Diplochory: are two seed dispersers better than one?

Diplochory is seed dispersal by a sequence of two or more steps or phases, each involving a different dispersal agent. Here, we describe five forms of diplochory and derive general characteristics of each phase of seed dispersal. The first and second phases of diplochory offer different benefits to plants. Phase one dispersal often results in escape from density-dependent seed and seedling mortality near the parent plant and can result in the colonization of habitat patches far from the parent. Phase two dispersal often moves seeds to discrete and predictable microsites, where the probability of seedling establishment is disproportionately high (i.e. directed dispersal). Diplochory appears to have evolved because combining two means of seed dispersal can increase the benefits of seed dispersal whilst reducing the likelihood of seed mortality.     

Masting,mixtures and modes: are two models better than one?

A key hypothesis in population ecology is that synchronous and intermittent seed production, known as mast seeding, is driven by the alternating allocation of carbohydrates and mineral nutrients between growth and reproduction in different years, i.e. ‘resource switching’. Such behaviour may ultimately generate bimodal distributions of long‐term flower and seed production, and evidence of these patterns has been taken to support the resource switching hypothesis. Here, we show how a widely‐used statistical test of bimodality applied by many studies in different ecological contexts may fail to reject the null hypothesis that focal probability distributions are unimodal. Using data from five tussock grass species in South Island, New Zealand, we find clear evidence of bimodality only when flowering patterns are analyzed with probabilistic mixture models. Mixture models provide a theory oriented framework for testing hypotheses of mast seeding patterns, enabling the different responses underlying medium‐ and high‐ versus non‐ and low‐flowering years to be modelled more realistically by associating these with distinct probability distributions. Coupling theoretical expectations with more rigorous statistical approaches will empower ecologists to reject null hypotheses more often.     

When are two heads better than one? Visual perception and information transfer affect vigilance coordination in foraging groups

Animals frequently raise their heads to check for danger. Ina group, individuals generally raise their heads independently.Earlier models suggest that all group members could gain bycoordinating their vigilance, i.e., each member raising itshead when others are not. We re-examine these suggestions, consideringgroups of different sizes, in light of empirical findings that:(1) animals can sometimes detect a predator without raisingtheir heads, and (2) when one member of a group detects a predator,the information does not always spread to other members of thegroup. Including these effects in models generally decreasesthe value of coordinated vigilance. Coordinated vigilance ishighly favored only when animals have a low probability of detectingpredators without lifting their heads but a high probabilityof being warned when another member of the group detects a predator.For other combinations, coordinated vigilance has little valueand may have a negative value. Group size has contrasting effectsdepending on how social information is obtained. Coordinationis favored in smaller groups when one or more detecting individualsprovide a constant amount of information to individuals unawareof the predator. On the other hand, coordination is favoredin larger groups if each detecting individual provides unawareindividuals with an independent source of information (i.e.,if the amount of information increases as the number of detectingindividuals increases). These results depend on the balanceof an escape due to social information and dilution of riskin groups with imperfect information spread. This frameworkcould be tested by examining species with different visual fieldsand in different environments.     

Are two strategies better than one? Manipulation of seed density and soil community in an experimental prairie restoration

Restoration practitioners have a variety of practices to choose from when designing a restoration, and different strategies may address different goals. Knowledge of how to best use multiple strategies could improve restoration outcomes. Here, we examine two commonly suggested strategies in a single tallgrass prairie restoration experiment: increased forb sowing density and prairie soil inoculation. We designed a study with two different forb seeding densities. Within these densities, we transplanted seedlings into 1‐m² plots that had been grown in either a whole prairie soil inoculum or sterilized prairie soil. After 4 years, we found positive effects of both high forb sowing density and inoculation treatments on the ratio of seeded to nonseeded plant cover in these plots, and negative effects of both treatments on nonseeded plant diversity. No effects of either treatment were seen on seeded plant diversity. Each strategy also affected the plant community in different ways: high forb sowing density increased seeded forb richness and decreased native nonseeded plant cover, while inoculation decreased non‐native cover, and tended to increase average successional stage of the community. These effects on restoration outcome were typically independent of each other, with the result that plots with both manipulations had the most positive effects on restoration outcomes. We thus advocate the combined use of these restoration strategies, and further studies which focus on both seeding and soil community manipulation in restoration.     

Problems of learning and memory: one or multiple memory systems?

