Transcript Caste determination in Melipona

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 : 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
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% queens
Workers control queen production by food manipulation
Melipona: 14-25% queens
Workers 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 selfdetermination
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
Interspecies comparison (from Wenseleers and
Ratnieks)
% males workers’
sons
0%
25
HIGHEST
>
41%
95%
INTERMEDIATE > LOWEST
3 / 1 / 2,476
20
10 / 12 / 8,162
15
Cols. / months / indiv’s.
*
10
6 / 2 / 3,989
-10
GLZ,
p
<
10
*
9 / 11 / 2,806
N.S.
*
5
78 / 10 / 13,514
Mean, 95% C.L.
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% of queens produced
predicted level
of queen production
34%
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
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