Evolution of Eusociality PDF

Evolution of eusociality Social insects represent the epitome of altruism Extreme form of reproductive altruism Eg. ants, bees, wasps  Group living ≠ sociality Many animals live together as a group, but they are not necessarily social (e.g. a school of fish) ...

Evolution of eusociality Social insects represent the epitome of altruism Extreme form of reproductive altruism Eg. ants, bees, wasps  Group living ≠ sociality Many animals live together as a group, but they are not necessarily social (e.g. a school of fish)  Degrees of sociality Solitary: living alone, almost no parental care (many insects) Subsocial: adults care for their offspring for some period of time (cockroaches) Communal: use the same composite nest without cooperation in brood care (digger bees) Quasisocial: use the same nest and show cooperative brood care (Euglossine bees) Semisocial: in addition to features of being quasisocial, also has a worker caste (Halictid bees) Eusocial: in addition to features of being semisocial, there is overlap in generations (Honey bees).  True sociality (eusociality) 1) There is cooperative brood-care. 2) Overlapping of generations so that the group (the colony) allowing offspring assist parents during their life. 3) There is a reproductive division of labor. There is one or a few reproductive(s) (Queen or King), and others are sterile. Castes differentiation in honey bees  How does eusociality work? Colonies behave like a single organism How did this organismal degree of cooperation evolve given that the Darwinian norm is closer to a struggle of everyone and against everyone else?  Colony as a superorganism Organism vs. superorganism Entities not physically connected Plasticity - Less fixed in function (with age, workers change from brood care to nest maintenance to foraging) Lack of centralised control centre (despite the controlling image attributed to the queen). No colonial brain. No individual perceives the state of the whole colony and sends out instructions Different parts are integrated by self-organisation using simple rules Different individuals have pieces of information which is then integrated by the colony as a whole eg. As in the mechanics of the waggle dance Waggle dance :successful foragers communicate symbolically information on food supply to other individuals (Show dance video if needed) https://www.youtube.com/watch?v=Cylim87fFgU https://www.youtube.com/watch?v=Cylim87fFgU  Superorganismal view of a eusocial Flowers colony of bees : The dance is a part of the signaling cascade of the larger Successful superorganism that then regulates work foragers according to supply and demand Waggle dance Tremble dance Recruit Recruit Inhibits waggling Recruits foragers processors by other processors returning foragers Storage, ripening of nectar A returning forager does not know the number of foragers or processors, must experience an indirect effect of those numbers through the time required to offload her nectar or pollen load. Eusociality: 1) Overlap in generations 2) Co-operative brood care 3) Specialized castes of non-reproductive individuals (sterile individuals) Taxa of insects and the number of times eusociality has evolved within each Insect Orders Common Names Frequency of Evolution of Eusociality Hymenoptera Ants, bees, wasps, and 11 Isoptera Termites 1 Homoptera Gall-forming aphids 1 Coleoptera Bark-nesting weavils 1 (beetles) Thysanoptera Gall-forming thrips 1 Non-insects Snapping shrimps and 2 naked mole rats Total 17 Eusocial species: those in order Hymenoptera Image:Wasps Building Nest 01.jpg Ants (all) Wasps (some) Bees (some) 15 Eusocial species: other Termites (all) Naked Mole Rat Damaraland Mole Rat Aphids (some) Thrips (some) Snapping shrimp (some) Alpheus digitalis 16 Sterile individuals display extreme altruistic behavior Bee-6 Honey bee workers Camponotus saundersi soldiers 17  Eusociality: why is it interesting? Natural selection thought to favour individuals that maximize their individual reproductive success Situations where individuals do not behave like this (e.g., forgo breeding, help others breed, dying for others) – are puzzling  How could eusociality evolve? Darwin, in his "Origin of Species" (1859) thought that sterile workers in a bee colony, being unable to transmit their genes, represent a special challenge to his theory of natural selection. This is because natural selection depends on the transmission of ‘traits’ that give selective advantages to the individuals, and these traits have to be determined genetically (heritable). If workers are sterile, how can they transmit the "helping genes" to the next generation? Why is eusociality rare? How does it evolve?  