Genetic evidence for posterior specification by convergent extension in the Xenopus embryo

Genetic studies substantiate that mesodermal convergent extension expressed behind the anteroposterior borderline, in the form of a gradient with the posterior apex after gastrulation, regulates morphogenesis of the posterior zone at the dorsal and dorso-lateral levels which is in full agreement with the model of dorsalization–caudalization. In contrast, how anteroposterior specification of mesodermal tissues occurs at the ventral and latero-ventral levels is not yet understood.     

CYTONUCLEAR THEORY FOR HAPLODIPLOID SPECIES AND X-LINKED GENES. II. STEPPING-STONE MODELS OF GENE FLOW AND APPLICATION TO A FIRE ANT HYBRID ZONE

We develop cytonuclear, hybrid zone models for haplodiploid species or X-linked genes in diploid species using a stepping-stone framework of migration, in which migration rates vary with both direction and sex. The equilibrium clines for the allele frequencies, cytonuclear disequilibria, and frequencies of pure parental types are examined for species with diagnostic markers, under four important migration schemes: uniform migration of both sexes in both directions, greater migration of both sexes from one direction, greater migration of females, and greater migration of males. Of the three cytonuclear variables examined, the allele frequency clines are the most informative in differentiating among the various migration patterns. The cytonuclear disequilibria and the frequency of the pure parental types tend to be useful only in revealing directional asymmetries in migration. The extent of hybrid zone subdivision has quantitative but not qualitative effects on the distribution of cytonuclear variables, in that the allele frequency clines become more gradual, the cytonuclear disequilibria decrease in magnitude, and the frequencies of pure parentals decline with increasing subpopulation number. Also, the only major difference between the X-linked and haplodiploid frameworks is that a higher frequency of pure parentals is found when considering haplodiploids, in which male production does not require mating. The final important theoretical result is that censusing after migration yields greater disequilibria and parental frequencies than censusing after mating. We analyzed cytonuclear data from two transects from a naturally occurring hybrid zone between two haplodiploid fire ant species, Solenopsis invicta and S. richteri, using our stepping-stone framework. The frequency of S. invicta mtDNA exceeds the frequency of the S. invicta nuclear markers through much of this hybrid zone, indicating that sex differences in migration or selection may be occurring. Maximum-likelihood estimates for the migration rates are very high, due to an unexpectedly large number of pure parental types in the hybrid zone, and differ substantially between the two transects. Overall, our model does not provide a good fit, in part because the S. invicta–S. richteri hybrid zone has not yet reached equilibrium.     

Aggregation as a cost-reducing strategy for lycaenid larvae

If a mutualistic relationship entails providing services ata cost, selection will favor individuals that maximize the netbenefits of the interaction and minimize the costs. Larvae ofmany species of lycaenid butterflies secrete nutritious foodrewards to attending ants and, in return, receive protectionagainst predators and parasitoids. Because ants typically recruitmore workers to larger resources, by forming groups the larvaemay ensure more reliable access to ants and thereby gain betterprotection. A further consequence of aggregating should be achange of the cost-benefit relationship for individual larvae.The larger the group, the smaller a single larva's influencewill be on total ant density, which could lead to a smallerinvestment in secretion, thus reducing the per capita cost ofcooperation. In this study, die influence of ant attendance,group size, and companion quality on larval investment was investigated.The interaction between the obligately ant-dependent lycaenid,Jalmanus evagoras, and its attendant Iridomyrmax ants was manipulatedand the effect on larval secretion measured. As the level ofant attendance increased, the delivery of food rewards increased,bodi for solitary and for aggregated larvae. When aggregated,larvae provided less food rewards to ants dun when solitary,and secretion rate decreased with increasing group size. Furthermore,larvae had lower secretion rates when paired with a bigger,more attractive larva than when paired with a smaller one. Theconsiderable reduction in secretion rates for larvae in groupssuggests that gaining protection at a lower secretion cost couldbe one factor that promotes aggregation in myrmecophilous lycaenids.     

The adaptive bases of ant-mimicry in a neotropical aphantochilid spider (Araneae: Aphantochilidae)

The aphantochilid spider Aphanlochilus rogersi accurately mimics black ants of tribe Cephalotini, and is commonly found in the neighbourhood of its models' nests. The mimic seems to be a specialized predator of this type of ant, rejecting any insect offered as prey other than cephalotines. In the field, A. rogersi was observed preying on the model species Zacryptocerus pusillus. In captivity, the spider preyed on the models Z. pusillus and Z. depressus, as well as on the yellow non-model Z. clypeatus. Recognition of correct prey by A. rogersi appears to be based primarily on visual and tactile stimuli. Capturing ant prey from behind was the most common attack tactic observed in A. rogersi, and is probably safer than frontal attacks, as in this case the spider can be bitten on the legs before the ant is immobilized. Aphanlochilus rogersi, when feeding on the hard-bodied ant models, uses the ant corpses as a ‘protective shield’ against patrolling ants of the victim's colony and resembles an ant carrying a dead companion. Certain types of mimetic traits in A. rogersi (close similarity to ant models in integument texture and pilosity of body and legs), together with ‘shielding behaviour’, are thought to function as ant-deceivers, facilitating the obligatory intimate contact the mimic must make with cephalotines in order to capture a prey among other ants. The close similarity in the arrangement of dorsal spines, body shape, integument brightness and locomotion, together with antennal illusion, is regarded as a strategy of A. rogersi for deceiving visually-hunting predators that avoid its sharp spined ant models. It is proposed that ant-mimicry in A. rogersi has both an aggressive and a Batesian adaptive component, and evolved as a result of combined selective pressures exerted both by Cephalotini ant models (through defensive behaviour towards the mimics which attack them) and predators that avoid cephalotines (through predatory behaviour toward imperfect mimics). This suggestion is schematized and discussed in terms of two tripartite mimicry systems.     

