Laura Beani

On status badges and quality signals in the paper wasp Polistes dominulus: body size, facial colour patterns and hierarchical rank

To establish a dominance order, social animals often rely on indicators of fighting to avoid costly aggressive encounters. In some species, individuals use colour patterns to signal their social status. Recent studies claimed that facial markings in the eusocial paper wasp Polistes dominulus are status badges that allow co-foundresses to form a linear hierarchy based on individual quality. Here, we evaluated facial patterns in natural populations of P. dominulus, in its native range, to observe whether the marks reflect overall wasp quality in different contexts. We used the same measures of clypeus patterns used by earlier studies, but did not find that they functioned as status badges. Our analyses showed no evidence that visual markers are related to: (i) size, (ii) probability of surviving winter, (iii) social rank in spring associations, or (iv) health status (assessed by the presence of strepsipteran endoparasites). Size, however, is important. Larger wasps are more likely to survive the winter and to acquire the dominant position in spring associations. Larvae infected with endoparasites become smaller adult wasps. These findings suggest that body size is a reliable quality indicator on which wasps build their social networks, and that clypeus patterning is not involved.

Tiny genomes and endoreduplication in Strepsiptera.

Using flow cytometry, the genome sizes of two species of Strepsiptera were studied: that of male Caenocholax fenyesi texensis Kathirithamby & Johnston (Myrmecolacidae) at 108 Mb, which is the smallest insect genome documented to date; and those of male and female Xenos vesparum Rossi (Stylopidae), which are 1C = 130 and 133 Mb, respectively. The genome sizes of the following were analysed for comparative purposes: (a) the Hessian fly, Mayetiola destructor (Say), which was previously reported to be the smallest among insects: the male measured at 1C = 121 Mb and the female at 1C = 158 Mb; and (b) the female parasitic, haplodiploid, microhymenopteran wasp, Trichogramma brassicae Bezdenko, which measured at 1C = 246 Mb. The hosts of the strepsipterans were also measured: male Solenopsis invicta Buren, the red imported fire ant (host of male C. f. texensis), which is 1C = 753.3 Mb, and female Polistes dominulus Christ, the paper wasp (host of X. vesparum), is 1C = 301.4 Mb. Endoreduplication (4C) of the genome of the thorax of the male strepsipteran, and higher levels of endoduplication (4, 8, 16C) in the body of the larger female was observed. In contrast, little or no endoreduplication was observed, either in the Hessian fly, or in the parasitic wasp.

Prevalence of the parasite Strepsiptera in Polistes as detected by dissection of immatures

Summary. hough the paper wasp genus, Polistes, is well studied, we know little of the incidence of parasitism in this group. Here we present details of 45 nest dissections for 4 species: P. dominulus (Christ), P. gallicus (L.), P. stabilinus Richards and P. carnifex (F.) to detail levels of parasitism of colony members by the obligate parasitic group of insects, the Strepsiptera. All 4 species showed evidence of parasitism among immature members. For 3 species, more than 50% of inspected nests were parasitized and the levels of parasitism among brood (larvae and pupae) was very high and did not differ significantly between parasitized nests. One species, P. stabilinus, suffered very low levels of parasitism, which may be related to its habitat choice. The number of parasites per host was positively related to the proportion of infected brood (parasite prevalence) and in some cases reached phenomenally high levels, which casts doubt on previ ously assumed mechanisms of infection for nest-making Hymenoptera, i.e. phoresy. We also document cases of egg parasitism and encapsulation in Polistes nests. Our data show that parasitism levels greatly varied among areas. Finally, the recent debate on the competitive advantage of P. dominulus in its introduced range, USA, has credited an absence of strepsipteran parasites of this species in facilitating its spread. For the first time, we document levels of parasitism for this species in its nature P range and this would appear to corroborate previous claims. We place our work in the context of other studies of parasitism of social insects and posit that the genus Polistes may have much to offer to this field.

Alternative mating tactics in males of Polistes dominulus (Hymenoptera: Vespidae)

SummaryIn summer, males of Polistes dominulus form large aggregations at sunny landmarks. We identified two size-correlated behavioural categories: residents (R) and transient (T). R males, which constitute 20%–25% of the total population, are larger than T males, territorial, aggressive, and more site-faithful, while T males range more widely, are non-aggressive, and show little site tenacity. Field and laboratory data suggest that R males have an advantage in mating, particularly if they engage in frequent flights while on their territories. These alternative mating tactics within the same population are combined with behavioural flexibility in some individuals, which switch from one option to the other.

Sexually selected vigilance behaviour of the grey partridge is affected by plasma androgen levels

In the grey partridge, Perdix perdixvigilance and calling activity are two sex-dimorphic behaviours that are critical for mate choice. To ascertain whether circulating levels of testosterone directly affect vigilance (i.e. the occurrence of upright alert posture), we compared vigilance scores of testosterone-implanted versus control males both during the normal activity of the flock and after the passage of a raptor silhouette; in the latter case, the calling activity was also recorded. Hormone-treated males were more vigilant than controls in both experimental situations. Vigilance was correlated with calling rate. Testosterone seems to act as a link, relating conspicuous behaviours involved in sexual selection to male quality and physical condition, because of the costs of having high levels of both signalling and androgens.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Testosterone-induced changes of call structure, midbrain and syrinx anatomy in partridges.

