Hydroxyurea (HU) is a DNA replication inhibitor that negatively affects both the elongation and initiation phases of replication and triggers the "intra-S phase checkpoint." Previous work with budding yeast has shown that, during a short exposure to HU, MEC1/RAD53 prevent initiation at some late S phase origins. In this study, we have performed microarray experiments to follow the fate of all origins over an extended exposure to HU. We show that the genome-wide progression of DNA synthesis, including origin activation, follows the same pattern in the presence of HU as in its absence, although the time frames are very different. We find no evidence for a specific effect that excludes initiation from late origins. Rather, HU causes S phase to proceed in slow motion; all temporal classes of origins are affected, but the order in which they become active is maintained. We propose a revised model for the checkpoint response to HU that accounts for the continued but slowed pace of the temporal program of origin activation.

Natural enemies can be significant sources of mortality for herbivorous insects and therefore important agents of natural selection. One might expect selection to favor herbivores that escape from their natural enemies into enemy-free space. Although this is an appealing idea, it has received little empirical support, and no studies have documented enemy-free space as part of a nonagricultural, nonartificial host shift. The Alaskan swallowtail butterfly, Papilio machaon aliaska, uses as host plants a species in the family Apiaceae (Cnidium cnidiifolium) along with two Asteraceae species (Artemisia arctica and Petasites frigidus). I analyzed growth and survival of P. m. aliaska larvae in the field on the three host plants in treatments that either exposed or protected them from predators. I found that, in the presence of predators, larval survival is greater on the novel hosts (Asteraceae) than on the ancestral host (Apiaceae), but that in the absence of predators survival and growth are greater on the ancestral host. These results are a demonstration of enemy-free space as a mechanism for maintaining a naturally occurring host shift.Papilio machaon aliaska ͉ Lepidoptera ͉ predation ͉ Formica podzolica S wallowtail butterflies from the Papilio machaon (Linnaeus) group use plants of the Apiaceae as their primary hosts (1-4). Behavioral and metabolic constraints evidently limit potential opportunities to switch to other co-occurring plant species (5). Apart from the occasional use of plants in the family Rutaceae, an ancestral host family for the genus Papilio (2), P. machaon swallowtails have rarely incorporated nonapiaceous plants into their diet. In Alaska and northwestern Canada, P. m. aliaska Scudder oviposits and feeds not only on the local apiaceous host, Cnidium cnidiifolium (Turczaninow) Schischkin, but also on Artemisia arctica Lessing and Petasites frigidus (Linnaeus) Franchet (6) in the distantly related family Asteraceae. This host-range expansion by P. m. aliaska appears to represent an intermediate step toward a complete host shift. There is at least one example of a species in the P. machaon group that is now restricted to the novel host genus Artemisia (2); Papilio oregonius Edwards, a close relative of P. m. aliaska (2), is monophagous on Artemisia dracunculus Linnaeus (7). It is unclear, however, whether P. m. aliaska and P. oregonius represent a single host shift or two independent host shifts to Artemisia.The P. m. aliaska system presents an ideal opportunity to examine the role of enemy-free space (EFS) in a naturally occurring host shift. Other swallowtail larvae are subject to attack by a range of invertebrate and vertebrate predators (8, 9); my observations over the past 5 years of P. m. aliaska near Fairbanks, AK, suggest that the two most important larval predators are Formica podzolica Francoeur, an ant species that is widely distributed throughout North America (10), and the ichneumonid parasitoid Trogus lapidator panzeri Carlson. Jeffries and Lawton (11) defined EFS as ''ways of living...

Primary consumers are under strong selection from resource ('bottom-up') and consumer ('topdown') controls, but the relative importance of these selective forces is unknown. We performed a meta-analysis to compare the strength of top-down and bottom-up forces on consumer fitness, considering multiple predictors that can modulate these effects: diet breadth, feeding guild, habitat/environment, type of bottom-up effects, type of top-down effects and how consumer fitness effects are measured. We focused our analyses on the most diverse group of primary consumers, herbivorous insects, and found that in general top-down forces were stronger than bottom-up forces. Notably, chewing, sucking and gall-making herbivores were more affected by top-down than bottom-up forces, top-down forces were stronger than bottom-up in both natural and controlled (cultivated) environments, and parasitoids and predators had equally strong top-down effects on insect herbivores. Future studies should broaden the scope of focal consumers, particularly in understudied terrestrial systems, guilds, taxonomic groups and top-down controls (e.g. pathogens), and test for more complex indirect community interactions. Our results demonstrate the surprising strength of forces exerted by natural enemies on herbivorous insects, and thus the necessity of using a tri-trophic approach when studying insect-plant interactions.

During the last decade, in response to the impact of collaborative learning theory (Johnson & Johnson, 1987) and a shift in the teaching of composition from an emphasis on product to an emphasis on process (Hairston, 1982), many L2 writing instructors began to use peer response groups in their writing classes. In peer response groups, students share their working drafts with others "as the drafts are developing in order to get guidance and feedback on their writing" (Leki, 1993, p. 22). The essence of peer response is students' providing other students with feedback on their preliminary drafts so that the student writers may acquire a wider sense of audience and work toward improving their compositions.First language studies (e.g., Nystrand & Brandt, 1989) provide a persuasive argument in favor of writing groups. However, the findings of L 1 studies do not necessarily apply to L2 students. With respect to peer response groups, L2 students differ from L1 students in at least two BRIEF REPORTS AND SUMMARIES 135

Numerous studies have examined relationships between primary production and biodiversity at higher trophic levels. However, altered production in plant communities is often tightly linked with concomitant shifts in diversity and composition, and most studies have not disentangled the direct effects of production on consumers. Furthermore, when studies do examine the effects of plant production on animals in terrestrial systems, they are primarily confined to a subset of taxonomic or functional groups instead of investigating the responses of the entire community. Using natural monocultures of the salt marsh cordgrass Spartina alterniflora, we were able to examine the impacts of increased plant production, independent of changes in plant composition and/or diversity, on the trophic structure, composition, and diversity of the entire arthropod community. If arthropod species richness increased with greater plant production, we predicted that it would be driven by: (1) an increase in the number of rare species, and/or (2) an increase in arthropod abundance. Our results largely supported our predictions: species richness of herbivores, detritivores, predators, and parasitoids increased monotonically with increasing levels of plant production, and the diversity of rare species also increased with plant production. However, rare species that accounted for this difference were predators, parasitoids, and detritivores, not herbivores. Herbivore species richness could be simply explained by the relationship between abundance and diversity. Using nonmetric multidimensional scaling (NMDS) and analysis of similarity (ANOSIM), we also found significant changes in arthropod species composition with increasing levels of production. Our findings have important implications in the intertidal salt marsh, where human activities have increased nitrogen runoff into the marsh, and demonstrate that such nitrogen inputs cascade to affect community structure, diversity, and abundance in higher trophic levels.

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This article discusses the teaching of oral communication skills (that is, speaking, listening, and pronunciation) in programs of English as a Second Language. The article is addressed to teachers who conduct courses in this area for ESL students in secondary schools, colleges, and universities although the guidelines presented can be adapted to other ESL contexts (e.g., continuing education, private tutorials). Speaking and listening are discussed as major skill areas; pronunciation is presented as a subset of both speaking and listening development. This article argues that attention to these three components of oral communication is viewed as indispensable to any coherent curriculum design. Although relative degrees of emphasis may vary for particular courses, speaking, listening, and pronunciation are characterized as reciprocally interdependent oral language processes.

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