Experimental conversion of colony social organization in fire ants (Solenopsis invicta): worker genotype manipulation in absence of queen effects

Colony social organization in the fire ant Solenopsis invicta appears to be under strong genetic control. In the invasive USA range, polygyny (multiple queens per colony) is marked by the presence of the Gp-9(b) allele in most of a colony's workers, whereas monogyny (single queen per colony) is associated with the exclusive occurrence of the Gp-9(B) allele. Ross and Keller, Behav Ecol Sociobiol 51:287-295 (2002) experimentally manipulated social organization by cross-fostering queens into colonies of the alternate form, thereby changing adult worker Gp-9 genotype frequencies over time. Although these authors showed that social behavior switched predictably when the frequency of b-bearing adult workers crossed a threshold of 5-10%, the possibility that queen effects caused the conversions could not be excluded entirely. We addressed this problem by fostering polygyne brood into queenright monogyne colonies. All such treatment colonies switched social organization to become polygyne, coincident with their proportions of b-bearing workers exceeding 12%. Our results support the conclusion that polygyny in S. invicta is induced by a minimum frequency of colony workers carrying the b allele, and further confirm that its expression is independent of queen genotype or history, worker genotypes at genes not linked to Gp-9, and colony genetic diversity.

Ecology and evolution of social-organization: insights from fire ants and other highly eusocial insects

Social organisms exhibit conspicuous intraspecific variation in all facets of their social organization. A prominent example of such variation in the highly eusocial Hymenoptera is differences in the number of reproductive queens per colony, Differences in queen number in ants are associated with differences in a host of reproductive and social traits, including queen phenotype and breeding strategy, mode of colony reproduction, and pattern of sex allocation. We examine the causes and consequences of changes in colony queen number and associated traits using the fire ant Solenopsis invicta as a principal model. Ecological constraints on mode of colony founding may act as important selective forces causing the evolution of queen number in this and many other ants, with social organization generally perpetuated across generations by means of the social environment molding appropriate queen phenotypes and reproductive strategies. Shifts in colony queen number have profound effects on genetic structure within nests and may also influence genetic structure at higher levels (aggregations of nests or local demes) because of the association of queen number with particular mating and dispersal habits. Divergence of breeding habits between populations with different social organizations has the potential to promote genetic differentiation between these social variants. Thus, evolution of social organization can be important in generating intrinsic selective regimes that channel subsequent social evolution and in initiating the development of significant population genetic structure, including barriers to gene flow important in cladogenesis.

