Caste Development Research Articles

The evolution of morphological castes under decoupled control.

Eusociality, where units that previously reproduced independently function as one entity, is of major interest in evolutionary biology. Obligate eusociality is characterized by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise in the Hymenoptera and factors constraining their evolution. Control over offspring morphology and behaviour seems likely to be decoupled. Provisioners (queens and workers) can influence offspring morphology directly through the nutrition they provide, while adult offspring control their own behaviour. Provisioners may, however, influence worker behaviour indirectly if offspring modify their behaviour in response to their morphology. If manipulation underlies helping, we should not see helping evolve before specialized worker morphology, yet empirical observations suggest that behavioural castes precede morphological castes. We use evolutionary invasion analyses to show how the evolution of a morphologically differentiated worker caste depends on the prior presence of a behavioural caste: specialist worker morphology will be mismatched with behaviour unless some offspring already choose to work. A mother's certainty about her offspring's behaviour is also critical-less certainty results in greater mismatch. We show how baseline worker productivity can affect the likelihood of a morphological trait being favoured by natural selection. We then show how under a decoupled control scenario, morphologically differentiated castes should be less and less likely to be lost as they become more specialized. We also suggest that for eusociality to be evolutionarily irreversible, workers must be unable to functionally replace reproductives and reproductives must be unable to reproduce without help from workers.

Major changes in domain arrangements are associated with the evolution of termites.

Domains as functional protein units and their rearrangements along the phylogeny can shed light on the functional changes of proteomes associated with the evolution of complex traits like eusociality. This complex trait is associated with sterile soldiers and workers, and long-lived, highly fecund reproductives. Unlike in Hymenoptera (ants, bees, and wasps), the evolution of eusociality within Blattodea, where termites evolved from within cockroaches, was accompanied by a reduction in proteome size, raising the question of whether functional novelty was achieved with existing rather than novel proteins. To address this, we investigated the role of domain rearrangements during the evolution of termite eusociality. Analysing domain rearrangements in the proteomes of three solitary cockroaches and five eusocial termites, we inferred more than 5,000 rearrangements over the phylogeny of Blattodea. The 90 novel domain arrangements that emerged at the origin of termites were enriched for several functions related to longevity, such as protein homeostasis, DNA repair, mitochondrial activity, and nutrient sensing. Many domain rearrangements were related to changes in developmental pathways, important for the emergence of novel castes. Along with the elaboration of social complexity, including permanently sterile workers and larger, foraging colonies, we found 110 further domain arrangements with functions related to protein glycosylation and ion transport. We found an enrichment of caste-biased expression and splicing within rearranged genes, highlighting their importance for the evolution of castes. Furthermore, we found increased levels of DNA methylation among rearranged compared to non-rearranged genes suggesting fundamental differences in their regulation. Our findings indicate the importance of domain rearrangements in the generation of functional novelty necessary for termite eusociality to evolve.

Socio-Economic conditions of Pourakarmikas – A Sociological Study in Mysore City Corporation

Untouchability is one of the worst features of the Hindu Society. In the process of evolution of caste on the principles of graded chathurvarna (four Varnas) such as Brahmana, Kshatriya, Vaisya and Shudra the vast body of people known as Panchama, exterior, Antyajas or scheduled castes fell outside the purview of chaturvarna. They were denied the rights enjoyed by Brahmin's, Kshatriya's. Vaishya's and Upanayana even they were prohibited to perorm upanayana to wear sacred thread, learning Vedas, doing trade and other clean occupations. They were forced to live on the outskirts of village, thus restricting their movement. In this way, they were segregated and become the Anthyajas or Untouchables. The untouchables (Panchamas or Avarnas) had no share in social, political and judiciary power.

Sexual dimorphism as a facilitator of worker caste evolution in ants.

Ant societies are primarily composed of females, whereby labor is divided into reproductive, or queen, and non-reproductive, or worker, castes. Workers and reproductive queens can differ greatly in behavior, longevity, physiology, and morphology, but queen-worker differences are usually modest relative to the differences in males. Males are short-lived, typically do not provide the colony with labor, often look like a different species, and only occur seasonally. It is these differences that have historically led to their neglect in social insect research, but also why they may facilitate novel phenotypic variation - by increasing the phenotypic variability that is available for selection. In this study, worker variation in multivariate size-shape space paralleled male-queen variation. As worker variation increased within species, so did sexual variation. Across species in two independent genera, using head width as a proxy for body size, sexual size dimorphism correlated with worker polymorphism regardless of whether the ancestral condition was large or small worker/sexual dimorphism. Mounting molecular data support the hypothesis that queen-worker caste determination has co-opted many genes/pathways from sex determination. The molecular evidence, coupled with the observations from this study, leads to the hypothesis that sexual selection and selection on colony-level traits are non-independent, and that sexual dimorphism may even have facilitated the evolution of the distinct worker caste.

