Evolution, phylogenetic distribution and functional ecology of division of labour in trematodes

Freshwater trematodes differ from marine trematodes in patterns connected with division of labor

Background

Prior research suggests that trematode rediae, a developmental stage of trematode parasites that reproduce clonally within a snail host, show evidence of division of labor (DOL). Single-species infections often have two morphologically distinct groups: small rediae, the 'soldiers', are active, aggressive, and do not appear to reproduce; large rediae, the 'reproductives', are larger, sluggish, and full of offspring. Most data supporting DOL come from trematodes infecting marine snails, while data from freshwater trematodes are more limited and generally do not supported DOL. The shorter lifespan typical of freshwater snails may partially explain this difference: defending a short-lived host at the expense of reproduction likely provides few advantages. Here, we present data from sixty-one colonies spanning twenty species of freshwater trematode exploring morphological and behavioral patterns commonly reported from marine trematodes believed to have DOL.

Methods

Trematode rediae were obtained from sixty-one infected snails collected in central Vermont, USA. A portion of the COI gene was sequenced to make tentative species identifications ('COI species'). Samples of rediae were photographed, observed, and measured to look for DOL-associated patterns including a bimodal size distribution, absence of embryos in small rediae, and pronounced appendages and enlarged pharynges (mouthparts) in small rediae. Additional rediae were used to compare activity levels and likelihood to attack heterospecific trematodes in large vs. small rediae.

Results

Many of the tests for DOL-associated patterns showed mixed results, even among colonies of the same COI species. However, we note a few consistent patterns. First, small rediae of most colonies appeared capable of reproduction, and we saw no indication (admittedly based on a small sample size and possibly insufficient attack trial methodology) that small rediae were more active or aggressive. This differs from patterns reported from most marine trematodes. Second, the small rediae of most colonies had larger pharynges relative to their body size than large rediae, consistent with marine trematodes. We also observed that colonies of three sampled COI species appear to produce a group of large rediae that have distinctly large pharynges.

Conclusions

We conclude that these freshwater species likely do not have a group of specialized non-reproductive soldiers because small rediae of at least some colonies in almost every species do appear to produce embryos. We cannot rule out the possibility that small rediae act as a temporary soldier caste. We are intrigued by the presence of rediae with enlarged pharynges in some species and propose that they may serve an adaptive role, possibly similar to the defensive role of small 'soldier' rediae of marine trematodes. Large-pharynx rediae have been documented in other species previously, and we encourage future efforts to study these large-pharynx rediae.

Social evolution: Diverse divisions of labor in trematode parasites

Some animal societies solve problems, like foraging or defense, by cooperatively dividing labor. A new discovery highlights that trematodes are unique in forming different societies at multiple parts of their complex life cycle with distinct divisions of labor to solve different problems.

Polymorphic parasitic larvae cooperate to build swimming colonies luring hosts

Parasites have evolved a variety of astonishing strategies to survive within their hosts, yet the most challenging event in their personal chronicles is the passage from one host to another. It becomes even more complex when a parasite needs to pass through the external environment. Therefore, the free-living stages of parasites present a wide range of adaptations for transmission. Parasitic flatworms from the group Digenea (flukes) have free-living larvae, cercariae, which are remarkably diverse in structure and behavior.1,2 One of the cercariae transmission strategies is to attain a prey-like appearance for the host.3 This can be done through the formation of a swimming aggregate of several cercariae adjoined together by their tails.4 Through the use of live observations and light, electron, and confocal microscopy, we described such a supposedly prey-mimetic colony comprising cercariae of two distinct morphotypes. They are functionally specialized: larger morphotype (sailors) enable motility, and smaller morphotype (passengers) presumably facilitate infection. The analysis of local read alignments between the two samples reveals that both cercaria types have identical 18S, 28S, and 5.8S rRNA genes. Further phylogenetic analysis of these ribosomal sequences indicates that our specimen belongs to the digenean family Acanthocolpidae, likely genus Pleorchis. This discovery provides a unique example and a novel insight into how morphologically and functionally heterogeneous individuals of the same species cooperate to build colonial organisms for the purpose of infection. This strategy bears resemblance to the cooperating castes of the same species found among insects.5.

