@unpublished{14478,
  abstract     = {Entire chromosomes are typically only transmitted vertically from one generation to the next. The horizontal transfer of such chromosomes has long been considered improbable, yet gained recent support in several pathogenic fungi where it may affect the fitness or host specificity. To date, it is unknown how these transfers occur, how common they are and whether they can occur between different species. In this study, we show multiple independent instances of horizontal transfers of the same accessory chromosome between two distinct strains of the asexual entomopathogenic fungus<jats:italic>Metarhizium robertsii</jats:italic>during experimental co-infection of its insect host, the Argentine ant. Notably, only the one chromosome – but no other – was transferred from the donor to the recipient strain. The recipient strain, now harboring the accessory chromosome, exhibited a competitive advantage under certain host conditions. By phylogenetic analysis we further demonstrate that the same accessory chromosome was horizontally transferred in a natural environment between<jats:italic>M. robertsii</jats:italic>and another congeneric insect pathogen,<jats:italic>M. guizhouense</jats:italic>. Hence horizontal chromosome transfer is not limited to the observed frequent events within species during experimental infections but also occurs naturally across species. The transferred accessory chromosome contains genes that might be involved in its preferential horizontal transfer, encoding putative histones and histone-modifying enzymes, but also putative virulence factors that may support its establishment. Our study reveals that both intra- and interspecies horizontal transfer of entire chromosomes is more frequent than previously assumed, likely representing a not uncommon mechanism for gene exchange.</jats:p><jats:sec><jats:title>Significance Statement</jats:title><jats:p>The enormous success of bacterial pathogens has been attributed to their ability to exchange genetic material between one another. Similarly, in eukaryotes, horizontal transfer of genetic material allowed the spread of virulence factors across species. The horizontal transfer of whole chromosomes could be an important pathway for such exchange of genetic material, but little is known about the origin of transferable chromosomes and how frequently they are exchanged. Here, we show that the transfer of accessory chromosomes - chromosomes that are non-essential but may provide fitness benefits - is common during fungal co-infections and is even possible between distant pathogenic species, highlighting the importance of horizontal gene transfer via chromosome transfer also for the evolution and function of eukaryotic pathogens.},
  author       = {Habig, Michael and Grasse, Anna V and Müller, Judith and Stukenbrock, Eva H. and Leitner, Hanna and Cremer, Sylvia},
  booktitle    = {bioRxiv},
  title        = {{Frequent horizontal chromosome transfer between asexual fungal insect pathogens}},
  doi          = {10.1101/2023.09.18.558174},
  year         = {2023},
}

@article{13127,
  abstract     = {Cooperative disease defense emerges as group-level collective behavior, yet how group members make the underlying individual decisions is poorly understood. Using garden ants and fungal pathogens as an experimental model, we derive the rules governing individual ant grooming choices and show how they produce colony-level hygiene. Time-resolved behavioral analysis, pathogen quantification, and probabilistic modeling reveal that ants increase grooming and preferentially target highly-infectious individuals when perceiving high pathogen load, but transiently suppress grooming after having been groomed by nestmates. Ants thus react to both, the infectivity of others and the social feedback they receive on their own contagiousness. While inferred solely from momentary ant decisions, these behavioral rules quantitatively predict hour-long experimental dynamics, and synergistically combine into efficient colony-wide pathogen removal. Our analyses show that noisy individual decisions based on only local, incomplete, yet dynamically-updated information on pathogen threat and social feedback can lead to potent collective disease defense.},
  author       = {Casillas Perez, Barbara E and Bod'Ová, Katarína and Grasse, Anna V and Tkačik, Gašper and Cremer, Sylvia},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{Dynamic pathogen detection and social feedback shape collective hygiene in ants}},
  doi          = {10.1038/s41467-023-38947-y},
  volume       = {14},
  year         = {2023},
}