Testosterone (T) treated Grey partridges (Perdix perdix) of both sexes uttered significantly longer and lower-pitched calls than controls; both these acoustic features play a critical role in mate choice. A morphometrical analysis of the midbrain nucleus intercollicularis showed a cell size increase in T-treated birds regardless of their sex. Histological study of the syrinx did not reveal any sexually dimorphic structure in experimental and control birds; the major T-induced change was a thickening of the external membranes, reported to be the main sound source in Galliforms. In conclusion, T appears able to modify not only some acoustic parameters, but also certain anatomical structures at the peripheral and central levels of the vocal system in a nonoscine species.

A new role for antennation in paper wasps (Hymenoptera,Vespidae): antennal courtship and sex dimorphic glands in antennomeres

Summary.Males of Polistes dominulus perform antennal vibrations and grasping of female antennae during pre-copulatory and copulatory phases. Male antennation plays a relevant role in mating success. In several antennomeres of males of P. dominulus and Vespa crabro, a further species in which we observed male antennation, we found secretory cells of class 1 and 3 associated to the same release site. In P. dominulus, class 3 cells with ampulla-like reservoirs and class 1 cells release their secretion through hairless multiporous areas. In V. crabro, tyloid-like structures are associated with large apodemes. The as yet unidentified secretion of these glands may act as a contact or low-volatile sex pheromone during courtship behaviour.

Strepsiptera, Phylogenomics and the Long Branch Attraction Problem

Insect phylogeny has recently been the focus of renewed interest as advances in sequencing techniques make it possible to rapidly generate large amounts of genomic or transcriptomic data for a species of interest. However, large numbers of markers are not sufficient to guarantee accurate phylogenetic reconstruction, and the choice of the model of sequence evolution as well as adequate taxonomic sampling are as important for phylogenomic studies as they are for single-gene phylogenies. Recently, the sequence of the genome of a strepsipteran has been published and used to place Strepsiptera as sister group to Coleoptera. However, this conclusion relied on a data set that did not include representatives of Neuropterida or of coleopteran lineages formerly proposed to be related to Strepsiptera. Furthermore, it did not use models that are robust against the long branch attraction artifact. Here we have sequenced the transcriptomes of seven key species to complete a data set comprising 36 species to study the higher level phylogeny of insects, with a particular focus on Neuropteroidea (Coleoptera, Strepsiptera, Neuropterida), especially on coleopteran taxa considered as potential close relatives of Strepsiptera. Using models robust against the long branch attraction artifact we find a highly resolved phylogeny that confirms the position of Strepsiptera as a sister group to Coleoptera, rather than as an internal clade of Coleoptera, and sheds new light onto the phylogeny of Neuropteroidea.

Geographic Variation in the Status Signals of Polistes dominulus Paper Wasps

Understanding intraspecific geographic variation in animal signals poses a challenging evolutionary problem. Studies addressing geographic variation typically focus on signals used in mate-choice, however, geographic variation in intrasexual signals involved in competition is also known to occur. In Polistes dominulus paper wasps, females have black facial spots that signal dominance: individuals wasps with more complex ‘broken’ facial patterns are better fighters and are avoided by rivals. Recent work suggests there is dramatic geographic variation in these visual signals of quality, though this variation has not been explicitly described or quantified. Here, we analyze variation in P. dominulus signals across six populations and explore how environmental conditions may account for this variation. Overall, we found substantial variation in facial pattern brokenness across populations and castes. Workers have less broken facial patterns than gynes and queens, which have similar facial patterns. Strepsipteran parasitism, body size and temperature are all correlated with the facial pattern variation, suggesting that developmental plasticity likely plays a key role in this variation. First, the extent of parasitism varies across populations and parasitized individuals have lower facial pattern brokenness than unparasitized individuals. Second, there is substantial variation in body size across populations and a weak but significant relationship between facial pattern brokenness and body size. Wasps from populations with smaller body size (e.g. Italy) tend to have less broken facial patterns than wasps from populations with larger body size (e.g. New York, USA). Third, there is an apparent association between facial patterns and climate, with wasp from cooler locations tending to have higher facial pattern brokenness than wasps from warmer locations. Additional experimental work testing the causes and consequences of facial pattern variation will be important, as geographic variation in signals has important consequences for the evolution of communication systems and social behavior.

Anti-predator behaviour of captive Grey partridges (Perdix perdix)

Different experiments of simulated predators (flights of a raptor model, playbacks of raptor whistles and of alarms by conspecifics) were carried out on groups of captive Grey partridges (Perdix perdix) from game-farm stock, either intensively- or parent-reared. Both flights and alarms elicited high levels of antipredator behaviour. Freezing and vigilance occurred significantly more in parentthan in intensively-reared coveys. Upright vigilant posture was performed significantly more by males than by females. Good sentinels, i.e. the more vigilant males, formed a stable pair bond earlier than the other males. Advanced reproductive timing might compensate for the increased risk of predation from a sustained vigilance.