Carlos R. Solis

Social and Genetic Structure of Paper Wasp Cofoundress Associations: Tests of Reproductive Skew Models

Recent models postulate that the members of a social group assess their ecological and social environments and agree a “social contract” of reproductive partitioning (skew). We tested social contracts theory by using DNA microsatellites to measure skew in 24 cofoundress associations of paper wasps, Polistes bellicosus. In contrast to theoretical predictions, there was little variation in cofoundress relatedness, and relatedness either did not predict skew or was negatively correlated with it; the dominant/subordinate size ratio, assumed to reflect relative fighting ability, did not predict skew; and high skew was associated with decreased aggression by the rank 2 subordinate toward the dominant. High skew was associated with increased group size. A difficulty with measuring skew in real systems is the frequent changes in group composition that commonly occur in social animals. In P. bellicosus, 61% of egg layers and an unknown number of non‐egg layers were absent by the time nests were collected. The social contracts models provide an attractive general framework linking genetics, ecology, and behavior, but there have been few direct tests of their predictions. We question assumptions underlying the models and suggest directions for future research.

Maternity Assignment and Queen Replacement in a Social Wasp

Assigning offspring to parents is important for understanding the evolution of reproductive conflicts and cooperation, particularly in the model systems represented by social insects. Molecular genetic markers are often used to exclude, and occasionally used to assign, candidate parents. However, their use in social insects has been unsatisfactory so far because candidate mothers are often highly related and candidate fathers are unknown. Here, we show that microsatellite loci can be scored from each mother’s stored sperm permitting effective maternity assignment. The theoretical power of this method is huge, and we demonstrate its practical utilization in this large-scale study of the wasp, Polistes annularis. All 219 genotyped daughters were either assigned to a unique mother or shown to be the progeny of an uncollected dead mother. The data reveal an unexpectedly high number of changes in reproductive dominance. Maternity assignments using this method should help solve many difficult questions in social evolution.

PHYLOGENETIC RELATIONSHIPS AMONG PAPER WASP SOCIAL PARASITES AND THEIR HOSTS (HYMENOPTERA: VESPIDAE; POLISTINAE)

Abstract— Cladistic analyses of data from allozyme polymorphisms in paper wasp social parasites and their hosts do not support the hypothesis that social parasites are most closely related to their hosts. Electrophoretic data are adduced for nine species of Polistes, including all three known species of social parasites (Sulcopolistes) and their hosts. Three different coding methods are investigated; in no case do the social parasites cluster most closely with their hosts. Rather, there is limited evidence that they form a monophyletic group. However, formal taxonomic recognition of Sulcopolistes is not justified, as it renders Polistes sensu stricto paraphyletic. Although the social parasites are not most closely related to their hosts, hosts and parasites belong in the same subgenus and share many characteristics that may have facilitated the exploitation and deception practised by the parasites on the hosts.

Relatedness and queen number in the Neotropical wasp, Parachartergus colobopterus

Abstract The maintenance of sociality is most difficult to explain under circumstances where non-reproducing helpers are physiologically capable of reproducing and distantly related to the brood they rear. The Neotropical swarm-founding wasps are likely to fulfil these conditions because most taxa lack morphological differences between workers and queens, and they have many queens per nest, which is expected to substantially lower worker to brood relatedness. No morphological differences between workers and queens in Parachartergus colobopterus were detected. Colonies contained an average of 27 queens. However, relatedness among nestmates in P. colobopterus was higher (r=0·31) than would be expected on the basis of queen number alone because the queens themselves are very closely related (r=0·67) and because of large variation in numbers of queens among colonies. This makes the harmonic mean of queen number (five queens), which is the appropriate measure for investigating the impact of queen number on relatedness, much lower than the arithmetic mean. Reproductive dominance of one or a few queens within colonies was not a factor that greatly increased relatedness among workers. Taken together, these results support the cyclical oligogyny hypothesis for the maintenance of sociality in Neotropical social wasps.