@article{12543,
  abstract     = {Treating sick group members is a hallmark of collective disease defence in vertebrates and invertebrates alike. Despite substantial effects on pathogen fitness and epidemiology, it is still largely unknown how pathogens react to the selection pressure imposed by care intervention. Using social insects and pathogenic fungi, we here performed a serial passage experiment in the presence or absence of colony members, which provide social immunity by grooming off infectious spores from exposed individuals. We found specific effects on pathogen diversity, virulence and transmission. Under selection of social immunity, pathogens invested into higher spore production, but spores were less virulent. Notably, they also elicited a lower grooming response in colony members, compared with spores from the individual host selection lines. Chemical spore analysis suggested that the spores from social selection lines escaped the caregivers’ detection by containing lower levels of ergosterol, a key fungal membrane component. Experimental application of chemically pure ergosterol indeed induced sanitary grooming, supporting its role as a microbe-associated cue triggering host social immunity against fungal pathogens. By reducing this detection cue, pathogens were able to evade the otherwise very effective collective disease defences of their social hosts.},
  author       = {Stock, Miriam and Milutinovic, Barbara and Hönigsberger, Michaela and Grasse, Anna V and Wiesenhofer, Florian and Kampleitner, Niklas and Narasimhan, Madhumitha and Schmitt, Thomas and Cremer, Sylvia},
  issn         = {2397-334X},
  journal      = {Nature Ecology and Evolution},
  pages        = {450--460},
  publisher    = {Springer Nature},
  title        = {{Pathogen evasion of social immunity}},
  doi          = {10.1038/s41559-023-01981-6},
  volume       = {7},
  year         = {2023},
}

@article{12696,
  abstract     = {Background: Fighting disease while fighting rivals exposes males to constraints and tradeoffs during male-male competition. We here tested how both the stage and intensity of infection with the fungal pathogen Metarhizium robertsii interfered with fighting success in Cardiocondyla obscurior ant males. Males of this species have evolved long lifespans during which they can gain many matings with the young queens of the colony, if successful in male-male competition. Since male fights occur inside the colony, the outcome of male-male competition can further be biased by interference of the colony’s worker force.
Results: We found that severe, but not yet mild, infection strongly impaired male fighting success. In late-stage infection, this could be attributed to worker aggression directed towards the infected rather than the healthy male and an already very high male morbidity even in the absence of fighting. Shortly after pathogen exposure, however, male mortality was particularly increased during combat. Since these males mounted a strong immune response, their reduced fighting success suggests a trade-off between immune investment and competitive ability already early in the infection. Even if the males themselves showed no difference in the number of attacks they raised against their healthy rivals across infection stages and levels, severely infected males were thus losing in male-male competition from an early stage of infection on.
Conclusions: Males of the ant C. obscurior have evolved high immune investment, triggering an effective immune response very fast after fungal exposure. This allows them to cope with mild pathogen exposures without cost to their success in male-male competition, and hence to gain multiple mating opportunities with the emerging virgin queens of the colony. Under severe infection, however, they are weak fighters and rarely survive a combat already at early infection when raising an immune response, as well as at progressed infection, when they are morbid and preferentially targeted by worker aggression. Workers thereby remove males that pose a future disease threat by biasing male-male competition. Our study thus revealed a novel social immunity mechanism how social insect workers protect the colony against disease risk.},
  author       = {Metzler, Sina and Kirchner, Jessica and Grasse, Anna V and Cremer, Sylvia},
  issn         = {2730-7182},
  journal      = {BMC Ecology and Evolution},
  publisher    = {Springer Nature},
  title        = {{Trade-offs between immunity and competitive ability in fighting ant males}},
  doi          = {10.1186/s12862-023-02137-7},
  volume       = {23},
  year         = {2023},
}