Sociogenetics of fire ants

RÉSUMÉ GRAND PUBLIC La complexité des sociétés d'insectes (telles que les abeilles, les termites ou les fourmis) a depuis longtemps fasciné l'Homme. Depuis le débfit du XIXème siècle, de nombreux travaux observationnels, comportementaux et théoriques leur on été consacrés afin de mieux les décrire et comprendre. L'avènement de la biologie moléculaire à la fin du XXèrne siècle a offert de nouveaux outils scientifiques pour identifier et étudier les gènes et molécules impliqués dans le développement et le comportement des êtres vivants. Alors que la majorité de ces études s'est focalisée sur des organismes de laboratoire tel que la mouche ou les nématodes, l'utilisation de ces outils est restée marginale jusqu'à présent dans l'étude des sociétés d'insectes. Lors de ma thèse, j'ai développé des outils moléculaires permettant de déterminer le niveau d'activité de zo,ooo gènes chez la fourmi de feu, Solenopsis invicta, ainsi qu'une base de données et un portail en ligne regroupant les informations relatives à l'étude génétique des fourmis: Fourmidable. J'ai ensuite utilisé ces outils dans le cadre d'une étude comportementale chez la fourmis S. invicta. Dans les sociétés d'insectes, une hiérarchie peut déterminer le statut reproducteur des individus. Suite à la mort d'un dominant, les subordonnés entrent en compétition en vue d'améliorer leur statut. Un tel phénomène se produit au sein des colonies de S. invicta contenant une unique reine mère, des milliers d'ouvrières et des centaines de reines vierges ailées. A la mort de la reine mère, un grand nombre de reines vierges tentent de la remplacer en arrachant leurs ailes et en activant leurs organes reproducteurs plutôt que de partir en vol nuptial. Ces tentatives sont le plus souvent arrêtées par les ouvrières qui exécutent la plupart de ces reines sur la base de signaux olfactifs produits lors de l'activation des organes reproducteurs. Afin de mieux comprendre les mécanismes moléculaires impliqués, j'ai étudié l'activité de gènes au sein des reines au début de ce processus. J'ai ainsi déterminé que des gènes impliqués dans communication olfactive, le développement des organes reproducteurs et la métabolisation de l'hormone juvénile sont activês à ce moment là. La vitesse à laquelle les reines perdent leurs ailes ainsi que les niveaux d'expression de gènes sont ensuite liés à leur probabilité de survie. ABSTRACT : Honeybees, termites and ants occupy the "pinnacle of social evolution" with societies of a complexity that rivals our own. Humans have long been fascinated by social insects, but studying them has been mostly limited to observational and behavioral experiments. The advent of molecular biology first made it possible to investigate the molecular-genetic basis of development in model systems such as the fruit fly Drosophila melarcogaster or the roundworm Caenorhabditis elegans and subsequently their behavior. Molecular and genomic tools are now becoming available for the study of social insects as well. To permit genomic research on the fire ant, Solenopsis invicta, we developed a cDNA microarray that can simultaneously determine the expression levels of approximately 1oooo genes. These genes were assembled and bioinformatically annotated using custom pipelines. The obtained data formed the cornerstones for Fourmidable, a web portal centralizing sequence, gene annotation and gene expression data as well as laboratory protocols for research on ants. In many animals living in groups the reproductive status of individuals is determined by their social status. In species with social hierarchies, the death of dominant individuals typically upheaves the social hierarchy and provides an opportunity for subordinate individuals to improve their social status. Such a phenomenon occurs in the monogyne form of S. invicta, where colonies typically contain a single wingless reproductive queen, thousands of workers and hundreds of winged non-reproductive virgin queens. Upon the death of the mother queen, many virgin queens shed their wings and initiate reproductive development instead of departing on a mating flight. Workers progressively execute almost all of them over the following weeks. The workers base their collective decision on pheromonal cues associated with the onset of reproductive development of the virgin queens which occurs after orphaning. We used the aforementioned tools to determine that genes putatively involved in processes including olfactory signaling, reproductive development and Juvenile Hormone metabolism are differentially expressed at the onset of competition. Additionally, we found that queens that initiate reproductive development faster and, to a certain extent, shed their wings faster after orphaning are more likely to become replacement queens. These results provide candidate genes that are putatively linked to competition outcome. To determine the extent to which specific genes affect different aspects of life in ant colonies, functional tests such as gene activation and silencing will still be required. We conclude by discussing some of the challenges and opportunities for molecular-genetic research on ants. RÉSUMÉ Les sociétés d'abeilles, de termites et de fourmis sont d'une complexité proche de celle de la nôtre et ont depuis longtemps fasciné l'Homme. Cependant, leur étude était jusqu'à présent limitée aux observations et expériences comportementales. L'avènement de la biologie moléculaire a d'abord rendu possible l'étude moléculaire et génétique du développement d'organismes modèles tels que la mouche Drosophila melanogaster ou le nématode Caenorhabditis elegans, puis dans un second temps de leur comportement. De telles études deviennent désormais possibles pour les insectes sociaux. Nous avons développé une puce à ADN permettant de déterminer simultanément les niveaux d'expression de 1oooo gènes de la fourmi de feu, Solenopsís invicta. Ces gènes ont été séquencés puis assemblés et annotés à l'aide de pipelines que nous avons développés. En se basant sur les informations obtenues, nous avons créé un portail web, Fourmidable. Ce portail vise à centraliser toutes les informations de séquence, d'annotation et d'expression de gènes, ainsi que les protocoles de laboratoire utilisés pour la recherche sur les fourmis. Par la suite, nous avons utilisé les outils développés pour étudier un aspect particulier de S. invicta. Chez les animaux grégaires, une hiérarchie sociale peut déterminer le statut reproducteur des individus. Suite à la mort d'un individu dominant, les individus subordonnés peuvent entrer en compétition en vue d'améliorer leur statut. Un tel phénomène se produit au sein des colonies monogynes de S. invicta, qui contiennent habituellement une unique reine mère, des milliers d'ouvrières et des centaines de reines vierges ailées. Suite à la mort de la reine mère, dominante, un grand nombre de reines vierges, subordonnées, perdent leurs ailes et activent leurs organes reproducteurs au lieu de partir en vol nuptial. Au cours des semaines suivantes, les ouvrières exécutent la plupart de ces reines sur la base de signaux olfactifs produits lors de l'activation des organes reproducteurs. Afin de mieux comprendre les mécanismes moléculaires impliqués, nous avons étudié l'expression de gènes au début de cette compétition. Nous avons identifié 297 gènes différemment exprimés, dont l'annotation indique qu'ils seraient impliqués dans des processus biologiques dont la communication olfactive, le développement des organes reproducteurs et la métabolisation de l'hormone juvénile. Par la suite, nous avons déterminé que la vitesse à laquelle les reines perdent leurs ailes en début de compétition ainsi que les niveaux d'expression de gènes sont corrélés à la probabilité de survie des reines. Nous concluons en discutant des opportunités offertes par la recherche génétique sur les fourmis ainsi que les défis qu'elle devra surmonter.

