Maria Cristina Lorenzi

Chemical usurpation of a nest by paper wasp parasites

The paper wasp Polistes atrimandibularis is an obligatory social parasite of another Polistes species, P. biglumis bimaculatus. To control the host nest, the parasite sequentially changes the composition of its chemical signature, the cuticular hydrocarbons, during the colonial cycle. Gas chromatography-mass spectrometry of the cuticular hydrocarbons at every stage of the cycle showed that the parasite can switch on and off an entire chemical family, namely, the unsaturated hydrocarbons. In this way the parasite can match the host signature at a critical moment of the colonial cycle.

Polistes biglumis bimaculatus epicuticular hydrocarbons and nestmate recognition (Hymenoptera, Vespidae)

Summary: An experimental analysis was conducted to determine if chemicals from the cuticle of a social wasp are used in nestmate recognition. These chemicals were also subsequently identified. Laboratory colonies of Polistes biglumis bimaculatus were presented with (1) dead nestmates and dead non‐nestmates, (2) dead nestmates and dead non‐nestmates that were subjected to solvent‐washing to eliminate epicuticular compounds and (3) dead nestmates and dead non‐nestmates that were treated again with extracts of nestmates or non-nestmates. Behavioural responses (acceptance or rejection of introduced wasps) by colony residents showed that they were able to discriminate between dead nestmates and dead non‐nestmates and that they used epicuticular substances to perform the discrimination process. The GC‐MS analyses of the epicuticular compounds showed that they consisted of a blend of hydrocarbons. Multivariate analysis of individual hydrocarbon profiles of wasps from different colonies showed that colonies had distinct hydrocarbon profiles.

Outcrossing hermaphroditic polychaete worms adjust their sex allocation to social conditions

Sex allocation theory predicts that simultaneous hermaphrodites shift sex allocation facultatively in response to variation in local group size. This study was performed to evaluate the relative investment in each sex function by the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema and to test whether allocation to each sex depends on the number of reproductive competitors. Four experimental groups were set up (in a 2 × 2 factorial design) with small or large group size and with small or large enclosures to control for potential confounding effects of density. We measured the proportion of female and male investment in focal individuals. Results revealed that individuals regulated their reproductive output so that when reproductive competitors were present, the number of female gametes was strongly reduced and the male function increased. In contrast, under monogamy, individuals in small groups produced lower numbers of sperm but had a higher egg output than worms in large groups. Density did not affect sex allocation in our experiment. Our findings provide qualitative support for Local Mate Competition theory, but also show that the pattern of sex allocation specific to this species is more complex than expected by current theory.

Dynamics of chemical mimicry in the social parasite wasp Polistes semenowi (Hymenoptera: Vespidae)

Chemical cues are so important in the recognition mechanism of social insects that most social parasites (which rely on hosts to rear their brood) have been documented as overcoming the mechanism by which colony residents recognize non-nestmates, by mimicking the odour of the usurped colony. We simulated in the laboratory the process by which the obligate social parasite, Polistes semenowi, invades nests of the host species, Polistes dominulus, in the field and analysed the epicuticular lipid layer before and after host nest usurpation. The experiment documents that P. semenowi social parasites have an epicuticular hydrocarbon pattern which is very similar to that of their host but, after entering host colonies, parasites mimic the odour of the colonies they invade, to the point that they perfectly match the hydrocarbon profile peculiar to the colony they entered. However, both before and after host nest invasion, parasites show a tendency to possess diluted recognition cues with respect to their hosts.

A measure of sexual selection in hermaphroditic animals: parentage skew and the opportunity for selection.

The role of sexual selection in shaping the mating system of hermaphrodites is currently widely accepted. However, a quantification of the intensity of sexual selection in hermaphroditic animals has never been accomplished. We evaluated the opportunity for sexual selection for both the female and the male functions in the simultaneous outcrossing hermaphrodite Ophryotrocha diadema by measuring focal hermaphrodites’ paternal and maternal offspring in experimental replicated monogamous and promiscuous populations, using genetic markers to estimate paternity. Opportunity for sexual selection for each of the two sexual functions was quantified by means of the Crow’s index, i.e. the ratio of variance in progeny number to the squared mean number of progeny. In addition, the extent to which the reproductive success was shared among competing individuals was estimated by means of the Nonacs’s B index. We documented that the strength of selection on the male and female function in hermaphrodites with external fertilization depends on the reproductive context. Under a promiscuous regime, hermaphrodites have higher opportunities for selection for both the male and the female function than under the monogamous regime. Moreover, the reproductive skew for the female function becomes greater than that for the male function, moving from monogamy to promiscuity. In our model system, allocation to one sexual function is opposed by any degree of allocation to the other, indicating that sex‐specific patterns of selection operate in this model species.

