Sociality in the phylogenetically basal allodapine bee genus Macrogalea (Apidae: Xylocopinae): implications for social evolution in the tribe Allodapini

Recent phylogenetic studies based on DNA sequence data indicate that the tropical African bee genus Macrogalea is the sister group to the remaining extant allodapine fauna, whereas previously it was thought to be a distal group. This leads to some fundamental changes in our understanding of social evolution in the allodapines. Earlier studies suggested that Macrogalea showed only weak forms of social behaviour and these were not well characterized. However, large samples of Macrogalea zanzibarica presented here show that this species exhibits marked social behaviour. Nearly half of nests collected contained two or more adult females, with up to 10 females per nest. Brood are reared progressively and brood ages within colonies are staggered, giving rise to colonies with very mixed age structures and therefore frequent opportunities for alloparental care. Ovarian dissections indicate non‐simple forms of reproductive partitioning within colonies and most multi‐female colonies show evidence that more than one female has contributed to egg production, though reproductive episodes among colony members are frequently asynchronous. Some females show signs of much higher wing wear than their nestmates, but always show signs of previous reproduction. Reproductive division of labour appears to be temporally marked, ovarian differentiation among nestmates is linked to relative body size, but permanent worker‐like or queen‐like castes appear to be absent. This is similar to the communal, continuously brooded and multivoltine behaviour of some tropical halictine species and may be due to the aseasonal nature of brood development in tropical regions. Patterns of per capita brood production indicate large benefits to multi‐female nest occupancy, and sex allocation is strongly female biased. These findings strongly suggest that the capacity for complex social interactions and alloparental care is an ancestral trait for all of the extant allodapine lineages. Therefore comparisons among extant allodapines are unlikely to throw light on the initial origin of social behaviour, though they may uncover origins of true caste behaviour and reversals to solitary nesting. Sex ratios in Macrogalea and most other allodapine genera, spanning a broad phylogenetic and ecological range, suggest that one or more allodapine traits have provided persistent selection for female‐biased sex allocation.     

Increased nest cofounding and high intra-colony relatedness in the bee Exoneura bicolor (Hymenoptera: Apidae): Results from an experimental situation

Abstract Identifying differences in a key social trait between two populations of die same species is important for understanding the evolution of sociality. Previous studies of new colonies in Exoneura bicolor, an Australian allodapine bee, have shown that there are high levels of kin cofounding in a montane population. The only study to examine intra-colony relatedness in a heathland population has found that new multifemale colonies are not formed by kin. In this study we used an experiment to investigate both cofounding behaviour and intra-colony relatedness in E. bicolor from a heathland population. Nest substrate was placed either 0.05 or 1 m distant from source nests in a novel environment. Although there were no differences in cofounding rate or intra-colony relatedness between die two treatments there was, overall, a high rate of cofounding: 53% of new nests were multifemale, approximately twice as high as found in previous field-based studies. Relatedness among cofoundresses was not only different from zero, ^r± SE = 0.597 ± 0.097, but almost identical to that found in montane populations. A constraint, such as nest substrate distribution, is suggested as a proximate factor affecting the expression of cofounding behaviour in E. bicolor. The implication of such proximate constraints for inferring the phylogenetic origins of social behaviour is discussed.     

Life cycle and social behavior in a heathland population of the allodapine bee,Exoneura bicolor (Hymenoptera: Apidae)

Summary Previous studies of the allodapine beeExoneura bicolor have utilised a predominantly social population from tall montane forests. In this report we examine aspects of the sociality and biology of a population from subcoastal heathland in western Victoria, Australia. Entire colonies, nesting inXanthorrhoea minor, were collected at monthly intervals over 12 months between February 1989 and January 1990. In addition, three collections fromMelaleuca squarrosa were made over three consecutive winter months. The heathland population differs from the montane population in several aspects associated with sociality: (i) it shows lower levels of co-occupancy in both newly founded and re-used nests; (ii) foundress dispersal occurs over a larger proportion of the year; and (iii) there may be a partical second brood produced in late summer, during and after adult eclosion of the first brood. The occurrence of a partial second brood gives rise to opportunities for sib-rearing and eusociality. Colony sizes tend to be larger inM. squarrosa thanX. minor, but still smaller than colonies from the montane population. Egg-laying commences several weeks earlier in multifemale colonies than in single female colonies, before spring dispersal begins. It appears that multifemale colonies are more successful at preventing brood loss than single females and that the mean number of brood per female is larger in multifemale colonies than in single female colonies. These two populations, which display differences in several life history traits, provide an ideal system for studying the mechanisms maintaining social behavior.     

Body Size Shapes Caste Expression,and Cleptoparasitism Reduces Body Size in the Facultatively Eusocial Bees <Emphasis Type="Italic">Megalopta</Emphasis> (Hymenoptera: Halictidae)

We used the facultatively social sweat bee Megalopta genalis (Halictidae) to test whether body size is associated with social caste. Behavioral observations showed that non-reproductive foragers were significantly smaller than reproductive nest mate queens, and foragers were also smaller than presumed pre-dispersal reproductives. Moreover, among females from field-collected nests without behavioral observations, relative body size correlated with relative ovary size. Reproductive status is not a direct result of body size, as body size was not significantly associated with either ovary size or fecundity among both solitary and social reproductives. Reproductive status is apparently an outcome of social competition for reproductive dominance, and status is influenced by size relative to nest mates. Our study is the first to demonstrate an association of body size with caste expression in a facultatively social species with relatively weak seasonal constraints on independent nesting. Larvae of a parasitic fly (Fiebrigella sp., Chloropidae) consume pollen provisions stored in nest cells of M. genalis and M. ecuadoria. We tested whether fly parasitism of M. genalis reduces body size. Parasitized females are significantly smaller as adults than their unparasitized nestmates. This reduction is of a similar magnitude to the size differences between castes, and has the potential to shape host reproductive options by influencing competition with nest mates. We present data on the prevalence of parasitism from four collections of M. genalis and two collections of M. ecuadoria from Barro Colorado Island, Panama, and La Selva, Costa Rica.     

Patterns of sexual size dimorphism in stingless bees: Testing Rensch’s rule and potential causes in highly eusocial bees (Hymenoptera: Apidae,Meliponini)

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non‐Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed a male‐biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non‐Melipona species. Our results in the stingless bees question the classical explanation of female‐biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size.     

Patterns of flowering and sex-ratio variation in the Mediterranean shrub Phillyrea angustifolia (Oleaceae): implications for the maintenance of males with hermaphrodites

The Mediterranean shrub Phillyrea angustifolia is one of the few plants that display androdioecy, a rare breeding system in which males co-occur with hermaphrodites. However, it has been difficult to account for male frequencies previously documented for this species, which have frequently been higher than 0.5. Here we present sex-ratio data from 13 sites in Spain and Portugal that throw further light on the maintenance of androdioecy in P. angustifolia . The inferred proportion of males in these populations was around 0.3, substantially lower that 0.5 and concurring with theoretical predictions for androdioecy. Differences in the proportion of hermaphrodites in flower between "stressed" and "nonstressed" sites suggest the existence of an important life-history trade-off between fruit production and the rate at which plants recover from recurrent disturbances such as fire. The reduced frequency of flowering by hermaphrodites relative to males may help to explain the maintenance of androdioecy in this long-lived, woody species.