On reconfirming the evidence for pre-imaginal caste bias in a primitively eusocial wasp

Caste is usually thought to be determined entirely in the adult stage in most primitively eusocial wasps and bees. A pre-imaginal caste bias has however been recently discovered in the primitively eusocial wasp Ropalidia marginata. This study also suggested that reigning queens and possibly other adults may influence the production of new queens and implied at least partial support to the parental manipulation or sub-fertility hypothesis for the evolution of insect sociality. The interest of these results prompted an attempt at their reconfirmation. Complete reconfirmation has now been obtained using data from an independent experiment and two additional methods of data analysis. We therefore conclude that caste is at least partly determined prior to eclosion in the primitively eusocial wasp Ropalidia marginata which lacks morphological differentiation between egg-layers and non-egg-layers

Seasonal variation in the onset of egg laying in a primitively eusocial wasp: implications for the evolution of sociality

When freshly eclosed females of the primitively eusocial wasp, Rapalidia marginata are isolated into individual cages, only about half of them build nests and lay eggs and those that do so take a long and variable amount of time (Mean ± SD = 66±37 days) before they lay their first egg. Part of the reason for this delay is because, when kept in isolation, no wasp begins to lay eggs during a period of approximately 82 days from mid - October to early January. Wasps maintained at a constant temperature of 26±1°C however initiate egg laying throughout the year, suggesting that the low temperatures during mid - October to early January may be at least one factor that makes this period unfavourable for wasps maintained at room temperature. Egg laying continues more or less normally throughout October-January however, in all natural and laboratory colonies studied. Natural colonies of R. marginata are initiated throughout the year and often by groups of females. Huddling together is a striking feature of the wasps especially on cold mornings. We therefore suggest that the isolated animals in our experiment are unable to lay eggs during the coldest part of the year because of their inability to huddle together, share metabolic heat and perform "co-operative thermoregulation". Such "co-operative thermoregulation" may thus be another factor that facilitates the evolution of socialitly

Serial polygyny in Ropalidia marginata: implications for the evolution of eusociality

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The role of larval nutrition in pre-imaginal biasing of caste in the primitively eusocial wasp Ropalidia marginata (Hymenoptera: Vespidae)

1. When freshly eclosed females of the primitively eusocial wasp Ropalidia marginata (Lep.) are isolated, only about 50% of them build nests and lay eggs thereby suggesting a pre-imaginal biasing of caste. 2. Wasps that lay eggs take a very variable amount of time after eclosion to start doing so. 3. Females eclosing from nests where larvae are fed at a relatively higher rate are more likely to become egg-layers and are likely to take less time after eclosion to begin to lay eggs. 4. Thus, both forms of pre-imaginal biasing of caste, namely, differences in egg laying capacity and differences in the time taken to attain reproductive maturity, appear to be influenced by larval nutrition

Serial polygyny in the primitively eusocial wasp Ropalidia marginata: implications for the evolution of sociality

Social insects usually live in colonies comprising one or a small number of reproductive individuals and a few or large number of sterile individuals. In termites only, both sexes are represented among the reproductives as well as among the sterile workers. In other social insects, namely ants, bees, and wasps, males do not participate significantly in the social life of colonies, which involves primarily the fertile queens and sterile female workers (Wilson 1971). The haplodiploid genetic system found universally in the Hymenoptera creates an asymmetry in genetic relatedness such that a female is more closely related to her full sister (coefficient of genetic relatedness, r = 0.75) than to her offspring (r = 0.5). This makes inclusive fitness theory (Hamilton, 19640, b) particularly applicable to the evolution of sterile worker castes in the social hymenoptera (Wilson 1971; Hamilton 1972)

Impaired odour discrimination on desynchronization of odour-encoding neural assemblies

Stimulus-evoked oscillatory synchronization of neural assemblies has been described in the olfactory and visual systems of several vertebrates and invertebrates. In locusts, information about odour identity is contained in the timing of action potentials in an oscillatory population response, suggesting that oscillations may reflect a common reference for messages encoded in time. Although the stimulus-evoked oscillatory phenomenon is reliable, its roles in sensation, perception, memory formation and pattern recognition remain to be demonstrated — a task requiring a behavioural paradigm. Using honeybees, we now demonstrate that odour encoding involves, as it does in locusts, the oscillatory synchronization of assemblies of projection neurons and that this synchronization is also selectively abolished by picrotoxin, an antagonist of the GABA_A (γ-aminobutyric acid) receptor. By using a behavioural learning paradigm, we show that picrotoxin-induced desynchronization impairs the discrimination of molecularly similar odorants, but not that of dissimilar odorants. It appears, therefore, that oscillatory synchronization of neuronal assemblies is functionally relevant, and essential for fine sensory discrimination. This suggests that oscillatory synchronization and the kind of temporal encoding it affords provide an additional dimension by which the brain could segment spatially overlapping stimulus representations

Perennial Indeterminate Colony Cycle in a Primitively Eusocial Wasp

The colony cycle of primitively eusocial wasps consists of three phases: the pre-emergence phase, the post-emergence phase and the declining phase. Most species of wasps in temperate regions follow a seasonai colony cycle. Being initiated synchronously in Spring by overwintered females, colonies grow through summer and are abandoned in Fall, after producing reproductives. Females produced in Fall mate and overwinter while the remaining individuals, including the males, die. In the tropics, however, colonies are aseasonal and may be initiated throughout the year. Colony cycles in most species, either temperate or tropical, may thus be termed determinate since they are abandoned after a fixed time after initiation. Ropalidia marginata follows a colony cycle which often encompasses multiple repeats of a typical determinate colony cycle. We therefore call it an indeterminate colony cycle. A major portion of the brood and cells are destroyed and a large fraction of adults leave the nest during the declining phase of each unit of the colony cycle but a small number of females may stay back on the nest and begin a new unit of the cycle. This leads to two interesting questions: why is there a decline if the colony is to continue? and why is the decline not complete as in the determinate colony cycle. We propose two alternate hypotheses.One is that the colony cycle is a response to predation by Vespa tropica. It may be adaptive to issue 'swarms' of dispersing wasps periodically to found new nests, before all is lost to the predator. This hypothesis predicts that queen replacements need not necessarily coincide with the beginning of every unit of the colony cycle. The second hypothesis is that the queen produces reproductives and dies at the end of each unit of the colony cycles. One of her daughters may however stay on and use her natal nest to produce her brood. This hypothesis predicts that queen replacements should necessarily coincide with the beginning of every unit of the colony cycle. Our present results are incapable of distinguishing between these hypotheses. In some colonies queen replacements always coincide with new units of the colony cycle but in others, this is not so. Further studies of such perennial indeterminate colony cycles are bound to be rewarding