An artificial chimeric derivative of Citrus viroid V involves the terminal left domain in pathogenicity

The recently described Citrus viroid V (CVd-V) induces, in Etrog citron, mild stunting and very small necrotic lesions and cracks, sometimes filled with gum. As Etrog citron plants co-infected with Citrus dwarfing viroid (CDVd) and CVd-V show synergistic interactions, these host–viroid combinations provide a convenient model to identify the pathogenicity determinant(s). The biological effects of replacing limited portions of the rod-like structure of CVd-V with the corresponding portions of CDVd are reported. Chimeric constructs were synthesized using a novel polymerase chain reaction-based approach, much more flexible than those based on restriction enzymes used in previous studies. Of the seven chimeras (Ch) tested, only one (Ch5) proved to be infectious. Plants infected with Ch5 showed no symptoms and, although this novel chimera was able to replicate to relatively high titres in singly infected plants, it was rapidly displaced by either CVd-V or CDVd in doubly infected plants. The results demonstrate that direct interaction(s) between structural elements in the viroid RNA (in this case, the terminal left domain) and as yet unidentified host factors play an important role in modulating viroid pathogenicity. This is the first pathogenic determinant mapped in species of the genus Apscaviroid .     

Impact of ocean barriers, topography, and glaciation on the phylogeography of the catfish Trichomycterus areolatus (Teleostei: Trichomycteridae) in Chile

We examined the role of several earth history events on the phylogeographic distribution of the catfish Trichomycterus areolatus in Chile using the cytochrome b gene. We explored three biogeographic hypotheses: that sea level changes have resulted in the isolation of populations by drainages; that glaciation has impacted genetic diversity; and that ichthyological subprovince boundaries correspond to phylogeographic breaks in our focal species. We found seven well-supported clades within T. areolatus with high levels of genetic divergence. The strongest signal in our data was for an important role of sea level changes structuring populations. Five of the seven clades mapped cleanly to the geographic landscape and breaks corresponded closely to areas of narrowest continental shelf. In addition, few haplotypes were shared between rivers within clades, suggesting that only limited local movement of individuals has occurred. There was no relationship between the levels of genetic diversity and the proportion of individual drainages covered by glaciers during the last glacial maximum. Two phylogeographic breaks within T. areolatus did match the two previously identified faunal boundaries, but we found three additional breaks, which suggests that faunal breaks have only limited utility in explaining phylogeographic patterns. These results imply that the narrow continental shelf coupled with sea level changes had a strong influence on the obligate freshwater fishes in Chile.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 876–892.     

Spatial expansion and population structure of the neotropical malaria vector, Anopheles darlingi (Diptera: Culicidae)

Extensive population structuring is known to occur in Anopheles darlingi , the primary malaria vector of the Neotropics. We analysed the phylogeographic structure of the species using the mitochondrial cytochrome oxidase I marker. Diversity is divided into six main population groups in South America: Colombia, central Amazonia, southern Brazil, south-eastern Brazil, and two groups in north-east Brazil. The ancestral distribution of the taxon is hypothesized to be central Amazonia, and there is evidence of expansion from this region during the late Pleistocene. The expansion was not a homogeneous front, however, with at least four subgroups being formed due to geographic barriers. As the species spread, populations became isolated from each other by the Amazon River and the coastal mountain ranges of south-eastern Brazil and the Andes. Analyses incorporating distances around these barriers suggest that the entire South American range of An. darlingi is at mutation–dispersal–drift equilibrium. Because the species is distributed throughout such a broad area, the limited dispersal across some landscape types promotes differentiation between otherwise proximate populations. Moreover, samples from the An. darlingi holotype location in Rio de Janeiro State are substantially derived from all other populations, implying that there may be additional genetic differences of epidemiological relevance. The results obtained contribute to our understanding of gene flow in this species and allow the formulation of human mosquito health protocols in light of the potential population differences in vector capacity or tolerance to control strategies.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 854–866.     

Seed survival and dispersal of an endemic Atlantic forest palm: the combined effects of defaunation and forest fragmentation

About 45 palm species occur in the Atlantic forest of Brazil, and most of them are affected by loss of seed dispersers resulting from forest fragmentation and hunting. Here we report the effects of habitat loss and defaunation on the seed dispersal system of an endemic palm, Astrocaryum aculeatissimum . We evaluated seed removal, insect and rodent seed predation, and scatter-hoarding in nine sites, ranging from 19 ha to 79 000 ha. We report the seedling, juvenile and adult palm densities in this range of sites. Endocarps remaining beneath the parent palm had a higher probability of being preyed upon by insects in small, mostly fragmented and more defaunated sites. The frequency of successful seed removal, scatter-hoarding and consumption by rodents increased in the larger, less defaunated sites. Successful removal and dispersal collapsed in small (< 1000 ha), highly defaunated sites and frequently resulted in low densities of both seedlings and juveniles. Our results indicate that a large fraction of Atlantic forest palms that rely on scatter-hoarding rodents may become regionally extinct due to forest fragmentation and defaunation. Current management practices including palm extraction and hunting pressure have a lasting effect on Atlantic forest palm regeneration by severely limiting successful recruitment of prereproductive individuals. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 141–149.     

