Developmental time and size-related traits in Drosophila buzzatii along an altitudinal gradient from Argentina

Clinal analysis for fitness-related traits provides a well-known approach to investigate adaptive evolution. Several fitness-related traits (developmental time, thorax length, wing length and wing loading) were measured at two laboratory generations (G7 and G33) of D. buzzatii from an altitudinal gradient from northwestern Argentina, where significant thermal differences persist. Developmental time (DT) was positively correlated with altitude of origin of population. Further, DT was negatively correlated with maximal mean temperature at the site of origin of population, and this thermal variable decreases with altitude. Wing loading tended to be larger in highland than in lowland populations, suggesting that flight performance is subject to stronger selection pressure in highland populations. Developmental time showed a significant increase with laboratory generation number. There was no significant correlation between developmental time and body size across populations along the altitudinal cline of DT. This result illustrates that developmental time and body size do not always evolve in the same direction, even though both traits are often positively and genetically correlated in a well-known tradeoff in Drosophila.     

An alkaloid fraction extracted from the cactus Trichocereus terscheckii affects fitness in the cactophilic fly Drosophila buzzatii (Diptera: Drosophilidae)

The host‐plant environment of phytophagous insects directly affects various aspects of an insect's life cycle. Interestingly, relatively few insect groups have specialized in the exploitation of plants in the Cactaceae family, potentially because of the chemical and ecological challenges imposed by these plants. The cactophilic Drosophila buzzatii Patterson & Wheeler, 1942 is a well‐studied model in evolutionary ecology, partially because of its ability to exploit toxic cactus hosts. Previous studies have shown a negative effect on performance when flies are reared in an alternative columnar cactus host of the genus Trichocereus, relative to its primary cactus host, Opuntia. These observations were attributed to the presence of alkaloids in Trichocereus tissues, a chemical deterrent to herbivores that indirectly affects Drosophila larvae; however, the putative toxic effect of alkaloids has never been tested directly in D. buzzatii. The present study is the first attempt to relate chemical extracts in Trichocereus terscheckii Britton & Rose, 1920 with detrimental effects on D. buzzatii. We assessed the effects of a crude alkaloid extract, rich in phenylethylamines, and a ‘non‐alkaloid fraction’ on viability and adult wing morphology. Our results indicate that rearing larvae on an artificial diet containing different concentrations of the crude alkaloid extract decreased pupal viability and adult size in a concentration‐dependent manner. We discuss the role of cactus alkaloids in the evolution of host‐plant use in cactophilic flies. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 342–353.     

Genetic variation for resistance and acclimation to high temperature stress in Drosophila buzzatii

Genetic variation for resistance to a high temperature stress under saturated humidity was examined within and among three Drosophila buzzatii populations from Australia. Further, the acclimation of this species to high temperatures was tested by prelreating flies for a shorter, sublethal, time period under conditions that lead to expression of heat shock proteins. Genetic variation for temperature resistance was present among lines for flies either pretreated to high temperature or not. Pro-treating increased survival, with the benefit significantly higher if pretreating was performed 24 h rather than 96 h before exposure to the potentially lethal stress. For (lies pretreated at both times, resistance to heat stress was even greater. The lack of a significant treatment by line interaction term suggested that all lines were similarly plastic for acclimation following previous exposure(s) to a high temperature. Significantly more males survived the heat stress than females, and, within each sex, larger flies were generally more heat resistant than smaller ones. Additionally, the lines from the population that naturally encounters the highest temperatures were generally more resistant to high temperature stress.     

Variation of life-history and morphometrical traits in Drosophila buzzatii and Drosophila simulans collected along an altitudinal gradient from a Canary island

Variation in three life‐history traits (developmental time, preadult viability and daily female productivity) and five morphometrical traits (thorax length, wing length, wing width, wing/thorax ratio and wing‐aspect ratio) was studied at three developmental temperatures (20, 25 and 30 °C) in Drosophila buzzatii and Drosophila simulans collected on the island of La Gomera (Canary Archipelago). The flies originated from five closely situated localities, representing different altitudes (from 20 to 886 m above sea level) and a range of climatic conditions. We found statistically significant population effects for all traits in D. buzzatii and for most of the traits in D. simulans. Although no correlations of trait values with altitude were detected, geographical patterns for three life‐history traits and body size in D. buzzatii indicated that short‐range geographical variation in this species could be maintained by local climatic selection. Five of eight traits showed population‐by‐temperature interactions either in D. buzzatii or in D. simulans, but in all cases except wing width in D. buzzatii this could not be interpreted as adaptive responses to thermal conditions in the localities. The range of plastic changes across temperatures for particular traits differed between species, indicating a possibility for different levels of environmental stress experienced by the natural populations. The reaction norm curves and the response of within‐population variability to thermal treatments suggested better adaptations to higher and lower temperatures for D. buzzatii and D. simulans, respectively. The levels of among‐population differentiation depended on developmental temperature, implying environmental effects on the expression of the genetic variance. At 20 and 25 °C, interpopulation variability in D. buzzatii was higher than in D. simulans, while at 30 °C the opposite trend was observed. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 119–136.     

