Presentation on theme: "Actual and potential tragedies: conflicts over female caste fate in Apis and Melipona bees Tom Wenseleers & Francis Ratnieks University of Sheffield, UK."— Presentation transcript:
Actual and potential tragedies: conflicts over female caste fate in Apis and Melipona bees Tom Wenseleers & Francis Ratnieks University of Sheffield, UK
Workers Give up reproduction for the benefit of their mother queen Darwinian puzzle ‘The sterile worker caste of the social Hymenoptera poses one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory.’ Darwin (1859) On the Origin of Species Why become a worker?
E.g. honey bee ò benefit of becoming a queen: ability to head daughter swarm ò SO WHY DO NOT MANY FEMALES OPT TO BECOME QUEENS? ò females benefit from becoming a queen, but colony would suffer if all would do so “caste fate conflict” (colony mostly needs workers for swarming) ò individual benefits but collective suffers = “tragedy of the commons” Bourke and Ratnieks 1999 BES
Tragedy of the commons Each individual gains by pursuing interests that increase returns relative to neighbours but decrease the value of the common goods. If all succumb to the temptation of free-riding, the outcome is a collective disaster. William Forster Lloyd 1832
Killing occurs quickly Colony% of queens in brood combs % N % of queens among adults % N Life expectancy virgin queens 4146150.35113125 hours 524210.2218438 3b185210.65122635 T1225601.5053262 T2237320.4961219 T31711840.0012730 Queens killed within 25 hours after eclosing Life expectancy adult workers = 48.5 days Melipona beecheii
Summary ò social insect females benefit from developing as a queen ò in Melipona, females have the ability to do this (’self determination’) ò results in excess queen production ò why do not all females develop as a queen? what limits exploitation within the group?
Costs to kin can limit exploitation ò when selfishness causes cost to kin exploitation becomes less profitable ò queen overproduction causes depletion of workforce and has two costs to kin: reduced ability to swarm reduced production of males ò prediction: less exploitation when group members are highly related ò has never been tested
Factors determining kinship ò multiple mating by queen: reduces relatedness among sisters does not occur in stingless bees ò worker laying –workers can sometimes produce sons –relatedness to worker’s sons = 0.75 –relatedness to queen’s sons = 0.25 can occur in stingless bees
Caste conflict model ò female should become a queen with a probability of (1-R f ) / (1+R m ) (self determination) with R f = sister-sister relatedness R m = relatedness to males = 20% under single mating, all males queen produced = 14% under single mating, all males worker produced ò assuming linear cost to total colony reproduction ò higher/lower ratios with other cost functions Ratnieks 2001 BES; Wenseleers & Ratnieks submitted
Test: interspecies comparison PREDICTION less queen overproduction when males are worker’s sons, since costs are then to closer relatives (nephews, r = 0.75, rather than brothers, r = 0.25)
% of males workers’ sons M. quadrifasciata 3 Various sites, Brazil M. subnitida 2 Rio Grande do Norte, Brazil M. beecheii 1 Yucatan, Mexico M. favosa 4 Tobago, West Indies * * 4 / 604 / OBS 4 / 1,338 / GEN+OBS 16 / 505 / GEN+OBS 13 / 108 / GEN #cols. / #males / study * GLZ, p < 10 -15 N.S. Male parentage in Melipona % of males LOW > INTERMEDIATE > HIGH workers’ sons 1 Paxton et al 2001; 2 Contel & Kerr 1976; Koedam et al 1999, 2002; 3 da Silva 1977; Toth et al 2002; 4 Sommeijer et al 1999 All species singly mated: Peters et al 1999, Contel & Kerr 1976, Paxton et al 2001, Kerr 1975, Kerr et al 1962 Mean, 95% C.L.
M. beecheii caste ratios Prop. of queens produced Max. = 21% Average = 14.6% 10 cols. 8,162 ind. 95% C.L. Moo-Valle, Quezada-Euan and Wenseleers 2001 Insectes Sociaux
% of queens produced % males workers’ 0% 34% 41% 95% sons predicted level HIGHEST > INTERMEDIATE > LOWEST of queen production M. beecheii 1 Yucatan, Mexico M. quadrifasciata 4 Various sites, Brazil M. subnitida 3 Rio Grande do Norte, Brazil M. beecheii 2 Yucatan, Mexico M. favosa 5 Tobago, West Indies * * 78 / 10 / 13,514 9 / 11 / 2,8066 / 2 / 3,989 10 / 12 / 8,162 3 / 1 / 2,476 Cols. / months / indiv’s. * GLZ, p < 10 -10 N.S. Test: interspecies comparison 1 Darchen & Delage-Darchen 1975; 2 Moo-Valle et al 2001; 3 Koedam et al 1999, 2002; 4 Kerr 1950; 5 Sommeijer et al 2002 Mean, 95% C.L.
% of queens produced N.S. * * Data from months with maximum queen production only M. quadrifasciata 3 Various sites, Brazil M. subnitida 2 Rio Grande do Norte, Brazil M. beecheii 1 Yucatan, Mexico M. favosa 4 Tobago, West Indies 1 Moo-Valle et al 2001; 2 Koedam et al 1999, 2002; 3 Kerr 1950; 4 Sommeijer et al 2002 % males workers’ 0% 34% 41% 95% sons predicted level HIGHEST > INTERMEDIATE > LOWEST of queen production Mean, 95% C.L. * GLZ, p < 10 -10 Test: interspecies comparison
MULTIPLE MOTHER QUEENS lowers relatedness should cause even greater queen overproduction Future test: Melipona bicolor
Summary ò Melipona females selfishly exploit colony by developing as queens ò causes “tragedy of the commons”: queen overproduction ò reduced exploitation when costs are to close kin (workers’ sons)
Alternative explanations for excess queen production in Melipona ?
