Population structure of Leptothorax ambiguus,a facultatively polygynous and polydomous ant species

We examined the genetic and spatial structure of Leptothorax ambiguus in a Vermont site. Nests of this tiny ant species have variable queen number and comprise larger polydomous colonies, as do their closest relatives in North America. Nests are patchily distributed in the forest, and sometimes occur in local abundance. We collected 121 nests in four years from plots in which all nests were mapped; furthermore, we subjected nests collected in two separate years to starch gel electrophoresis and estimated relatedness according to the Queller—Goodnight (1989) algorithm. Queens that share a nest site also share 33% of their alleles on average, and relatedness among worker nestmates is about 0.5. The existence of diploid males and nonzero F-values demonstrate inbreeding in this species, an unusual phenomenon for social insects in general. Mapping data showed that nests with like genotypes tended to cluster in space, forming polydomous colonies. Colonies consisted of 1–6 nest subunits, and about half of all colonies were polygynous. We compare these features of L. ambiguus to its close relative L. longispinosus and a European congener L. acervorum. These comparisons allow us to conclude that an interplay between ecological and genetic factors produces the observed pattern of multiple queening and nest spatial distribution in this species.     

The fate of Corydalis cava elaiosomes within an ant colony of Myrmica rubra: elaiosomes are preferentially fed to larvae

Summary. The diaspores of many plant species are adapted to dispersal by ants (myrmecochory). The ants carry the diaspores into their nests where the appendages of the diaspores (elaiosomes) are consumed. Little is known, however, about the fate of diaspores in the colony, i.e., whether elaiosomes are consumed only by the larvae or also by the workers. In this study, the distribution of Corydalis cava (Fumariaceae) elaiosomes was monitored between the larvae and the working caste of laboratory colonies of Myrmica rubra (Formicidae, Myrmicinae). In the first part of the study, ¹⁵N labelled elaiosomes were found to be mainly consumed by larvae. To find out whether elaiosomes are a more attractive food source than an artificial, nutritionally optimized diet for ants (Bhatkar diet), a combined ¹³C and ¹⁵N labelling experiment was conducted. Ants were offered ¹⁵N labelled elaiosomes for two days (pulse labelling), while being continuously fed with ¹³C labelled Bhatkar diet over 19 days. Under the given laboratory conditions, elaiosomes proved to be a far more attractive diet for the larvae than Bhatkar diet, contributing 87 ± 2% of the daily nitrogen and 79 ± 3% of daily carbon incorporation of larvae during the first four days of the experiment. The mean elaiosome incorporation met 73 ± 5% of nitrogen and 63 ± 6% of carbon demand of workers during the first four days of the experiment. Generally, incorporation rates in workers (per dry body mass) were lower both for carbon and nitrogen during the experiment – by a factor of 6.8 for nitrogen and by a factor of 6.2 for carbon compared to larvae. On a colony basis, workers received 39% and larvae 61% of the elaiosome nitrogen. The results indicate that elaiosomes are a major food source for growing larvae and thus support the hypothesis that elaiosomes play an important role in the life cycle of temperate ants.Received 22 December 2003; revised 16 June 2004; accepted 22 June 2004.     

Seasonal variation in the sex allocation of a neotropical solitary bee

We carried out a field study on the life history and sex allocationof theground-nesting solitary bee Diadasina distincta (Hymenoptera:Anthophoridae).This species is multivoltine, undergoing five generations ayearbetween February and September. The numerical sex ratio of thisspecieswas female biased overall (approximately 38% males)and showed a strong andconsistent seasonal pattern. The numericalsex ratio was extremely femalebiased (approximately 20% males)from February until May, and then slightlymale biased (approximately60% males) from June until September. Females were3.26 timesthe size of males, and so the overall investment ratio was femalebiasedthroughout the year. The overall female bias and seasonal variationinsex allocation is unlikely to be explained by models thatinvoke overlappinggenerations or competition between brothersfor mates (local matecompetition). We suggest that a possibleexplanation for the female bias inthe early part of the seasonis local resource enhancement (LRE): nesting nearlarger numbersof sisters reduces parasitism. LRE is likely to decrease inimportancein the later part of the season, when the biased numerical andinvestmentratios may be explained by models in which male and femaleoffspringgain different fitness returns from resources invested.     