Helminth-host-environment interactions: Looking down from the tip of the iceberg

In 1978, the theory behind helminth parasites having the potential to regulate the abundance of their host populations was formalized based on the understanding that those helminth macroparasites that reduce survival or fecundity of the infected host population would be among the forces limiting unregulated host population growth. Now, 45 years later, a phenomenal breadth of factors that directly or indirectly affect the host-helminth interaction has emerged. Based largely on publications from the past 5 years, this review explores the host-helminth interaction from three lenses: the perspective of the helminth, the host, and the environment. What biotic and abiotic as well as social and intrinsic host factors affect helminths? What are the negative, and positive, implications for host populations and communities? What are the larger-scale implications of the host-helminth dynamic on the environment, and what evidence do we have that human-induced environmental change will modify this dynamic? The overwhelming message is that context is everything. Our understanding of second-, third-, and fourth-level interactions is extremely limited, and we are far from drawing generalizations about the myriad of microbe-helminth-host interactions.Yet the intricate, co-evolved balance and complexity of these interactions may provide a level of resilience in the face of global environmental change. Hopefully, this albeit limited compilation of recent research will spark new interdisciplinary studies, and application of the One Health approach to all helminth systems will generate new and testable conceptual frameworks that encompass our understanding of the host-helminth-environment triad.

Association between parasite microbiomes and caste development and colony structure in a social trematode

Division of labour through the formation of morphologically and functionally distinct castes is a recurring theme in the evolution of animal sociality. The mechanisms driving the differentiation of individuals into distinct castes remain poorly understood, especially for animals forming clonal colonies. We test the association between microbiomes and caste formation within the social trematode Philophthalmus attenuatus, using a metabarcoding approach targeting the bacterial 16S SSU rRNA gene. Clonal colonies of this trematode within snail hosts comprise large reproductive individuals which produce dispersal stages, and small, non-reproducing soldiers which defend the colony against invaders. In colonies extracted directly from field-collected snails, reproductives harboured more diverse bacterial communities than soldiers, and reproductives and soldiers harboured distinct bacterial communities, at all taxonomic levels considered. No single bacterial taxon showed high enough prevalence in either soldiers or reproductives to be singled out as a key driver, indicating that the whole microbial community contributes to these differences. Other colonies were experimentally exposed to antibiotics to alter their bacterial communities, and sampled shortly after treatment and weeks later after allowing for turnover of colony members. At those time points, bacterial communities of the two castes still differed across all antibiotic treatments; however, the caste ratio within colonies changed: after antibiotic disruption and turnover of individuals, new individuals were more likely to become reproductives than in undisturbed control colonies. Our results reveal that each caste has a distinct microbiome; whether the social context affects the microbiota, or whether microbes contribute to modulating the phenotype of individuals, remains to be determined.

When being flexible matters: Ecological underpinnings for the evolution of collective flexibility and task allocation

A central problem in evolutionary biology is explaining variation in the organization of task allocation across collective systems. Why do human cells irreversibly adopt a task during development (e.g., kidney vs. liver cell), while sponge cells switch between different cell types? And why have only some ant species evolved specialized castes of workers for particular tasks? Although it seems reasonable to suppose that such differences reflect, at least partially, the different ecological pressures that systems face, there is no general understanding of how a system’s dynamic environment shapes its task allocation. To this end, we develop a general mathematical framework that reveals how simple ecological considerations could potentially explain cross-system variation in task allocation—including in flexibility, specialization, and (in)activity.

Task allocation is a central feature of collective organization. Living collective systems, such as multicellular organisms or social insect colonies, have evolved diverse ways to allocate individuals to different tasks, ranging from rigid, inflexible task allocation that is not adjusted to changing circumstances to more fluid, flexible task allocation that is rapidly adjusted to the external environment. While the mechanisms underlying task allocation have been intensely studied, it remains poorly understood whether differences in the flexibility of task allocation can be viewed as adaptive responses to different ecological contexts—for example, different degrees of temporal variability. Motivated by this question, we develop an analytically tractable mathematical framework to explore the evolution of task allocation in dynamic environments. We find that collective flexibility is not necessarily always adaptive, and fails to evolve in environments that change too slowly (relative to how long tasks can be left unattended) or too quickly (relative to how rapidly task allocation can be adjusted). We further employ the framework to investigate how environmental variability impacts the internal organization of task allocation, which allows us to propose adaptive explanations for some puzzling empirical observations, such as seemingly unnecessary task switching under constant environmental conditions, apparent task specialization without efficiency benefits, and high levels of individual inactivity. Altogether, this work provides a general framework for probing the evolved diversity of task allocation strategies in nature and reinforces the idea that considering a system’s ecology is crucial to explaining its collective organization.

Somatic Dimorphism in Cercariae of a Bird Schistosome

Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. “peregra”, in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.

