• RAGHAVENDRA GADAGKAR

      Articles written in Journal of Genetics

    • Evolution of sex ratios in social hymenoptera: kin selection, local mate competition, polyandry and kin recognition

      N. V. Joshi Raghavendra Gadagkar

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      A model is constructed to study the effects of local mate competition and multiple mating on the optimum allocation of resources between the male and female reproductive brood in social hymenopteran colonies from the ‘points of view’ of the queen (parental manipulation theory) as well as the workers (kin selection theory). Competition between pairs of alleles specifying different sex investment ratios is investigated in a game theoretic frame work. All other things being equal, local mate competition shifts the sex allocation ratio in favour of females both under queen and worker control. While multiple mating has no effect on the queen’s optimum investment ratio, it leads to a relatively male biased investment ratio under worker control. Under queen control a true Evolutionarily Stable Strategy(ess) does not exist but the ‘best’ strategy is merely immune from extinction. A trueess exists under worker control in colonies with singly mated queens but there is an asymmetry between the dominant and recessive alleles so that for some values of sex ratio a recessive allele goes to fixation but a dominant allele with the same properties fails to do so. Under multiple mating, again, a trueess does not exist but a frequency dependent region emerges. The best strategy here is one that is guaranteed fixation against any competing allele with a lower relative frequency. Our results emphasize the need to determine levels of local mate competition and multiple mating before drawing any conclusions regarding the outcome of queen-worker conflict in social hymenoptera. Multiple mating followed by sperm mixing, both of which are known to occur in social hymenoptera, lower average genetic relatedness between workers and their reproductive sisters. This not only shifts the optimum sex ratio from the workers’ ‘point of view’ in favour of males but also poses problems for the kin selection theory. We show that kin recognition resulting in the ability to invest in full but not in half sisters reverts the sex ratio back to that in the case of single mating and thus completely overcomes the hurdles for the operation of kin selection.

    • Evidence for multiple mating in the primitively eusocial waspRopalidia marginata (Lep.) (Hymenoptera: Vespidae)

      K. Muralidharan M. S. Shaila Raghavendra Gadagkar

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      Asymmetries in genetic relatedness created by haplodiploidy have been considered to be crucially important for the evolution of worker behaviour in Hymenoptera. Multiple mating by the queens destroys this asymmetry and should make kin selection less powerful. The number of males that social insect queens mate with is thus of considerable theoretical interest especially in primitively eusocial species. The results presented here provide evidence for multiple mating by foundresses of the primitively eusocial waspRopalidia marginata (Lep.)

    • Origin and evolution of eusociality: a perspective from studying primitively eusocial wasps

      Raghavendra Gadagkar

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      Eusocial insects are those that show overlap of generations, cooperative brood care and reproductive caste differentiation. Of these, primitively eusocial insects show no morphological differences between reproductive and worker castes and exhibit considerable flexibility in the social roles that adult females may adopt. This makes them attractive model systems for investigations concerning the origin of eusociality. The rapidly accumulating information on primitively eusocial wasps suggests that haplodiploidy is unlikely to have an important role in the origin of eusociality. General kin selection (without help from haplodiploidy) could however have been an important factor due to the many advantages of group living. Pre-imaginal caste bias leading to variations in fertility is also likely to have some role. Because workers often have some chance of becoming reproductives in future, mutualism and other individual selection models suggest themselves as important factors. A hypothesis for the route to eusociality which focuses on the factors selecting for group living at different stages in social evolution is presented. It is argued that group living originates owing to the benefit of mutualism (the ‘Gambling Stage’) but parental manipulation and subfertility soon become important (the ‘Manipulation Stage’) and finally the highly eusocial state is maintained because genetic asymmetries created by haplodiploidy are exploited by kin recognition (the ‘Recognition Stage’).

    • On testing the role of genetic asymmetries created by haplodiploidy in the evolution of eusociality in the Hymenoptera

      Raghavendra Gadagkar

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      The haplodiploid genetic system found in all Hymenopterans creates an asymmetry in genetic relatedness so that full-sisters are more closely related to each other than a mother is to her daughters. Thus Hymenopteran workers who rear siblings can obtain higher inclusive fitness compared to individuals who rear offspring. However, polyandry and polygyny reduce relatedness between workers and their sisters and thus tend to break down the genetic asymmetry created by haplodiploidy. Since the advent of electrophoretic analysis of variability at enzyme loci, several estimates of intra-colony genetic relatedness in the Hymenoptera have been published. To test the role of the genetic asymmetry created by haplodiploidy in the evolution of eusociality, I assume that workers are capable of investing in their brothers and sisters in their ratio of relatedness to them. I then compute ahaplodiploidy threshold, which is the threshold relatedness to sisters required for workers to obtain a weighted mean relatedness of 0.5 to siblings and thus break even with solitary foundresses. When workers rear mixtures of sisters and brothers in an outbred population, the value of this threshold is 0.604. An examination of the distribution of 185 estimates of mean genetic relatedness between sisters in Hymenopteran colonies shows that the values are well below the expected 0.75 for full sisters, both in higly eusocial as well as in primitively eusocial species although relatedness values in the latter are higher than in the former. Of the 177 estimates with standard error, 49 are significantly lower than the haplodiploidy threshold and 22 are significantly higher. Of the 35 species studied only 6 have one or more estimates that are significantly higher than the haplodiploidy threshold. For more than half the estimates, the probability of the relatedness value being above the haplodiploidy threshold is less than 0.5. Reanalysis of these data using 0.5 as the threshold does not drastically alter these conclusions. I conclude that the genetic asymmetry created by haplodiploidy is, in most cases, insufficient by itself either topromote the origin of eusociality or tomaintain the highly eusocial state.