Current status of a model system: the gene Gp-9 and its association with social organization in fire ants.

The Gp-9 gene in fire ants represents an important model system for studying the evolution of social organization in insects as well as a rich source of information relevant to other major evolutionary topics. An important feature of this system is that polymorphism in social organization is completely associated with allelic variation at Gp-9, such that single-queen colonies (monogyne form) include only inhabitants bearing B-like alleles while multiple-queen colonies (polygyne form) additionally include inhabitants bearing b-like alleles. A recent study of this system by Leal and Ishida (2008) made two major claims, the validity and significance of which we examine here. After reviewing existing literature, analyzing the methods and results of Leal and Ishida (2008), and generating new data from one of their study sites, we conclude that their claim that polygyny can occur in Solenopsis invicta in the U.S.A. in the absence of expression of the b-like allele Gp-9(b) is unfounded. Moreover, we argue that available information on insect OBPs (the family of proteins to which GP-9 belongs), on the evolutionary/population genetics of Gp-9, and on pheromonal/behavioral control of fire ant colony queen number fails to support their view that GP-9 plays no role in the chemosensory-mediated communication that underpins regulation of social organization. Our analyses lead us to conclude that there are no new reasons to question the existing consensus view of the Gp-9 system outlined in Gotzek and Ross (2007).

A Y-like social chromosome causes alternative colony organization in fire ants.

Intraspecific variability in social organization is common, yet the underlying causes are rarely known. In the fire ant Solenopsis invicta, the existence of two divergent forms of social organization is under the control of a single Mendelian genomic element marked by two variants of an odorant-binding protein gene. Here we characterize the genomic region responsible for this important social polymorphism, and show that it is part of a pair of heteromorphic chromosomes that have many of the key properties of sex chromosomes. The two variants, hereafter referred to as the social B and social b (SB and Sb) chromosomes, are characterized by a large region of approximately 13 megabases (55% of the chromosome) in which recombination is completely suppressed between SB and Sb. Recombination seems to occur normally between the SB chromosomes but not between Sb chromosomes because Sb/Sb individuals are non-viable. Genomic comparisons revealed limited differentiation between SB and Sb, and the vast majority of the 616 genes identified in the non-recombining region are present in the two variants. The lack of recombination over more than half of the two heteromorphic social chromosomes can be explained by at least one large inversion of around 9 megabases, and this absence of recombination has led to the accumulation of deleterious mutations, including repetitive elements in the non-recombining region of Sb compared with the homologous region of SB. Importantly, most of the genes with demonstrated expression differences between individuals of the two social forms reside in the non-recombining region. These findings highlight how genomic rearrangements can maintain divergent adaptive social phenotypes involving many genes acting together by locally limiting recombination.

