A Genetic Analysis of Variation for the Ability to Fly After Exposure toThermal Stress in Drosophila mojavensis

To help us understand how adaptive tolerance to heat stress has evolved, we made F1 hybrid crosses and backcrosses amongst populations of Drosophila mojavensis, and tested their ability to fly after exposure to a debilitating, but non-lethal, heat stress. Previous work identified that these populations vary in thermotolerance as measured for a variety of traits. Hybrid superiority was observed when crossing all four pairs of strains. Patterns of inheritance in flight ability after stress varied depending on which strains were used to set up complete reciprocal backcrosses, and, for both population pairs, results supported a multigenic model. This quantitative inheritance may be an outcome of the many different physiological and biochemical systems recently shown to influence muscle activity during heat stress. Therefore, the ability to maintain flight in the presence of high temperatures has the potential to vary considerably among populations in nature. As effects occur at temperatures well below those causing mortality, variation in this trait may greatly impact organismal fitness

Direct and Correlated Effects of Selection on Flight After Exposure to Thermal Stress in Drosophila melanogaster

To demonstrate how insects may adapt to ecologically relevant levels of heat stress, we performed artificial selection on the ability of Drosophila melanogaster to fly after an exposure to a high but non-lethal thermal stress. Both tolerance and intolerance to heat stress arose very quickly, as only a few generations of selection were necessary to cause significant separation between high and low lines for heat tolerance. Estimates of heritability based on the lines artificially selected for increased flight ability ranged from 0.024 to 0.052, while estimates of heritability based on the lines selected for the inability to fly after heat stress varied between 0.035 and 0.091. Reciprocal F1 crosses among these lines revealed strong additive effects of one or more autosomes and a weaker X-chromosome effect. This variation apparently affected flight specifically; neither survival to a more extreme stress nor knockdown by high temperature changed between lines selected for high and low heat tolerance as measured by flight ability. As the well-studied heat-shock response is associated with heat tolerance as measured by survival and knockdown, the aspects of the stress physiology that actually affect flight ability remains unknown

Developmental Variation and Distyly in Hedyotis caerulea (Rubiaceae)

The development of distyly is thought to arise from differential growth patterns in the pin and thrum morphs. However, few detailed studies exist on the early floral development of distylous flowers, and fewer still look at variation in these traits among populations. Buds at multiple stages of development were collected from five populations of Hedyotis caerulea to quantify how pins and thrums diverge with respect to the initiation, rate, and termination of growth between the stamens and stigmas. The growth rate of anthers varied little spatially across five populations and temporally in both pins and thrums, although thrum anthers grew faster than pin anthers. Dimorphy in stigma height was more complex. Pin stigmas first grew at a faster rate than those of thrums, and late in bud development, growth of thrum styles slowed. These rate changes varied among populations, and they differed from the congeneric H. salzmanii. Similar differences between morphs are known in other heterostylous species, and such variation in growth pattern among related species has been used to infer independent evolution of distylous systems

Quantitative Evaluation of Reciprocal Herkogamy in the Distylous Species, Hedyotis caerulea (Rubiaceae)

Hedyotis caerulea possesses two distinct floral morphs that are generally found in equal numbers in naturally occurring populations. Flowers either possess a relatively long style and short anthers, called a “pin,” or a short style and long anthers, called a “thrum.” This placement of reproductive organs is considered herkogamous and distylous, as it encourages outcrossing by restricting pollination to individuals of the alternate morph. Numerous species have been described as distylous without quantitative data establishing stigma-anther reciprocity. Here we assess those assumptions in H. caerulea by measuring stigma height, anther height and a suite of additional floral traits across multiple localities. All populations surveyed were isoplethic, although variation among them was present in all floral traits measured as well as for pollen diameter, pollen count, flower dry weight, and seed set. Pins produced smaller pollen than thrums, but made more of them. Thus, the total volume of pollen was similar for pins and thrums, and seed set was similar, suggesting that each morph has equal male and female fitness with no movement towards dioecy. Given a significant degree of variation found in the morphometric analysis, and that two of the three measures used to assess reciprocity were not consistent with predictions of precise symmetry, extensive change is possible where selection is acting on these traits. Even so, the distylous mating system in H. caerulea appears to be stable

Reduced Enzyme Activity Following Hsp70 Overexpression in Drosophila melanogaster

Acclimation to environmental change can impose costs to organisms. One potential cost is the change in cell metabolism that follows a physiological response, e.g., high expression of heat shock proteins may alter specific activity of important enzymes. We examined the significance of this cost in a pair of Drosophila melanogaster lines transformed with additional copies of a gene that encodes the heat shock protein, Hsp70. Heat shock induces Hsp70 expression in all lines, but lines with extra copies produce much more Hsp70 than do excision control strains. The consequence of this supranormal Hsp70 expression is to reduce specific activity of both enzymes analyzed, adult alcohol dehydrogenase (ADH), which is heat sensitive, and lactate dehydrogenase, which is not. Strain differences were most pronounced under those conditions where Hsp70 expression was maximized, and not where the heat stress denatured proteins. That result supported the idea that Hsp70 expression is constrained evolutionarily by its tendency to bind nascent peptides when overabundant within the cell

