Evolution, origin and age of lineages in the Californian and Mediterranean floras

This paper addresses some of the conceptual issues involved in the analysis of the age and origin of mediterranean‐climate plant taxa, paying particular attention to three topics: (1) the importance of an explicit time frame in the definition of biogeographical origins, (2) the distinction between the age of traits and the age of taxa, and (3) the idea of mediterranean‐type ecosystems as environmental islands. (1) In California, recent analyses demonstrate that the diversity of species derived from different biogeographical origins is significantly correlated with temperature and precipitation gradients. These patterns support the hypothesis that niche conservatism is an important factor structuring modern diversity gradients. However, depending on how far back in time one looks, a species may be assigned to different origins; future discussions of biogeographical origins need to address the appropriate time frame for analysis. (2) Past research has demonstrated distinctive trait syndromes among woody plants of the Mediterranean, Chile, California and Mexico, and proposed that the syndromes are associated with lineages of different age in these floras. Reanalysis of individual traits demonstrates greater variability among regions than previously reported. The classification of plants into ‘old’ and ‘new’ genera is re‐evaluated, and it is suggested that greater attention be paid to the age of traits, rather than to the age of taxa, especially at an arbitrary rank such as genus. (3) The idea of mediterranean‐climate regions as ‘climatic islands’ is examined. Space–time diagrams of climate enable one to view the emergence of distinctive climatic regions in a continental context. The terms ‘synclimatic’ and ‘anticlimatic’ are proposed, referring to migration routes that parallel climate contours in space and time versus those that cross contours (including the case of geographic stasis in the face of climate change), respectively. Mediterranean‐climate regions have served as important case studies in plant ecology and evolution, and merit continued close examination in the light of continued advances in phylogenetics and palaeoecology.     

Range edges in heterogeneous landscapes: Integrating geographic scale and climate complexity into range dynamics

The impacts of climate change have re‐energized interest in understanding the role of climate in setting species geographic range edges. Despite the strong focus on species' distributions in ecology and evolution, defining a species range edge is theoretically and empirically difficult. The challenge of determining a range edge and its relationship to climate is in part driven by the nested nature of geography and the multidimensionality of climate, which together generate complex patterns of both climate and biotic distributions across landscapes. Because range‐limiting processes occur in both geographic and climate space, the relationship between these two spaces plays a critical role in setting range limits. With both conceptual and empirical support, we argue that three factors—climate heterogeneity, collinearity among climate variables, and spatial scale—interact to shape the spatial structure of range edges along climate gradients, and we discuss several ways that these factors influence the stability of species range edges with a changing climate. We demonstrate that geographic and climate edges are often not concordant across species ranges. Furthermore, high climate heterogeneity and low climate collinearity across landscapes increase the spectrum of possible relationships between geographic and climatic space, suggesting that geographic range edges and climatic niche limits correspond less frequently than we may expect. More empirical explorations of how the complexity of real landscapes shapes the ecological and evolutionary processes that determine species range edges will advance the development of range limit theory and its applications to biodiversity conservation in the context of changing climate.     

Topographic heterogeneity lengthens the duration of pollinator resources

The availability of sufficient and diverse resources across time is important for maintenance of biodiversity and ecosystem functioning. In this study, we examine the potential for variation in environmental conditions across topographic gradients to extend floral resource timing. Flowering time on a landscape may vary across topography due to differences in abiotic factors, species turnover, or genotypic differences. However, the extent to which this variation in phenology affects overall flowering duration on a landscape, and the components of diversity that influence flowering duration, are unexplored. We investigate whether differences in flowering time due to topography yield an overall extension in duration of flowering resources in a northern California grassland. We recorded flowering time of pollinator resource species across four successive spring growing seasons (2015–2018) on paired north and south aspects. Flowering time differences were evaluated both at the community level and within species present on both paired aspects. The role of plasticity was examined in an experimental case study using genotypes of Lasthenia gracilis. We found that aspect is a strong determinant of phenology, with earlier flowering on warmer south‐facing slopes. Aspect differences resulted in complementarity in timing of flowering resources across sites, as aspects that started flowering earlier also ended earlier. Complementarity between north and south aspects served to extend the flowering time of pollinator resources by an average of 4–8 days (8%–15%), depending on the year. This extension can be attributed to both within‐species responses to aspect differences and species turnover. Flowering of L. gracilis genotypes was distinct across aspects, demonstrating that plasticity can drive the extension of flowering duration. Our findings indicate that heterogeneous topography can extend overall flowering time of pollinator resources, which may support pollinator biodiversity. Extension was most pronounced at the community level, which incorporates species turnover as well as plastic and genotypic differences within species.     

