The biology of Brevineura froggatti and phylogenetic conservatism in Australian allodapine bees (Apidae,Allodapini)

Summary: We present the first data on the social biology of the allodapine bee, Brevineura froggatti. Colony sizes are small, and only 12.5% of nests contained more than two females. Brood rearing occurs throughout the year, including winter, as it does in the other species of Brevineura studied to date. In both Brevineura species, per capita brood production is much higher in multi-female nests than single female nests, raising the question of why more colonies aren't multifemale. The occurrence of small colony sizes, despite large benefits to group living, differs strongly from species of the sister clade Exoneura. These findings, combined with previous allodapine studies, indicate conservatism in voltinism, brood phenology and colony size within, but not between, Australian allodapine genera.     

Sex Ratios in a Socially Parasitic Bee and Implications for Host-Parasite Interactions

Obligate social parasites of Hymenoptera, known as inquilines, have received enormous attention due to the elaborate adaptations they exhibit for exploiting their hosts, and because they have frequently been used to infer sympatric speciation. Their population biology can be difficult to infer as they are both rare and difficult to extract from host nests. Sex allocation has been studied for very few inquilines of social Hymenoptera. Here we report sex ratio patterns in the allodapine bee Inquilina schwarzi, which is an obligate social parasite of another allodapine, Exoneura robusta. We show that the sex ratio of this inquiline varies with its brood number, it is female-biased in the smallest broods, but becomes more even in larger broods, where the population-wide sex ratio is close to parity. We argue that this pattern of bias is consistent with local resource competition, where inquiline females compete to inherit their natal colony. We also argue that extremely female-biased sex ratios of the host species, combined with overall sex ratio parity in the parasite, may help ameliorate disparity in effective population sizes between these two species which are locked in an evolutionary arms race.     

Molecular phylogenetics of the allodapine bee genus Braunsapis: A-T bias and heterogeneous substitution parameters

Extreme AT bias in Hymenopteran mitochondrial genes have created difficulties for molecular phylogenetic analyses, especially for older divergences where multiple substitutions can erode signal. Heterogeneity in the evolutionary rates of different codon positions and different genes also appears to have been a major problem in resolving ancient divergences in allodapine bees. Here we examine the phylogeny of relatively recent divergences in the allodapine bee genus Braunsapis. We examined heterogeneity in nucleotide substitution parameters for one nuclear gene and codon positions in two mitochondrial genes, exploring various phylogenetic analyses for recovering relationships among species from Africa, Madagascar, southern Asia, and Australia. We explored maximum parsimony, maximum likelihood, Log determinant and Bayesian analyses. Broad topological features of best fit trees tended to be similar for equivalent data sets (e.g., total, or with 3rd mt positions excluded), regardless of the analytic method used (e.g., maximum likelihood or Bayesian). Analyses that used the total data set without modelling partitions separately gave unlikely results, indicating that the Malagasy species was most closely related to Australian species. However, analyses that excluded 3rd mitochondrial positions, or modelled partitions separately, suggested that the Malagasy species falls within the African clade. The unlikely topologies apparently result from long branch attraction, and this problem is ameliorated where modelling allows more realistic estimates of base composition and evolutionary rates for 3rd mitochondrial positions. However, we found that even when codon positions are modelled separately, estimated evolutionary rates for 3rd mitochondrial positions are likely to underestimate true rates. Long branch attraction and multiple substitutions are likely to be much more difficult to circumvent in analyses that explore older, generic-level, divergences in allodapine bees where overwriting is expected to be much more extreme. Our results indicate an African origin for Braunsapis, followed by a single, very early, dispersal event into Asia and then by a later dispersal event into Australia. The Malagasy species is derived from within the African clade.     

Biogeographical origins and diversification of the exoneurine allodapine bees of Australia (Hymenoptera,Apidae)

