Social insects and social amoebae. - Centre for Ecological Sciences

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222 Raghavendra Gadagkar and J T Bonnerand workers are lacking and there is considerable flexibility in the social roles thata freshly eclosing adult individual c;an adopt. More importantly, in many primitivelyeusocial species, unlike in the highly eusocial species, new colonies or nests arefounded either by a group of individuals or by a single individual. In the lattercase, the solitary foundress can successfully raise her offspring to adulthood, unaidedby other members of her species. For these reasons, primitively eusocial speciesare often considered ideal model systems for understanding the evolutionary forcesthat may have been responsible for the development of eusociality. Thus the problemof the evolution of eusociality, for practical purposes, can be usefully rephrasedas the problem of how some individuals in primitively eusocial species are favouredby natural selection to accept sterile worker roles in spite of being potentiallycapable of starting their own solitary foundress nests. But the fact that such aselection has taken place suggests that social life, even if it means partial or totalsterility, must sometimes be more "advantageous" than solitary life.How can social life be genetically more advantageous than solitary life if itmeans sterility? This question seemed impossible to answer until the advent of theconcept of inclusive fitness (Hamilton 1964). Today, in any calculation of fitness,it is common practice to include both direct transmission of genes to futuregenerations via offspring production and indirect transmission via helping the survivalof relatives. The inclusive fitness of an individual is thus computed as the summedproduct of the number of individuals so helped (be they offspring or geneticrelatives) and the coefficient of genetic relatedness between each individual andthose he or she helps. Given this definition, there is no reason why the inclusivefitness of a worker in a social insect colony should not be greater than that of asolitary foundress. In other words natural selection can, in principle, promote theadoption of the worker strategy. Indeed there can be a variety of reasons why theinclusive fitness of a worker can be greater than that of a solitary foundress. Theproblem then is not so much to show that this can be so but to show for whatreasons it is so. And that is what various theories proposed for the evolution ofeusociality attempt to do.2. I The role of haplodiploidyOne way in which the inclusive fitness of a worker may be greater than that ofa solitary foundress is for genetic relatedness between the worker and the broodunder her care to be greater than the genetic relatedness. between a solitary foundressand her offspring (which of course will be 0.5 under outbreeding). This may beachie~ed with a haplodiploid genetic system, where genetic relatedness between afemale and her full-sister is 0.75, which is greater than the value between a femaleand her daughters (r= 0.5). Since many origins of eusociality have occurred in theinsect order Hymenoptera, in which haplodiploidy is nearly universal, much attentionhas been paid to the possibility that haplodiploidy predisposes Hymenopterans tothe development of eusociality. In order to be validated, however, this so-canedHaplodiploidy Hypothesis requires several conditions. Colonies should consist of asingle egg-layer mated to a single male, so that workers do not rear half-sistersor more distantly related individuals. Workers must either succeed in skewinginvestment in favour of their sisters or must be able to rear their own sons instead
Social insects alld social amoebae 223of brothers. The latter condition is necessary because workers are related to theirbrothers by only 0.25 while they are related to their sons by 0.5.Many features of the biology of R. marginata suggest that these conditions maynot always be met. Foundress associations may not consist exclusively of formernestmates; there is considerable drifting of individuals during colony foundation sothat cofoundresses probab.ly are not often closely related (M Shakarad and RGadagkar, in preparation). Even when they are, subordinate cofoundresses are atbest sisters of the egg-layer and are thus helping raise nieces and nephews (r =0.375). Queens mate multiply and simultaneously use sperm from two or moremales so that the average genetic relatedness among their daughters is reduced toabout 0.5 (Muralidharan etal1986; Gadagkar 1990d). Even more important, frequentqueen supersedures .lead to serial polygyny, further reducing intra-colony geneticrelatedness to values that may be as low as 0.2 (Gadagkar et al 1991c, 1993).Polygyny or multiple mating by the queen should pose no great difficulty forthe haplodiploidy hypothesis if workers discriminate between full-sisters and lessrelated individuals and preferentially aid the former (Gadagkar 1985). However,studi~s of nestmate discrimination in R. marginata suggest that the labels andtemplates used in discrimination are not produced individually, but rather areacquired from a common external source, namely the natal nest or nestmates,making it unlikely that different levels of genetic relatedness can be effectivelyrecognised among members of the same colony (Gadagkar 1985; Yenkataraman etal 1988). Thus R. marginata workers often rear complex mixtures of full-sisters,half-sisters, nieces, daughters, brothers, nephews, sons, and cousins, and even moredistantly related individuals, and probably do so without the ability to discriminateamong them on the basis of genetic relatedness. It seems reasonable to concludetherefore that the genetic asymmetry created by haplodiploidy is broken down tothe extent that there is no longer any predisposition for worker behaviour onaccpunt of it.2.2 The roles of parental manipulation and sub fertilityThere is another way by which the inclusive fitness of workers can be higher thanthat of solitary foundresses. Ecological conditions may exist in which a parent whomanipulates a fraction of her offspring into being sterile and helps rear her remaining(fertile) offspring leaves behind more grandchildren than a parent that does notmanipulate offspring (Alexander 1974). A significant problem with this parentalmanipulation hypothesis is whether counter-selection .on the offspring would notbe successful in making them overcome parental manipulation. A related idea whichgets around this problem is the subfertility hypothesis of V/est-Eberhard (1975),which simply reminds us that subfertile females produced by whatever cause (evenby accidental variation in the quantity of food obtained as -larvae) will find it'easier' to give up reproduction and I}ccept a worker role. The general ideasembodied in the parental manipulation and subfertility hypotheses have found supportin theoretical modelling (Craig 1979, 1983; Stubblefield and Charnov 1986), andin empirical studies (Michen~r and Brothers 1974), and there seems to be littledoubt that subfertility caused by parental manipulation or other factors plays amajorlole in maintaining eusociality in highly social insects.
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Social insects and social amoebae. - Centre for Ecological Sciences

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