Defences against brood parasites from a social immunity perspective

Nest architecture influences host use by avian brood parasites and is shaped by coevolutionary dynamics

Brood (social) parasites and their hosts exhibit a wide range of adaptations and counter-adaptations as part of their ongoing coevolutionary arms races. Obligate avian brood parasites are expected to use potential host species with more easily accessible nests, while potential hosts are expected to evade parasitism by building more concealed nests that are difficult for parasites to enter and in which to lay eggs. We used phylogenetically informed comparative analyses, a global database of the world's brood parasites, their host species, and the design of avian host and non-host nests (approx. 6200 bird species) to examine first, whether parasites preferentially target host species that build open nests and, second, whether host species that build enclosed nests are more likely to be targeted by specialist parasites. We found that species building more accessible nests are more likely to serve as hosts, while host species with some of the more inaccessible nests are targeted by more specialist brood parasites. Furthermore, evolutionary-transition analyses demonstrate that host species building enclosed nests frequently evolve to become non-hosts. We conclude that nest architecture and the accessibility of nests for parasitism represent a critical stage of the ongoing coevolutionary arms race between avian brood parasites and their hosts.

The Microbiota of a Mite Prey-Predator System on Different Host Plants Are Characterized by Dysbiosis and Potential Functional Redundancy

Microbiota has diverse roles in the life cycles of their hosts, affecting their growth, development, behavior, and reproduction. Changes in physiological conditions of the host can also impact the assemblage of host-associated microorganisms. However, little is known of the effects of host plant-prey-predatory mite interactions on mite microbiota. We compared the microbial communities of eggs and adult females of the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), and of adult females of the predatory mite Neoseiulus californicus (McGregor) (Acari: Phytoseiidae) on four different host plants (cotton, maize, pinto bean, and tomato) by metabarcoding sequencing of the V3-V4 region of the 16S ribosomal RNA gene (16S rRNA), using the Illumina MiSeq platform. Only the egg microbiota of T. urticae was affected by the host plant. The microbiota of the predatory mite N. californicus was very different from that of its prey, and the predator microbiota was unaffected by the different host plant-prey systems tested. Only the microbiota of the eggs of T. urticae carried Serratia as a high fidelity-biomarker, but their low abundance in T. urticae adult females suggests that the association between Serratia and T. urticae is accidental. Biomarker bacteria were also detected in the microbiota of adult females of T. urticae and N. californicus, with different biomarkers in each host plant species. The microbiota associated with eggs and adult females of T. urticae and adult females of N. californicus differed in their functional potential contributions to the host mite.

Group intrusions by a brood parasitic fish are not cooperative

Brood parasites relegate all parental duties to unrelated hosts. Host resistance against brood parasitism is most effective during egg laying and is best countered by surreptitious oviposition. This may be aided through distraction of host attention by the male partner or a larger cooperative group. Cuckoo catfish (Synodontis multipunctatus) parasitize the broods of mouthbrooding cichlids, which collect their eggs immediately after oviposition. Cuckoo catfish must time their intrusion precisely, as the temporal window for parasitism lasts only a few seconds. As the cuckoo catfish typically intrude host spawning as a group, we tested whether groups of catfish distract spawning cichlid pairs more successfully than a single catfish pair. We found that larger catfish groups were not more effective in parasitism, as parasitism success by groups of three catfish pairs increased only proportionally to single catfish pairs. The number of cichlid eggs in host clutches decreased at high catfish abundance, apparently due to elevated cuckoo catfish predation on the eggs. Hence, group intrusions do not represent cooperative actions, but incur an increased cost to the host cichlid from greater egg predation by cuckoo catfish.

A model of infection in honeybee colonies with social immunity

Honeybees (Apis mellifera) play a significant role in the pollination of various food crops and plants. In the past decades, honeybee management has been challenged with increased pathogen and environmental pressure associating with increased beekeeping costs, having a marked economic impact on the beekeeping industry. Pathogens have been identified as a contributing cause of colony losses. Evidence suggested a possible route of pathogen transmission among bees via oral-oral contacts through trophallaxis. Here we propose a model that describes the transmission of an infection within a colony when bee members engage in the trophallactic activity to distribute nectar. In addition, we examine two important features of social immunity, defined as collective disease defenses organized by honeybee society. First, our model considers the social segregation of worker bees. The segregation limits foragers, which are highly exposed to pathogens during foraging outside the nest, from interacting with bees residing in the inner parts of the nest. Second, our model includes a hygienic response, by which healthy nurse bees exterminate infected bees to mitigate horizontal transmission of the infection to other bee members. We propose that the social segregation forms the first line of defense in reducing the uptake of pathogens into the colony. If the first line of defense fails, the hygienic behavior provides a second mechanism in preventing disease spread. Our study identifies the rate of egg-laying as a critical factor in maintaining the colony's health against an infection. We propose that winter conditions which cease or reduce the egg-laying activity combined with an infection in early spring can compromise the social immunity defenses and potentially cause colony losses.

Sickness and the Social Brain: Love in the Time of COVID

As a highly social species, inclusion in social networks and the presence of strong social bonds are critical to our health and well-being. Indeed, impaired social functioning is a component of numerous neuropsychiatric disorders including depression, anxiety, and substance use disorder. During the current COVID-19 pandemic, our social networks are at risk of fracture and many are vulnerable to the negative consequences of social isolation. Importantly, infection itself leads to changes in social behavior as a component of "sickness behavior." Furthermore, as in the case of COVID-19, males and females often differ in their immunological response to infection, and, therefore, in their susceptibility to negative outcomes. In this review, we discuss the many ways in which infection changes social behavior-sometimes to the benefit of the host, and in some instances for the sake of the pathogen-in species ranging from eusocial insects to humans. We also explore the neuroimmune mechanisms by which these changes in social behavior occur. Finally, we touch upon the ways in which the social environment (group living, social isolation, etc.) shapes the immune system and its ability to respond to challenge. Throughout we emphasize how males and females differ in their response to immune activation, both behaviorally and physiologically.

The coevolutionary biology of brood parasitism: a call for integration

Obligate brood-parasitic cheats have fascinated natural historians since ancient times. Passing on the costs of parental care to others occurs widely in birds, insects and fish, and often exerts selection pressure on hosts that in turn evolve defences. Brood parasites have therefore provided an illuminating system for researching coevolution. Nevertheless, much remains unknown about how ecology and evolutionary history constrain or facilitate brood parasitism, or the mechanisms that shape or respond to selection. In this special issue, we bring together examples from across the animal kingdom to illustrate the diverse ways in which recent research is addressing these gaps. This special issue also considers how research on brood parasitism may benefit from, and in turn inform, related fields such as social evolution and immunity. Here, we argue that progress in our understanding of coevolution would benefit from the increased integration of ideas across taxonomic boundaries and across Tinbergen's Four Questions: mechanism, ontogeny, function and phylogeny of brood parasitism. We also encourage renewed vigour in uncovering the natural history of the majority of the world's brood parasites that remain little-known. Indeed, it seems very likely that some of nature's brood parasites remain entirely unknown, because otherwise we are left with a puzzle: if parental care is so costly, why is brood parasitism not more common? This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.