@phdthesis{10727,
  abstract     = {Social insects are a common model to study disease dynamics in social animals. Even though pathogens should thrive in social insect colonies as the hosts engage in frequent social interactions, are closely related and live in a pathogen-rich environment, disease outbreaks are rare. This is because social insects have evolved mechanisms to keep pathogens at bay – and fight disease as a collective. Social insect colonies are often viewed as “superorganisms” with division of labor between reproductive “germ-like” queens and males and “somatic” workers, which together form an interdependent reproductive unit that parallels a multicellular body. Superorganisms possess a “social immune system” that comprises of collective disease defenses performed by the workers - summarized as “social immunity”. In social groups immunization (reduced susceptibility to a parasite upon secondary exposure to the same parasite) can e.g. be triggered by social interactions (“social immunization”). Social immunization can be caused by (i) asymptomatic low-level infections that are acquired during caregiving to a contagious individual that can give an immune boost, which can induce protection upon later encounter with the same pathogen (active immunization) or (ii) by transfer of immune effectors between individuals (passive immunization).
In the second chapter, I built up on a study that I co-authored that found that low-level infections can not only be protective, but also be costly and make the host more susceptible to detrimental superinfections after contact to a very dissimilar pathogen. I here now tested different degrees of phylogenetically-distant fungal strains of M. brunneum and M. robertsii in L. neglectus and can describe the occurrence of cross-protection of social immunization if the first and second pathogen are from the same level. Interestingly, low-level infections only provided protection when the first strain was less virulent than the second strain and elicited higher immune gene expression.
In the third and fourth chapters, I expanded on the role of social immunity in sexual selection, a so far unstudied field. I used the fungus Metarhizium robertsii and the ant Cardiocondyla obscurior as a model, as in this species mating occurs in the presence of workers and can be studied under laboratory conditions. Before males mate with virgin queens in the nest they engage in fierce combat over the access to their mating partners.
First, I focused on male-male competition in the third chapter and found that fighting with a contagious male is costly as it can lead to contamination of the rival, but that workers can decrease the risk of disease contraction by performing sanitary care.
In the fourth chapter, I studied the effect of fungal infection on survival and mating success of sexuals (freshly emerged queens and males) and found that worker-performed sanitary care can buffer the negative effect that a pathogenic contagion would have on sexuals by spore removal from the exposed individuals. When social immunity was prevented and queens could contract spores from their mating partner, very low dosages led to negative consequences: their lifespan was reduced and they produced fewer offspring with poor immunocompetence compared to healthy queens. Interestingly, cohabitation with a late-stage infected male where no spore transfer was possible had a positive effect on offspring immunity – male offspring of mothers that apparently perceived an infected partner in their vicinity reacted more sensitively to fungal challenge than male offspring without paternal pathogen history.},
  author       = {Metzler, Sina},
  issn         = {2663-337X},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Pathogen-mediated sexual selection and immunization in ant colonies}},
  doi          = {10.15479/AT:ISTA:10727},
  year         = {2022},
}

@article{10284,
  abstract     = {Infections early in life can have enduring effects on an organism's development and immunity. In this study, we show that this equally applies to developing ‘superorganisms’––incipient social insect colonies. When we exposed newly mated Lasius niger ant queens to a low pathogen dose, their colonies grew more slowly than controls before winter, but reached similar sizes afterwards. Independent of exposure, queen hibernation survival improved when the ratio of pupae to workers was small. Queens that reared fewer pupae before worker emergence exhibited lower pathogen levels, indicating that high brood rearing efforts interfere with the ability of the queen's immune system to suppress pathogen proliferation. Early-life queen pathogen exposure also improved the immunocompetence of her worker offspring, as demonstrated by challenging the workers to the same pathogen a year later. Transgenerational transfer of the queen's pathogen experience to her workforce can hence durably reduce the disease susceptibility of the whole superorganism.},
  author       = {Casillas Perez, Barbara E and Pull, Christopher and Naiser, Filip and Naderlinger, Elisabeth and Matas, Jiri and Cremer, Sylvia},
  issn         = {1461-0248},
  journal      = {Ecology Letters},
  number       = {1},
  pages        = {89--100},
  publisher    = {Wiley},
  title        = {{Early queen infection shapes developmental dynamics and induces long-term disease protection in incipient ant colonies}},
  doi          = {10.1111/ele.13907},
  volume       = {25},
  year         = {2022},
}