Opportunistic discrimination of alien eggs by social wasps (Polistes biglumis, Hymenoptera Vespidae): a defense against social parasitism?

Abstract Foundresses of the social wasp Polistes biglumis were tested to see whether they were able to recognize alien eggs experimentally introduced into their own nests. Foundresses removed alien conspecific reproductive-destined eggs while they accepted worker-destined eggs. The results indicate that social wasps discriminate among eggs and that they discriminate against alien eggs destined to produce unrelated reproductives. P. biglumis is a strictly solitary founding species, with no reproductive competition within colonies; thus, brood discrimination abilities could have evolved as a counteradaptation against intra- and inter-specific brood parasitism.

Behaviour in usurpers and late joiners ofPolistes biglumis bimaculatus (Hymenoptera, Vespidae)

SummaryDuring the late pre-emergence phase, a foundress of the paper waspPolistes biglumis bimaculatus may be expelled by a conspecific female from her own nest (usurpation) or, less frequently, joined by another female of the same species (late association). The behaviour of femalePolistes biglumis bimaculatus, when usurping a conspecific colony or joining another foundress, is compared with that of foundresses on non-usurped colonies. The most conspicous difference is the intense abdomen stroking behaviour the usurper performs over the comb surface on the first days after usurpation. As observed in otherPolistes species, once usurpers and joiners arrive on a strange nest they will destroy most of the immature brood of the previous nest owner. Although host workers are not aggressive towards the intruder females, reproductive success of usurpers and joiners is low compared with that of legitimate foundresses. The same behaviours observed on usurped colonies are found in the obligate social parasites ofPolistes. These behaviours are therefore discussed in the context of the evolution of intra- and inter-specific parasitism.

Social wasps without workers: geographic variation of caste expression in the paper wasp Polistes biglumis

In primitively eusocial insects, caste expression is flexible. Even though Polistes species are well known to show social trait variation (e.g., worker vs. gyne) depending on ecological context, loss of worker caste in some populations of a eusocial, worker-containing species has never been documented. We report first data on geographic variation in caste expression in Polistes biglumis. We compared physiological and behavioural traits of the first female offspring from four populations that experience different climatic conditions and social parasite prevalence. We demonstrated that the first female offspring to emerge in cold areas with high parasite prevalence had more abundant, gyne-like fat bodies and exhibited lower foraging effort, in comparison to the first female offspring produced in warm areas with low parasite prevalence. Thus, the populations under severe environmental conditions produced a totipotent female offspring and suppressed worker production, whereas the population living in less extreme environmental conditions produced worker-like females as first female offspring and gyne-like females as offspring that emerged later. The existence of mixed social strategies among populations of primitively eusocial species could have important consequences for the study of social evolution, shedding light on the sequence of steps by which populations evolve between the extremes of solitary state and eusocial state.

Epicuticular hydrocarbons of Polistes biglumis bimaculatus (Hymenoptera Vespidae): preliminary results

The cuticular hydrocarbon components of foundresses of Polistes biglumis bimaculatus and of eggs, larvae, pupae and nest paper of the same species were examined by gas-chromatography/mass spectrometry. All the profiles obtained contained a mixture of n—and branched alkanes of various chain lengths; n-pentacosane, n-heptacosane and n-nonacosane have been identified. Profiles of different individuals showed quantitative differences. Hexane extracts obtained from eggs, larvae, pupae and nest paper presented almost the same hydrocarbon composition found in adult female extracts.

Effects of social parasitism of polistes atrimandibularis on the colony cycle and brood production of polistes biglumis bimaculatus (Hymenoptera, Vespidae)

Abstract The cycle of colonies of Polistes biglumis bimaculatus invaded by the obligate and permanent social parasite Polistes atrimandibularis differs from that of normal colonies of the same species as an effect of the presence of the inquiline. In parasitized nests the host foundresses disappear sooner than they do from non parasitized ones and P. b. bimaculatus emergences are both limited in time and reduced in number. After a brief temporal gap from the end of the host emergence period parasite offspring begin to appear in invaded colonies and continue to emerge until the end of the season. In parasitized colonies the number of P. b. bimaculatus emergences is reduced by half with respect to non‐parasitized colonies, while the total number of emerged individuals (host plus parasite brood) is greater than that observed in normal colonies (only P. b. bimaculatus brood). Changes in colony cycle and loss in host brood production in parasitized nests are discussed.