that of the X chromosome is much smaller. The genetic architecture of copulation duration and the larger effect of male than female genotype suggest that females can detect and discriminate against differences in male genitalia.     

COMPLEX TRADE-OFFS AND THE EVOLUTION OF STARVATION RESISTANCE IN DROSOPHILA MELANOGASTER

The measurement of trade-offs may be complicated when selection exploits multiple avenues of adaptation or multiple life-cycle stages. We surveyed 10 populations of Drosophila melanogaster selected for increased resistance to starvation for 60 generations, their paired controls, and their mutual ancestors (a total of 30 outbred populations) for evidence of physiological and life-history trade-offs that span life-cycle stages. The directly selected lines showed an impressive response to starvation selection, with mature adult females resisting starvation death 4–6 times longer than unselected controls or ancestors—up to a maximum of almost 20 days. Starvation-selected flies are already 80% more resistant to starvation death than their controls immediately upon eclosion, suggesting that a significant portion of their selection response was owing to preadult growth and acquisition of metabolites relevant to the stress. These same lines exhibited significantly longer development and lower viability in the larval and pupal stages. Weight and lipid measurements on one of the starvation-selected treatments (SB_1–5), its control populations (CB_1–5), and their ancestor populations (B_1–5) revealed three important findings. First, starvation resistance and lipid content were linearly correlated; second, larval lipid acquisition played a major role in the evolution of adult starvation resistance; finally, increased larval growth rate and lipid acquisition had a fitness cost exacted in reduced viability and slower development. This study implicates multiple life-cycle stages in the response to selection for the stress resistance of only one stage. Our starvation-selected populations illustrate a case that may be common in nature. Patterns of genetic correlation may prove misleading unless multiple pleiotropic interconnections are resolved.     

RESOURCE ACQUISITION AND THE EVOLUTION OF STRESS RESISTANCE IN DROSOPHILA MELANOGASTER

Resistance to environmental stress is one of the most important forces molding the distribution and abundance of species. We investigated the evolution of desiccation stress resistance using 20 outbred Drosophila melanogaster populations directly selected in the laboratory for adult desiccation resistance (D), postponed senescence (O), and their respective controls (C and B). Both aging and desiccation selection increased desiccation resistance relative to their controls, creating a spectrum of desiccation resistance levels across selection treatments. We employed an integrative approach, merging data on the life histories of these populations with a detailed physiology of water balance. The physiological basis of desiccation resistance may be mechanisms enhancing either resource conservation or resource acquisition and allocation. Desiccation-resistant populations had increased water and carbohydrate stores, and showed age-specific patterns of desiccation resistance consistent with the resource accumulation mechanism. A significant proportion of the resources relevant to resistance of the stress were accumulated in the larval stage. Males and females of desiccation-selected lines exhibited distinctly different patterns of desiccation resistance and resource acquisition, in a manner suggesting intersexual antagonism in the evolution of stress resistance. Preadult viability of stress-selected populations was lower than that of controls, and development was slowed. Our results suggest that there is a cost to preadult resource acquisition, pointing out a complex trade-off architecture involving characters distributed across distinct life-cycle stages.     

IMMUNE RESPONSE TO PARASITISM REDUCES RESISTANCE OF DROSOPHILA MELANOGASTER TO DESICCATION AND STARVATION

Abstract In natural populations, organisms experience simultaneously biotic (e.g., competitors and parasites) and abiotic (e.g., temperature and humidity) stresses. Thus, species must have the capacity to respond to combinations of stressors. How does interaction between biotic and abiotic stress affect organismal performance? To address this question, I studied stress resistance of adult Drosophila melanogaster that survived parasitic attack (as larvae) by the parasitoid Asobara tabida. To determine the impact of genotype on stress resistance, I measured survival under desiccation and starvation of flies within isofemale (genetic) lines. Survivors of parasitism had slightly reduced survivorship compared to unparasitized relatives when both were unstressed, and this difference was exacerbated by desiccation and starvation. These results indicate multiple stressors can compound each other's individual negative effects on fitness. Moreover, isofemale lines differed in their sensitivity to environmental stress and to parasitism. Consequently, genotypic differences in sensitivity to stress may reflect differences in investment priorities between traits that promote survival over other life‐history characters.     

