The eusocial insects:

Isoptera: Termites

Hymenoptera: Ants, bees, wasps

• Insect behavior has a strong genetic component.

• It often depends on simple external cues.

• However, there is some flexibility in workers’ behavior.

• Task allocation is often based on:environmental factors what others are doing.

Eusocial insects

• Many adults live together in a group• Overlapping generations• Cooperation among adults in nest-

building, care of young• Reproductive dominance• Sterile castes (in some species) • Division of labor

Two possible routes to sociality:

Familial route: Staying home to helpHigh r will permit extreme skew

Parasocial route: Sharing a nest

In both cases, sociality will be determined by ecological conditions.

HaplodiploidyHaplodiploidy

Females develop from fertilized eggs, likebirds and mammals.

Males develop from unfertilized eggs.They have no father.When a male forms gametes, there is no

meiosis, so all gametes are identical.Each of a male’s daughters receives

identical sets of genes.Males cannot have sons.

Females develop from fertilized eggs, males from unfertilized eggs.

Males have mothers, but no fathers.Males have daughters, but no sons

Female Male

gametes

meiosis

Male Female

Diploid species:Diploid species:

Offspring receive ½ of genetic material from mother, ½ from father.

Full siblings: (½ x ½)(from mother) + (½ x ½)(from father) = r = 0.5

Haplodiploid species:Haplodiploid species:

Father is haploid, so 50% of a female’s genome has 100% chance of being shared with her sister (i.e. is identical with her sister’s).

Mother is diploid, so 50% of a female’s genome has a a 50% chance of being shared with her sister.

r of sisters: .5 (from father) + (.5 x .5)(from mother) = 0.75

F

Sister

.5 1.0 .5 .5

A receives ½ of her alleles from her father. The probability that her sister shares them is 1.0.

From mother: .5 x .5 = .25From father: .5 x 1.0 = .5 So r = 0.75r = 0.75

Probability that 2 antant or beebee sisters will share the same allele:

A

M

F M

Sister

.5 .5

Brother to sister:

Sister to brother:

Brother

.5 1.0

F M

r = 0.25

Only 1 path, since males have no father

r = 0.50

Sister Brother

Haplodiploid species:Haplodiploid species:

r to: Female Male

Mother 0.50 1.00Father 0.50 0Sister 0.75 0.50Brother 0.25 0.50Son 0.50 0Daughter 0.50 1.00

Conflicts:Conflicts:

• Between colonies• Between queens• Between workers and unrelated queens• Between workers and their own queen (mother)

- over sex ratio of offspring- over workers’ reproduction

• Between workers

0 5 10 15 20 25 3010

Se

x r

ati

o (

Ma

le/F

em

ale

)0

5

1:1 investment

3:1 investment

Ratio of investment in 21 species of ants

Weight ratio (Female:Male)

Requirements of eusociality:Requirements of eusociality:• Overlapping generations• High effort expended in parental care• Non-reproductive can aid in parental care

Ecological pressures:Ecological pressures:• Nest defense• Low opportunity for young to disperse• Need to cooperate to rear young

HaplodiploidyHaplodiploidy only predisposes individuals to be eusocial:

• Not all eusocial species are haplodiploid (e.g. termites, naked mole rats) • Not all haplodiploid species are eusocial (e.g. many wasps)

Crucial determinant: Ecological factors.

If a female can breed without help, she does better if her daughters also breed.

Haplodiploidy helps to explain the maintenance of eusociality once it has arisen through ecological factors.