Some factors influencing dealation by virgin queen fire ants

Virgin queens of the fire ant,Solenopsis invicta Buren, that were removed from the influence of the inhibitory queen pheromone, dealated more readily in the presence of workers than in their absence. During 72 hours after disinhibition, a significantly greater number of overwintered virgin queens than spring-reared virgin queens dealated when they were isolated, but the numbers that dealated in the presence of workers were very similar. Some sexually immature virgin queens dealated after disinhibition. Virgin dealates were found to be capable of preventing other virgin queens from dealating. The various factors that influence dealation by virgin queens were used to develop a bioassay for the inhibitory queen pheromone ofS. invicta. Lorsque des reines vierges de la fourmi de feu sont soustraites à l'influence de la phéromone inhibitrice produite par la reine, elles perdent leurs ailes plus facilement en présence d'ouvrières qu'en leur absence. Lorsqu'elles sont isolées pendant 72 heures après la levée de l'inhibition, un nombre significativement plus grand de reines vierges ayant hiverné perdent leurs ailes, par rapport aux reines vierges élevées au printemps. Toutefois, les nombres d'individus perdant leurs ailes en présence d'ouvrières sont similaires. Après désinhibition, quelques reines vierges immatures perdent leurs ailes. Les sexués vierges désailés sont capables de prévenir la perte des ailes chez d'autres sexués vierges. Les divers facteurs influençant la déalation chez les reines vierges ont été utilisés afin de développer un essai biologique pour la phéromone inhibitrice produite par la reine deS. invicta.

Molecular and social regulation of worker division of labour in fire ants.

Reproductive and worker division of labour (DOL) is a hallmark of social insect societies. Despite a long-standing interest in worker DOL, the molecular mechanisms regulating this process have only been investigated in detail in honey bees, and little is known about the regulatory mechanisms operating in other social insects. In the fire ant Solenopsis invicta, one of the most studied ant species, workers are permanently sterile and the tasks performed are modulated by the worker's internal state (age and size) and the outside environment (social environment), which potentially includes the effect of the queen presence through chemical communication via pheromones. However, the molecular mechanisms underpinning these processes are unknown. Using a whole-genome microarray platform, we characterized the molecular basis for worker DOL and we explored how a drastic change in the social environment (i.e. the sudden loss of the queen) affects global gene expression patterns of worker ants. We identified numerous genes differentially expressed between foraging and nonforaging workers in queenright colonies. With a few exceptions, these genes appear to be distinct from those involved in DOL in bees and wasps. Interestingly, after the queen was removed, foraging workers were no longer distinct from nonforaging workers at the transcriptomic level. Furthermore, few expression differences were detected between queenright and queenless workers when we did not consider the task performed. Thus, the social condition of the colony (queenless vs. queenright) appears to impact the molecular pathways underlying worker task performance, providing strong evidence for social regulation of DOL in S. invicta.

A simple genetic basis for complex social behaviour mediates widespread gene expression differences.

A remarkable social polymorphism is controlled by a single Mendelian factor in the fire ant Solenopsis invicta. A genomic element marked by the gene Gp-9 determines whether workers tolerate one or many fertile queens in their colony. Gp-9 was recently shown to be part of a supergene with two nonrecombining variants, SB and Sb. SB/SB and SB/Sb queens differ in how they initiate new colonies, and in many physiological traits, for example odour and maturation rate. To understand how a single genetic element can affect all these traits, we used a microarray to compare gene expression patterns between SB/SB and SB/Sb queens of three different age classes: 1-day-old unmated queens, 11-day-old unmated queens and mated, fully reproductive queens collected from mature field colonies. The number of genes that were differentially expressed between SB/SB and SB/Sb queens of the same age class was smallest in 1-day-old queens, maximal in 11-day-old queens and intermediate in reproductive queens. Gene ontology analysis showed that SB/SB queens upregulate reproductive genes faster than SB/Sb queens. For all age classes, genes inside the supergene were overrepresented among the differentially expressed genes. Consistent with the hypothesized greater number of transposons in the Sb supergene, 13 transposon genes were upregulated in SB/Sb queens. Viral genes were also upregulated in SB/Sb mature queens, consistent with the known greater parasite load in colonies headed by SB/Sb queens compared with colonies headed by SB/SB queens. Eighteen differentially expressed genes between reproductive queens were involved in chemical signalling. Our results suggest that many genes in the supergene are involved in regulating social organization and queen phenotypes in fire ants.

The role of queen pheromones in social insects: queen control or queen signal?