ALTERNATIVE ROUTES TO THE EVOLUTION OF COMPETITIVE ABILITY IN TWO COMPETING SPECIES OF DROSOPHILA

The evolution of components of interspecific competitive ability was examined in three different environments for three replicate pairs of Drosophila melanogaster and D. simulans populations. Populations evolved enhanced competitive ability in three ways: (1) increased effectiveness at reducing the numbers of the competitor species; (2) increased resistance to the inhibitory effects of the competitor species; or (3) a combination of the two. Considerable variation was seen in the evolutionary outcomes of competition among environments, and also among replicate populations within environments. Significant replicate-by-environment interactions were also observed. The results suggest that the evolutionary routes to the evolution of enhanced competitive abilities can potentially differ among subdivided populations, creating a geographic mosaic of outcomes.     

INFLUENCE OF SUCCESSIVE AND COMBINED ULTRAVIOLET A AND B IRRADIATIONS ON MATRIX METALLOELASTASES PRODUCED BY HUMAN DERMAL FIBROBLASTS IN CULTURE

To study the cumulative influence of UV irradiations on skin matrix alterations, human skin fibroblasts were irradiated successively three-fold, at 24h intervals, with UVA (3×5J/cm²), UVB (3×8mJ/cm²), UVA plus UVB (3×5J/cm²and 3×8mJ/cm²) and the levels of 92kDa gelatinase (pro-MMP₉), 72kDa gelatinase (pro-MMP₂) and plasma-membrane elastase type protease were determined, following subsequent 24-h culture in 10% serum-containing medium. UV irradiations had only minor influence (1.4-fold increase for UVB) on secreted levels of pro-MMP₂and decreased the amount of plasma membrane elastase produced by cells. It did however, for UVA and UVB alone, induce a significant increase of 66kDa activated MMP₂production: 2.5- and 1.7-fold respectively. Such enhancement was not observed when combined irradiations were administered. UV exposure possessed a much higher influence on pro-MMP₉secretion by dermal fibroblast enhancing enzyme levels by 2.5-, 6.5- and 5-fold for UVA, UVB and UVA+UVB, respectively.     

DESICCATION AND STARVATION TOLERANCE OF ADULT DROSOPHILA: OPPOSITE LATITUDINAL CLINES IN NATURAL POPULATIONS OF THREE DIFFERENT SPECIES

Desiccation and starvation tolerance were measured along latitudinal transects in three Drosophilid species (Drosophila ananassae, D. melanogaster, and Zaprionus indianus) of the Indian subcontinent. In each case, significant latitudinal clines were observed; desiccation tolerance increased with latitude while starvation tolerance decreased. Such field observations suggest that desiccation and starvation tolerance are fitness related traits that are independently selected in nature and genetically independent. It was, however, difficult to relate these genetic changes with precise climatic variables, except winter temperature. The overall negative correlation between the two traits, which was evidenced in natural populations, contrasts with a positive correlation generally observed in various laboratory selection experiments and that also seems to exist between different species. These observations point to the difficulty of interpreting correlations among fitness-related traits when different evolutionary levels are compared, and also different sets of data, that is, field versus laboratory studies.     

INTERSPECIFIC LABORATORY COMPETITION OF THE RECENTLY SYMPATRIC SPECIES DROSOPHILA SUBOBSCURA AND DROSOPHILA PSEUDOOBSCURA

Abstract.—Drosophila subobscura and D. pseudoobscura are closely related species coexisting on the West Coast of North America, which was recently colonized by D. subobscura. In competition experiments with overlapping generations, D. subobscura is eliminated by D. pseudoobscura in a few generations at all four temperatures and two initial frequencies tested. Yet in one-species cultures, D. subobscura thrives at all experimental conditions. Single-generation competition experiments reveal lower survivorship and productivity of D. subobscura at all temperatures and frequencies. Productivity per female is dependent on the initial frequencies: greater for D. subobscura as its initial frequency becomes higher, but lower for D. pseudoobscura as its frequency becomes higher. Strains of D. subobscura from three disparate geographic origins yield similar results.     