A field test of the stress‐gradient hypothesis along an aridity gradient

Aims: The stress‐gradient hypothesis (SGH) predicts how plant interactions change along environmental stress gradients. We tested the SGH in an aridity gradient, where support for the hypothesis and the specific shape of its response curve is controversial. Location: Almería, Cáceres and Coimbra, three sites in the Iberian Peninsula that encompass the most arid and wet habitats in the distribution range of a nurse shrub species –Retama sphaerocarpa L. (Boiss) – in Europe. Methods: We analysed the effect of Retama on its understorey plant community and measured the biomass and species richness beneath Retama and in gaps. We estimated the frequency (changes in species richness), importance and intensity of the Retama effects, and derived the severity–interaction relationship pattern, analysing how these interaction indices changed along this aridity gradient. Results and conclusions: The intensity and frequency of facilitation by Retama increased monotonically with increasing environmental severity, and the importance tended to have a similar pattern, overall supporting the SGH. Our data did not support predictions from recent revisions of the SGH, which may not apply to whole plant communities like those studied here or when interactions are highly asymmetrical. Facilitation by Retama influenced community composition and species richness to the point that a significant fraction of species found at the most arid end of the gradient were only able to survive beneath the nurse shrub, whereas some of these species were able to thrive in gaps at more mesic sites, highlighting the ecological relevance of facilitation by nurse species in mediterranean environments, especially in the driest sites.     

Developmental acclimation affects clinal variation in stress resistance traits in Drosophila buzzatii

Patterns of clinal genetic variation in Drosophila are often characterized after rearing at constant temperatures. However, clinal patterns might change after acclimation if populations differ in their plastic response to fluctuating environments. We studied longevity, starvation and heat knock‐down resistance after development at either constant or fluctuating temperatures in nine Drosophila buzzatii populations collected along an altitudinal gradient in Tenerife, Spain. Flies that developed at fluctuating temperatures had higher stress resistance despite experiencing a slightly lower average temperature than those at constant temperatures. Genetic variation along the gradient was found in both stress‐resistance traits. Because Q_ST values greatly exceeded F_ST values, genetic drift could not explain this diversification. In general, differences among populations were larger after rearing at fluctuating temperatures, especially in heat knock‐down, for which clinal patterns disappeared when flies were reared at constant temperatures. This result emphasizes the importance of determining whether populations originating from different environments differ in their plastic responses to stress.     

Trends in savanna structure and composition along an aridity gradient in the Kalahari

Abstract. The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m².ha^‐1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP.     

Patterns of variation in wing morphology in the cactophilic Drosophila buzzatii and its sibling D. koepferae

Drosophila buzzatii and D. koepferae are two sibling species that breed on the necrotic tissues of several cactus species and show a certain degree of niche overlap. Also, they show differences in several life history traits, such as body size and developmental time, which probably evolved as a consequence of adaptation to different host plants. In this work we investigate the ecological and genetic factors affecting wing morphology variation both within and between species. Three wing traits were scored, distal and proximal wing length and width in isofemale lines reared in two of the most important host cacti: Opuntia sulphurea and Trichocereus terschekii. Our results revealed that differences between species and sexes in wing size and shape were significant, whereas the cactus factor was only significant for wing size. Intraspecific analyses showed that differences among isofemale lines were highly significant for both size and shape in both species, suggesting that an important fraction of variation in wing morphology has a genetic basis. Moreover, the line by cactus interaction, which can be interpreted as a genotype by environment interaction, also accounted for a significant proportion of variation. In summary, our study shows that wing size is phenotypically plastic and that populations of D. buzzatii and D. koepferae harbour substantial amounts of genetic variation for wing size and shape. Interspecific differences in wing size and shape are interpreted in terms of spatial predictability of the different host plants in nature.     