Queens and workers in social insect colonies can differ in reproductive goals such as colony-level sex allocation and production of males by workers. That the presence of queen(s) often seems to affect worker behaviour in situations of potential conflict has given rise to the idea of queen control over reproduction. In small colonies queen control is possible via direct aggression against workers, but in large colonies queens cannot be effectively aggressive towards all the workers. This, plus evidence that queen-produced chemicals affect worker behaviour, has led to the conclusion that physical intimidation has been replaced by pheromonal queen control, whereby queen(s) chemically manipulate workers into behaving in ways that increase the queen's fitness at the worker's expense. It is argued in this paper, however, that pheromonal queen control has never conclusively been demonstrated and is evolutionarily difficult to justify. Proposed examples of pheromonal control are more likely to be honest signals, with workers' responses increasing their own inclusive fitness. A series of experimental and field studies in which positive results would give prima facie evidence for pheromonal queen control is suggested. Finally, three terms are defined: (1) pheromonal queen control for workers or subordinate queens being chemically manipulated into acting against their own best interests; (2) pheromonal queen signal for situations where workers or subordinate queens react to queen pheromones in ways that increase their, and possibly the queens', inclusive fitness; and (3) pheromonal queen effect where changes in the workers' or subordinate queens' behaviour have an unknown consequence on their inclusive fitness.

Introduced ants in Galápagos (Floreana Island) : importance of competition, coexistence and agressive behaviors