1. Kerr’s theory of genetic caste determination Kerr (1950) proposed 2-locus 2-allele system for Melipona females heterozygous at both loci develop into queens results in 25% queens
Different levels of explanation ò not an alternative hypothesis – different level of explanation (Alcock 1993) : Kerr’s hypothesis suggests HOW the observed caste ratios could come about (PROXIMATE) Caste conflict theory explains WHY the caste ratios are as observed (ULTIMATE) ò cf. XY-sex determination as an efficient mechanism to attain optimal 1:1 sex ratio in mammals
2. Insurance against queen loss? ò queen are overproduced to ensure that continuous stock of queens is present ò bet-hedging argument ò queen overproduction is far too high queen replacement takes 10 days in this period up to 70 queens are produced ò there are other ways to ensure a continuous stock of queens
Queen stocks kept in prisons In Trigonini stingless bees, e.g. Plebeia remota ensures that continuous stock of queens is present without having to overproduce them
What about other social insects? ò other swarming social insects: queen-worker size dimorphism –army ants –honey bees –trigonine (non-Melipona) stingless bees ò caste fate enforced via food control ò results in few queens being produced ò makes individuals work for the benefit of society and develop as a worker, even when not in best interests of individuals themselves
Queen rearing in honey bees Policing of caste fate: food control
Honey bee ò multiply mated: R f =0.3, R m =0.25 ò females would like to become queens with prob. of (1-R f ) / (1+R m ) = 56% ò only 0.02% actually become queens ò strong divergence between individual and colony optimum ò females are coerced into a working role
Queen rearing in trigonine bees Policing of caste fate: food control
Evasion of caste policing: dwarf queens ò observations –occur in ants and trigonine bees –same size as workers –produced in excess –can reproduce, although usually less fecund ò hypothesis –selfish strategy to overcome worker feeding control? ò support –overproduced relative to normal queens
Frequency of dwarf queens ò overproduced relative to normal queens –E.g. Schwarziana quadripunctata –1 in 85 worker cells (1.2%) yield dwarf queens –only 1 in 620 females reared as normal queens (0.16%) –i.e. 88% of all queens produced are dwarf queens & produced in 7-fold excess relative to normal queens ò excess queens are killed by workers as in Melipona ò as predicted by caste conflict theory !
Termite caste conflict ò when colony loses royal pair it may be replaced by replacement reproductives (‘neotenics’) ò in lower termites most individuals are totipotent ò any individual would like to be a replacement reproductive ò but just one pair is needed = another example of a ToC ò should result in excess production of replacements
Termite caste conflict model ò ESS is to molt into a replacement reproductive with a probability of 1-relatedness = 1 – 0.5 = 50% cf. Frank 1995 (assuming outbreeding) ò predicts development of excess replacements Wenseleers, Korb & Ratnieks, in prep.
Cryptotermes brevis ò 50% of all individuals develop as neotenics (Lenz et al. 1985) ò all but one pair killed ò as predicted by caste conflict theory
Development and killing of excess reproductives TERMOPSIDAEPorotermes adamsoniMensa-Bonsu 1976 Lenz 1985 KALOTERMITIDAEKalotermes flavicollisRuppli 1969, Lüscher 1952, 1956, 1974 Neotermes connexusMyles & Chang 1984 Neotermes jouteliNagin 1972 Cryptotermes brevisLenz et al. 1985 RHINOTERMITIDAEReticulitermes lucifugusBuchli 1956 Reticulitermes santonensis Wenseleers, Korb & Ratnieks, in prep.
Summary ò social insect caste system provides scope for conflict ò caste conflict may cause significant costs to the society (Melipona – queen overproduction) ò coercion is more effective than kinship in suppressing caste conflict
Insight into conflict resolution Self determination 20% queen production stingless bees Policing of caste fate 0.02% queen production honey bees Individual Freedom Causes a Cost to Society But females prefer to become queen with probability of 56% ! Efficient Society but No Individual Freedom THE SAME TENSION OCCURS IN HUMAN SOCIETY !
Bourke A.F.G., Ratnieks F.L.W. 1999. Kin conflict over caste determination in social Hymenoptera. Behavioral Ecology & Sociobiology 46: 287-297. Moo-Valle H., Quezada-Euán J.J.G., Wenseleers T. 2001. The effect of food reserves on the production of sexual offspring in the stingless bee Melipona beecheii (Apidae, Meliponini). Insectes Sociaux 48: 398-403. Ratnieks F.L.W., Monnin T., Foster K.R. 2001. Inclusive fitness theory: novel predictions and tests in eusocial Hymenoptera. Annales Zooogici Fennici 38: 201- 214. Ratnieks F.L.W. 2001. Heirs and spares: caste conflict and excess queen production in Melipona bees. Behavioral Ecology & Sociobiology 5: 467-473. Wenseleers T., Ratnieks F.L.W., Billen J. 2002. Conflict over caste fate in social insects: a tragedy of the commons examined. Submitted. Wenseleers T., Ratnieks F.L.W. 2002. Tragedy of the commons in bees. Submitted. PDF reprints and talk at www.shef.ac.uk/projects/taplab/twpub.html References
Acknowledgements ò Collaborators V-L. Imperatriz-Fonseca, M. de F. Ribeiro, D. de A. Alves (SP, Brazil) H. Moo-Valle, J. Quezada-Euán and Luis Medina-Medina (Dept. of Apiculture, Merida, Mexico) R. Paxton (Tübingen, Germany) ò Funding British Council FWO-Vlaanderen Vlaamse Leergangen EU ‘INSECTS’ and ‘Social Evolution’ Networks Marie Curie Fellowship