Learning, and hence memory, is ubiquitous not only throughout the animal kingdom, but apparently throughout many regions of the brain. Is all learning reducible to a single common form? Neuropsychological dissociations suggest that the mammalian brain possesses a number of different and potentially independent memory systems, with different mechanisms and anatomical dispositions, some of which are neurally widely dispersed and others of which are narrowly organized. Among the types considered are: (i) short-term memory; (ii) knowledge and skills; (iii) stable associative memory; (iv) event memory; and (v) priming. As double or multiple dissociations do not lead to logically inevitable conclusions, it has been argued that an alternative to multiple memory systems is variable modes of processing. But these, too, would be dissociable on the same lines of evidence. Dissociations, if strong and absolute, have strong pragmatic power when they are combined with evolutionary and neuroscientific evidence. Multiple memory systems may possibly share some common cellular mechanisms, but such mechanisms do not define the separate properties at the systems level.     

Should young ever be better off with one parent than with two?

We analyze models of parental care, providing the first systematiccomparison of the care given to young by one parent versus bytwo parents. In the Houston-Davies model of care, young alwaysdo better with two parents rather than with one parent. Whenone parent decides about its level of care before the other,then the young may do better with one parent when the costsof care for the parents are asymmetric. When the level of parentaleffort is reached by negotiation, there are cases in which youngdo better with one parent, even when costs are symmetric. Theanalysis suggests empirical ways to differentiate between differentresponse rules.     

Evolution of cooperation: two for one?

How can cooperation thrive in a selfish world? Recent evolution experiments show how bacteria themselves can generate conditions that make cooperation a winning strategy. At least in the short term.     

Are two better than one? Analysis of an FtsK/Xer recombination system that uses a single recombinase

Bacteria harbouring circular chromosomes have a Xer site-specific recombination system that resolves chromosome dimers at division. In Escherichia coli, the activity of the XerCD/dif system is controlled and coupled with cell division by the FtsK DNA translocase. Most Xer systems, as XerCD/dif, include two different recombinases. However, some, as the Lactococcus lactis XerS/dif_SL system, include only one recombinase. We investigated the functional effects of this difference by studying the XerS/dif_SL system. XerS bound and recombined dif_SL sites in vitro, both activities displaying asymmetric characteristics. Resolution of chromosome dimers by XerS/dif_SL required translocation by division septum-borne FtsK. The translocase domain of L. lactis FtsK supported recombination by XerCD/dif, just as E. coli FtsK supports recombination by XerS/dif_SL. Thus, the FtsK-dependent coupling of chromosome segregation with cell division extends to non-rod-shaped bacteria and outside the phylum Proteobacteria. Both the XerCD/dif and XerS/dif_SL recombination systems require the control activities of the FtsKγ subdomain. However, FtsKγ activates recombination through different mechanisms in these two Xer systems. We show that FtsKγ alone activates XerCD/dif recombination. In contrast, both FtsKγ and the translocation motor are required to activate XerS/dif_SL recombination. These findings have implications for the mechanisms by which FtsK activates recombination.     

Carbonic anhydrase inhibitors: X-ray crystallographic structure of the adduct of human isozyme II with a bis-sulfonamide-two heads are better than one?

The X-ray crystal structure for the adduct of human carbonic anhydrase II (hCA II) with 4-(4-sulfamoylphenylcarboxamidoethyl)benzenesulfonamide, a topically acting antiglaucoma sulfonamide has been resolved at a resolution of 1.8 A. Its binding to the enzyme is similar with that of other sulfonamides, considering the interactions of the sulfonamide zinc anchoring group, but differs considerably when the organic part of the inhibitor is analyzed. This part of the inhibitor interacts only within the hydrophobic half of the CA active site, leaving the hydrophilic half able to accomodate several water molecules not present in the uncomplexed enzyme. Furthermore, the second head (sulfonamide moiety) participates in two strong hydrogen bonds with amino acid residues (Gly 132 and Gln 136) situated on the rim of the entrance to the active site cleft. Thus, the answer to the question in the title of this paper is that two heads are better than one, since the two sulfamoyl moieties of the inhibitor allow its proper orientation within the active site, with only one head binding in ionized form to the zinc ion, the organic part lying within the hydrophobic half of the active site, and the terminal, carboxamido containing phenylsulfamoyl head participating in strong hydrogen bonds with amino acid residues located at the entrance of it. All these findings are important for the design of better carboxamido CA inhibitors with applications in clinical medicine.     