Biosynthesis of lysosomal hydrolases: their synthesis in bound polysomes and the role of co- and post-translational processing in determining their subcellular distribution

By in vitro translation of mRNA’s isolated from free and membrane-bound polysomes, direct evidence was obtained for the synthesis of two lysosomal hydrolases, β-glucuronidase of the rat preputial gland and cathespin D of mouse spleen, on polysomes bound to rough endoplasmic reticulum (ER) membranes. When the mRNA’s for these two proteins were translated in the presence of microsomal membranes, the in vitro synthesized polypeptides were cotranslationally glycosylated and transferred into the microsomal lumen. Polypeptides synthesized in the absence of microsomal membranes were approximately 2,000 daltons larger than the respective unglycosylated microsomal polypeptides found after short times of labeling in cultured rat liver cells treated with tunicamycin. This strongly suggests that nascent chains of the lysosomal enzymes bear transient amino terminal signals which determine synthesis on bound polysomes and are removed during the cotranslational insertion of the polypeptides into the ER membranes. In the line of cultured rat liver cells used for this work, newly synthesized lysosomal hydrolases showed a dual destination; approximately 60 percent of the microsomal polypeptides detected after short times of labeling were subsequently processed proteolytically to lower molecular weight forms characteristic of the mature enzymes. The remainder was secreted from the cells without further proteolytic processing. As previously observed by other investigations in cultured fibroblasts (A. Gonzalez-Noriega, J.H. Grubbs, V. Talkad, and W.S. Sly, 1980, J Cell Biol. 85: 839-852; A. Hasilik and E.F. Neufeld, 1980, J. Biol. Chem., 255:4937-4945.) the lysosomotropic amine chloroquine prevented the proteolytic maturation of newly synthesized hydrolases and enhanced their section. In addition, unglycosylated hydrolases synthesized in cells treated with tunicamycin were exclusively exported from the cells without undergoing proteolytic processing. These results support the notions that modified sugar residues serve as sorting out signals which address the hydrolases to their lysosomal destination and that final proteolytic cleavage of hydrolase precursors take place within lysosome itself. Structural differences in the carbohydrate chains of intracellular and secreted precursors of cathespin D were detected from their differential sensitivity to digestion with endoglycosidases H and D. These observations suggest that the hydrolases exported into the medium follow the normal secretory route and that some of their oligosaccharides are subject to modifications known to affect many secretory glycoproteins during their passage through the Golgi apparatus.     

Why are evergreen leaves so contrary about shade?

Leaf mass per area (LMA) is one of the most widely measured of all plant functional traits. In deciduous forests, there is similarity between plastic and evolutionary responses of LMA to light gradients. In evergreens, however, LMA is lower in shaded than sunlit individuals of the same species, whereas shade-tolerant evergreens have higher LMA than light-demanders grown under the same conditions. We suggest that this pattern of 'counter-gradient variation' results from some combination of (i) close evolutionary coordination of LMA with leaf lifespan, (ii) selection for different leaf constitutions (relative investment in cell walls versus cell contents) in sun and shade environments and/or (iii) constraints on plasticity as a result of genetic correlations between phenotypes expressed in sun and shade.     

Correlated evolution of chloroplast heat shock protein expression in closely related plant species

Interspecific variation in chloroplast low molecular weight (cLMW) HSP (heat shock protein) expression was examined with respect to phylogeny, species specific leaf area, chlorophyll fluorescence, and mean environmental conditions within species ranges. Eight species of Ceanothus (Rhamnaceae) were heat shocked for 4 h at several different temperatures. Leaf samples were collected immediately after the heat shock, and cLMW HSP expression was quantified using Western blots. At 45°C species from the subgenus Cerastes had significantly greater cLMW HSP expression than species from the subgenus Ceanothus. Specific leaf area was negatively correlated with cLMW HSP expression after the 45°C heat treatment. In addition, chlorophyll fluorescence (F(v)/F(m)) 1 h after the heat shocks was positively correlated with cLMW HSP expression. Contrary to our prediction, there was no correlation between July maximum temperature within species ranges and cLMW HSP expression. These results suggest that evolutionary differentiation in cLMW HSP expression is associated with leaf physiological parameters and related aspects of life history, yet associations between climatic conditions within species ranges and cLMW HSP expression require further study.     