Trinucleotide microsatellite loci for a social wasp, Polistes

that build open-faced paper nests in which offspring are reared. The lack of distinct morphological castes and great plasticity in social behaviour among females, combined with the ease of observing individually paintmarked adults, has made this a model genus for behavioural studies (Reeve 1991; Turillazzi & WestEberhard 1996). All species are eusocial, but at a relatively rudimentary level: colonies are relatively small (Reeve 1991), workers are not consistently different from queens (Haggard & Gamboa 1980), social relations are often characterized by strife (West-Eberhard 1969; Strassmann 1981a; Noonan 1981), and the queen in some species must act as the pacemaker, physically manipulating workers into working (Reeve & Gamboa 1983). Polistes has served as a model system for a wide variety of topics including dominance hierarchies (WestEberhard 1969), advantages of group living (Strassmann et al. 1988; Strassmann & Queller 1989; Strassmann 1991), kin selection (Noonan 1981; Strassmann 1981b; Queller & Strassmann 1988), kin recognition (Gamboa et al. 1987), usurpation (Klahn 1988), and social control by queens (Reeve & Gamboa 1983). To date, it has been difficult to combine detailed studies of behavioural interactions with studies of genetic relatedness because of the difficulty in obtaining precise estimates of relatedness for individual pairs of interactants with the available genetic markers (e.g. Strassmann et al. 1989). Clearly, a more polymorphic set of markers is needed. Microsatellite loci are highly polymorphic, codominant, and can be genotyped from many sources of DNA including alcohol-preserved tissues and sperm in a female’s spermatheca (Evans 1993; Queller et al. 1993; Peters et al. 1995). Here we present 18 new microsatellite loci derived from Polistes bellicosus and eight new loci from Polistes annularis. All of these loci contain trinucleotide repeat regions which are much easier to score unambiguously than are dinucleotide repeats. To obtain these loci we constructed very large partial genomic libraries in plasmids with inserts (not enriched for microsatellites) averaging 500 bp (Hughes & Queller 1993; see Strassmann et al. 1996, for the rationale of our approach and specific protocols). We separately probed replicate membranes with synthesized oligonucleotides containing 10 or 12 repeats of either AAT, AAC, AAG, CAT or TAG, five of the 10 possible trinucleotide repeats. We picked up hundreds of potential positives. Reprobing a Southern blot of the inserts indicated that most of these sequences contained repeats. We sequenced part or all of about 80 clones of P. bellicosus. Of these, 59 contained at least one repeat region. We designed polymerase chain reaction (PCR) primers for clones containing uninterrupted repeats, five or more repeats long for which we could read the flanking regions (Table 1). We did not design PCR primers around all repeats for reasons including the nature of the flanks, absence of sufficient flanking sequence, or because they were dinucleotide repeats, not the trinucleotides we were after. We evaluated these primers for heterozygosity on 3–24 unrelated individuals. Genomic DNA was prepared as described in Hughes & Queller (1993) or using protocol Strassmann.1 in Strassmann et al. (1996). PCR was carried out under oil in a 10 μL volume made up of 2 μL diluted genomic DNA (about a nanogram), 2 μL of primer mix (2.5 μM), 0.1 μL 10 mM dNTP mix, 1 μL 10× buffer (provided with Taq), 4.08 μL dH2O, 0.62 μl 25 mM MgCl2, 0.05 μL Taq polymerase (5 units/μL, Promega), 0.15 μL 35S dATP (12.5 μCi/μL). After an initial denaturing for 5 min at 95 °C, we carried out 30–35 cycles of 60-s denaturing at 92 °C, 60-s annealing (at a temperature optimized for the primers used: see Table 1) and 45-s extension at 72 °C. After that, 5 extra minutes at 72 °C allowed for the completion of the extension. PCR products were run on 6% denaturing acrylamide gels (Strassmann et al. 1996). All of the 18 microsatellite loci proved to be polymorphic in P. bellicosus though heterozygosity varied from 0.05 to 1 (Table 1). Seven of the eight loci from P. annularis proved to be polymorphic, with heterozygosities ranging from 0.17 to 1 (Table 1). We have published other microsatellites for Polistes annularis and for another social wasp, Parachartergus PRIMER NOTE

Microsatellite variation in a social insect

We undertook a study to explore the potential of microsatellite loci as genetic markers for investigating kinship patterns in a social waspParachartergus colobopterus. A plasmid library with small inserts was screened for several oligonucleotide repeat motifs. Positive clones were sequenced and several were selected for further work. PCR primers were constructed that would amplify the tandem repeat region and a number of female wasps were scored for variation in the number of tandem repeats at these loci. The five amplified loci were far more variable than allozymes, with an average heterozygosity of 0.35.