Local resource competition and local resource enhancement shape primate birth sex ratios

Sex ratio theory provides a powerful source of testable predictions about sex allocation strategies. Although studies of invertebrates generally support predictions derived from the sex ratio theory, evidence for adaptive sex ratio biasing in vertebrates remains contentious. This may be due to the fact that most studies of vertebrates have focused on facultative adjustment in relation to maternal condition, rather than processes that might produce uniform sex biases across individuals. Here, we examine the effects of local resource enhancement (LRE) and local resource competition (LRC) on birth sex ratios (BSRs). We also examine the effects of sex differences in the costs of rearing male and female offspring on BSRs. We present data from 102 primate species and show that BSRs are skewed in favour of the dispersing sex in species that do not breed cooperatively, as predicted by the LRC model. In accordance with the LRE model, BSRs are generally skewed in favour of the more beneficial sex in cooperatively breeding primate species. There is no evidence that BSRs reflect the extent of sexual size dimorphism, an indirect measure of the costs of rearing male and female offspring. These analyses suggest that adaptive processes may play an important role in the evolution of BSRs in vertebrates.     

The interaction of food distribution and the caste composition of an ant colony (Myrmica rubra L.)

The distribution of food between members of a Myrmica rubra L. Society was investigated by varying the ratios of queens, workers and medium-sized larvae. Observations revealed patterns in colony behaviour which could be of importance in a polygyne system.
Queens had little effect upon the rate of food transmission, but the worker/larva ratio was of significance. Many workers effectively fed all larvae present in a colony, but a small number of workers fed only a few. If larvae and/or queens were in abundance, the workers were partly deprived of access to them. Competition between the queens and larvae for food and worker attention occurred when their numbers were high. In this situation, queens fed themselves while the workers cared for the larvae. The significance of overcrowding, not only upon the administration of food, but upon the queen effect acting on the workers to stimulate or inhibit worker egg-laying and brood-rearing, is discussed.     

Phylogeography of the ant Myrmica rubra and its inquiline social parasite

Widely distributed Palearctic insects are ideal to study phylogeographic patterns owing to their high potential to survive in many Pleistocene refugia and-after the glaciation-to recolonize vast, continuous areas. Nevertheless, such species have received little phylogeographic attention. Here, we investigated the Pleistocene refugia and subsequent postglacial colonization of the common, abundant, and widely distributed ant Myrmica rubra over most of its Palearctic area, using mitochondrial DNA (mtDNA). The western and eastern populations of M. rubra belonged predominantly to separate haplogroups, which formed a broad secondary contact zone in Central Europe. The distribution of genetic diversity and haplogroups implied that M. rubra survived the last glaciation in multiple refugia located over an extensive area from Iberia in the west to Siberia in the east, and colonized its present areas of distribution along several routes. The matrilineal genetic structure of M. rubra was probably formed during the last glaciation and subsequent postglacial expansion. Additionally, because M. rubra has two queen morphs, the obligately socially parasitic microgyne and its macrogyne host, we tested the suggested speciation of the parasite. Locally, the parasite and host usually belonged to the same haplogroup but differed in haplotype frequencies. This indicates that genetic differentiation between the morphs is a universal pattern and thus incipient, sympatric speciation of the parasite from its host is possible. If speciation is taking place, however, it is not yet visible as lineage sorting of the mtDNA between the morphs.     

Size‐dependent sex allocation in a simultaneous hermaphrodite parasite

Most models of sex allocation distinguish between sequential and simultaneous hermaphrodites, although an intermediate sexual pattern, size‐dependent sex allocation, is widespread in plants. Here we investigated sex allocation in a simultaneous hermaphrodite animal, the tapeworm Schistocephalus solidus, in which adult size is highly variable. Sex allocation was determined using stereological techniques, which allow measuring somatic and reproductive tissues in a common currency, namely volume. We investigated the relationships between individual volume and allocation to different reproductive tissues using an allometric model. One measure of female allocation, yolk gland volume, increased more than proportionally with individual volume. This is in contrast to the measure of male allocation, testis volume, which showed a strong tendency to increase less than proportionally with individual volume. Together these patterns led to sex allocation being strongly related to individual volume, with large individuals being more biased towards female allocation. We discuss these findings in the light of current ideas about size‐dependent sex allocation in, primarily, plants and try to extend them to simultaneous hermaphrodite animals.     

Behavioural activity of pure chiral 3-octanol for the ants Myrmica scabrinodis Nyl. and Myrmica rubra L.

ABSTRACT. The mandibular glands of Myrmica ants contain, among other substances, 3-octanol, a chiral substance, 90% of it being the R -enantiomer in M. scabrinodis Nyl., ruhra L. and ruginodis Nyl. (Cammaerts et al. , 1985). Pure R and S 3-octanol has been prepared and tested on M. rubra and scabrinodis , workers. Both species react specifically only to the R enantiomer; in M. rubra this constituent arrests foragers briefly, and in M. scahrinodis it attracts them and incites them to walk more quickly. It is shown that, in M. scahrinodis , the naturally produced mixture of R and S 3-octanol (proportions of 9:1, v/v) is more active than the pure R enantiomer or a mixture of R and S 3-octanol (5:5 v/v).     