Leaf size, sapling allometry, and Corner's rules: phylogeny and correlated evolution in maples (Acer)

We studied the evolution of leaf size, sapling canopy allometry, and related traits in 17 Acer species growing in the understory of temperate deciduous forests, using parsimony methods, randomization tests, and independent contrasts calculated on idena phylogeny inferred from nuclear ribosomal internal transcribed spacer (ITS) sequences. Bivariate correlations and multivariate analyses indicated two independent suites of coevolving traits, and the results were robust over a range of alternative phylogenies. The first suite consisted of strong positive correlations among twig thickness, leaf size, inflorescence length, and branch spacing (Corner's rules). Seed size and mature height were also weakly corre- lated with these traits. The second suite reflected aspects of sapling crown allometry, including crown size, stem diameter, and total leaf area, which appear to be related to shade tolerance. There was a weak negative correlation between sapling crown size and mavegetative ture height, but no correlation with leaf or seed size. Most correlattion were similar in magnitude for ahistorical and independent contrasts analyses, and discrepancies between these two measures were greater in traits with lower levels of convergent evolution. The evolutionary correlations among twig, leaf, seed, inflorescence, and canopy dimensions emphasize the need for integrated theories of evolution and function of these disparate traits.     

Evolution of hydraulic traits in closely related species pairs from Mediterranean and nonMediterranean environments of North America

Chaparral shrubs in California experience cool, wet winters and hot, dry summers characteristic of mediterranean-type climates; by contrast, morphologically similar close relatives in central Mexico experience summer rainfall. A comparison of closely related species pairs was conducted to examine whether evolutionary divergences in plant hydraulic conductivity were associated with contrasting seasonality of precipitation. Six species pairs in Santa Barbara, California and Tehuacan, Mexico were chosen to test for repeated directional divergences across the habitat contrast. Additionally, evolutionary correlations were examined using phylogenetically independent contrasts (PICs) among a suite of hydraulic traits, including stem- and leaf-specific conductivity, resistance to embolism, wood density, inverse Huber value, and minimum seasonal water potential. Leaf-specific conductivity was generally higher in California, but for most hydraulic traits the species pairs exhibited varied evolutionary trajectories across the climate contrast. A significant correlation was found between divergences in xylem resistance to embolism and minimum seasonal water potential, but no evolutionary trade-off was found between resistance and stem conductivity. Higher leaf-specific conductivity may be adaptive in California, where soil and atmospheric droughts coincide during summer months. This response is consistent with a hydraulic strategy of high leaf water supply under high evaporative demand to prevent excessive drops in water potential.     

Phylogeny,niche conservatism and the latitudinal diversity gradient in mammals

Biologists have long searched for mechanisms responsible for the increase in species richness with decreasing latitude. The strong correlation between species richness and climate is frequently interpreted as reflecting a causal link via processes linked to energy or evolutionary rates. Here, we investigate how the aggregation of clades, as dictated by phylogeny, can give rise to significant climate–richness gradients without gradients in diversification or environmental carrying capacity. The relationship between climate and species richness varies considerably between clades, regions and time periods in a global-scale phylogenetically informed analysis of all terrestrial mammal species. Many young clades show negative richness–temperature slopes (more species at cooler temperatures), with the ages of these clades coinciding with the expansion of temperate climate zones in the late Eocene. In carnivores, we find steeply positive richness–temperature slopes in clades with restricted distributions and tropical origins (e.g. cat clade), whereas widespread, temperate clades exhibit shallow, negative slopes (e.g. dog–bear clade). We show that the slope of the global climate–richness gradient in mammals is driven by aggregating Chiroptera (bats) with their Eutherian sister group. Our findings indicate that the evolutionary history should be accounted for as part of any search for causal links between environment and species richness.