A colony-level optimization model provides a potential mechanism for the evolution of novel castes in eusocial ant colonies

Ant species often have multiple morphologically distinct ‘castes’ within a single colony. Given that most of these castes are involved in non-reproductive tasks, and since such individuals thus never reproduce, the question of how ant castes can evolve is a non-trivial one. Over the years, several models have been proposed in order to explain the evolution of castes in ant colonies. Here, we attempt to answer this question using an economics-based approach, developing an optimization model that implements adaptation and selection at the colony level. We argue that due to the nature of ant colonies, selection is shifted to the group level, and, due to this, individual ants are sheltered from negative selection. We show that our framework can explain the evolution of novel castes in ant colonies, and discuss the novelty of our model with regard to previous models that have been proposed. We also show that our model is consistent with several empirical observations of ant colonies.

Division of foraging behaviour: Assessments of pollinator traits when visiting a model plant species

Diverse animals visit flowers but not all engage in pollination, and only behaviours involving stigmal contact result in successful pollination events. Ethological indices developed for sociobiological contexts (caste evolution) have unrealized potential to provide insights on pollination services but have not been applied in this context before. In the present study, we characterized foraging behaviours as functional traits, which allowed us to predict whether ecosystems services are robust to species-specific trait loss. We aimed to (1) document the variety, duration and frequency of on-flower behaviours, (2) determine which taxa engage in stigmal contact and (3) investigate vulnerabilities of pollination service to loss of pollinator taxa. We used video recordings to document on-flower behaviours of animals visiting a single apple cultivar (Malus domestica var. ‘Pink Lady’), which removed confounding factors to enable a robust assessment of functional pollination behaviour. We adapted division of labour statistics (designed to assess caste-specific behavioural task fidelity within social insect colonies) to investigate taxon-specific foraging behaviours within this assemblage of pollinators – a novel application. An ethogram of 11 behaviours was derived from 19,518 min of observations (2350 visits; 19 taxa). Most (53%) flower visits involved stigmal contact (termed ‘contact visits’), but the percentage of contact visits varied between taxa. We introduce division of foraging behaviour (DFB) indices – which imply the focal pollinator assemblage consists of a mix of behaviourally generalized and specialized taxa with a slight bias towards specialization: division of taxa into behaviours DFBY|X = 0.59; division of behaviours into taxa DFBX|Y = 0.76. Simulated extinction of taxa from this insect assemblage significantly shifted DFBY|X in four instances. These qualitative assessments of behavioural traits imply moderate functional redundancy in pollination services provided by 11 insect taxa to a model plant, even under simulated extinction events, suggesting pollination services may be somewhat resilient to animal–plant network disruptions in this Australian agroecological study system.

The role of epigenetics, particularly DNA methylation, in the evolution of caste in insect societies.

Eusocial insects can be defined as those that live in colonies and have distinct queens and workers. For most species, queens and workers arise from a common genome, and so caste-specific developmental trajectories must arise from epigenetic processes. In this review, we examine the epigenetic mechanisms that may be involved in the regulation of caste dimorphism. Early work on honeybees suggested that DNA methylation plays a causal role in the divergent development of queen and worker castes. This view has now been challenged by studies that did not find consistent associations between methylation and caste in honeybees and other species. Evidence for the involvement of methylation in modulating behaviour of adult workers is also inconsistent. Thus, the functional significance of DNA methylation in social insects remains equivocal. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Hourglass Model for Developmental Evolution of Ant Castes

Classic models for the development and evolution of ant castes struggle to explain recent empirical results. Here, we propose an hourglass model compatible with all existing data, providing a formal, falsifiable framework for future study. This illustrates how phenotypic variation can be used to infer underlying developmental and genetic architecture.