Abstract Invasive species represent with fragmentation of habitat the most serious threats to biodiversity in the world. Galápagos Archipelago, as most oceanic islands, suffers a high rate of introduced animals and plants that affect equilibrium and biodiversity of this unique biota. Ants rank among the most devastating invaders. Their social organization confer them a high ability to adapt and to spread in new environments forming rapidly populous communities. We studied the ant community of Floreana Island composed mainly of introduced species (at least 1 S). Introduction events occurred successively during last century. The last record is Monomorium destructor arrived in the eighties. Our aim is to investigate the modalities of interaction and coexistence of these introduced species. We highlighted the competition hierarchy of the coexisting species using attractive food baits. Two species behave as competitively dominant by monopolizing an important part of resources. They are M. destructor restricted to a small area and the fire ant Solenopsis geminata widely distributed on the island. Then we evaluated the relative importance of abiotic factors and interspecific competition in structuring the community. Ecological data were collected and presence and abundance of species were estimated using different methods in a wide range of habitats. Several species showed preferences either for arid or for humid areas. The little fire ant Wasmannia auropunctata, awell-known devastating species when introduced, was exclusively found in moist habitat in and around the agricultural area situated in the upper and central part of thé island. It coexists with other species in several parts but in a restricted perimeter it excludes all other ants and worker's density on the ground is nearly 70 times higher than ant's density in similar habitats occupied by several species. But most opportunist tramp species establish everywhere without particular ecological requirement. Analyses of species co-occurrences at various levels didn't reveal any marked effect of competition in structuring ant's assemblages. We supposed that the lack ofcompetition-derived structure has to be attributed to the dynamic of the system. Indeed, across the successive census of 1996, 2003, 2004 and 2005, species distributions and abundances appeared to be highly variables. In particular harsh conditions occurring in dry season in certain parts seem to be limiting to S. geminatai. We suggest that huge variations in the local distribution of the dominant S. geminata disrupt the community organization. Finally we conducted artificial ant confrontations to evaluate to what extend an aggressive behavior at the worker level maybe linked to the ecological success of a species on the island. S. geminata was rather indifferent when confronted to a submissive species on food sources, suggesting that its competitive dominance is largely due to a numerical superiority. On the other hand M. destructor exhibits a strong agonistic behavior in similar confrontations. As soon as the presence of a competitor is detected, most workers were observed to abandon foraging and to take part in physical aggressions. Since it is still restricted nearby its introduction spot two decades after its arrival, we suggest that the energetic cost of such an aggressive behavior prevent it to spread on that island already highly colonized. Dominant invasive species such as the fire ants S. geminata and W. auropunctata have negative impacts on Galápagos fauna, disturbing the hatching of land tortoises and birds. But very little is known about the impact of other exotic ants. Indeed, impact on arthropods and generally on ground-dwelling organisms is very diffcult to evaluate. As a consequence of the dynamic character of Floreana I. ant community it is difficult to build models or to málce predictions on evolution of introduced ant fauna. But Camponotus macilentus, an abundant endemic species seems today to be little affected by introduced ant species thanks to its strong interference competition ability and its preference for arid and harsh environments. Résumé Les espèces envahissantes représentent, avec la fragmentation du paysage, la plus grande menace pour la biodiversité. L'archipel des Galápagos, comme la plupart des îles du Pacifique, compte un grand nombre d'espèces introduites qui menacent la biodiversité de ce milieu unique.. Les fourmis sont parmi les envahisseurs les plus dévastateurs. Leur organisation sociale leur permet de s'adapter et de se propager pour devenir rapidement abondantes. Nous avons étudié la communauté de fourmis sur l'île de Floreana principalement composée d'espèces introduites (au moins 15). Les introductions se sont succédées au cours du siècle précédent. La dernière espèce recensée est Monomorium destructor introduite dans les années 80. Notre objectif est de mettre à jour les modalités des interactions et de la coexistence de ces espèces introduites. Nous avons mis en évidence la hiérarchie de compétition des différentes espèces à l'aide d'appâts de nourriture. Deux espèces se comportent de façon dominante en monopolisant une part importante des ressources. Ce sont M. destructor, restreintes à un petit périmètre, et la fourmi de feu Solenopsis geminata, largement distribuée sur l'île. Nous avons évalué l'importance relative des facteurs abiotiques et de la compétition interspécifique dans la structuration des peuplements. Des données écologiques ont été collectées et la présence et l'abondance des espèces ont été estimées à l'aide de trois méthodes au sein d'une grande diversité d'habitats. Plusieurs espèces .montrent des préférences soit pour les milieux humides, soit pour les milieux arides. La petite fourmi de feu Wasmannia auropunctata, une espèce connue pour être dévastatrice dans ses sites d'introduction, est présente exclusivement dans les habitats humides dans et à proximité de la zone agricole située dans la partie centrale de l'île. Elle coexiste en plusieurs points avec d'autres espèces mais au sein d'un périmètre restreint elle exclut toute autre fourmi et atteint des densités record au sol presque 70 fois supérieures aux densités de fourmis observées sur les sites voisins occupés par plusieurs espèces. Mais la plupart des espèces vagabondes opportunistes s'établissent partout sans exigences écologiques particulières. Des analyses de cooccurrence d'espèces à plusieurs niveaux n'ont pas révélé de rôle marqué de la compétition dans la structuration des communautés. Nous supposons que l'absence d'une telle structure doit être attribuée à la dynamique