Biotic resistance to invasion along an estuarine gradient

Biotic resistance is the ability of native communities to repel the establishment of invasive species. Predation by native species may confer biotic resistance to communities, but the environmental context under which this form of biotic resistance occurs is not well understood. We evaluated several factors that influence the distribution of invasive Asian mussels (Musculista senhousia) in Mission Bay, a southern California estuary containing an extensive eelgrass (Zostera marina) habitat. Asian mussels exhibit a distinct spatial pattern of invasion, with extremely high densities towards the back of Mission Bay (up to 4,000 m⁻²) in contrast with near-complete absence at sites towards the front of the bay. We established that recruits arrived at sites where adult mussels were absent and found that dense eelgrass does not appear to preclude Asian mussel growth and survival. Mussel survival and growth were high in predator-exclusion plots throughout the bay, but mussel survival was low in the front of the bay when plots were open to predators. Additional experiments revealed that consumption by spiny lobsters (Panulirus interruptus) and a gastropod (Pteropurpura festiva) likely are the primary factors responsible for resistance to Asian mussel invasion. However, biotic resistance was dependent on location within the estuary (for both species) and also on the availability of a hard substratum (for P. festiva). Our findings indicate that biotic resistance in the form of predation may be conferred by higher order predators, but that the strength of resistance may strongly vary across estuarine gradients and depend on the nature of the locally available habitat.     

Hypolithic and soil microbial community assembly along an aridity gradient in the Namib Desert

The Namib Desert is considered the oldest desert in the world and hyperarid for the last 5 million years. However, the environmental buffering provided by quartz and other translucent rocks supports extensive hypolithic microbial communities. In this study, open soil and hypolithic microbial communities have been investigated along an East–West transect characterized by an inverse fog-rainfall gradient. Multivariate analysis showed that structurally different microbial communities occur in soil and in hypolithic zones. Using variation partitioning, we found that hypolithic communities exhibited a fog-related distribution as indicated by the significant East–West clustering. Sodium content was also an important environmental factor affecting the composition of both soil and hypolithic microbial communities. Finally, although null models for patterns in microbial communities were not supported by experimental data, the amount of unexplained variation (68–97 %) suggests that stochastic processes also play a role in the assembly of such communities in the Namib Desert.     

Direct and correlated responses to selection for longevity in Drosophila buzzatii

The possible associations between longevity, early fecundity, and stress-resistance traits were explored using artificial selection on longevity in a laboratory population of Drosophila buzzatii . Three replicated lines were selected for increased lifespan (L lines) and compared with the respective unselected controls (C lines) after the 14th generation of selection. Mean longevity exhibited a significant response to selection. The baseline mortality tended to decrease in the L lines and a negative correlated response to longevity selection was found for early fecundity. Egg-to-adult developmental time increased in L lines. Longevity selection increased stress resistance for both high and low temperatures, as measured by heat knockdown resistance and chill-coma recovery. Starvation resistance also tended to be higher in L than in C lines. The results obtained are consistent with the hypothesis of trade-offs between longevity and early fecundity, and also suggest a trade-off association between adult longevity and developmental time. Correlated selection responses were generally consistent with correlations among the traits previously inferred from altitudinal clines for longevity and stress-resistance phenotypes.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 738–748.     

Life history variation in an annual plant under two opposing environmental constraints along an aridity gradient

Environmental gradients represent an ideal framework for studying adaptive variation in the life history of plant species. However, on very steep gradients, largely contrasting conditions at the two gradient ends often limit the distribution of the same species across the whole range of environmental conditions. Here, we study phenotypic variation in a winter annual crucifer Biscutella didyma persisting along a steep gradient of increasing rainfall in Israel. In particular, we explored whether the life history at the arid end of the gradient indicates adaptations to drought and unpredictable conditions, while adaptations to the highly competitive environment prevail at the mesic Mediterranean end. We examined several morphological and reproductive traits in four natural populations and in populations cultivated in standard common environment. Plants from arid environments were faster in phenological development, more branched in architecture and tended to maximize reproduction, while the Mediterranean plants invested mainly in vertical vegetative growth. Differences between cultivation and field in diaspore production were very large for arid populations as opposed to Mediterranean ones, indicating a larger potential to increase reproduction under favorable conditions. Our overall findings indicate two strongly opposing selective forces at the two extremes of the aridity gradient, which result in contrasting strategies within the studied annual plant species.     

Evolvability of between‐year seed dormancy in populations along an aridity gradient

Under global climate change, adaptation to new conditions is crucial for plant species persistence. This requires the ability to evolve in traits that are correlated with changing climatic variables. We studied between‐year seed dormancy, which correlates with environmental variability, and tested for clinal trends in its evolvability along an aridity gradient in Israel. We conducted a germination experiment under five irrigation levels with two dryland winter annuals (Biscutella didyma, Bromus fasciculatus) from four sites along the gradient. Species differed in means and evolvability of dormancy. Biscutella had high dormancy, which significantly increased with aridity but decreased with higher irrigation. In Bromus, dormancy was low, similar among populations, and only marginally affected by irrigation. Evolvability in Biscutella was high and varied among populations, without a clinal trend along the gradient. Conversely, in Bromus, trait evolvability was low and declined with increasing aridity. We argue that changes in evolvability along climatic gradients depend on the relative intensity of stabilizing selection. This may be high in Bromus and not only depends on environmental stress, but also on variability. Our findings point to the importance of measuring evolvability of climate‐related traits across different natural and artificial environments and for many coexisting species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 924–934.     