Influence of queen weight and colony origin on worker response in Solenopsis geminata

Abstract. The influence of weight and colony origin of the queen of Solenopsis geminata (F.) (Hymenoptera: Formicidae) on worker attraction is studied under laboratory conditions. In the first experiment, worker response to individual queens of different weight from the same colony is evaluated. Heavier queens are more attractive than smaller queens to their own workers. In subsequent experiments, the colony origin effect is investigated and worker response to a pair of queens of the same weight from the same or different colonies is compared. When queens are from the same colony, workers do not show a significant preference between queens. However, when queens are from a different colony, workers are significantly more attracted to their own queen than to the foreign queen. Finally, the response of workers to queens of different weight from the same or different colonies is investigated. In both cases, workers are significantly more attracted to a heavier queen than a lighter queen, even if the lighter queen is their own queen. A putative pheromonal component (E)‐6‐(1‐pentenyl)‐2H‐2‐pyranone, is not positively correlated with queen weight.     

Testing ant predation on the coffee berry borer in shaded and sun coffee plantations in Colombia

Soil‐dwelling ants, many of which are generalist predators, are more diverse in shaded than in sun coffee plantations without trees. We compared ant predation on the coffee berry borer, Hypothenemus hampei (Ferrari) (Coleoptera: Curculionidae: Scolytinae) in three shaded and three sun coffee plantations in Apía, Colombia, in both the wet and the dry seasons. We found that H. hampei adults exposed to ants for 5 days suffered higher removal in shaded plantations and in the wet season. In the laboratory, we observed that ants killed 74–99% of H. hampei adults over the course of 5 days. Ants appear to be important predators of H. hampei, particularly in shaded coffee plantations and in the wet season.     

Role of social and individual experience in interaction of the meadow ant Formica pratensis (Hymenoptera: Formicidae) with ladybird imagines and hoverfly larvae

The ability to recognize aphidophages is one of the key points in the protection ants provide aphids against their natural enemies. Behavior of honeydew collectors from nature (“field,” control) and laboratory reared “naive” ants of Formica pratensis Retzius, which had never met either “mature” workers or aphids and aphidophages, was observed during their pairwise interactions with ladybird imagines and hoverfly larvae. The majority of the “naive” ants perceived ladybirds as an enemy at their first encounter attacking them immediately without any prior antennation. Ants seem to have a certain innate “enemy image” that lets them react very quickly to protect aphids. Hoverfly larvae were rarely attacked by both “field” and “naive” ants (>15%). During tests with ladybirds ants from nature attacked them and also demonstrated the most aggressive reactions (series of bites and “death grip”) less frequently than the “naive” ants. The percentage of ants avoiding aphidophages after a contact with their chemical defense (reflex bleeding and glue‐like saliva) was significantly higher in the control group. Whereas the “naive” ants did not learn to avoid danger, foragers from nature usually tried to avoid negative experience and used tactics of “short bites.” Overall, experience has been proved to be unimportant for displaying key behavioral reactions underlying ant–ladybird interaction. However, accumulation of experience has been assumed to play an important role in the formation of behavioral strategy that allows honeydew collectors to drive aphidophages away with lower energy costs and avoid or minimize negative consequences of aphidophages’ chemical defense.     

Pathogenic interaction between Escovopsis weberi and Leucoagaricus sp.: mechanisms involved and virulence levels

Attini are the only ants that use fresh plant material to cultivate species of Leucoagaricus, which are their source of nutrition. Escovopsis species are specialized mycoparasites of Leucoagaricus sp. and Escovopsis parasitism has a negative impact on the health of the ants' colonies. The goals of this work were: to test if the virulence of different isolates of Escovopsis weberi were the same across Leucoagaricus sp. and to analyze if structural mechanisms were related to variation in the virulence of E. weberi isolates. All E. weberi isolates were able to parasitize isolates of Leucoagaricus spp. but with striking differences in virulence, and it was shown that the contact between hyphae of both fungi was the main process that generates the degradation of Leucoagaricus isolates. Additionally, the two most virulent isolates produced hook-like protuberances, increasing the damage caused to its target. Finally, E. weberi was re-classified as a destructive biotrophic parasite.     

Plant invasions differentially affected by diversity and dominant species in native‐ and exotic‐dominated grasslands

Plant invasions are an increasingly serious global concern, especially as the climate changes. Here, we explored how plant invasions differed between native‐ and novel exotic‐dominated grasslands with experimental addition of summer precipitation in Texas in 2009. Exotic species greened up earlier than natives by an average of 18 days. This was associated with a lower invasion rate early in the growing season compared to native communities. However, invasion rate did not differ significantly between native and exotic communities across all sampling times. The predictors of invasion rate differed between native and exotic communities, with invasion being negatively influenced by species richness in natives and by dominant species in exotics. Interestingly, plant invasions matched the bimodal pattern of precipitation in Temple, Texas, and did not respond to the pulse of precipitation during the summer. Our results suggest that we will need to take different approaches in understanding of invasion between native and exotic grasslands. Moreover, with anticipated increasing variability in precipitation under global climate change, plant invasions may be constrained in their response if the precipitation pulses fall outside the normal growing period of invaders.