Insectes soc. 47 (2000) 94–95 0020-1812/00/010094-02 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 2000
Insectes Sociaux
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Colony relatedness in aggregations of Apis dorsata Fabricius (Hymenoptera, Apidae) B.P. Oldroyd 1, K.E. Osborne 1 and M. Mardan 2 1 2
School of Biological Sciences A12, University of Sydney, Sydney NSW 2066, Australia, e-mail:
[email protected] Plant Protection Department, Faculty of Agriculture, 43400 UMP Serdang, Selangor Dural Ehsan, Malaysia
Received 8 December 1998; revised 15 July and 18 October 1999; accepted 3 November 1999.
Summary. Apis dorsata colonies often form dense aggregations, with over 100 colonies sometimes seen in the same tree. Reasons for these aggregations are unknown, but one reasonable hypothesis is that colonies form a related family group. Here we show that 7 adjacent colonies sampled from a single branch of a tree (near Alor Setar in northern peninsular Malaysia) containing over 120 colonies were not related as mother/daughter. Thus the notion that aggregations arise through splitting of the first-arriving colonies can be rejected. Key words: Relatedness, swarming, aggregations, migration, Apis dorsata.
Introduction Apis dorsata is the largest bee in the genus. Its range extends from the Indian subcontinent to south-east Asia, including the Philippines (Ruttner, 1988). Like the dwarf bees A. florea and A. andreniformis, its colonies are characterised by single combs, which are usually suspended from tree branches (Morse, 1970; Seeley et al., 1982). Less commonly, colonies nest on cliff overhangs or on human-built structures (Ruttner, 1988). A. dorsata (and the related species A. laboriosa) is unique in that colonies are often found in dense aggregations (Lindauer, 1956; Seeley et al., 1982; Roubik et al., 1985; Underwood, 1990; Dyer and Seeley, 1994). While A. mellifera nests tend to be aggregated (Oldroyd et al., 1995; McNally and Schneider, 1996), aggregation is much more pronounced in A. dorsata. Over 120 individual colonies may occur on a single tree or rock face (pers. obs.). Aggregations are facultative, and individual colonies or small aggregations of 2–3 colonies are more common than mass aggregations (pers. obs.).
A second important feature of A. dorsata biology is frequent colony migration (Butani, 1950; Thaker and Tonapi, 1961; Morse, 1970; Koeniger and Koeniger, 1980; Dyer and Seeley, 1994). Migration follows a seasonal pattern, and may be related to available forage, or to predation (particularly by humans). A further intriguing possibility is that migration helps control levels of the parasitic mite Tropilaelaps clareae (Rinderer et al., 1994). A third feature is the process of reproductive swarming. This appears to be of two kinds (Lindauer, 1956). In the first (which seems akin to that observed in A. mellifera) a queen will leave the nest, and, as she flies slowly away, she is followed by a swarm of workers. It is not known whether these swarms form a temporary combless cluster, or whether they move directly to a new nest site. The distance that these swarms generally move is also unknown, but at least one such swarm has settled more than 500 m from the natal nest (Lindauer, 1956). In the second mechanism, known as “budding”, a group of workers gradually separates from the nest, and forms a new colony about 1 m from the old (Lindauer, 1956). Here we report on the relatedness of adjacent colonies in a very large aggregation. If the “budding” observed by Lindauer (1956) is common, then we would predict that daughter colonies would be found adjacent to mother colonies. Another potential reason why we might expect closely related colonies to group together is the inclusive fitness benefits of cooperative defence, which has been postulated as the reason for aggregations of A. dorsata (Seeley et al., 1982). Materials and methods We studied the relatedness of 7 adjacent A. dorsata colonies to determine whether neighbouring colonies are unrelated. The null hypothesis that two queens are related as mother daughter can be unambiguously rejected if the two queens do not share an allele at any one locus. The null hypothesis that two queens are related as half sisters cannot be