Aim Early diversification of allodapine bees occurred in Africa c. 50 Ma. They are most abundant in sub‐Saharan Africa and Australia, and one of the oldest phylogenetic divergences in the tribe involves a split between an African + Malagasy clade and an Australian clade. The historical biogeographical scenario for this has been highly problematic, entailing an Eocene dispersal from Africa to Australia, followed by an unresolved, and apparently rapid, set of bifurcations leading to the Australian ‘exoneurine’ genera. Here we use an expanded taxon set of Australian species to explore the timing and historical biogeography of the exoneurine radiation. Location Australia, Africa, Madagascar. Methods One nuclear gene (F2 copy of elongation factor 1α) and two mitochondrial genes (cytochrome c oxidase subunit I and cytochrome b) were sequenced for 33 Australian exoneurine species from all five genera found on the continent, as well as for an additional 37 species from all non‐parasitic genera in the remainder of the tribe. We used Bayesian inference analyses to study phylogenetic topology and penalized likelihood analyses to infer key dates of divergence within the tribe. We also used lineage‐through‐time (LTT) analyses and Bayesian analyses to explore the tempo of radiations and biogeographical history of the exoneurines. Results Results from the phylogenetic analyses were congruent with previous studies, indicating a single colonization event c. 34 Ma, too late for Gondwanan vicariance models, and too early for a Laurasian dispersal route. In contrast to earlier studies, we show that this colonization event did not result in an ancient rapid radiation. However, LTT patterns indicated a rapid radiation of the temperate‐adapted genera Exoneura and Brevineura, but not of the xeric‐adapted genus Exoneurella, from 10 to 6 Ma. Main conclusions Our results indicate a trans‐oceanic dispersal event from Africa to Australia, most likely via Antarctica, with an accelerated diversification of temperate‐adapted lineages during the major Late Miocene event referred to as the ‘Hill Gap’. This is the first study to link radiations in Australian bee faunal elements to changing climate, and differs from many other plant and insect phylogenetic studies by showing increased radiation of temperate clades, rather than xeric clades, with increasing aridification of Australia.     

Molecular phylogenetics of the exoneurine allodapine bees reveal an ancient and puzzling dispersal from Africa to Australia

Previous phylogenetic studies of the bee tribe Allodapini suggested a puzzling biogeographic problem: one of the key basal divergences involved separation of the southern African and southern Australian clades at a very early stage in allodapine evolution, but no taxa occur in the Palaearctic or Asian regions that might suggest a Laurasian dispersal route. However, these studies lacked sufficient sequence data and appropriate maximum likelihood partition models to provide reliable phylogenetic estimates and enable alternative biogeographic hypotheses to be distinguished. Using Bayesian and penalized likelihood approaches and an expanded sequence and taxon set we examine phylogenetic relationships between the Australian, African, and Malagasy groups and estimate divergence times for key nodes. We show that divergence of the three basal Australian clades (known as the exoneurines) occurred at least 25 Mya following a single colonization event, and that this group diverged from the African + Madagascan clade at least 30 Mya, but actual divergence dates are likely to be much older than these very conservative limits. The bifurcation order of the exoneurine clades was not resolved and analyses could not rule out the existence of a hard polytomy, suggesting rapid radiation after colonization of Australia. Their divergence involved major transitions in life history traits and these placed constraints on the kinds of social organization that subsequently evolved in each lineage. Early divergence between the African, Malagasy, and Australian clades presents a major puzzle for historical biogeography: node ages are too recent for Gondwanan vicariance hypotheses, but too early for Laurasian dispersal scenarios. We suggest a scenario involving island hopping across the Indian Ocean via a series of now largely submerged elements of the Kergulen Plateau and Broken Ridge provinces, both of which are known to have had subaerial formations during the Cenozoic. [Bayesian; biogeography; dispersal; Gondwana; Kerguelen Plateau; penalized likelihood.].     

Decisions,decisions, decisions: the host colony choices of a social parasite

Many factors contribute to the success of a socially parasitic strategy, especially the ability of the parasite to invade a host colony. However, little research has focused on the choices that may be made by an invading parasite, specifically whether parasites actively discriminate between different host colonies and if they have a preference for colonies of a particular size. When an allodapine social parasite, Inquilina schwarzi, was presented with colonies of their host species, Exoneura robusta, the parasites were found to invade the larger host colonies. However, it could not be ascertained from this study whether the parasites were making an active decision concerning which colony to invade, or whether they were simply more attracted to the larger colonies due to potentially stronger odour cues. Regardless of the cause, the larger host colonies are more at risk of being invaded by a social parasite, which would give parasites greater resources for exploitation and could also provide selection against the large host colony sizes.     

