Eusociality Flashcards
What is the biology of naked mole rats?
- Neither moles nor rats – more closely related to porcupines, guinea pigs and chinchillas
- Wild colonies range in size form 20-300 individuals, with an average colony of 75
- Strict hierarchical caste, at the top the queens harem plus 1-3 mating males
- Male and female soldiers defend the colony against predators and foreign mole rats
- Odours distinguish colony members from intruding NMRs. Roll about in the burrow toilet chamber, coating their body with the familiar scent of the colony’s faeces and urine.
- NMRs attack unfamiliar intruders when one colony breaks into the burrow of another.
Only mammal species thought to be eusocial – like termites and ants, and with a caste system.
• Only 1 breeding female (Queen)
• 1 to 3 breeding males
• Other males and females do not breed – they are either workers or soldiers.
What is eusociality and how might it have evolved?
- Extreme form of reproductive specialisation in which only one (or very few) individual breeds in a colony
- All other individuals are biologically sterile
- Evolves if individuals are closely related, and thus benefit from the reproductive effort of one individual (kin selection)
- Genetic variation within a colony is low
- Although only the queen gives birth and nurses pups, most individuals actively participate in pup care
How does this evolve?
The Aridity Food Distribution Hypothesis
• Evolves in environments where animals live in a dry habitat with patchy food distribution, and where the food items themselves may be very large (eg, tubers of large plants)
• They could be as far as 12 feet underground – you need to dig tunnels to access the food
− In hard soil, the animals have to dig through and mine these tubers, and bring the food back to the nest site, which is then consumed by the colony
− This required massive colonial worker effort → thought to have lead to eusociality
• Where food sources are easy to gain access too , you can live solitary (because you can easily gan access to the food yourself) or socially (because there is easy access for all to share).
Predation also plays a role
• Gives an advantage to subterranean life (high ambient temp and low unpredictable rainfall in an arid environment also gives this an advantage)
• High burrow temp with high CO2 and low O2 give selection to drive low metabolic rates, thermolability and small body size (patchy food resource in an arid environment also gives an advantage to this).
• This gives slow growth rates, increased longevity and low recruitment.
How do non-breeders repress reproductive function?
- Only the queen breeds
- Ovarian quiescence in other females
- Most of the colony is sterile because GnRH is switched off
- If you take a non-breeding female and put her in a new colony so she becomes queen, she will begin to ovulate
Smith, 2010
• In the other females in the colony, they are in a ‘prepuberty’ like state, as they do not ovulate or display breeding behaviours
• They are not however, infertile, and are capable of rising to the breeding position
• GnRH is the ‘master hormone’ of reproduction
• GnRH neurons found in the hypothalamus and anterior pituitary of both queens and subordinates, however in the brain of the queen, there is a significantly larger area of immunoreactivity
• This suggests that GnRH is inhibited at the level of production in subordinates
• Unlikely that GnRH is the sole factor involved → many other hormones such as kisspeptin may influence GnRH and sexual maturity)
There are also breeding and non-breeding males in the colony
Faulkes et al, 1991
• Breeding males have larger reproductive tract masses, but spermatogenesis is active in both breeding and non-breeding
• Breeding males had higher urinary testosterone levels
• Breeding males have higher LH in response to administration of GnRH
• Seems none-breeding males have reduced sensitivity to GnRh
How is the brain of mole rats structured?
- If you map the sensory motor cortex to various structures, you can see the mole rats devote large areas of the brain to mouth and teeth, at the expense of the visual system.
- The somatocortex devoted to the head occupies cortical territory normally taken up by the visual system in other mammals
- The hindpart of the animal is deemed unimportant
- Seems the brain is organised to reflect lifestyle.
What are disperser morphs
How can a species that lives in hard spoil spread through Africa?
Disperser morphs are individuals that leave a colony and go off to form a new colony
• In insects such as termites, disperser morphs establish new colonies
• A similar morph has been described for NMRs
• Body size, growth and fat reserves are different in these morphs
− Laden with fat
− Elevated levels of LH
− Strong urge to disperse
− Only solicit matings with non-colony members
• In eusocial colonies, there is a lot of inbreeding. This reduces fitness. Dispersal allows for occasional outbreeding.
How is oxytocin expressed in naked mole rats?
Reminder:
• Oxytocin linked to sociality
• Structures in the NAc important
• Transgenic mice over-expressing oxytocin in these structures exhibit altered social behaviour
Oxytocin expression in the brain of the naked mole rat:
• Strong expression in the NAc
• The only other rodents where such expression is observed are in the social, monogamous Prairie voles
Comparative studies with Cape Mole Rats
• Solitary and intolerant of conspecifics, except during fleeting seasonal copulation or minimal maternal behaviour
• OT express differs in this solitary species compared to the NMR – there is much less expression of the receptor in the NAc → much like the montane vole.
→ In NMRs, an abundance of OT receptor binding in the NAc may reflect sociality and alloparenting.
→ Paucity of OT in the NAc of cape mole rats may reflect paucity of social behaviours.
What is unusual about NMR ageing?
• Live longer than many other mammals and orders of magnitude longer than mammals of equivalent size
• Mortality curve is unusual – declines almost linearly
− Usually, initial mortality rates are high when you are young vulnerable, then decrease, then increase with age.
• As a naked mole rat ages, factors which normally change in other rodents remain unchanged in the naked mole rat, eg)
− Basal metabolic rate
− Bone mineral density
− Articular cartilage
• In addition to longevity, NMRs have an extraordinary resistance to cancer – tumours have never been observed in these rodents.
• They have hypersensitivity to contact inhibition – which provides a clue to cancer resistance, as it arrests cell division when cells reach a high density.
• NMR fibroblasts arrest at a much lower density than mouse fibroblast.
• NMR cells also resistant to cadmium, heat, low-glucose medium
• NMR cells more sensitive to UV light.
So NMRs have:
• Reproductive mode analogous to social insets
• Other adaptations for life in extreme environments including fundamental differences in:
− Metabolism
− Life expectancy
− Physiology
− Cellular responses
What does genome sequencing tell us about eusocial animals adaptations to life?
• Can look more closely at their relationships with other rodents.
Abstract:
• NMRs show negligible senescence, no age-related increases in mortality and high fecundity until death
• In addition to delayed ageing, resistant to spontaneous cancer and experimentally induced tumourigenesis
• Pose a challenge to the theories that link ageing, cancer and redox homeostasis.
• Although characterised by significant oxidative stress, the NMR proteome does not show age-related susceptibility to oxidative damage or increased ubiquitination.
• Reported sequence and analysis of NMR genome reveals unique molecular adaptaitons consistence with cancer resistance, poikilothermy, hairlessness and insensitivity to low oxygen, and altered visual function.
− Altered telomerase → may be related to long life expentancy
− Very few genes show altered expression with age in the brain
− Altered UCP-1 gene → thermoregulation
− Loss of melatonin receptors
− Altered TAC1 regulation → pain regulation
− Loss of cone photoreceptors, but retains rods
− Altered hairless gene → naked phenotype?
→ Have genomic remodelling to reflect their lifestyle!