du système. En effet, au cours des différents recensements de 1996-1997, 2003, 2004 et 2005, la distribution et l'abondance des espèces était très variable. En particulier, les conditions rudes qui règnent dans la zone aride durant la saison sèche semblent affecter particulièrement S. geminata. Nous suggérons que de fortes variations dans la distribution de l'espèce dominante perturbent l'orgaiùsation des communautés. Finalement nous avons effectué des confrontations artificielles pour évaluer dans quelle mesure un comportement agressif au niveau de l'ouvrière peut être lié au succès écologique d'une espèce sur l'île. S. geminata montre très peu de réaction face à une espèce subordonnée sur une même source de nourriture, ce qui laisse supposer que sa dominance est largement due à sa supériorité numérique. Par contre, dans des conditions similaires, M. destructor est fortement agressive. En présence d'un compétiteur, la plupart des ouvrières renoncent très vite à leur activité de fourragement pour agresser les individus de l'autre espèce. Puisque deux décennies après son introduction elle est toujours confinée à son point d'arrivée, nous supposons que le coût en énergie et en ouvrières de ce comportement très agressif est un obstacle à son expansion sur cette île déjà fortement colonisée. Les espèces envahissantes dominantes comme les fourmis de feu S. geminata et W. auropunctata sont connues pour leur impact négatif sur la faune des Galápagos, entre autre sur les jeunes des tortues terrestres et des oiseaux. Mais nous savons très peu de choses sur l'impact des autres espèces de fourmis introduites. En effet, l'impact sur les arthropodes, et plus généralement sur la faune du sol, est très difficile à évaluer. En raison du caractère dynamique de la communauté de fourmi de Floreana, il est difficile de construire des modèles et de faire des prédictions sur l'évolution des peuplements de fourmis introduites. Mais Camponotus macilentus, une espèce endémique abondante, semble aujourd'hui peu affectée par les espèces introduites grâce à ses capacités de compétition par interférence et sa préférence pour les milieux arides. Resumen Las, especies invasoras representan, junto con la fragmentación del paisaje, la mayor amenaza para la biodiversidad. El archipiélago de Galápagos, como la mayoría de las islas del Pacífico, cuenta con un gran número de especies introducidas que amenazan la biodiversidad de este lugar único. Las hormigas son uno de los invasores más devastadores. Su organización social les permite adaptarse y propagarse para ser rápidamente abundante. Estudiamos la comunidad de hormigas de la isla Floreana principalmente compuesta de especies introducidas (al menos 15). Las introducciones se sucedieron durante el siglo anterior. La última especie contabilizada es Monomorium destructor introducida en los años 80. Nuestro objetivo es poner al día las modalidades de las interacciones y de la coexistencia de estas especies introducidas. Pusimos de relieve la jerarquía de competencia de las distintas especies con ayuda de cebos de comida. Dos especies se implican de manera dominante monopolizando una parte importante de los recursos. Son M. destructor, limitado a un pequeño perímetro, y la hormiga de fuego Solenopsis geminata; ampliamente distribuida por la isla. Evaluamos la importancia relativa de los factores abióticos y de la competencia interespecífica en la estructuración de la communidad. Se recogieron algunos datos ecológicos y se consideraron la presencia y la abundancia de las especies con ayuda de tres métodos en una gran diversidad de hábitats. Varias especies muestran preferencias o por los medios húmedos, o por los medios áridos. La pequeña hormiga de fuego Wasmannia auropunctata, una especie conocida por ser devastadora en sus lugares de introducción, está presente exclusivamente en los hábitats húmedos y cerca de la zona agrícola situada en la parte central de la isla. Coexiste en varios puntos con otras especies pero en un perímetro limitado excluye a cualquier otra hormiga y alcanza densidades en el suelo casi 70 veces superiores a las densidades de hormigas observadas en los lugares vecinos ocupados por varias especies. Pero la mayoría de las especies vagabundas oportunistas se establecen por todas partes sin exigencias ecológicas particulares. Análisis de cooccurrencía de las especies a varios niveles no revelaron una grande importancía de la competencia en la estructuración de las comunidades. Suponemos que la ausencia de tal estructura Bebé ser por la dinámica del sistema. Efectivamente, durante los distintos censos de 1996-1997, 2003,.2004 y 2005, la distribución ? la abundancia de las especies eran muy variables. En particular, las condiciones drásticas que reinan en la zona árida durante la temporada seca parecen afectar especialmente a S. geminata. Sugerimos que fuertes variaciones en la distribución de la especie dominante perturben la organización de las comunidades. Finalmente efectuamos confrontaciones artificiales para evaluar hastá que punto un comportamiento agresivo a nivel de la obrera puede explicar el éxito ecológico de una especie en la isla. S. geminata muestra muy poca reacción ante una especie subordinada mientras comparten la misma comida, lo que deja suponer que su dominancia se debe a su superioridad numérica. Por el contrario, en condiciones similares, M. destructor es muy agresivo. En presencia de otra especie, la mayóría de las obreras renuncian muy rápidamente a alimentarse para atacar a los individuos de la otra especie. Puesto que dos décadas después de su introducción todavía se confina en su punto de llegada, suponemos que el coste en energía y en obreras de este comportamiento muy agresivo es un obstáculo a su extensión en esta isla ya muy colonizada. Las especiés invasoras dominantes como las hormigas de fuego S. geminata y W. auropunctata son conocidas por su impacto negativo en la fauna de Galápagos, entre otras cosas sobre los juveniles de las tortugas terrestres y pájaros. Pero sabemos muy poco sobre el impacto de las otras especies de hormigas introducidas. Efectivamente es muy difïcil de evaluar el impacto en los artrópodos, y más generalmente en la fauna del suelo. Debido al carácter dinámico de la comunidad de hormiga de Floreana, es diEcil construir modelos y hacer predicciones sobre la evolución de las poblaciones de hormigas introducidas. Pero Camponotus macilentus, una especie endémica abundante, parece poco afectadá hoy por las especies introducidas gracias a sus capacidades de competencia por interferencia y su preferencia por los medios áridos.