Routes to eusociality: two hypotheses Extended family Mutualism Solitary Several (often sisters) build nests close together Nest guarding by Co-operate in defence, but each reproduces Young stay at home herself and help defend nest/enlarge it Domination by one , others lose chance to Young stay at home reproduce permanently and Overlap of generations never breed and young females become workers Why might sterile individuals display such altruism? There is a donor/actor who gives help There is a recipient/s who gets helped by the donor/actor R=relatedness between donor/actor and recipient Hamilton’s C=cost to donor/actor Rule: B=benefit to recipient R x B > C 22 Calculating the coefficients of relatedness – Hamilton’s coefficient of relatedness, r Hamilton’s rule Altruistic behaviour will spread if Br – Ca > 0 Br = benefit to the recipient Ca = cost to the actor Both B and C are measured in terms of surviving offspring numbers Hamilton’s rule states that altruism will spread when benefits to the recipient are great, the cost to the actor are low and the participants are closely related How did eusociality originate? Haplodiploid hypothesis 24  Eusocial colonies - Honeybees Members of the colony are all relatives; consists of a single queen and her progeny/offspring Honeybees are haplodiploid animals What is haplodiploidy? Haplodiploidy 1. Some thrips 2. Some aphids 3. Bees, ants, wasps, Fertilize egg? thrips yes no Female Male (Diploid) (Haploid) Haplodiploidy Gender is determined by a single gene If there is only one copy of that gene ➔ Haploid male If there are copies from 2 chromosomes and they differ in their DNA sequences (heterozygous) ➔ Heterozygous Diploid Female Coefficients of genetic relatedness ( r) in diploid organisms (eg. Humans, lions etc) In diploid animals the average degree of relatedness between Parent – offpring = 0.5 Between Siblings = 0.5  Coefficient of genetic relatedness ( r) in haplodiploidy An offspring formed by the fusion of male and female gametes is diploid and female An offspring formed from an unfertilized egg is haploid and male Queens and workers (females) have 32 chromosomes (diploid), drones (males) have 16 (haploid) Drones are identical to each other (except for mutations) Coefficient of genetic relatedness (r) in haplodiploidy Offspring Offspring (female) (male) Offspring  Genetic relatedness in haplodiploid animals Haploid Male has one set of chromosomes so he passes on 100% of his genes in each sperm. Diploid female has two sets of chromosomes, and each egg will contain one of these two sets. Between Drones (Sons) and Queen (Mother) Sons (drones; formed from unfertilised eggs) will have 100% of their genes in common with their mother(queen). But the queen passed on only half her genes to her son so she is related to him by 50%.  Between Workers and Queens Between Workers and their father A Worker receives one set of the possible two of the queen's chromosomes so they will be 50% related to her A Worker gets the only set that the male has so is 50% related to her father  Between Workers (sisters) Sisters get the same set of genes from the haploid father. So half their genes are exactly the same. Each sister receives one set out of the two sets of chromosomes from the queen, and these make up half her genes, so there is a 50% chance that 50% of her genes will be the same as her sister's. So that's 50% x 50% = 25%, or 1/2 x 1/2 = 1/4. And the coefficient of relatedness between sisters is 75% or ¾ (also called the 3/4th hypothesis)  Plausible coefficients for relatedness (r) in Haplodiploid colonies of honeybees Sex Daughter Son Mother Father Sister Brother Female ½ ½ ½ ½ ¾ ¼ (worker) Sisters are more genetically valuable for a worker than offspring or brothers.  Hamilton introduced the term ‘inclusive fitness’ Workers do not reproduce, but they share genes with their mother (the queen) and if they help to raise more sisters, their genes would be transmitted to the next generation through their mother (queen)  Therefore, a worker can pass on more of her genes by helping to produce more sisters (r = 0.75) instead of producing her own offspring ( r= 0.5) A worker can gain more inclusive fitness by raising a sister than a son or a daughter Or she gets more fitness /benefit for the same amount of work she does altruistically (helping mother) than she would as a selfish individual Haplodiploidy opens up a way for the evolution of a worker caste Stinging behaviour: A gene for ‘self-sacrifice’ can be favoured by NS if it helps relatives who might bear copies of genes But the help must be large enough to compensate for the loss of their own lives and the helped individuals must be relatives Honey bee workers that sting intruders can benefit by ‘sacrificing’ themselves because they are aiding survival of a huge number of kin/relatives who carry their genes (kin selection) This explained why eusociality has evolved in Hymenoptera  Or so people thought until………….. Later models incorporated reproductive value sex ratios high relatedness to sisters ( r = 0.75) is countered by unusually low relatedness to brothers ( r = 0.25) If workers raise equal numbers of males and females the average r becomes 0.5 = r in diploidy BUT countered by Worker policing, removal of male larvae. So, the haplodiploid hypothesis was still sound.