Experimental Manipulation of the Cost of Thermal Acclimation in Drosophila melanogaster

Acclimation to environmental change can impose costs on organisms. One potential cost is the energy and nutrients consumed by a physiological response, e.g. the resources required for expression of heat-shock proteins (Hsps). We examined the significance of this cost by genetic manipulation. We isolated four isofemale lines from a Drosophila melanogaster population previously transformed with a hsp70-kcZ fusion. Lines were similar in Hsp70 expression but differed in P-galactosidase expression upon heat shock, and replicates of each line were reared on a high quantity and low quantity medium. Multiple heat shock reduced survival in all lines, but did not increase developmental time. Variation in expression of β-galactosidase among lines, which differed more than 4-fold in response to heat treatment, was unrelated to the decreased survival. Thus the predicted effects of β-galactosidase expression on components of fitness were not evident. The superimposition of costs upon those normal for acclimation had no effect on mortality or developmental time, even when resources were especially limiting

Phylogeography and Postglacial Dispersal of Smallmouth Bass (Micropterus dolomieu) into the Great Lakes

Refugia and dispersal routes of smallmouth bass (Micropterus dolomieu) into the Great Lakes were identified using 427 mitochondrial sequences from across their native range. Overall, smallmouth bass accessed the Great Lakes via long-distance dispersal from multiple refugia, and the distribution of genetic variation reflected the consequences of vicariant, dispersal, and paleogeological events. Proximity of lakes to glacial outlets had a greater predictive power on the resulting distribution of mitochondrial diversity relative to interbasin migration. Populations in the Eastern and Interior Highlands contained the most divergent and oldest haplotypes, reflecting their role as glacial refugia and subsequent high incidence of endemicity. Lake Superior, Lake Michigan, and northern Lake Huron were colonized by bass of a single mitochondrial clade that accessed the Brule–Portage (Mississippi and St. Croix rivers) and Chicago (Illinois and Fox rivers) outlets. Lakes Huron and Erie contained admixed mitochondrial lineages in part due to numerous access points, including the Fort Wayne (Wabash and Maumee rivers), Lower Peninsula of Michigan (Grand River valley), and Kirkfield (Kawartha Lakes) outlets. In contrast, populations in Lake Ontario, Georgian Bay of Lake Huron, and the St. Lawrence River were monomorphic, indicating a single but unidentified source. These patterns were consistent with many examples from the North American freshwater ichthyofauna

Evolution of Thermotolerance and Variation in the Heat Shock Protein, Hsp70

Low to moderate levels of stress induce a class of molecular chaperones called heat shock proteins (Hsps), which protect cells, tissues and whole organisms from more severe stress. In higher Eukaryotes, Hsp70 is one of the principle heat-induced chaperones. This response is general, and how much Hsp70 an animal produces correlates with the level of stress to which it is exposed. Nonetheless, definitively linking high Hsp70 expression as an adaptation to stress tolerance is problematic, because organisms and cells respond to stress in many ways. By molecular manipulation of Hsp70 in one animal group, Drosophila, differences in hsp70 copy number are shown to directly influence heat-induced expression of Hsp70 and tolerance of heat. However, too high an expression level of Hsp70 can harm individuals during periods of rapid growth. This strong physiological relationship between Hsp70 concentration and thermotolerance, along with Hsp70\u27s remarkable degree of interspecific coding sequence conservation, suggest that hsp70 regulatory elements may evolve as an adaptation in diverse species to their thermal environments. To examine this possibility, correlative studies within species and research on phylogenetic covariation between these traits is reviewed with a focus on Drosophila species. However, the techniques and results discussed should broadly apply to other animal groups where evolutionary approaches can be used to test whether genetic variation in both thermotolerance and Hsp expression within and among species select locally on either hsp70 sequence and/or expression

Hsp70 and Larval Thermotolerance in Drosophila melanogaster: How Much is Enough and When is More Too Much?

Heat shock proteins (Hsps) and other molecular chaperones perform diverse cellular roles (e.g., inducible thermotolerance) whose functional consequences are concentration dependent. We manipulated Hsp70 concentration quantitatively in intact larvae of Drosophila melanogaster to examine its effect on survival, developmental time and tissue damage after heat shock. Larvae of an extra-copy strain, which has 22 hsp70 copies, produced Hsp70 more rapidly and to higher concentrations than larvae of a control strain, which has the wild-type 10 copies of the gene. Increasing the magnitude and duration of pretreatment increased Hsp70 concentrations, improved tolerance of more severe stress, and reduced delays in development. Pretreatment, however, did not protect against acute tissue damage. For larvae provided a brief or mild intensity pretreatment, faster expression of Hsp70 in the extra-copy strain improved survival to adult and reduced tissue damage 21 h after heat shock. Negative effects on survival ensued in extra-copy larvae pretreated most intensely, but their overexpression of Hsp70 did not increase tissue damage. Because rapid expression to yield a low Hsp70 concentration benefits larvae but overexpression harms them, natural selection may balance benefits and costs of high and low expression levels in natural populations