@misc{13061,
  abstract     = {Infections early in life can have enduring effects on an organism’s development and immunity. In this study, we show that this equally applies to developing “superorganisms” – incipient social insect colonies. When we exposed newly mated Lasius niger ant queens to a low pathogen dose, their colonies grew more slowly than controls before winter, but reached similar sizes afterwards. Independent of exposure, queen hibernation survival improved when the ratio of pupae to workers was small. Queens that reared fewer pupae before worker emergence exhibited lower pathogen levels, indicating that high brood rearing efforts interfere with the ability of the queen’s immune system to suppress pathogen proliferation. Early-life queen pathogen-exposure also improved the immunocompetence of her worker offspring, as demonstrated by challenging the workers to the same pathogen a year later. Transgenerational transfer of the queen’s pathogen experience to her workforce can hence durably reduce the disease susceptibility of the whole superorganism.},
  author       = {Casillas Perez, Barbara E and Pull, Christopher and Naiser, Filip and Naderlinger, Elisabeth and Matas, Jiri and Cremer, Sylvia},
  publisher    = {Dryad},
  title        = {{Early queen infection shapes developmental dynamics and induces long-term disease protection in incipient ant colonies}},
  doi          = {10.5061/DRYAD.7PVMCVDTJ},
  year         = {2021},
}

@phdthesis{6435,
  abstract     = {Social insect colonies tend to have numerous members which function together like a single organism in such harmony that the term ``super-organism'' is often used. In this analogy the reproductive caste is analogous to the primordial germ
cells of a metazoan, while the sterile worker caste corresponds to somatic cells. The worker castes, like tissues, are
in charge of all functions of a living being, besides reproduction. The establishment of new super-organismal units
(i.e. new colonies) is accomplished by the co-dependent castes. The term oftentimes goes beyond a metaphor. We invoke it when we speak about the metabolic rate, thermoregulation, nutrient regulation and gas exchange of a social insect colony. Furthermore, we assert that the super-organism has an immune system, and benefits from ``social immunity''.

Social immunity was first summoned by evolutionary biologists to resolve the apparent discrepancy between the expected high frequency of disease outbreak amongst numerous, closely related tightly-interacting hosts, living in stable and microbially-rich environments, against the exceptionally scarce epidemic accounts in natural populations. Social
immunity comprises a multi-layer assembly of behaviours which have evolved to effectively keep the pathogenic enemies of a colony at bay. The field of social immunity has drawn interest, as it becomes increasingly urgent to stop
the collapse of pollinator species and curb the growth of invasive pests. In the past decade, several mechanisms of
social immune responses have been dissected, but many more questions remain open.

I present my work in two experimental chapters. In the first, I use invasive garden ants (*Lasius neglectus*) to study how pathogen load and its distribution among nestmates affect the grooming response of the group. Any given group of ants will carry out the same total grooming work, but will direct their grooming effort towards individuals
carrying a relatively higher spore load. Contrary to expectation, the highest risk of transmission does not stem from grooming highly contaminated ants, but instead, we suggest that the grooming response likely minimizes spore loss to the environment, reducing contamination from inadvertent pickup from the substrate.

The second is a comparative developmental approach. I follow black garden ant queens (*Lasius niger*) and their colonies from mating flight, through hibernation for a year. Colonies which grow fast from the start, have a lower chance of survival through hibernation, and those which survive grow at a lower pace later. This is true for colonies of naive
and challenged queens. Early pathogen exposure of the queens changes colony dynamics in an unexpected way: colonies from exposed queens are more likely to grow slowly and recover in numbers only after they survive hibernation.

In addition to the two experimental chapters, this thesis includes a co-authored published review on organisational
immunity, where we enlist the experimental evidence and theoretical framework on which this hypothesis is built,
identify the caveats and underline how the field is ripe to overcome them. In a final chapter, I describe my part in
two collaborative efforts, one to develop an image-based tracker, and the second to develop a classifier for ant
behaviour.},
  author       = {Casillas Perez, Barbara E},
  issn         = {2663-337X},
  keywords     = {Social Immunity, Sanitary care, Social Insects, Organisational Immunity, Colony development, Multi-target tracking},
  pages        = {183},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Collective defenses of garden ants against a fungal pathogen}},
  doi          = {10.15479/AT:ISTA:6435},
  year         = {2019},
}