    • The evolution of caste polymorphism in social insects:0 Genetic release followed by diversifying evolution

      Raghavendra Gadagkar

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      Caste polymorphism, defined as the presence within a colony of two or more morphologically differentiated individuals of the same sex, is an important character of highly eusocial insects both in the Hymenoptera (ants, bees and wasps) and in the Isoptera (termites), the only two groups in the animal kingdom where highly eusocial species occur. Frequently, caste polymorphism extends beyond mere variations in size (although the extent of variations in size can be in the extreme) and is accompanied by allometric variations in certain body parts. How such polymorphism has evolved and why, in its extreme form, it is essentially restricted to the social insects are questions of obvious interest but without satisfactory answers at the present time. I present a hypothesis entitled ‘genetic release followed by diversifying evolution’, that provides potential answers to these questions. I argue that genetic release followed by diversifying evolution is made possible under a number of circumstances. One of them I propose is when some individuals in a species begin to rely on the indirect component of inclusive fitness while others continue to rely largely on the direct component, as workers and queens in social insects are expected to do. Thus when queens begin to rely on workers for most of the foraging, nest building and brood care, and workers begin to rely increasingly on queens to lay eggs—when queen traits and worker traits do not have to be expressed in the same individual—I postulate the relaxation of stabilizing selection and new spurts of directional selection on both queen-trait genes and worker-trait genes (in contrasting directions) leading to caste polymorphism.

    • Genetically engineered monogamy in voles lends credence to theModus Operandi of behavioural ecology

      Raghavendra Gadagkar

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    • Ernst Mayr - 1904–2005

      Raghavendra Gadagkar

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    • Karyotype instability in the ponerine ant genus Diacamma

      Nutan Karnik H. Channaveerappa H. A. Ranganath Raghavendra Gadagkar

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      The queenless ponerine ant Diacamma ceylonense and a population of Diacamma from the Nilgiri hills which we refer to as ‘nilgiri’, exhibit interesting similarities as well as dissimilarities. Molecular phylogenetic study of these morphologically almost similar taxa has shown that D. ceylonense is closely related to ‘nilgiri’ and indicates that ‘nilgiri’ is a recent diversion in the Diacamma phylogenetic tree. However, there is a striking behavioural difference in the way reproductive monopoly is maintained by the respective gamergates (mated egg laying workers), and there is evidence that they are genetically differentiated, suggesting a lack of gene flow. To develop a better understanding of the mechanism involved in speciation of Diacamma, we have analysed karyotypes of D. ceylonense and ‘nilgiri’. In both, we found surprising inter-individual and intra-individual karyotypic mosaicism. The observed numerical variability, both at intra-individual and inter-individual levels, does not appear to have hampered the sustainability of the chromosomal diversity in each population under study. Since the related D. indicum displays no such intra-individual or inter-individual variability whatsoever under identical experimental conditions, these results are unlikely to be artifacts. Although no known mechanisms can account for the observed karyotypic variability of this nature, we believe that the present findings on the ants under study would provide opportunities for exciting new discoveries concerning the origin, maintenance and significance of intra-individual and inter-individual karyotypic mosaicism.

    • The evolution of culture (or the lack thereof): mapping the conceptual space

      RAGHAVENDRA GADAGKAR

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      This short essay is based on a lecture that I gave at short notice on a subject in which I am by no means an expert. The combination of lack of expertise and time for preparation, created an unexpectedly unique opportunity for thinking outside the box. I decided not to try to read up (as there was no time in any case) but instead to organize the little that I already knew about cultural evolution in a systematic schema—I attempted to create a scaffolding, on which I could hang everything I knew about cultural evolution, and hopefully, everything I might ever discover about cultural evolution in the future. I considered three dimensions ofthe study of cultural evolution, namely (i) the phenomenon of cultural evolution, (ii) production of knowledge in the field of cultural evolution, and (iii) the consequences or applications of an understanding of the evolution of culture.

    • Genetic relatedness does not predict the queen’s successors in the primitively eusocial wasp, Ropalidia marginata

      SAIKAT CHAKRABORTY SHANTANU P. SHUKLA K. P. ARUNKUMAR JAVAREGOWDA NAGARAJU RAGHAVENDRA GADAGKAR

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      Ropalidia marginata is a social wasp in which colonies consist of a single fertile queen and several sterile workers. If the queen is removed, one of the workers, potential queen (PQ), becomes hyperaggressive and becomes the next queen. The identity of the PQ cannot be predicted in the presence of the queen. The probability of a worker succeeding the queen is uncorrelated with herbody size, dominance rank, ovarian or mating status, but imperfectly correlated with her age. Here, we investigate whether genetic relatedness help to predict the queen’s successors. We constructed models based on successors being (i) most closely related to the queen, (ii) most closely related to the immediate predecessor queen/PQ, or (iii) having the highest relatedness to the majority of theworkers; and (iv) having the highest average relatedness to all theworkers.We predicted five successors fromeach of these models using pair-wise genetic relatedness estimated from polymorphic microsatellite loci. We independently performed serial queen/PQ removal experiments and compared the observed sequence of successors with the predictions from the models. The predictions of none of the models matched the experimental results; on an average 5-6 individuals predicted by the models were bypassed in the experiment.Thus, genetic relatedness is inadequate to predict the queen’s successors in this species.We discuss why relatedness sometimes predicts the patterns of altruistic behaviour and sometimes not, and argue that the cost and benefit terms in Hamilton’s rule, i.e. ecology, should be vigorously investigated when relatedness does not have adequate explanatory power.

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