Effects of sandy beach cusps on wrack accumulation,sediment characteristics and macrofaunal assemblages

Beach‐cast marine macroalgae and seagrasses, collectively termed wrack, provide shelter and habitat for beach fauna and can often provide a large input of energy and nutrients to sandy beaches. Wrack deposition on sandy beaches varies spatially and could be affected by morphological features on the beach face such as cusps. This study tested a series of hypotheses regarding the differences in wrack deposits, sediments and macrofaunal assemblages between cusp bays and horns on two beaches in South Australia. Bays had greater cover and larger pieces of wrack than horns. Sediment organic matter content was greater on horns than in bays but mean particle size did not differ consistently between bays and horns. Macrofaunal diversity was higher in bays and this pattern was probably driven by differences in the cover of wrack between bays and horns. Cusp morphology thus influences the distribution of wrack on the beach face, which in turn influences the distribution of macrofauna. Studies of sandy beaches with cusps should therefore be explicitly designed to sample cusp features and associated wrack deposits.     

Epistatic effects of promoter and repressor functions of the Tn10 tetracycline-resistance operon on the fitness of Escherichia coli

We have been studying the effects of expression of plasmid-borne, Tn10-encoded, tetracycline resistance on the fitness of Escherichia coli K12. We previously demonstrated large reductions in fitness resulting from induced or constitutive expression of the resistance protein; however, any residual expression by the repressed operon was so slight that possession of an inducible resistance function imposed essentially no burden in the absence of antibiotic. Here, we demonstrate two distinct disadvantages for inducible genotypes relative to isogenic constitutive constructs. During the transition from antibiotic-free to antibiotic-containing media, the inducible genotype experiences a longer lag phase prior to growth. In the sustained presence of antibiotic, full induction of the resistance function in the inducible genotype is prevented by the continued action of its repressor. However, these disadvantages may be reduced by increasing the strength of the promoter for the resistance gene in the inducible genotype. Simultaneous consideration of the mode of gene regulation (i.e. constitutive or inducible) and the strength of the resistance-gene promoter (i.e. maximum level of expression) indicates an adaptive landscape with very strong epistasis and, perhaps, multiple fitness peaks.     

Inter‐specific gene flow dynamics during the Pleistocene‐dated speciation of forest‐dependent mosquitoes in Southeast Asia

Tropical forests have undergone repeated fragmentation and expansion during Pleistocene glacial and interglacial periods, respectively. The effects of this repeated forest fragmentation in driving vicariance in tropical taxa have been well studied. However, relatively little is known about how often this process results in allopatric speciation, since it may be inhibited by recurrent gene flow during repeated secondary contact, or to what extent Pleistocene‐dated speciation results from ecological specialization in the face of gene flow. Here, divergence times and gene flow between three closely‐related mosquito species of the Anopheles dirus species complex endemic to the forests of Southeast Asia, are inferred using coalescent based Bayesian analysis. An Isolation with Migration model is applied to sequences of two mitochondrial and three nuclear genes, and 11 microsatellites. The divergence of An. scanloni has occurred despite unidirectional nuclear gene flow from this species into An. dirus. The inferred asymmetric gene flow may result from the unique evolutionary adaptation of An. scanloni to limestone karst habitat, and therefore the fitness advantage of this species over An. dirus in regions of sympatry. Mitochondrial introgression has led to the complete replacement of An. dirus haplotypes with those of An. baimaii through a recent (～62 kya) selective sweep. Speciation of An. baimaii and An. dirus is inferred to have involved allopatric divergence throughout much of the Pleistocene. Secondary contact and bidirectional gene flow has occurred only within the last 100 000 years, by which time the process of allopatric speciation seems to have been largely completed.     

Ontogeny of worker body size distribution in bumble bee (Bombus impatiens) colonies

1. Bumble bees exhibit worker size polymorphisms; highly related workers within a colony may vary up to 10‐fold in body mass. As size variation is an important life history feature in bumble bees, the distribution of body sizes within the colony and how it fluctuates over the colony cycle were analysed. 2. Ten commercially purchased colonies of Bombus impatiens (Cresson) were reared in ad libitum conditions. The size of all workers present and newly emerging workers (callows) was recorded each week. 3. The average size of bumble bee workers did not change with colony age, but variation in body size tended to decrease over time. The average size of callows did not change with population size, but did tend to decrease with colony age. In all measures, there was considerable variation among colonies. 4. Colonies of B. impatiens usually produced workers with normally distributed body sizes throughout the colony life cycle. Unlike most polymorphic ants, there was no increase in worker body size with colony age or colony size. This provides the first, quantitative data on the ontogeny of bumble bee worker size distribution. The potential adaptive significance of this size variation is discussed.     

DEVELOPMENT OF A PCR ASSAY SUITABLE FOR CAMPYLOBACTER SPP. MASS SCREENING PROGRAMS IN BROILER PRODUCTION

Campylobacter is the most common cause of human acute bacterial gastroenteritis worldwide. In order to comply with the demands of consumers for food free of bacterial pathogens, a mass screening program for Campylobacter in Danish broilers has been carried out based on conventional bacterial culture techniques since 1998. However, using conventional culture methods is time consuming and laborious, and therefore a Polymerase Chain Reaction (PCR) Campylobacter detection assay suitable for mass screening of cloacal swab samples from broilers was developed. By comparing the PCR detection with conventional culture methods, significantly more samples were found positive for Campylobacter with the PCR method. The PCR method is rapid, sensitive and suitable for mass screening for Campylobacter in poultry. Using this PCR method Campylobacter can be detected within 15 h. Notably, the method can be applied to detect Campylobacter directly from chicken feces at the species level.     

美国海滨桤木和薄叶桤木水分生理特性的比较

采取盆栽、人工控水的方式, 研究并比较了美国本土海滨桤木(Alnus maritima)和薄叶桤木(A. incana)的气孔导度(G_s)、叶片水势(ψ_leaf)以及渗透调节能力对土壤水分条件的响应, 以探讨引起两种桤木生态分布差异巨大的生理生态原因。结果表明: 1)正常水分条件下, 海滨桤木的G_s低于薄叶桤木, 其与大气温度、相对湿度和水蒸气亏缺等气象因子的相关性低于薄叶桤木; 干旱胁迫下, 海滨桤木的G_s对其自身ψ_leaf下降信号的敏感度低于薄叶桤木; 复水后, 其G_s恢复更为缓慢。2)正常水分条件下, 海滨桤木的ψ_leaf高于薄叶桤木, 且引起气孔关闭的ψ_leaf临界值较高; 干旱胁迫下, 海滨桤木的ψ_leaf下降幅度高于薄叶桤木。3)正常水分条件下, 海滨桤木和薄叶桤木的渗透调节能力无显著差异; 干旱胁迫下, 尽管两种桤木均表现出饱和状态渗透势(ψ_s^sat)下降、膨压与水势关系的最大变化率降低、初始失膨点渗透势(ψ_s^tlp)增加、细胞渗透调节能力范围(ψ_s^sat-ψ_s^tlp, Dψ_s)减小的趋势, 但与薄叶桤木相比, 海滨桤木的ψ_s^tlp较高, Dψ_s较小。从以上生理生态指标可以看出, 较高的叶片水势、较低的气孔调节能力、干旱下较低的渗透调节能力是造成海滨桤木分布范围狭小的重要原因。