INFLUENCE OF DESICCATION ON THE MECHANICAL PROPERTIES OF IRIDAEA CORDATA (RHODOPHYTA)1

The influence of desiccation on the mechanical properties of the intertidal macroalga Iridaea cordata (Turner) Bory (Rhodophyta) was investigated over a range of water losses (0–83%) that bracketed in situ levels (26–67%). The tissue modulus (stiffness) remained constant for water losses up to about 70%, but increased sharply with losses between 70–83%. Tissue strength of desiccated samples did not fall below the range measured for undesiccated samples. There was a significant increase in breaking strain up to water losses of about 50%, after which breaking strain decreased. The relationship between toughness and desiccation resembled that for breaking strain. Overall, the mechanical properties of Iridaea cordata did not deteriorate when desiccated to levels consistent with those observed in the field.     

EFFECTS OF DESICCATION AND ILLUMINATION ON PHOTOSYNTHESIS AND PIGMENTATION OF AN EDAPHIC POPULATION OF TRENTEPOHLIA ODORATA (CHLOROPHYTA)1

The photosynthetic rates of Trentepohlia odorata (L.) Martius growing on wall surfaces in Singapore changed throughout the day with a maximum in midmorning and decreasing thereafter during the day. Optimum temperature for photosynthesis was 25° C. Different levels of air humidity also affected photosynthetic rates with low relative humidity reducing the rates and efficiency of photosynthesis. Our results suggested that T. odorata was able to maximize its rate of photosynthesis before photoinhibitory light levels were reached and that its growth might be dependent on high levels of atmospheric relative humidity, which may serve as a source of water supply for the alga.     

EVOLUTION OF STARVATION RESISTANCE IN DROSOPHILA MELANOGASTER: ASPECTS OF METABOLISM AND COUNTER-IMPACT SELECTION

An artificial selection experiment for increased female starvation resistance employed five selected lines and five control lines of Drosophila melanogaster. Females responded to selection within the first five generations, but a substantial male response was not observed until starvation resistance was assessed at generation 15. By measuring respiration rate in selected and control lines, it was possible to test the hypothesis that reduced metabolic rate is a general mechanism for stress resistance. There was no association between starvation resistance and respiration rate and thus no support for the hypothesis. Studies using vertebrates have shown that starvation causes a decrease in intermediary metabolism enzyme activity, but this relationship is not well documented in invertebrates. In the present study, intermediary metabolism enzyme activities decreased in response to starvation in control-line females and males, and in selected-line males. However, the selected females showed no overall decrease in enzyme activities in response to starvation. One interpretation is that selected females evolved to resist the phenotypic impact of stress. The concept of “counter-impact selection” is discussed in relationship to the use of phenotypic manipulations for the study of evolution.     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

THE SEX-RATIO TRAIT IN DROSOPHILA SIMULANS: GEOGRAPHICAL DISTRIBUTION OF DISTORTION AND RESISTANCE

The sex-ratio trait we describe here in Drosophila simulans results from X-linked meiotic drive. Males bearing a driving X chromosome can produce a large excess of females (about 90%) in their progeny. This is, however, rarely the case in the wild, where resistance factors, including autosomal suppressors and insensitive Y chromosomes, prevent the expression of the driver. In this study, we searched for drive and resistance factors in strains of Drosophila simulans collected all over the world. Driving X chromosomes were found in all populations whenever a good sample size was available. Their frequency may reach up to 60%. However, the presence of driving X chromosomes never results in an excess of females, due to the systematic co-occurrence of resistance factors. The highest frequencies of driving X chromosomes were observed in islands, while populations from East and Central Africa (the supposed center of origin of the species) showed the highest level of resistance. The geographical pattern of drive and resistance factors, as well as the results of crosses between strains from different geographical areas, suggest that the sex-ratio system described here has a unique and ancient origin in the species.     