The biodiversity of marine trematodes: then, now and in the future

Trematodes are the richest class of platyhelminths in the marine environment, infecting all classes of marine vertebrates as sexual adults and many phyla of marine invertebrates as part of their life cycles. Despite the cryptic nature of their existence (almost all marine trematodes are internal parasites), they have been the focus of study for almost 250 years, with the first species described in 1774. Here we review progress in the study of the "biodiversity" of these parasites, contrasting the progress made in the last 50 years (post-1971) to that in the almost 200 years before it (pre-1972). We consider an understanding of biodiversity to require knowledge of the species present in the system, an understanding of their evolutionary relationships (which informs higher classification), and, specifically for trematodes, an understanding of their complex life cycles. The fauna is now large, comprising well over 5,000 species. Although species description continues, we see evidence of a slow-down in all aspects of discovery. There has been only one completely new family identified since 1984 and the proposal of new genera is in decline as is the description of new species, especially for those of tetrapods. However, the extent to which this slow-down reflects an approach to the richness asymptote is made uncertain by changes in the field; reduced effort and difficulty of study may be important components of the effect. Regardless of how close we are to a complete description of the fauna, we infer that the outline is well-understood although the details are not. Adoption of molecular methodologies over the last 40 years have complemented morphometric analyses to facilitate objective recognition of species; however, despite these objective data, there is still inconsistency between authors on species delimitation. Molecular methodologies have also completely revolutionised inference of relationships at all levels, from within genera to between orders, and underpinned elucidation of novel life cycles. We expect the next 50 years to produce further dividends from technological innovations. The backdrop to the field will be global environmental concerns and the growing problem of funding for basic biodiversity studies.

The weird eusociality of polyembryonic parasites

Some parasitoid wasps possess soldier castes during their parasitic larval stage, but are often neglected from our evolutionary theories explaining caste systems in animal societies. This is primarily due to the polyembryonic origin of their societies. However, recent discoveries of polyembryonic trematodes (i.e. flatworms) possessing soldier castes require us to reconsider this reasoning. I argue we can benefit from including these polyembryonic parasites in eusocial discussions, for polyembryony and parasitism are taxonomically vast and influence the evolution of social behaviours and caste systems in various circumstances. Despite their polyembryony, their social evolution can be explained by theories of eusociality designed for parent–offspring groups, which are the subjects of most social evolution research. Including polyembryonic parasites in these theories follows the trend of major evolutionary transitions theory expanding social evolution research into all levels of biological organization. In addition, these continued discoveries of caste systems in parasites suggest social evolution may be more relevant to parasitology than currently acknowledged.

Cost of interspecific competition between trematode colonies

In a range of trematode species, specific members of the parthenitae colony infecting the molluscan first intermediate host appear specialized for defence against co-infecting species. The evolution of such division of labour requires that co-infection entails fitness costs. Yet, this premise has very rarely been tested in species showing division of labour. Using Himasthla elongata (Himasthlidae) and Renicola roscovita (Renicolidae) infecting periwinkles Littorina littorea as study system, we show that the size of emerged cercariae is markedly reduced in both parasite species when competing over host resources. Cercarial longevity, on the other hand, is negatively influenced by competition only in R. roscovita. Season, which may impact the nutritional state of the host, also affects cercarial size, but only in H. elongata. Hence, our study underlines that cercarial quality is, indeed, compromised by competition, not only in the inferior R. roscovita (no division of labour) but also in the competitively superior H. elongata (division of labour).

Clonemate cotransmission supports a role for kin selection in a puppeteer parasite

Host manipulation by parasites is a fascinating evolutionary outcome, but adaptive scenarios that often accompany even iconic examples in this popular field of study are speculative. Kin selection has been invoked as a means of explaining the evolution of an altruistic-based, host-manipulating behavior caused by larvae of the lancet fluke Dicrocoelium dendriticum in ants. Specifically, cotransmission of larval clonemates from a snail first host to an ant second host is presumed to lead to a puppeteer parasite in the ant's brain that has clonemates in the ant abdomen. Clonal relatedness between the actor (brain fluke) and recipients (abdomen flukes) enables kin selection of the parasite's host-manipulating trait, which facilitates transmission of the recipients to the final host. However, the hypothesis that asexual reproduction in the snail leads to a high abundance of clonemates in the same ant is untested. Clonal relationships between the manipulator in the brain and the nonmanipulators in the abdomen are also untested. We provide empirical data on the lancet fluke's clonal diversity within its ant host. In stark contrast to other trematodes, which do not exhibit the same host-manipulating behavioral trait, the lancet fluke has a high abundance of clonemates. Moreover, our data support existing theory that indicates that the altruistic behavior can evolve even in the presence of multiple clones within the same ant host. Importantly, our analyses conclusively show clonemate cotransmission into ants, and, as such, we find support for kin selection to drive the evolution and maintenance of this iconic host manipulation.