Why do leafcutter bees cut leaves? New insights into the early evolution of bees

Stark contrasts in clade species diversity are reported across the tree of life and are especially conspicuous when observed in closely related lineages. The explanation for such disparity has often been attributed to the evolution of key innovations that facilitate colonization of new ecological niches. The factors underlying diversification in bees remain poorly explored. Bees are thought to have originated from apoid wasps during the Mid-Cretaceous, a period that coincides with the appearance of angiosperm eudicot pollen grains in the fossil record. The reliance of bees on angiosperm pollen and their fundamental role as angiosperm pollinators have contributed to the idea that both groups may have undergone simultaneous radiations. We demonstrate that one key innovation--the inclusion of foreign material in nest construction--underlies both a massive range expansion and a significant increase in the rate of diversification within the second largest bee family, Megachilidae. Basal clades within the family are restricted to deserts and exhibit plesiomorphic features rarely observed among modern bees, but prevalent among apoid wasps. Our results suggest that early bees inherited a suite of behavioural traits that acted as powerful evolutionary constraints. While the transition to pollen as a larval food source opened an enormous ecological niche for the early bees, the exploitation of this niche and the subsequent diversification of bees only became possible after bees had evolved adaptations to overcome these constraints.     

Effects of multi-year droughts on fish assemblages of seasonally drying Mediterranean streams

1. This study analysed changes occurring in Mediterranean stream fish assemblages over a sequence of dry years followed by a generally wet period (1991–98). Variations in assemblage attributes were quantified at the basin and stream reach scales, and related to variables reflecting the occurrence of unusually dry or wet conditions. 2. Assemblage variability increased along with the resolution of analysis, with little changes in species richness, composition and rank abundances, but significant variation in individual species abundances. Fluctuations in relative abundances were significantly affected by variables reflecting the severity of summer droughts and the occurrence of rainy springs. These patterns were evident at the basin scale, while variability at individual stream reaches tended to be higher and less related to rainfall patterns. 3. At least three response guilds to rainfall variation could be identified: two of the four abundant and widespread species (chub and loach) declined following dry years, whereas the two other core species (nase and eel) declined after rainy spring; one scarce native species (stickleback) increased in dry years. 4. Except at the two most upstream reaches, the assemblages tended to recover quickly to previous configuration after the changes occurring during the sequence of dry years. 5. Temporal variability of local assemblages was concordant among reaches but did not follow any consistent spatial pattern. Instead, spatial patterns in assemblage attributes changed over time in response to environmental variability, with a tendency for a disruption of upstream–downstream gradients following dry years. 6. Results supported the view that present‐day droughts cause relatively small and transient changes to Mediterranean stream fish assemblages. However, longer and more severe droughts expected under altered future climates, may result in declines or local extinctions of the most sensitive species and their potential replacement by more resistant species. Changing drought regimes thus need to be duly considered in the development of conservation strategies for Mediterranean stream fish.     

Phylogenetic analysis of Micrathena and Chaetacis spiders (Araneae: Araneidae) reveals multiple origins of extreme sexual size dimorphism and long abdominal spines

The phylogenetic relationships amongst the New World spiny orb‐weaving spiders Micrathena and Chaetacis were assessed through parsimony and Bayesian analyses of morphological characters. A total of 146 characters was scored for ten outgroup taxa and 37 Micrathena and four Chaetacis species. The results indicate that Chaetacis nests within Micrathena and we propose Chaetacis as a junior synonym of Micrathena. Twelve subgeneric species groups of Micrathena are recognized and diagnosed. Species with extremely long spines evolved at least eight times in the genus and we suggest that this may be related to antipredator defences. Micrathena is primitively sexually monomorphic and extreme sexual size dimorphism has arisen at least six times in the genus. Most of these events are because of enlargement of the female in relation to the ancestral size, although in two cases sexual dimorphism was attained through male reduction, adding more data to the ‘giant females’ vs. ‘dwarf males’ controversy. The genus is probably of South American origin and has repeatedly invaded Central and North America. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 166 , 14–53.     

Specific lipid recognition is a general feature of CD300 and TREM molecules

CD300, triggering receptor expressed on myeloid cells (TREM), and TREM-like (TREML) receptors are important regulators of the mammalian immune response. Homologs of these receptors, which occur in activating and inhibitory transmembrane forms as well as soluble variants, are found throughout the jawed vertebrates. Specific ligands for most members of these receptor families remain elusive. We report here that at least 11 separate receptors from the CD300, TREM, and TREML families engage in robust and specific interactions with major polar lipids found in prokaryotic and eukaryotic cell membranes. Both soluble and membrane-bound receptor forms exhibit lipid interactions in the solid phase as well as in a physiological signaling context. Overlapping but distinctive patterns of receptor specificity suggest that the CD300/TREM system as a whole may discriminate immunological stimuli based on lipid signatures, thereby influencing downstream responses.     