Brood insurance via protogyny: a source of female-biased sex allocation

Sex allocation patterns and colony productivity are examined in Exoneura nigrescens, a social allodapine bee. As for previous studies on Australian allodapine bees, numerical sex ratios were strongly female biased in the smallest broods, but neared equality in larger broods. Local fitness enhancement has been suggested previously to explain female-biased allocation in allodapine bees. Here, we propose an alternative model, the 'insurance model', which predicts protogyny and, as a consequence, female-biased sex allocation in small broods with decreasing bias in larger broods. Because allodapine broods are reared progressively in an open burrow, broods require that adult females be present throughout their development in order to survive to maturity. If mothers invest in daughters (alloparents) first, these daughters can rear younger, dependent brood in cases in which orphaning occurs. If such daughters behave as surrogate mothers, then investment in them by mothers should not be regarded as investment in female sex allocation per se, giving rise to apparently female-biased broods. The model predicts a pattern of sex ratios as a function of total brood size that very closely match empirical data from E. nigrescens.     

The Problems of a Priori Categorisation of Agonism and Cooperation: Circle‐Tube Interactions in Two Allodapine Bees

Circle‐tube experiments have been widely used to both examine nestmate recognition in social and solitary insects, as well as to characterise interactions in terms of agonism, cooperation, and avoidance. Despite their use in studies of halictid bees, carpenter bees, adrenid bees, and wasps, they have never been used to explore social interactions in the bee tribe Allodapini, where sociality varies widely in terms of complexity. Here, we investigate behaviours displayed within the highly eusocial allodapine Exoneurella tridentata and the facultatively social Exoneura robusta, using a standardised circle‐tube apparatus. We present two broad and important results: (i) Discrimination between nestmates and non‐nestmates is much more strongly expressed in the facultatively social species and (ii) principal components analyses do not indicate suites of behaviours that permit clear interpretations as being agonistic, cooperative, or avoidance. Our first set of results suggests that nestmate recognition is not an essential ability for social species. Our second set of results raise important questions about a priori functional categorisations of behaviours and whether these can be used to reliably classify observed behaviours as either avoidance, cooperation, or aggression. Our analyses highlight the risks of interpreting social insect interactions and call for a more cautious approach.     

Sociality and sex allocation in an Australian allodapine bee Braunsapis protuberans

Abstract  Herein the first detailed study of sociality and life history in an Australian species of the allodapine bee genus Braunsapis is provided. Colonies of B. protuberans were sampled from the Great Sandy National Park in Queensland from late winter until later summer. Single-female colonies appear to have suffered substantially higher rates of total brood loss, or failure to produce brood, than multifemale colonies, but rearing efficiency in colonies with at least some brood did not differ between these two colony types. There was marked reproductive skew in multifemale colonies and ovary size was strongly linked to relative body size. However, females with smaller body size and smaller ovaries also had lower levels of wing wear, suggesting that they do not specialise in foraging. These patterns are similar to an African species, Braunsapis vitrea , and suggest that colonies may principally comprise a dominant reproductive female who provides food for her juvenile offspring, along with a variable number of subordinate females who may be waiting to inherit their natal nest in the event of their mother's death. However, it is likely that in such events, the remaining female(s) will care for orphaned brood as some single-female colonies contained substantial older brood, even though the sole female had little or no wing wear. Such alloparental care in the case of orphaning may help explain the high level of female bias in sex ratios.     

Strong constraints to independent nesting in a facultatively social bee: quantifying the effects of enemies-at-the-nest

Constraints to independent nesting play a key role in the understanding of social evolution in insects, but the source and the magnitude of such constraints are poorly known for many key taxa. In allodapine bees it is known that solitary nesting females have low rates of successful brood rearing and that this drives selection for cooperative nesting. It has been hypothesized that these constraints are due to the presence of enemies-at-the-nest, such as ants, but no direct link has been demonstrated between such enemies and colony failure. We set up an experiment in which solitary founded nests of an Australian allodapine bee, Exoneura nigrescens, were either protected from non-flying predators or left unprotected, and compared the resulting colony survival and brood production rates. We found that protected colonies have much higher rates of survival and that the constraints to independent nesting are extreme, with a mean of less than one offspring per nest at the end of the brood rearing period. This means that cooperative nesting is essential for this species to persist in its habitat. Received 6 July 2007; revised 5 November 2007; accepted 12 November 2007.     

Sociality in the Australian Allodapine Bee Brevineura elongata: Small Colony Sizes Despite Large Benefits to Group Living

Allodapine bees (family Apidae, subfamily Xylocopinae) provide substantial material for investigating the evolution of sociality because of their wide variation in colony size, life history traits, and caste differentiation. Two recent studies have shown that the Australian allodapine genus Brevineura exhibits a strong increase in per capita brood production (PCBP) in social colonies compared to single-female nests. However both species previously examined, B. xanthoclypeata and B. froggatti, show relatively few multi-female nests, which is puzzling considering the apparently large advantages for group-living in these species. Here we show that in a third species, B. elongata, there are also substantial benefits for group living, involving increased PCBP and a greatly reduced likelihood of nests without brood. As expected from these observations, we also found strongly female biased sex allocation. Nevertheless only a small percentage of nests contained more than one adult female, similar to the other two Brevineura species, raising the question of why multifemale colonies are not more common in this genus. Solving this puzzle will throw light on conditions that constrain sociality when group living apparently provides major advantages.     