Sex allocation within broods: the intrabrood sharing-out hypothesis

Selection is expected to cause parents to adjust the sex oftheir offspring when the environment is predictable during development,and it is expected to affect each sex differently. When severaloffspring compete for limited resources, the environmental conditionsacting on the brood are not a good predictor of the conditionsaffecting individual offspring. There is evidence for some speciesthat, regardless of any bias in brood sex ratio, the sex ofindividual offspring within a brood may be related to its positionin the hatching/birth/weight rank, in ways that might correlatewith the expected share of available resources. Here I proposethat parents may be selected to adjust offspring sex withinthe brood, provided that some depreciable environmental qualityis unequally distributed among siblings in a predictable manner.I call this the "intrabrood sharing-out" hypothesis and presenta graphical model to derive predictions about the relationshipbetween offspring sex and positions within the brood. The modelconsiders that sibling competition not only produces differencesin the mean share of resources among siblings, but it also increasesthe predictability of the share obtained by high-ranking sibsand decreases the predictability of the share for low-rankingones. Consequently, parents should be selected to deal withsuch a distribution by promoting the conditions to make it morepredictable and then adaptively adjust the sex of particularsiblings, especially in high-ranking positions within the brood,rather than to modify the sex ratio of the brood as a whole.     

New fungal pathogens of the red ant, Myrmica rubra, from the UK and implications for ant invasions in the USA

The red ant, Myrmica rubra, is an increasingly invasive pest species in north-eastern USA, where it is known as the European fire ant. During surveys for natural enemies in part of its native range in the UK, three previously unreported fungal pathogens developed on ants when incubated in the laboratory. These are described and illustrated: Paraisaria myrmicarum sp. nov., Hirsutella stilbelliformis var. myrmicarum var. nov., and Hirsutella subramanianii var. myrmicarum var. nov. Based on analyses of the protein coding region EF-1α and LSU?rDNA, all three described taxa are shown to be affiliated with the hypocrealean family?Ophiocordycipitaceae. The implications for the management of M. rubra in its exotic North American range using classical biological control are discussed.     

Intercolony aggression within and among local populations of the invasive ant, Myrmica rubra (Hymenoptera: Formicidae), in coastal Maine

Myrmica rubra L. was introduced into New England in the early 20th century, and at present, has a patchy distribution in parts of northeastern North America, including records from 31 communities in Maine. M. rubra is highly polygynous, and colonies reproduce vegetatively, forming dense local populations where conditions are favorable. Using mobile nests and baited arenas in a series of field aggression bioassays, we tested patterns of internest tolerance within and among local populations on Mt. Desert Island, ME. We found that foragers originating from fragments of the same colony or from neighboring nests retained a high level of intraspecific tolerance over several months, whereas significant intercolony aggression among workers was present between colonies within the same local patch separated by approximately 10 m. Within populations, aggression score values were found to increase linearly with internest distance within a site. Aggression was highest between colonies from spatially different populations on the island and was higher still when nests were assayed against colonies at an off-island site 70 km away in Castine, ME. These data strongly suggest a multicolonial organization within and among local populations of M. rubra in parts of its introduced range. These findings contradict the loss of intraspecific aggression and unicolonial social structure over large geographic areas that have previously been observed in other invasive ant species, particularly Linepithema humile Mayr.     

Male parentage does not vary with colony kin structure in a multiple-queen ant

Kin selection theory predicts that, in social Hymenoptera, the parentage of males should be determined by within-colony relatedness. We present a model showing that, when sex ratios are split (bimodal) as a function of colony kin structure, the predictions of kin selection theory regarding the occurrence of worker reproduction and policing (prevention of worker reproduction) require modification. To test the predictions of kin selection theory and our model, we estimated using microsatellites the frequency of worker-produced male eggs and adults in the facultatively polygynous (multiple-queen) ant Leptothorax acervorum. Analysis of 210 male eggs and 328 adult males from 13 monogynous (single-queen) and nine polygynous colonies demonstrated that the frequency of worker-produced males was low (2.3-4.6% of all males) and did not differ significantly between colony classes or between eggs and adults. This suggested workers' self-restraint as the cause of infrequent worker reproduction in both colony classes. Such an outcome is not predicted either by comparing relatedness values or by our model. Therefore, it appears that factors other than colony kin structure and sex ratio effects determine the pattern of male parentage in the study population. A likely factor is a colony-level cost of worker reproduction.     