Packing characteristics of highly unsaturated bilayer lipids: Raman spectroscopic studies of multilamellar phosphatidylcholine dispersions

The thermotropic properties and acyl chain packing characteristics of multilamellar dispersions of highly unsaturated lipids were examined by Raman spectroscopy. Bilayer assemblies were composed of POPC (1-palmitoyl-2-oleoylphosphatidylcholine), PAPC (1-palmitoyl-2-arachidonylphosphatidylcholine), and PDPC (1-palmitoyl-2-docosahexaenoylphosphatidylcholine), lipid systems possessing saturated sn-1 chains and unsaturated sn-2 chains with one, four, and six double bonds, respectively. Raman spectra were recorded in the acyl chain 2800-3100-cm-1 carbon-hydrogen (C-H) stretching and 1100-1200-cm-1 carbon-carbon (C-C) stretching mode regions, spectral intervals reflecting both the inter- and intrachain order/disorder properties of the various lipid dispersions. In order to obtain C-H stretching mode spectra relevant solely to the sn-1 chains of PAPC and PDPC, liquid-phase spectra of arachidonic and docosahexaenoic acid, respectively, were subtracted from the observed phospholipid spectra. The unsaturated sn-2 chains of PAPC and PDPC undergo minimal conformational reorganizations as the bilayers pass from the gel to liquid-crystalline phases. Phase transition temperatures, Tm, derived from statistically fitting the temperature-dependent Raman spectral data are approximately -2.5, -22.5, and -3 degrees C for POPC, PAPC, and PDPC, respectively. As the degree of unsaturation increases from POPC to PAPC and PDPC, the cooperativity of the phase transition, as measured by its breadth, decreases. Estimates of the transition widths from the temperature profiles are approximately 15 degrees C for PAPC and 20 degrees C for PDPC. The behavior of various Raman spectral parameters for the lipid gel phase reflects the formation of lateral microdomains, or clusters, whose packing properties maximize the van der Waals interactions between sn-1 chains.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of sn-1-saturated,sn-2-polyunsaturated phospholipids in control of membrane receptor conformational equilibrium: effects of cholesterol and acyl chain unsaturation on the metarhodopsin I in equilibrium with metarhodopsin II equilibrium.

The effect of phospholipid bilayer acyl chain packing free volume on the equilibrium concentration of the form of photolyzed rhodopsin which initiates visual signal transduction, metarhodopsin II (meta II), is examined in reconstituted systems formed from the saturated phospholipid dimyristoylphosphatidylcholine (DMPC) and in the polyunsaturated phospholipid sn-1-palmitoyl-sn-2-arachidonoylphosphatidylcholine (PAPC) with and without 30 mol% cholesterol. The extent of meta II formation is determined from both flash photolysis measurements and rapidly acquired absorbance spectra. Equilibrium and dynamic properties of the lipid bilayer are characterized by the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH). DPH orientational properties are characterized by fv, a parameter which reflects the volume available for probe reorientation in the bilayer, relative to that available in an unhindered, isotropic environment [Straume, M., & Litman, B. J. (1987) Biochemistry 26, 5121-5126]. The metarhodopsin I in equilibrium with meta II equilibrium constant, Keq has a linear relationship with fv for rhodopsin in PAPC vesicles with and without cholesterol as well as for rhodopsin in DMPC vesicles, and these two correlation lines have different slopes. The correlations between Keq and fv in PAPC and DMPC systems are compared with a similar correlation in the native rod outer segment disk membrane and one reported previously in an egg phosphatidylcholine (egg PC) system [Mitchell, D. C., Straume, M., Miller, J. L., & Litman, B. J. (1990) Biochemistry 29, 9143-9149].(ABSTRACT TRUNCATED AT 250 WORDS)     

Gradients in stream fish assemblages across a Mediterranean landscape: contributions of environmental factors and spatial structure