New species and unexpected diversity of socially parasitic bees in the genus Inquilina Michener (Hymenoptera: Apoidea: Apidae)

Abstract  Allodapine bees present particular problems for taxonomy due to a high level of morphological conservatism in adults, even between genera. However, this tribe of bees also presents a unique opportunity to explore the evolution of social parasitism because of the comparatively large number of origins of socially parasitic species. Morphological differences presented here, along with DNA sequence data and molecular phylogenetic analyses, indicate a much larger number of Australian social parasite species in the genus Inquilina than previously anticipated, and suggest that the final number of socially parasitic species may be considerable. We describe five new species and present sequence data that will help elucidate the delineation of further new species. Inquilina provides a unique opportunity to study the evolution of social parasitism in social insects, but further studies will need to encompass both population genetic and phylogenetic approaches.     

Molecular phylogenetics of allodapine bees,with implications for the evolution of sociality and progressive rearing

Allodapine bees have long been regarded as providing useful material for examining the origins of social behavior. Previous researchers have assumed that sociality arose within the Allodapini and have linked the evolution of sociality to a transition from mass provisioning to progressive provisioning of brood. Early phylogenetic studies of allodapines were based on morphological and life-history data, but critical aspects of these studies relied on small character sets, where the polarity and coding of characters is problematic. We used nucleotide sequence data from one nuclear and two mitochondrial gene fragments to examine phylogenetic structure among nine allodapine genera. Our data set comprised 1506 nucleotide positions, of which 402 were parsimony informative. Maximum parsimony, log determinant, and maximum likelihood analyses produced highly similar phylogenetic topologies, and all analyses indicated that the tropical African genus Macrogalea was the sister group to all other allodapines. This finding conflicts with that of previous studies, in which Compsomelissa + Halterapis formed the most basal group. Changing the basal node of the Allodapini has major consequences for understanding evolution in this tribe. Our results cast doubt on the previous hypotheses that progressive provisioning and castelike social behavior evolved among lineages leading to the extant allodapine taxa. Instead, our results suggest that mass provisioning in Halterapis is a derived feature and that social behavior is an ancestral trait for all allodapine lineages. The forms of social behavior present in extant allodapines are likely to have resulted from a long evolutionary history, which may help explain the complexity of social traits found in many allodapine bees.     

Fine-scale genetic structure, co-founding and multiple mating in the Australian allodapine bee (Exoneura robusta)

Despite the role of Australian native bees in important ecological processes, surprisingly little is known of their population structuring. In this study five microsatellite loci were used to investigate genetic structuring of the allodapine bee Exoneura robusta sampled from four locations (maximum pairwise distance c . 35 km) in the Mountain Ash forests of Victoria. Although E. robusta would seem to have high dispersal ability, several analyses show significant population subdivision and a strong pattern of isolation-by-distance from which limited gene flow was inferred. Limited gene flow was not associated with inbreeding at the within-colony level, and within-colony genetic structure implied co-founding, multiple breeding pairs and some degree of reproductive skew. Strong population structure at such fine scales suggests that substantially divergent populations are likely within the extensive distribution currently ascribed to E. robusta .     

Rearing of Non-Descendant Offspring in an Allodapine Bee, Exoneura bicolor Smith (Hymenoptera: Apidae: Xylocopinae): A Preferred Strategy or Queen Coercion?

In heathland populations of the Australian allodapine bee Exoneura bicolor , the presence of overlapping broods in some colonies creates opportunities for eusocial-like sib rearing. By artificially orphaning colonies in observation nests, we investigated whether sib-rearing by some older daughters occurs in the absence of any potential for coercion by parental females. We found strong evidence for sib-rearing in the absence of parental females in 40% of our nests and weaker evidence in another 40%. of those colonies with strong evidence of sib-rearing, 65% of such rearing occurred in nests where there was also no opportunity for coercion by adult siblings. These results indicate that sib-rearing in E. bicolor does not require coercion by adult nestmates. A variety of factors that may make sib-rearing a preferred strategy in the absence of coercion are discussed.     