Colony life history and demography of a swarm-founding social wasp

Abstract  Colonies of social insects are sometimes viewed as superorganisms. The birth, reproduction, and death of colonies can be studied with demographic measures analogous to those normally applied to individuals, but two additional questions arise. First, how do adaptive colony demographies arise from individual behaviors? Second, since these superorganisms are made up of genetically distinct individuals, do conflicts within the colony sometimes modify and upset optima for colonies? The interplay between individual and superindividual or colony interests appears to be particularly complex in neotropical, swarm-founding, epiponine wasps such as Parachartergus colobopterus. In a long-term study of this species, we censused 286 nests to study colony-level reproduction and survivorship and evaluated individual-level factors by assessing genetic relatedness and queen production. Colony survivorship followed a negative exponential curve very closely, indicating type II survivorship. This pattern is defined by constant mortality across ages and is more characteristic of birds and other vertebrates than of insects. Individual colonies are long-lived, lasting an average of 347 days, with a maximum of over 4.5 years. The low and constant levels of colony mortality arise in part from colony initiation by swarming, nesting on protected substrates, and an unusual expandable nest structure. The ability to requeen rapidly was also important; relatedness data suggest that colonies requeen on average once every 9–12 months. We studied whether colony optima with respect to the timing of reproduction could be upset by individual worker interests. In this species, colonies are normally polygynous but new queens are produced only after a colony reaches the monogynous state, a result which is in accord with the genetic interests of workers. Therefore colony worker interests might drive colonies to reproduce whenever queen number happens to cycled down to one rather than at the season that is otherwise optimal. However, we found reproduction to be heavily concentrated in the rainy season. The number of new colonies peaked in this season as did the percentages of males and queens. Relatedness among workers reached a seasonal low of 0.21–0.27, reflecting the higher numbers of laying queens. This seasonality was achieved in part by a modest degree of synchrony in the queen reduction cycle. Worker relatedness reached peaks of around 0.4 in the dry season, reflecting a decrease to a harmonic mean queen number of about 2.5. Thus, a significant number of colonies must be approaching monogyny entering the rainy season. Coupled with polygynous colonies rearing only males (split sex ratios), this makes it possible for a colony cycle driven by selfish worker interests to be consistent with concentrating colony reproduction during a favorable season.

Control of reproduction in social insect colonies : individual and collective relatedness preferences in the paper wasp, Polistes annularis

Abstract Social insect colonies often have one or a few queens. How these queens maintain their reproductive monopoly, when other colony members could gain by sharing in the reproduction, is not generally known. DNA microsatellite genotyping is used to determine reproductive interests of various classes of colony members in the paper wasp, Polistes annularis. The relatedness estimates show that the best outcome for most individuals is to be the reproductive egg-layer. For workers, this depends on the sex of offspring: they should prefer to lay their own male eggs, but are indifferent if the queen lays the female eggs. The next-best choice is usually to support the current queen. As a rule, subordinates and workers should prefer the current queen to reproduce over other candidates (though subordinates have no strong preference for the queen over other subordinates, and workers may prefer other workers as a source of male eggs). This result supports the theory that reproductive monopoly stems from the collective preferences of non-reproductives, who suppress each other in favor of the queen. However, we reject the general hypothesis of collective worker control in this species because its predictions about who should succeed after the death of the present queen are not upheld. The first successor is a subordinate foundress even though workers should generally prefer a worker successor. If all foundresses have died, an older worker succeeds as queen, in spite of a collective worker preference for a young worker. The results support the previous suggestion that age serves as a conventional cue serving to reduce conflict over queen succession.

Genetic relatedness and population structure in the social wasp,Mischocyttarus mexicanus (Hymenoptera: Vespidae)

SummaryIn primitively eusocial wasps workers often retain the ability to become queens, so their continued performance in the worker role is partly dependent on elevated genetic relatedness between workers and the brood they rear. In colonies of the social wasp,Mischocyttarus mexicanus, workers were related to female pupae by 0.29±0.12, a value that is significantly below the full sister value of 0.75, but not significantly below 0.50, worker relatedness to daughters. Though individuals often build new nests within meters of their natal nest, there was no genetic population structure discernable among four nest clusters, or inbreeding of any kind.

Trinucleotide Microsatellite Loci and Increased Heterozygosity in Cross-Species Applications in the Social Wasp, Polistes

Though microsatellite loci are usually found to be most polymorphic in the species in which they are first identified, we have found significant increases in polymorphisms in some cross-species applications. We present eight new trinucleotide microsatellite loci derived from two species of social wasps, Polistes annularis and Polistes bellicosus. We assessed the primers designed from these species and the degree of polymorphism in two additional species, P. dorsalis, which is very closely related to P. bellicosus, and P. dominulus, which is an Old World congener, thought to have diverged from New World Polistes over 80 million years ago. Cross-species applications for these microsatellite loci indicate that the priming sites from P. bellicosus loci are conserved in P. dorsalis and amplified similarly sized products with higher heterozygosities than the original species in two of three cases. A locus that was monomorphic in P. annularis had a heterozygosity of 1.0 in the distantly related P. dominulus. Cross-species applications of these loci indicated that alleles were generally of similar lengths in the new and original species when they retained their heterozygosity.