1. Despite wide recognition that fish assemblages are influenced by factors operating over a range of spatial scales, little effort has been devoted to quantifying large‐scale variation and the multiscale dependencies of assemblage patterns and processes. This is particularly true for Mediterranean streams, where seasonally predictable drying‐up may lead to a strong association between assemblage attributes and large‐scale factors affecting the distribution of population sources and extinction likelihood. 2. The contribution of large‐scale factors to stream fish assemblage variation was quantified across a Mediterranean landscape, in south‐west Portugal. Fish abundance and species composition were estimated at 166 sites across third‐ to sixth‐order streams, in March–July 1998. Variance partitioning by redundancy analyses was used to analyse assemblage variation against three sets of predictor variables: environmental (catchment position, and geomorphic and hydrological factors), large‐scale spatial trends and neighbourhood effects. 3. Environmental variables and spatial trends accounted for 34.6% of the assemblage variation across the entire region, and for 36.6 and 57.8% within the two largest catchments (Mira and Seixe). Neighbourhood effects were analysed at the catchment scale, increasing the explained variation to 56.1% (Mira) and 70.7% (Seixe). 4. A prevailing environmental gradient was reflected in an increase in the abundance of all species and size‐classes in relation to catchment position, with more fish present in larger streams and in downstream reaches. Variables describing geomorphic and hydrological settings were less important in explaining assemblage variation. 5. Spatial trends always accounted for the smallest fraction of assemblage variation, and they were probably associated with historical barriers to fish dispersal. The strong neighbourhood effects may be related to spatially autocorrelated habitat conditions, but they are also a likely consequence of fish emigration/extinction and colonisation processes. 6. These results emphasise that a substantial proportion of fish assemblage variation in Mediterranean streams may be explained by large‐scale factors, irrespective of microhabitats and local biotic interactions. It is suggested that this pattern results to a large extent from the seasonal drying‐up, with the summer shortage of surface water limiting fish occurrence in headwaters, and consequently the key core areas for fish concentrating in larger streams and tributaries adjacent to large streams because of neighbourhood effects.     

Reproductive barriers between two sympatric beetle species specialized on different host plants

Knowledge on interspecific pre‐ and post‐zygotic isolation mechanisms provides insights into speciation patterns. Using crosses (F₁ and backcrosses) of two closely related flea beetles species, Altica fragariae and A. viridicyanea, specialized on different hosts in sympatry, we measured: (a) the type of reproductive isolation and (b) the inheritance mode of preference and host‐specific performance, using a joint‐scaling test. Each species preferred almost exclusively its host plant, creating strong prezygotic isolation between them, and suggesting that speciation may occur at least partly in sympatry. Reproductive isolation was intrinsic between females of A. fragariae and either A. viridicyanea or F₁ males, whereas the other crosses showed ecologically dependent reproductive isolation, suggesting ecological speciation. The genetic basis of preference and performance was at least partially independent, and several loci coded for preference, which limits the possibility of sympatric speciation. Hence, both ecological and intrinsic factors may contribute to speciation between these species.     

Generation of a P1 artificial chromosome library of the Southern pufferfish

We describe the generation of a P1 artificial chromosome genomic library from the Southern pufferfish, Spheroides nephelus. The arrayed library consists of approximately 30,000 clones and has an average insert size of 125-150 kb. The coverage is estimated to encompass seven to eight genome equivalents. The library has been used for isolating numerous genomic clones and for establishing contigs of several multigene families. Analysis of several of the clones from this library suggests a preponderance of CA repeat tracts relative to their abundance in humans. The library and high-density filters have been made available to the scientific public through genomics distribution companies.     

Manipulation of cholesterol levels in rod disk membranes by methyl-beta-cyclodextrin: effects on receptor activation

The effect of cholesterol on rod outer segment disk membrane structure and rhodopsin activation was investigated. Disk membranes with varying cholesterol concentrations were prepared using methyl-beta-cyclodextrin as a cholesterol donor or acceptor. Cholesterol exchange followed a simple equilibrium partitioning model with a partition coefficient of 5.2 +/- 0.8 in favor of the disk membrane. Reduced cholesterol in disk membranes resulted in a higher proportion of photolyzed rhodopsin being converted to the G protein-activating metarhodopsin II (MII) conformation, whereas enrichment of cholesterol reduced the extent of MII formation. Time-resolved fluorescence anisotropy measurements using 1,6-diphenyl-1,3,5-hexatriene showed that increasing cholesterol reduced membrane acyl chain packing free volume as characterized by the parameter f(v). The level of MII formed showed a positive linear correlation with f(v) over the range of 4 to 38 mol % cholesterol. In addition, the thermal stability of rhodopsin increased with mol % of cholesterol in disk membranes. No evidence was observed for the direct interaction of cholesterol with rhodopsin in either its agonist- or antagonist-bound form. These results indicate that cholesterol mediates the function of the G protein-coupled receptor, rhodopsin, by influencing membrane lipid properties, i.e. reducing acyl chain packing free volume, rather than interacting specifically with rhodopsin.