A preliminary study on relationships among selected Australian members of the tribe Spilomelini (Lepidoptera: Crambidae: Pyraustinae)

A preliminary study was conducted on phylogenetic relationships among some selected genera of the Australian Spilomelini, focusing on relationships among the Australian Glyphodes group (Glyphodes Guenée, 1854, Dysallacta Lederer, 1863, Talanga Moore, 1885 and Agrioglypta Meyrick, 1932) and the 17 genera which are morphologically similar to it. Representatives of three genera of the Pyarustini were used as outgroups. Cladistic analysis of morphological data from the adult moths produced 10 equally MP trees (length = 221, CI=0.294, and RI=0.608). The clade formed by the 21 selected genera of the Australian Spilomelini had low bootstrap support even though a good apomorphy supported the monophyly of this group, namely, a strong, bilobed praecinctorium of abdominal tympanal organs. The analysis showed that the Glyphodes group is not monophyletic because the genus Chrysothyridia Snellen appears within it in the 10 MP trees. The concept of the Glyphodes group should be expanded to include Chrysothyridia and also the Synclera Zeller and Didymostoma (Walker) since the Synclera + Didymostoma clade, as the hypothesised sister group of the Glyphodes group, is not sufficiently supported as a separate monophyletic group. The analysis also showed that genus Glyphodes is not a monophyletic group, while Metallarcha Meyrick is a monophyletic group.     

Molecular Phylogeny and Biogeography of the Native Rodents of Madagascar (Muridae: Nesomyinae): A Test of the Single-Origin Hypothesis

Complete nucleotide sequences from the mitochondrial cytochrome b gene (1143 bp) were used to investigate the phylogenetic relationships among the native rodents of Madagascar. Specifically, this study examines whether the nine genera of nesomyines form a monophyletic group relative to other Old World murids. All nine of the nesomyine genera, including multiple individuals from 15 of the 21 described species, were included in the analysis, and their monophyly was assessed relative to the murid subfamilies Mystromyinae, Petromyscinae, Dendromurinae, Cricetomyinae, Murinae, Rhizomyinae, and Calomyscinae. Phylogenetic analysis of the resulting 95 taxa and 540 characters resulted in 502 equally parsimonious cladograms. The strict consensus tree weakly refutes the monophyly of Nesomyinae and suggests that the Malagasy rodents form a clade with dendromurines (as represented by Steatomys ) and the African rhizomyine Tachyoryctes . The cladogram strongly refutes the association of the South African genus Mystromys with the Malagasy genera and suggests that Petromyscus and Mystromys form a monophyletic group. We provide the first explicitly phylogenetic scenario for the biogeographic history of nesomyine rodents. Our phylogenetic hypothesis indicates: (1) rodents invaded Madagascar only once, (2) they came from Asia not from Africa as is commonly assumed, and (3) there was a secondary invasion of rodents from Madagascar into Africa.     

FEMALE-BIASED SEX RATIOS IN A FACULTATIVELY SOCIAL BEE AND THEIR IMPLICATIONS FOR SOCIAL EVOLUTION

Montane populations of the Australian allodapine bee, Exoneura bicolor, are characterized by high levels of cooperative nesting and strongly female-biased sex ratios. A conspecific population from heathland shows much lower levels of cooperative nesting and lower levels of female bias. In both habitats, sex-ratio bias is greatest in the smallest brood sizes and becomes successively less biased in larger broods. Parity is approached in the largest heathland colonies, but not for any brood-size category in montane areas. Adult intracolony relatedness is moderately high for colonies in both reused and newly founded nests in the montane habitat, but probably low or zero for newly founded nests in heathland. Colony efficiency, measured as the number of brood per adult, increases with colony size in both habitats, suggesting that cooperation between females increases mean female fitness. It is argued that patterns of sex allocation are consistent with nonlinear fitness-return models, in which the mean reproductive value of daughters increases with the number of daughters produced in a brood. Such increases probably arise from a number of social interactions, including cooperative brood defense, increased task efficiency, and lower per capita costs in nest construction. The term “local fitness enhancement” is introduced here to describe these effects collectively. The female-biased ratios should lower selective thresholds for sib-directed altruism, at least in the earlier stages of colony development. It is argued that local fitness enhancement facilitates eusociality in allodapine bees and could also play a role in other haplodiploid taxa, provided cooperative nesting largely involves sisters, colony efficiency increases with colony size, and optimal colony sizes are only achieved after two or more generations after founding.     

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.