Visualization and quality assessment of de novo genome assemblies.

Recent technological progress has greatly facilitated de novo genome sequencing. However, de novo assemblies consist in many pieces of contiguous sequence (contigs) arranged in thousands of scaffolds instead of small numbers of chromosomes. Confirming and improving the quality of such assemblies is critical for subsequent analysis. We present a method to evaluate genome scaffolding by aligning independently obtained transcriptome sequences to the genome and visually summarizing the alignments using the Cytoscape software. Applying this method to the genome of the red fire ant Solenopsis invicta allowed us to identify inconsistencies in 7%, confirm contig order in 20% and extend 16% of scaffolds.Scripts that generate tables for visualization in Cytoscape from FASTA sequence and scaffolding information files are publicly available at https://github.com/ksanao/TGNet.

Convergent genetic architecture underlies social organization in ants.

Complex adaptive polymorphisms are common in nature, but what mechanisms maintain the underlying favorable allelic combinations [1-4]? The convergent evolution of polymorphic social organization in two independent ant species provides a great opportunity to investigate how genomes evolved under parallel selection. Here, we demonstrate that a large, nonrecombining "social chromosome" is associated with social organization in the Alpine silver ant, Formica selysi. This social chromosome shares architectural characteristics with that of the fire ant Solenopsis invicta [2], but the two show no detectable similarity in gene content. The discovery of convergence at two levels-the phenotype and the genetic architecture associated with alternative social forms-points at general genetic mechanisms underlying transitions in social organization. More broadly, our findings are consistent with recent theoretical studies suggesting that suppression of recombination plays a key role in facilitating coordinated shifts in coadapted traits [5, 6].

Relaxed selection is a precursor to the evolution of phenotypic plasticity.

Phenotypic plasticity allows organisms to produce alternative phenotypes under different conditions and represents one of the most important ways by which organisms adaptively respond to the environment. However, the relationship between phenotypic plasticity and molecular evolution remains poorly understood. We addressed this issue by investigating the evolution of genes associated with phenotypically plastic castes, sexes, and developmental stages of the fire ant Solenopsis invicta. We first determined if genes associated with phenotypic plasticity in S. invicta evolved at a rapid rate, as predicted under theoretical models. We found that genes differentially expressed between S. invicta castes, sexes, and developmental stages all exhibited elevated rates of evolution compared with ubiquitously expressed genes. We next investigated the evolutionary history of genes associated with the production of castes. Surprisingly, we found that orthologs of caste-biased genes in S. invicta and the social bee Apis mellifera evolved rapidly in lineages without castes. Thus, in contrast to some theoretical predictions, our results suggest that rapid rates of molecular evolution may not arise primarily as a consequence of phenotypic plasticity. Instead, genes evolving under relaxed purifying selection may more readily adopt new forms of biased expression during the evolution of alternate phenotypes. These results suggest that relaxed selective constraint on protein-coding genes is an important and underappreciated element in the evolutionary origin of phenotypic plasticity.

The assessment of reproductive success of queens in ants and other social insects

Recently considerable research has focused on the causes of evolution of multiple-queen (polygynous) colonies. In order to better understand the factors which may have led to these polygynous associations it is vital to compare the reproductive success of queens in monogynous (one queen per colony) and polygynous colonies as well as the relative fitness of queens in polygynous colonies. This paper addresses the difficulties arising from such comparisons and their implications with regard to the methods commonly used to assess reproductive success in queens. The relative reproductive success of queens in monogynous and polygynous colonies is commonly assessed by comparing the relative number of reproductives they produce during a single reproductive season. However, shift in queen number seems to be only one aspect of a profound shift in social structure and reproductive strategy that constitutes, in effect, a ''polygyny syndrome''. For example, female reproductives produced in polygynous colonies frequently use a different mode of colony founding, which in turn affects the probability of their survival. Furthermore, queens from monogynous and polygynous colonies frequently differ in their life-span and the number of sexual broods they produce. As a result, the reproductive success of queens in monogynous and polygynous colonies may not be directly related to the relative number of sexuals they produce during a single reproductive season.