Morphological caste differences in primitively eusocial insects: the Van der Vecht organ ofMischocyttaruspaper wasps

AbstractNests of independent-founding primitively eusocial paper wasps are founded by one or a few females. Before worker emergence, foundresses must forage, sometimes leaving the nest unattended. Chemical defence - the application of ant repellent secretions of the Van der Vecht organ (VdVO) on the nest surface - allows brood protection when active defence is weak or not possible. After worker emergence, it is more likely that some wasps stay in the nest, so active nest defence may be more common. Given this constraint on the nest defence, selection for increasing the cuticular secretory area of the VdVO, to improve chemical defence, may be stronger in foundresses (queens) than in workers - morphological castes are hypothesized to evolve. We investigated the occurrence of morphological castes in the Neotropical paper wasp Mischocyttarus. Confirming our prediction, we found that in some (but not all) species, foundresses have a distinct increase in the secretory area of the VdVO in relation to workers; even though there is a strong overlap between caste phenotypes (incipient morphological castes). Implications of these findings for the evolution of morphological castes in primitively eusocial wasps are discussed.

Trait evolution is reversible, repeatable, and decoupled in the soldier caste of turtle ants

The scope of adaptive phenotypic change within a lineage is shaped by how functional traits evolve. Castes are defining functional traits of adaptive phenotypic change in complex insect societies, and caste evolution is expected to be phylogenetically conserved and developmentally constrained at broad phylogenetic scales. Yet how castes evolve at the species level has remained largely unaddressed. Turtle ant soldiers (genus Cephalotes), an iconic example of caste specialization, defend nest entrances by using their elaborately armored heads as living barricades. Across species, soldier morphotype determines entrance specialization and defensive strategy, while head size sets the specific size of defended entrances. Our species-level comparative analyses of morphotype and head size evolution reveal that these key ecomorphological traits are extensively reversible, repeatable, and decoupled within soldiers and between soldier and queen castes. Repeated evolutionary gains and losses of the four morphotypes were reconstructed consistently across multiple analyses. In addition, morphotype did not predict mean head size across the three most common morphotypes, and head size distributions overlapped broadly across all morphotypes. Concordantly, multiple model-fitting approaches suggested that soldier head size evolution is best explained by a process of divergent pulses of change. Finally, while soldier and queen head size were broadly coupled across species, the level of head size disparity between castes was decoupled from both queen head size and soldier morphotype. These findings demonstrate that caste evolution can be highly dynamic at the species level, reshaping our understanding of adaptive morphological change in complex social lineages.

Comparative advantage and caste evolution.

Most of the theory for the evolution of caste specialization in social insects assumes that increased efficiency in worker labor leads to specialization and increased worker efficiency gives colonies with behavioral specialists an advantage. However, there are an increasing number of studies that show that the task specialists within social insect colonies do not have the highest efficiency. Indeed, some studies show that some groups of workers are uniformly better than all other groups at every task. In this note, I adapt the principle of comparative advantage from economics to show that, rather than maximizing the payoff, specialization is advantageous when minimizing opportunity costs. This leads to the prediction that caste specialization should be associated with reduced opportunity costs rather than increased task efficiency.

Caste-specific morphological modularity in the ant tribe Camponotini (Hymenoptera, Formicidae)

BackgroundThe morphological structures of organisms form tightly integrated but mutually independent character complexes (modules) linked through common development and function. Even though their abundance, diversity, and complex caste systems make camponotine ants ideal subjects to research developmental modularity and phenotypic integration, no studies investigating these phenomena have been conducted in this taxonomic group. This study attempts to identify and visualize integrated character complexes in 14 taxa from the genera Camponotus and Colobopsis using statistical analyses of morphometry.ResultsThe identified modules differ between castes: Minor workers have small heads and long extremities, while major workers have enlarged heads modified for defence, and short, thick appendages; extremities (legs and antennae) are strongly correlated in both worker castes. Gynes show weaker integration of extremities, but a strong correlation of mesosoma and eyes, and highly variable median ocellus size. Gynes infested by mermithid nematodes exhibit reduction of gyne-specific characters and altered patterns of phenotypic integration.ConclusionThe integrated character complexes described herein can largely be interpreted as functional, caste-specific modules related to behavioural ecology and task allocation within ant colonies. This modular nature of the body plan is hypothesized to facilitate the evolution of novel phenotypes and thus contributes to the tremendous evolutionary success of ants. The study of these modules can help to further elucidate the evolution and ontogeny of castes in camponotine ants, as well as the effects of parasite infestation on the phenotype.

