Caste Determination in Melipona

1. Caste Determination in Melipona Darelyn David

2. Overview • Eusocial insects • Caste determination in Melipona • Confounding factors • Conclusions

3. “Why do we study these insects? Because, together with man, hummingbirds and the bristlecone pine, they are among the great achievements of organic evolution.” (Wilson 1971)

4. Cooperative brood care • Overlap between generations • Reproductive division of labour

5. Reproductive castes

6. Trophogenic Caste is determined by quality/quantity of food provided to larvae Very common Genetic Inherent larval feature Quite rare Mechanisms of caste determination

7. Honeybees (Apis) royal jelly at all instars : queen pollen and nectar at last instars: workers 0.02% of females become queens Bumblebees (Bombus) Small colonies->few workers-> little food: workers Large colonies-> many workers-> lots of food: queens Most stingless bees (Meliponini: Trigona and Lestrimellita) Workers place little or a lot of food in cells Queen lays eggs in individual cells Lot of food : queen Little food: worker

8. In Melipona • Worker and queen cells are the same size (Kerr 1950) • Mass provisioning in cells • Cells are sealed after egg laying • 25% queens in some Melipona, 12.5% in others

9. Genetic caste determination? Kerr’s hypothesis • Mendelian ratios • 3:1 ratio • Queens heterozygous for 2 pairs of genes (AaBb) • All homozygotes are workers • 7:1 ratio • Queens heterozygous for 3 pairs of genes (AaBbCc)

10. Suggested mechanism of evolution • Ancestor is AABBCC • Mutation to Aa • Heterozygote is fertile, with higher adaptive value than AA or aa • Differential feeding superfluous, genetic mechanism becomes the norm

11. Confounding factors • Queen frequency is usually much lower than the perfect 25% • Excess queens are killed off by workers “…great agitation was noted in the nest. After three days much activity was noted in at the entrance and finally a young queen flew, apparently followed by some workers. She was not observed to return during one hour of observation. At that time the nest was opened and found still in great agitation, and two young queens were found being killed by covering with wax…” (From Moure, Nogueira-Neto and Kerr 1958)

12. Environmental effects Are genes enough to make a queen? • Nutritional effects: Low pupal weights do not yield queens (Kerr et al. 1966,Velthuis 1976) • <70 mg: 26 workers, no queens • 70-86 mg: 96 workers, 25 queens • >86 mg: 133 workers, 25 queens

13. Finding wolves in sheep’s clothing: evidence for GCD • Queens disguised as workers? • Morphological markers (Kerr and Nielson 1966)

14. Fusion of ventral ganglia correlated with increase in pupal ecdysteroid levels (Pinto et al. 2003) • Dose dependent shortening of connectives • Pinto et al. 2002 • Queens have higher early titer peak of ecdysteroids than workers • Workers have higher late titer peak than queens

15. Genetic markers that segregate with caste (Hartfelder et al. 2006)

16. Why so many queens? • Both Apis and Melipona found new colonies by swarming • Very few reproductive opportunities for virgin queens • Apis: 0.02% queens • Workers control queen production by food manipulation • Melipona: 14-25% queens • Workers control queen production by killing excess queens

17. Insurance hypothesis • Provide spare queens in case of queen failure • Provide stock from which to pick best queens • Have a continuous stock of queens handy

18. Caste conflict hypothesis • Potentially totipotent larvae • Queen is more related to own offspring than sister’s • Increased queen production • Depletes workforce • Reduces male population • Conflict between colony and individual interests • Support from selfish strategies in Trigona • Dwarf queens • Larval voracity

19. Caste conflict or insurance? M. Beecheii (Wenseleers et al. 2004) • Caste conflict hypothesis: • Killed rapidly after eclosion • no selection • Insurance hypothesis: • Not killed immediately • If killed, selective killing of queens

20. Queens are killed aggressively by workers as quickly as they eclose • Decapitation • Pulling apart of limbs • Heavier/ larger queens do not survive longer (insurance hypothesis)

21. Survival curve of queens not significantly different from an exponential decline: constant rate of killing • Av. Life expectancy of queens is 47 hrs • In natural colonies: 14-23% develop as queens, only 0-1.5% of adult females are queens Support for selfish self-determination

22. Caste conflict model (Ratnieks 2001) • Individual exploitation can be limited by degree of relatedness to kin • Invasion of rare allele (F) forming heterozygote females • Queens appear with probability of (1-Rf)/(1+Rm) • Rf = relatedness of females • Rm = relateness to males • If all males are produced by queens (rm=0.25): p= 0.25 • If all males are produced by workers (rm=0.75): p= 0.14

23. Interspecies comparison (from Wenseleers and Ratnieks) % males workers’ 0% 34% 41% 95% sons predicted level HIGHEST > INTERMEDIATE > LOWEST of queen production 3 / 1 / 2,476 10 / 12 / 8,162 * GLZ, p < 10-10 Cols. / months / indiv’s. * % of queens produced 6 / 2 / 3,989 9 / 11 / 2,806 N.S. * 78 / 10 / 13,514 Mean, 95% C.L. M. beecheii1 Yucatan, Mexico M. beecheii2 Yucatan, Mexico M. favosa5 Tobago, West Indies M. subnitida3 Rio Grande do Norte, Brazil M. quadrifasciata4 Various sites, Brazil 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

24. Conclusions • Genetic caste determination is present in Melipona • Queen production is further dependent on sufficient nutrition • High queen production is a selfish strategy, leading to caste conflict within a colony • Exploitation by self-determination decreases with increasing relatedness