A morphometric analysis of a putative hybrid between Agave marmorata Roezl and Agave kerchovei Lem.: Agave peacockii Croucher

A supposed hybrid population ( Agave peacockii ) between Agave marmorata and Agave kerchovei in a semiarid ecosystem at Zapotitlán Salinas, in the Mexican State of Puebla, was investigated, and its hybrid status corroborated in morphometric terms. A Stepwise Discriminant Analysis of the ratios: Number of leaved Rosette diameter, Leaf width/Leaf length, Number of spines/Leaf length, Distance between the upper spine and the tip of the leaf/Leaf length, and Distance between the upper spine and the tip of the leaf/Number of spines, supported our hypothesis of the existence of these naturally occurring hybrids, which are intermediate and distinct from the parental species. A null model was constructed to contrast the real case with a possible artifact, and the results also supported our hypothesis. The possible meaning of this kind of hybridization is discussed.     

Regulation of Nitrate Reductase in Acetabularia mediterranea

Nitrate reductase (NR) activity has been characterized on cell-freeextracts of the marine unicellular green alga, Acetabulariamediterranea. Reduced methyl viologen and bromophenol blue aswell as NAD(P)H were effective electron donors for the enzyme. NADH-NR activity increased during cell development reachingits maximum level when the cells approached their maximum length.A substantial increase was also seen in enucleated cells. Theenzyme activity was only present in the green parts of the celland became diminished as the stalks bleached in ageing cells.Under a 12 h light: 12 h darkness photoperiod, NADH-NR activityexhibited pronounced daily fluctuations reaching its maximumon the first hour of illumination (this increase was impairedby cycloheximide) and a minimum in the middle of the dark period. In N-starved algal cultures, NR was constitutively present atappreciable levels and new synthesis took place in the presenceof either NO₃ or low concentrations of NH⁺₄ . However, in thepresence of this cation the enzyme remained mostly in its inactiveform, and could be reactivated in vitro with ferrycyanide. Key words: Nitrate reductase, Acetabularia, daily rhythms, nitrate, ammonium     

Sabotaging behaviour and minimal latex of Asclepias curassavica incur no cost for larvae of the southern monarch butterfly Danaus erippus

1. The southern monarch, Danaus erippus, uses mainly Asclepias curassavica as its host in the Neotropics, a plant species bearing articulated anastomosing laticifers. When artificially severed, A. curassavica has been shown to release significantly less latex than other Asclepias species. 2. The present study tested the hypothesis that sabotaging behaviour changes during the ontogeny of D. erippus and recorded latex outflow of A. curassavica during sabotaging and feeding. Larvae displayed vein‐cutting behaviour, which was initially observed in the second instar, became more pronounced in the third and fourth instars, and less frequent in the fifth instar. When present, latex outflow was never more than 1 µl at a time during either vein cutting or feeding, regardless of the instar. 3. Mandibular and midrib morphometrics revealed that larvae selected thicker midrib sites for severing as instars progressed; however, no correlation between mandibular size and midrib size severed was found within instars. 4. Costs of sabotaging behaviour and the effects of A. curassavica latex outflow on D. erippus larvae were also examined. Sabotaging behaviour did not incur growth costs for larvae, and only latex exudation volumes at least 10‐fold greater than those observed due to D. erippus sabotaging or feeding, caused significantly higher larval mortality than controls. 5. Since latex outflow is minimal or non‐existent in A. curassavica, sabotaging behaviour in D. erippus is mostly limited by morphological constraints and is probably driven by chemical stimulants rather than latex defence. In turn, latex does not constitute a major defence of A. curassavica against D. erippus.     

A new Mississippian problematicum from Ireland and Britain: comparison with the problematicum Draffania

Upper Viséan platform limestones from Ireland and northern England contain occasionally hemispherical to bean-shaped structures attached to fenestellid bryozoan colonies. These problematic structures are described and compared with another Mississippian problematicum, Draffania. Both problematic taxa exhibit a similar skeletal structure, but have some different morphological features. They also have comparable stratigraphic records and occupied similar palaeoecological niches. Possible affinities of this new problematicum with foraminiferans, crinoids, algae, and bryozoans are discussed, but its definite taxonomic status remains to be elucidated.