Molecular and morphological recognition of species boundaries in the neglected ant genus Brachymyrmex (Hymenoptera: Formicidae): toward a taxonomic revision

Brachymyrmex is a neglected genus of Formicinae because of its small body size, soft mesosoma, and superficially monotonous external morphology. These features have complicated the documentation of morphological variation, resulting in poorly defined and incompletely described species. Consequently, the taxonomy of the genus is complex and problematic, which has impeded research and conservation efforts. Here, we integrate molecular and morphological data to recognize species boundaries in Brachymyrmex and to guide its long-overdue revision. Specifically, we (1) redefine the limits of all described species, subspecies, and varieties based on intra- and interspecific morphological variation in workers; (2) document this variation quantitatively by constructing morphospace occupation and statistically analyzing measurements; (3) synthesize our findings on diagnostic traits in a dichotomous, illustrated identification key; and (4) examine the significance of our morphological identification system with molecular evidence from four gene fragments (EF1aEF1, EF1aEF2, WG, and COI). We recognize 40 species, of which four are new to science: Brachymyrmex bahamensis, Brachymyrmex bicolor, Brachymyrmex iridescens, and Brachymyrmex sosai. Furthermore, Brachymyrmex attenuatus and Brachymyrmex bonariensis are raised to species, and we propose 25 new synonyms. Morphometrics indicated that even poorly distinguishable species pairs show statistically significant differences in some traits, and that taxonomically problematic cases relate to taxa that demonstrate large intraspecific trait variance. Our molecular analysis supports the monophyly of the genus based on increased taxon sampling, and of the 19 species that were included 18 were retrieved as monophyletic. The single case of incongruence was also flagged in morphological analyses and requires extended geographic sampling before it can be resolved. In conclusion, the molecular work corroborates the morphologically recognized species boundaries. We also document the presence of worker dimorphism and putative worker-queen intercastes in several Brachymyrmex species, which indicates that the genus may present a promising study system to understand caste evolution in ants.

Social evolution and the two elements of causation

The kin selection theory has recently been criticised on the basis of claiming that genetic relatedness does not play a causal role in the social evolution among individuals of insect societies. We outline here a line of criticism of this view by demonstrating two things. First, there are strong conceptual, theoretical and empirical reasons to think that close genetic relatedness has been necessary for the rise of the helper castes of social insects. And second, once we understand how causal explanation itself results from an interplay of two logically distinct elements, necessity and sufficiency, we can also understand the scenarios in which relatedness does not seem to play a causal role for evolution of helper castes. The result of this analysis is that we should be more careful about the way we frame the empirical data on the evolution of social behaviour.

Conflict and major transitions - why we need true queens.

In contrast to human societies, where kings and queens can be sources of conflict, we argue that the morphologically distinct queens of insect colonies are central to the minimization of conflict within their societies. Thus, the evolution of irreversible queen and worker castes represents a major transition in social evolution. Queens are selected to become better reproducers, and workers are selected to become better workers. The reproductive success of queens and workers are, therefore, inextricably linked. Workers achieve reproductive success by assisting the queen, whereas the queen needs her workers to provide her with the wherewithal to raise her brood. The tighter the mutual dependence, the lower conflict, and the larger insect societies can become. As the queen becomes a better breeder, workers are selected to become better at raising their siblings. Yet, nothing in nature is ever free of conflict and with the evolution of a true worker caste a new set of conflicts arises. Multiple mating by queens in particular opens the door to a new set of conflicts. Ironically, multiple mating can only evolve once within-colony conflict is reduced by evolving a true worker caste.

Caste-biased genes in a subterranean termite are taxonomically restricted: implications for novel gene recruitment during termite caste evolution

The caste system of social insects presents a classic polyphenism in which widely divergent reproductive and non-reproductive phenotypes are expressed from the same genome. In termites, the sterile soldier caste is particularly divergent in phenotype and presumably evolved under selection for defensiveness. In this study, we use genomic phylostratigraphy to show that genes with soldier- and other caste-biased expression from the Eastern subterranean termite Reticulitermes flavipes are more taxonomically restricted on the tree of life than genes with no caste-biased expression. This pattern suggests that caste-biased genes are relatively young and implies past selection for novel gene recruitment during termite caste evolution. Moreover, a soldier-biased set of 74 genes contains a higher proportion of orphan genes with no known homology than does a nymph-biased set or any null gene sets. This again suggests that the termite caste—and soldiers in particular—makes disproportionate use of evolutionarily novel genes that are potentially recruited from non-coding regions of the genome. Given that Reticulitermes and most termite soldiers are sterile, any past selection for genetic novelty of this caste must have been indirect and mediated through reproducing relatives.