Resistance to thermal stress in preadult Drosophila buzzatii: variation among populations and changes in relative resistance across life stages

Resistance to a short term exposure to a high temperature stress was examined in eggs, larvae and pupae of Drosophila buzzfltii from seven localities. Across development, pupae were most resistant, followed by eggs, and then first and third-instar larvae. Variation among populations for resistance to heat stress was significant in all life stages. However, there was much less variation among populations where measured as eggs and pupae than for both first and third instar larvae. Older larvae showed large changes both in viability and developmental time, while exposure of young larvae to heat stress led to a decline in viability without delayed development. Populations that had the shortest developmental time at 25^oC were relatively the most resistant to heat stress as larvae. High relative resistance at one preadult life stage was not necessarily associated with relatively high resistance at another, or with previous measurements of resistance for adults from these populations. Comparison of populations that were more similar in their pattern of change in resistance across development suggested a relationship with the climate of origin. The possibility that developmental variation in the expression of heat shock proteins may cause variation in resistance to thermal stress for different life stages is discussed.     

Resistance to thermal stress in adult Drosophila buzzatii: acclimation and variation among populations

Survival after heat stress was examined in adult Drosophila buzzatii from seven localities. Males and females were conditioned by a non-lethal high temperature before exposure to a severe thermal stress. Variable times elapsed between conditioning, either as adults or pupae, and exposure to the stress were used, and experimental times to the stress ranged from 2 to 96 hours. Survival after stress varied among populations, and differences generally were consistent across conditioning treatments and across experiments. Resistance to thermal stress was much higher following one conditioning bout 2–4 hours before exposure to a severe stress than when the time elapsed between conditioning and exposure was increased to 24 or more hours. Significantly more adults survived the stress if conditioned 4 days before exposure to the thermal stress, either as adults or as pupae, than if not conditioned. The rank order of resistance roughly followed that predicted from the climatic conditions of the localities of origin.     

Comparative molecular population genetics of the Xdh locus in the cactophilic sibling species Drosophila buzzatii and D. koepferae

The Xdh (rosy) gene is one of the best studied in the Drosophila genus from an evolutionary viewpoint. Here we analyze nucleotide variation in a 1875-bp fragment of the second exon of Xdh in Argentinian populations of the cactophilic D. buzzatii and its sibling D. koepferae. The major electrophoretic alleles of D. buzzatii not only lack diagnostic amino acids in the region studied but also differ on average from each other by four to 13 amino acid changes. Our data also suggest that D. buzzatii populations belonging to different phytogeographic regions are not genetically differentiated, whereas D. koepferae exhibits a significant pattern of population structure. The Xdh region studied is twice as polymorphic in D. buzzatii as in D. koepferae. Differences in historical population size or in recombinational environment between species could account for the differences in the level of nucleotide variation. In both species, the Xdh region exhibits a great number of singletons, which significantly departs from the frequency spectrum expected under neutrality for nonsynonymous sites and also for synonymous sites in D. buzzatii. These excesses of singletons could be the signature of a recent population expansion in D. buzzatii, whereas they may be simply explained as the result of negative selection in D. koepferae.     

Altitudinal variation for stress resistance traits and thermal adaptation in adult Drosophila buzzatii from the New World

Multiple stress resistance traits were investigated in the cactophilic fly Drosophila buzzatii. Adults from seven populations derived from North-Western Argentina were compared with respect to traits relevant for thermal stress resistance and for resistance to other forms of environmental stress. The populations were collected along an altitudinal gradient spanning more than 2000 m in height, showing large climatic differences. The results suggest that knock-down resistance to heat stress, desiccation resistance and Hsp70 expression at a relatively severe stressful temperature best reflect thermal adaptation in this species. Furthermore, cold resistance seemed to be of less importance than heat resistance, at least for the adult life stage, in these populations. Clinal variation in thermal resistance traits over short geographical distances suggests relatively strong adaptive differentiation of the populations. This study provides the first evidence for altitudinal differentiation in stress-related traits, and suggests that Hsp70 expression level can be related to altitudinal clines of heat-stress resistance.     

Rooting depth,water availability,and vegetation cover along an aridity gradient in Patagonia

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44–45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80–0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2–3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2–3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.