Caste Determination

in Melipona

Darelyn David

Overview

• Eusocial insects• Caste determination in Melipona• Confounding factors• Conclusions

“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)

• Cooperative brood care

•Overlap between generations

•Reproductive division of labour

Reproductive castes

Mechanisms of caste determination

• Trophogenic– Caste is determined

by quality/quantity of food provided to larvae

– Very common

• Genetic– Inherent larval feature– Quite rare

Honeybees (Apis)royal jelly at all instars : queenpollen and nectar at last instars:

workers0.02% of females become queens

Bumblebees (Bombus)Small colonies->few workers-> little food: workersLarge colonies-> many workers-> lots of food: queens

Most stingless bees (Meliponini: Trigona and Lestrimellita)Workers place little or a lot of food in cellsQueen lays eggs in individual cellsLot of food : queenLittle food: worker

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

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)

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

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)

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

Finding wolves in sheep’s clothing: evidence for GCD

• Queens disguised as workers?– Morphological

markers (Kerr and Nielson 1966)

• 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

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

Why so many queens?• Both Apis and Melipona found new colonies by swarming• Very few reproductive opportunities for virgin queens

Apis: 0.02% queensWorkers control queen production by food manipulation

Melipona: 14-25% queensWorkers control queen production by killing excess queens

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

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

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

•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)

• 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

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

0

5

10

15

20

25%

of q

ueen

s pr

oduc

ed

0

5

10

15

20

25

% males workers’ 0% 34% 41% 95% sons

predicted level HIGHEST > INTERMEDIATE > LOWESTof queen production

M. bee

cheii

1

Yucatan

, Mex

ico

M. quad

rifas

ciata

4

Various s

ites,

Brazil

M. subnitid

a3

Rio Gran

de do N

orte, B

razil

M. bee

cheii

2

Yucatan

, Mex

ico

M. favo

sa5

Tobago, W

est In

dies

**

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-

10N.S

.

Interspecies comparison (from Wenseleers and Ratnieks)

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.

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