Cultural Differences and Negotiations in Inter-Caste Marriages: A Study in Bengaluru

B R Ambedkar (1936) had suggested inter-caste marriages as one of the potential remedies to annihilate caste system. He later contradicted this stance in the latter half of his academic journey by comparing inter-caste marriages to „force-feeding and artificial ways. 'Even after 80 years, the society is still divided between the effects of inter-caste marriages on the centuries-old caste system. Inter-caste marriage in the country was not a very common event till the 2000s after which its instances have been steadily increasing as per the reports from both the IHDS (2011) and NFHS (2005-2006). The more critical aspect of this uptrend of inter-caste marriage should be the interaction and negotiation of cultural differences among couples during the process of union. This qualitative study among 20 individuals (10 couples) in Bengaluru, looks into the various ways in which the inter-caste couples adjust their lifestyles, make decisions about their cultural practices and their children‟s socialization. The assumptions are laid in the backdrop of B. R. Ambedkar‟s work on the caste system and the study attempted to understand the subtle evolution of caste in the exogamous marriages. The study has also attempted to preempt the variety of bearings such inter-caste marriages can have on the future of caste system.

CASTE DEVELOPMENTAL PATHWAYS OF Cortaritermes fulviceps (BLATTODEA, TERMITIDAE).

The study of the developmental pathways in termites contributes to the understanding of caste evolution and resources utilization, as well as other ecological aspects. In this paper, the caste developmental pathways of Cortaritermes fulviceps (Silvestri) were analyzed in natural colonies from the Province of Corrientes, Argentina. Five measurements were taken of different body parts of larvae (L), workers (W), pre-soldiers (PS), soldiers (S), nymphs (N) and alates (A) for morphometric analyses. Dissections were also performed to determine the sex of workers and soldiers. The collected data were analyzed using principal component analysis, analysis of variance and Tukey test. In the nymphal line, four nymphal instars and alates were identified. The apterous line included two larval instars. Major workers were females and minor workers, pre- soldiers and soldiers were males. The results suggest that the developmental pathways of C. fulviceps is similar to the one of the genus Nasutitermes Dudley.

Caste-biased gene expression in a facultatively eusocial bee suggests a role for genetic accommodation in the evolution of eusociality.

Developmental plasticity may accelerate the evolution of phenotypic novelty through genetic accommodation, but studies of genetic accommodation often lack knowledge of the ancestral state to place selected traits in an evolutionary context. A promising approach for assessing genetic accommodation involves using a comparative framework to ask whether ancestral plasticity is related to the evolution of a particular trait. Bees are an excellent group for such comparisons because caste-based societies (eusociality) have evolved multiple times independently and extant species exhibit different modes of eusociality. We measured brain and abdominal gene expression in a facultatively eusocial bee, Megalopta genalis, and assessed whether plasticity in this species is functionally linked to eusocial traits in other bee lineages. Caste-biased abdominal genes in M. genalis overlapped significantly with caste-biased genes in obligately eusocial bees. Moreover, caste-biased genes in M. genalis overlapped significantly with genes shown to be rapidly evolving in multiple studies of 10 bee species, particularly for genes in the glycolysis pathway and other genes involved in metabolism. These results provide support for the idea that eusociality can evolve via genetic accommodation, with plasticity in facultatively eusocial species like M. genalis providing a substrate for selection during the evolution of caste in obligately eusocial lineages.

Juvenile hormone, but not nutrition or social cues, affects reproductive maturation in solitary alkali bees (Nomia melanderi).

Eusocial insect colonies are defined by extreme variation in reproductive activity among castes, but the ancestral conditions from which this variation arose are unknown. Investigating the factors that contribute to variation in reproductive physiology among solitary insects that are closely related to social species can help to fill this gap. We experimentally tested the role of nutrition, juvenile hormone (JH) and social cues on reproductive maturation in solitary alkali bees (Halictidae: Nomia melanderi). We found that alkali bee females emerge from overwintering with small Dufour's glands and small ovaries, containing oocytes in the early stages of development. Oocyte maturation occurs rapidly, and is staggered between the two ovaries. Lab-reared females reached reproductive maturity without access to mates or nesting opportunities, and many had resorbed oocytes. Initial activation of these reproductive structures does not depend on pollen consumption, though dietary protein or lipids may be necessary for long-term reproductive activity. JH is likely to be a limiting factor in alkali bee reproductive activation, as females treated with JH were more likely to develop mature oocytes and Dufour's glands. Unlike for related social bees, the effects of JH were not suppressed by the presence of older, reproductive females. These results provide valuable insight into the factors that influence reproductive activity in an important native pollinator, and those that may have been particularly influential in the evolution of reproductive castes.