Social situation,sperm competition and sex allocation in a simultaneous hermaphrodite parasite,the cestode Schistocephalus solidus

Evolutionary theory predicts an influence of mating group size on sex allocation in simultaneous hermaphrodites. We experimentally manipulated the social situation during reproduction in a simultaneous hermaphrodite parasite, the tapeworm Schistocephalus solidus, by placing worms as singles, pairs or triplets into an in vitro system that replaces the final host. We then determined the reproductive allocation patterns after 24 h (i.e. before the start of egg release) and after 72 h (i.e. around the peak of egg release rate) using stereology. After 24 h, sex allocation strongly depended on worm volume (which is determined in the second intermediate host), but was not significantly affected by the social situation experienced during reproduction. After 72 h, worms in groups had less vesicular sperm (i.e. sperm to be used in future inseminations) than singles. They also stored significantly more received sperm in their seminal receptacles than singles, suggesting that more sperm had been transferred in groups. Moreover, worms in triplets stored significantly more received sperm than worms in pairs, suggesting that they either mated more often and/or transferred more sperm per mating. This suggests a behavioural response to the increased risk of sperm competition in triplets. We further discuss the relative importance of sex allocation decisions at different life‐history stages.     

Evolution of habitat-dependent sex allocation in plants: superficially similar to, but intrinsically different from animals

Because pollen disperses and ovules do not, a basic difference in dispersal abilities of male and female gametes exists in plants. With an analytical model, we show that the combination of such sex-biased dispersal of gametes and variation of habitat quality results in two opposite selective forces acting on the evolution of sex allocation in plants: (i) a plant should overproduce pollen in good patches and overproduce ovules in poor patches in order to equilibrate secondary sex ratios of gametes after pollen dispersal; (ii) a plant should overproduce ovules in good patches and overproduce pollen in poor patches in order to increase the likelihood that its progeny establishes in good patches. Our theoretical results indicate that the evolution of habitat-dependent sex allocation should be favoured in plants, in a direction that depends on the relative dispersal ability of pollen and seeds. We also show that superficially similar predictions obtained for habitat-dependent evolutionarily stable sex allocation in animals actually result from a completely different balance between the two underlying evolutionary forces.     

Sources of stochasticity in models of sex allocation in spatially structured populations

There are many ways to include stochastic effects in models of sex allocation evolution. These include variability in the number of mating partners and fecundity in a rich literature that goes back 20 years. The effects of variance in the fecundity and number of mating partners have typically been considered separately from the stochastic effects of mortality. However, I show that these processes produce mathematically equivalent models with subtly different biological details. These scenarios differ in the way that information becomes available to individuals because the parents often have information on mating partners while they are making sex allocation decisions, but must make these decisions before brood mortality takes place. This makes it possible to test which mechanism, stochastic mortality or variation in mating partners, is responsible for observed sex ratios. Alternatively, asymmetric variance between sexual functions can cause skewed sex allocation, even in the absence of local mate competition. This allows the evolution of either female- or male-biased sex ratios depending on which sexual function is more variable.     

Sex ratios, local fitness enhancement and eusociality in the allodapine bee Exoneura richardsoni

Previous studies of a facultatively eusocial allodapine bee, Exoneura richardsoni Rayment, indicated that high levels of cooperative nesting among close relatives seem to be maintained by benefits that lead to increases in per capita brood production. These traits could lead to local fitness enhancement, which in turn could select for female-biased sex ratios. We show here that sex investment ratios in this species are female-biased in small colony sizes, becoming progressively male-biased in larger colonies, consistent with expectations for local fitness enhancement, but not explainable by alternative models. Our results support previous suggestions that local fitness enhancement can lead to sex ratio bias in primitively social Hymenoptera, but differ from previous studies by suggesting that patterns of bias could lower selective thresholds for sib-directed altruism in small colonies, but have an opposing effect in large colonies.     

Genetic variation in sex allocation in a parasitic wasp: variation in sex pattern within sequences of oviposition

In order to maximize their fitness under Local Mate Competition (LMC), arrhenotokous female wasps have to produce a precise sex ratio when encountering hosts. Recent progress in the theory of hymenopterous parasitoid reproduction suggest that they manage to do it by laying male and female eggs in a particular order and that such reproductive strategies are adaptive. Therefore, the determinism of such sequential patterns would be regulated by genetic control on which natural selection could act. To test this hypothesis, sequences of oviposition were recorded in a set ofTrichogramma brassicae Bezdenko (Hymenoptera; Trichogrammatidae) females and in their daughters by providing themEphestia kuehniella Zeller (Lepidoptera; Pyralidae) eggs. In order to describe accurately sex pattern within these oviposition sequences, I present a joined non-parametric and multivariate statistical method. It is shown thatT. brassicae females do not produce male and female eggs in random sequences. Moreover, the way they organize the sequence of the sexes in their progeny seems to be under a strong genetic control. The evolutionary consequences of such results are discussed.     

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above‐mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two‐dimensional spatial model female‐biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male‐biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.