Insectes soc. Vol. 47, 2000 rejected so simply, and because of the limited scope of this study, we do not consider this possibility further here. Samples were collected from a “bee tree” situated in rainforest at Pedu Lake near Alor Setar in northern peninsular Malaysia. This particular tree (Kompassia excelsa) was extremely large (estimated as 90 m), and in 1995 was occupied by 127 colonies. Honey bees have occupied this tree since at least 1964, and colonies are harvested annually by Mr. Salleh Mohd. Nor, a local honey hunter, using traditional methods. Mr. Nor reports that the tree is usually occupied by migrating A. dorsata swarms during October and November. These swarms build combs and establish colonies, and some reproduction of colonies may occur. Colonies begin to abscond in March, and the tree is usually deserted by April. Mr. Nor provided us with brood from 7 sequential colonies along the lowest branch (about 40 m above ground) of this tree. Samples were frozen in liquid nitrogen for transport to the laboratory where they were stored at –70°C. The microsatellite genotype of each of 10 bees was determined for 4 loci (A14, A88, A107 and B124 identified by Estoup et al., 1994). To do this, we extracted DNA from the leg of an adult by boiling the ground tissue in 5% Chelex 100 resin for fifteen minutes (Walsh et al., 1991). We then amplified these DNA extractions using the polymerase chain reaction (PCR) with primers specific to the honey bee microsatellite loci. In each case the reverse primer had been labelled by the manufacturer (Bresatec, Adelaide, Australia) with the flourescent dye HEX. PCRs were conducted in 10 ml volumes, of which 2 ml was the DNA extraction. Each reaction contained 400 nM of each primer, 150 mM of each dNTP, 1.5 mM MgCl2, 1 ¥ reaction buffer, and 0.45 units of Tth polymerase (Fisher Biotech, Perth, Australia). After an initial denaturing of the template DNA for 4 min at 94°C, samples underwent 30 cycles of denaturation (30 s at 94°C), annealing (30 s at 55 °C), and extension (30 s at 72°C) followed by final elongation for 10 min at 72°C. PCR products were electrophoresed on an automated DNA fragment analyser (Corbett Research, Sydney), which, by reference to standard lanes, called the allele length in base pairs (bp). Each lane was manually checked, and if necessary the automated call was adjusted to take account of gel warping. Queen genotype was inferred from the genotypes of the 10 workers via the following rules: (1) Queens carry only two alleles and all workers must carry one of those two alleles. (2) If any worker is homozygous, then her mother carries that allele. (3) If all 10 workers carry an allele, then the queen is likely to be homozygous for that allele. (4) If all 10 workers carry one of two alleles, then the queen is likely to be heterozygous for those two alleles. In combination, the rules will almost always unambiguously define the queen genotype. Violations will occur when the queen carries a null allele, but by reference to other loci, this condition is readily distinguished from the alternative possibility of multiple maternity (Oldroyd et al., 1996).
Results and discussion No queens were related as mother/daughter, because all pairs of individuals carried different alleles at at least one locus (Table 1). Although we cannot exclude the possibility that some queens studied were half sisters, none were mother/ daughter, and it is most likely that colonies were unrelated. This indicates that there is no tendency for daughter colonies to remain close to the mother colony. Thus reproductive swarming via “budding”, as observed by Lindauer (1956) may not be common. Further, mutual defence via aggregation of highly related colonies also appears unlikely. We conclude that aggregations of A. dorsata arise by unrelated colonies gathering at particular nest sites, not by budding of colonies.
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Table 1. Inferred genotype (microsatellite length in base pairs) of queens heading 7 adjacent A. dorsata colonies Colony
A14
A88
A107
B124
1 2 3 4 5 6 7
164/167 167/170 167/169 164/169 169/Null 170/173 164/164
97/124 97/146 118/131 97/124 97/104 106/124 99/102
212/284 173/216 223/297 189/268 207/209 187/212 189/205
216/216 216/218 216/218 218/218 216/218 216/? 216/218
Acknowledgments Thanks to K. Parker for technical assistance, D. Low and A. Barron for critically reading the manuscript, and the Australian Research Council for funding. Two anonymous reviewers and the Editor straightened out some woolly thinking. Thanks for that.
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