Obesity and hypertension: two epidemics or one?

The association between obesity and hypertension is well documented, although the exact nature of this relation remains unclear. Sympathetic nervous and renin-angiotensin-aldosterone system activation appear to play an important role in the sodium and water retention, rightward shift in the pressure-natriuresis, and blood pressure elevation observed in obese individuals. Visceral obesity and the ectopic deposition of adipose tissue may be important in the activation of these systems and in the target organ damage that ensues. Weight loss is critical in the effective management of obesity hypertension and the accompanying target organ damage, although recidivism rates are high. However, prevention of weight gain should be the major priority for combating hypertension and its consequences in the future. The present review will provide an overview of our understanding of the etiology, pathophysiology, and treatment of obesity hypertension. Our focus is on the state of knowledge in humans. The potential role of abdominal obesity is considered throughout our review. We refer to relevant animal literature for supportive evidence and where little or no data in humans are available.     

Oxysterol binding proteins: in more than one place at one time?

Oxysterols are potent signalling lipids that directly bind liver X receptors (LXRs) and a subset of oxysterol binding protein (OSBP) related proteins (ORPs). It is relatively well established that the oxysterol-regulated function of LXRs is to control the expression of genes involved in reverse cholesterol transport, catabolism of cholesterol, and lipogenesis. In contrast, the mechanisms by which oxysterols and ORPs affect cellular lipid metabolism have remained poorly understood. In this review, we summarize the information available on function of the ORPs and compare the two families of proteins binding oxysterol to demonstrate the different responses that similar lipids can elicit within cells. The other focus is on the membrane targeting determinants and the protein interaction partners of ORPs, which provide interesting clues to the mode(s) of ORP action. Specifically, we suggest a model in which a general property of ORPs is to function at membrane contact sites, specialized zones of communication between two different organelles.     

Evolution of competitive fitness in experimental populations of E. coli: What makes one genotype a better competitor than another?

An important problem in microbial ecology is to identify those phenotypic attributes that are responsible for competitive fitness in a particular environment. Thousands of papers have been published on the physiology, biochemistry, and molecular genetics of Escherichia coli and other bacterial models. Nonetheless, little is known about what makes one genotype a better competitor than another even in such well studied systems. Here, we review experiments to identify the phenotypic bases of improved competitive fitness in twelve E. coli populations that evolved for thousands of generations in a defined environment, in which glucose was the limiting substrate. After 10000 generations, the average fitness of the derived genotypes had increased by 50% relative to the ancestor, based on competition experiments using marked strains in the same environment. The growth kinetics of the ancestral and derived genotypes showed that the latter have a shorter lag phase upon transfer into fresh medium and a higher maximum growth rate. Competition experiments were also performed in environments where other substrates were substituted for glucose. The derived genotypes are generally more fit in competition for those substrates that use the same mechanism of transport as glucose, which suggests that enhanced transport was an important target of natural selection in the evolutionary environment. All of the derived genotypes produce much larger cells than does the ancestor, even when both types are forced to grow at the same rate. Some, but not all, of the derived genotypes also have greatly elevated mutation rates. Efforts are now underway to identify the genetic changes that underlie those phenotypic changes, especially substrate specificity and elevated mutation rate, for which there are good candidate loci. Identification and subsequent manipulation of these genes may provide new insights into the reproducibility of adaptive evolution, the importance of co-adapted gene complexes, and the extent to which distinct phenotypes (e.g., substrate specificity and cell size) are affected by the same mutations.     

On hotshots and lotteries: are the best males on larger leks better than expected?

Leks are clusters of males that hold territories containingno special resources except for the males themselves (Höglundand Alatalo, 1995). Among hypotheses put forth to explain whysuch clusters form, the hotshot hypothesis posits that leksform by males that would otherwise receive little attentionfrom females taking up positions next to males that would receivea great deal of female attention in any case (Beehler and Foster,1988; Höglund and Alatalo, 1995). Although this ideamight well be true, we still face the task of evaluating theevidence. In this note I question the efficacy of testing forhotshots by observing the success of males at leks of different sizes. Several authors have noted that the males most successful inattracting mates on larger leks are more successful than thebest males on smaller leks (Westcott and Smith, 1997