Castes in Social Insects

(1)

Castes in Social Insects

Module Chemical Ecology, Ayasse

1. definition

2. castes in social insects 3. caste determination

A. psychophysiological caste determination

B. blastogenic und trophogenic caste determination

- effect of juvenile hormone C. genetic caste determination 4. advantages of castes

5. summary

(2)

castes: individuals of a colony of one species that differ

in morphology, physiology and behaviour.

(ecological caused polymorphisms) (Wilson 1971)

caste polymorphism: individuals with a functionally different form (e.g. workers and queen).

further differentiation in (Wilson, 1985):

physical castes: different function and morphology, same genotype (termites, ants).

temporal castes:(=age polyethism) age dependent division of labour (honeybees, wasps)

caste differentiation:

different specification of the

characteristics of individuals with identical genetic

information

caste determination:

causes and mechanisms of

caste differentiation (

preimaginal or imaginal

).

caste regulation:

caste determination in adult instars

intercastes:

morphological intermediates of two

clearly distinguishable extreme forms

(3)

further differentiation mainly in ants:

gynomorph:

winged females or wingless females after

mating (queenlike)

ergatomorph:

animal of a typical worker´s form

intermorph:

commonly intermediates in ants

all of the three forms can functionally be a queen

(mated and fertile) or a worker

Castes in social insects

Isoptera

Termitidae, Nasutitermes

• larvae • nymphs

• workers (higher termites:

wingless, rudimentary genital apparatus, eyes reduced or absent)

(4)

• larvae • nymphs

• workers (higher termites:

wingless, rudimentary genital apparatus, eyes reduced or absent)

• workers of lower termites:

immature stages, the worker function is being filled by nymphs and pseudergates

• pseudergates

regressed from

a.) nymphal stages by reduction of wing buds or b.) larvae by non-differentiating molts Isoptera • soldier

with morph. features specialized for defence different types within the

caste of soldiers: “Mandibulate” soldiers, “Snapping” soldiers, “Nasute soldiers“

Castes in social insects

(5)

• primary reproductives (colony founder) • replacement reproductives (sec. developed)

Castes in social insects

Isoptera

• queens • workers

different size, morphology and function in bees and wasps

no subcastes

division of labour is mostly based on temporal polyethism only

ants with morphological subcastes

Castes in social insects

(6)

Hymenoptera

Halictidae

Castes in social insects

Hymenoptera

(7)

Castes in social insects

Age polyethism Hymenoptera

Stingless bees and honeybees

Castes in social insects

Hymenoptera

(8)

Hymenoptera

Ants

Isoptera (termites)

•hemimetabol

•larvae contribute to caste system (lower termites)

•bisexual colony, castes consist of males and females

•XX –XY – mechanism of sex determination

•long-living males

Hymenoptera (bees, ants, wasps)

•holometabol

•castes consist of adult animals only

•castes consist of females only •haploid-diploid-mechanism of

sex determination •short-living males

Social insects

Characteristics of

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1. definition

5. summary

Castes in social insects

Epigenetic

1. Psychophysiological caste determination (often acting

on adult individuals)

• pheromones and behaviour

• sexual predisposition (importance of mating) • temporal predisposition (attractiveness and rank)

Castes in social insects

(10)

pheromones

Caste determination

e.g. lower termites (Kalotermes)

Complex caste system and

determination in a higher termite. What determines caste? Depending on

species:

•

Condition of colony (stress factors)

•

Contact between nestmates

•

Hormones, especially JH

•

Pheromones, especially from reproductives

Castes in social insects

(11)

pheromones

Castes in social insects

Caste determination Fire ants • behaviour • temporal predisposition: commonly the oldest female is fertile with developed ovaries

Castes in social insects

(12)

sexual predisposition:

mated female becomes a queen

Sweat bees

1. definition

5. summary

(13)

Epigenetic

2. Blastogenic and trophogenic caste determination

• nutrition factors in larval development (e.g. honeybee, royal jelly)

• egg ingredients (reserve substances, RNA content) • colony size (important in determination of young

queens)

further: nest temperature during breeding

Castes in social insects

nutrition factors in larval development

Castes in social insects

Caste determination in bumblebees

castes similar size

(14)

Caste determination in bumblebees

castes similar size

castes different size

(trophogenic determination)

Castes in social insects

(15)

Castes in social insects

Caste determination in stingless bees

nutrition amount: larva fed with a high quantity of food develop into queens

egg size: queens often hatch from large, yolk-rich eggs

pheromones: reproductive queens can inhibit production of new queens

colony size: important for the determination of young queens

Castes in social insects

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nest temperature during breeding: in species that

overwinter its brood in the larval stage females develop into queens in winter and into workers in summer age of the queen: younger queens produce a lot of

workers, old queens more reproductives

Caste determination in ants

Influence of juvenile hormone? on caste determination in ants

Castes in social insects

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Influence of juvenile hormone on caste determination in honeybees

Castes in social insects

JH mediator between environmental impacts (nutrition, pheromones, … and morphological specification)

1. definition

- effect of juvenile hormone

C. genetic caste determination

4. advantages of castes 5. summary

(18)

3. Genetic caste determination

contribution of genetic factors to caste determination is poorly understood

indications in: A. Higher termites

certain castes consist of the same sex

Genetic caste determination in higher termites

Castes in social insects

JH application causes development of soldiers of the false sex⇒ epigenetic mechanisms

(19)

certain castes consist of the same sex B. Bees – Meliponinae

queens develop from diploid eggs with paired heterozygous loci that determine the caste

Castes in social insects

Genetic caste determination in Meliponinae

two-locus model

Castes in social insects

• JH appl. causes development of queens • nutrition factors

(20)

certain castes consist of the same sex B. Bees – Meliponinae

queens develop from diploid eggs with paired heterozygous loci that determine the caste C. Ants

Genetic caste determination in Harpagoxenus Crossing experiments

Castes in social insects

• carriers of allele E are more frequently workers, of allele e more frequently queens • E suppresses wing building • Only ee can develop into

queens

• ee, eE and EE develop into workers under presence of a nutrition factor

(21)

• division of labour and cooperative nest defence

• specialisation (tasks can be performed more

effectively and at the same time)

• less mistakes because of the specialisation

Castes in social insects

Advantages of castes

description of castes caste determination regulation of reproduction

(morph./phys./behav.) (larval) (adult)

bees minimal morph. food quantity physical

(primitive eusocial) phys. + dominance pheromone

behaviour mating, age

bees morphological food quantity physical

(highly eusocial) phys. +quality + dominance pheromone behaviour

wasps minimal morph. food quantity physical

(primitive eusocial) phys. + dominance pheromone behaviour age

wasps morph. food quantity physical

(highly eusocial) phys. (+ quality?) + dominance pheromone behaviour

ants morph. food quantity physical

(highly eusocial) phys. (quality) + dominance pheromone behaviour temp., egg factors lower termites morph. food

(highly eusocial) phys. pheromones dominance pheromone behaviour further abiot. factors higher termites morph. food

(highly eusocial) phys. pheromones dominance pheromone