COSTS OF COUNTERDEFENSES TO HOST RESISTANCE IN A PARASITOID OF DROSOPHILA

Abstract The ability of a parasitoid to evolve enhanced counterdefenses against host resistance and its possible costs were studied in a Drosophila -parasitoid system. We reared Asobara tabida (Braconidae, Hymenoptera) exclusively on D. melanogaster to impose artificial selection for improved counterdefenses against cellular encapsulation, the main host defense against parasitism. Controls were reared on D. subobscura , the main host of the population of wasps from which the laboratory culture was derived and a species that never encapsulates parasitoids. We observed improved survival and avoidance of encapsulation in all five selection lines compared to their paired control lines, although there was unexpected variation among pairs. Improved survival was associated with parasitoid eggs becoming embedded in host tissue, where they were protected from circulating haemocytes. There were no differences among lines in average adult size, fat content, egg load, or performance on D. subobscura . However, the duration of the egg stage in selection lines was longer than that of control lines, probably because of reduced nutrient and/or oxygen supply when eggs are embedded in host tissue. We suggest that this delay in hatching reduces the probability of parasitoid survival if another parasitoid egg is laid in the same host (superparasitism or multiparasitism) and hence is a cost of enhanced counterdefenses against host resistance.     

PHYLOGENETIC CONSTRAINTS IN KEY FUNCTIONAL TRAITS BEHIND SPECIES’ CLIMATE NICHES: PATTERNS OF DESICCATION AND COLD RESISTANCE ACROSS 95 DROSOPHILA SPECIES

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92–95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate‐to‐strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids’ high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate‐to‐high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.     

四种植物种子萌发及苗期抗旱性差异的研究

对四种豆科植物种子萌发及苗期抗旱性作了比较研究,结果表示：种子萌发期抗旱笥强的植物其苗期抗旱性亦强。干旱引起组织脱水,植株生长受到抑制,幼苗出现衩始萎蔫时间与细胞膜受害时间一致。四种植物显示萎蔫时土壤含水量明显不同,红豆草为４．８０％、羊柴２．２％、花棒１．７４％、柠条１．５１％．据种子相对发芽率、膜透性变化等生理指标综合评定四种植物种子萌发及苗期抗旱能力次序为：柠条＞花棒＞羊柴＞红豆草。     

THE GENETICS OF CENTRAL AND MARGINAL POPULATIONS OF DROSOPHILA SERRATA. II. HYBRID BREAKDOWN IN FITNESS COMPONENTS AS A CORRELATED RESPONSE TO SELECTION FOR DESICCATION RESISTANCE

Multiple-peak epistasis is one of the four premises that underlie Wright's shifting-balance theory of evolution. A selection experiment was conducted in an attempt to push different geographic populations to different fitness peaks as a correlated response to selection for an additively controlled character (desiccation resistance). Four populations of Drosophila serrata, sampled from central and marginal areas of its distribution along a 3000-km stretch of Australia's east coast, underwent selection for desiccation resistance for 14 generations. After selection had ceased, control lines from each of the populations were crossed to determine the amount of hybrid breakdown that existed before selection and selected lines were crossed to determine the amount of hybrid breakdown after selection. Hybrid breakdown was measured in three fitness traits: developmental time, viability, and fecundity. When the individual crosses were examined, virtually no evidence was found for hybrid breakdown between these populations. However, the level of hybrid breakdown in development time in the control lines increased as the distance between the populations in the field increased. This relationship was lost in the selected lines. Therefore, selection for desiccation resistance influenced the level of hybrid breakdown in a fitness trait, although selection may need to be maintained for longer than 14 generations if a new relationship between hybrid breakdown and distance is to be formed.     

THE INFLUENCE OF SEA ANEMONES ON THE MORPHOLOGY AND PRODUCTIVITY OF TWO INTERTIDAL SEAWEEDS1

The intertidal seaweeds Corallina vancouveriensis Yendo and Gelidium coulteri Harv. respond to the experimental removal of associated mats of the clonal anemone Anthopleura elegantissima Brandt with morphological changes that affect their photosynthetic performances. With the removal of anemones, these seaweeds develop a tight compaction of thalli which aids in the retention of moisture during aerial exposure. The morphological response, however, results in greatly reduced net photosynthesis in C. vancouveriensis (–98%) and G. coulteri (–73%) on an areal basis. Association with the clonal anemones allows these seaweeds to maintain a morphology that is more highly productive yet inconsistent with the exposure stresses in many microhabitats at similar tidal levels.