Final Report Summary - CHEMDOC (The Chemical Code of Cooperation, Conflict and Competition)
Aims of the project
- To identify the first ant queen pheromone regulating worker reproduction, and investigate its proximate functions and ultimate consequences
To investigate the mechanisms and consequences of sexual selection in ants
Primary results
Identification and functional tests of the first ant queen pheromone
We first narrowed down the list of candidate queen pheromones by investigating which of the many chemicals produced by queens are associated with fertility, using the black garden ant Lasius niger as a model. By manipulating the amount of brood present in incipient ant colonies, we were able to non-invasively increase and decrease queen fecundity, and identify hydrocarbons whose expression was changed correspondingly. We also compared the hydrocarbons of queens at different stages of reproductive maturity to identify those that increase as queens begin to produce eggs. Lastly, we compared the hydrocarbons of queens that were killed or spared by the workers in colonies with supernumerary queens. The hydrocarbon 3-methylhentriacontane, or 3-MeC31, was a strong predictor of all three traits (fertility, maturity and surviving the cull by workers).
we next used samples of pure 3-MeC31, synthesized by chemist collaborators at the University of Copenhagen, to experimentally investigate whether this chemical is a queen pheromone. Workers were less aggressive to glass models treated with 3-MeC31 compared to controls, demonstrating that 3-MeC31 affects worker behaviour. Moreover, workers in the control groups developed their ovaries (as is normal when their queen is not present), while those supplied with 3-MeC31 did not. 3-MeC31 is responsible for the inhibitory effect of queens on worker reproduction, making it the first "primer pheromone" (one that affects physiology) to be identified in any social insect other than the well-studied honey bee and one species of termite.
we further showed that 3-MeC31 is present on ant brood (eggs, larvae and pupae), and experimentally demonstrated that 3-MeC31 lowers the reproductive output of queens just as it does in workers. This suggests that 3-MeC31 has a multi-faceted role in colony organisation. As well as inhibiting worker reproduction and aggression and facilitating "election" of the most fertile queen by workers, the pheromone may allow queens to assess the amount of brood in the colony and thereby regulate their own productivity by negative feedback.
insights into queen pheromone evolution
Our recent experiments and a comparative analysis suggest that 3-methylalkanes are queen pheromones throughout the ant genus Lasius, suggesting that queen pheromones evolve slowly, and therefore that there is little queen-worker conflict over queen pheromones. We are currently expanding the search for queen pheromones to distantly-related taxa such as wasps (in collaboration with the group of Dr. Tom Wenseleers, in Leuven).
the evolutionary conservation of the queen pheromone suggests that it is somehow uniquely linked to egg production, so we investigated the links between fertility and egg production using quantitative genetics. 3-MeC31 is positively genetically correlated with egg production, in contrast to all other hydrocarbons. This suggests that genes that cause increased production of 3-MeC31 also tend to increase fertility and vice versa, suggesting that fertility and queen pheromone production cannot evolve independently.
we also manipulated egg production by treating queens with insect "juvenile hormone". The hormone inhibited queens'fertility but stimulated their pheromone production. The study demonstrates that queen pheromone production is traded-off against another important trait because of a shared, antagonistic dependence on juvenile hormone. These results shed light on the evolutionary mechanisms that keep animal signals honest, and the fellow is currently engaged in reconciling the "handicap" model of honest signalling with one of its main rivals, the index hypothesis.
mechanisms and consequences of sexual selection in ants
Using a novel form of genetic analysis (competitive PCR, which allows quantification of the relative contributions of different individuals to a mixed sample of DNA), we investigated the mechanics of sperm competition in the leafcutter ant Atta colombica. We showed that the relative paternity of different males mating with the same queen was directly related to the relative number of sperm stored by the queen from each male (e. g. if 30 % of the sperm belonged to one male, he sired around 30 % of the offspring). This suggests the absence of overt sperm competition between males after the sperm have been stored, and shows that all sperm in storage have an equal chance of being used in fertilisation. Sexual selection also had important sociogenetic consequences: some males'offspring are more likely to develop into one worker morph over another (either giant "soldiers" or tiny "farmer" workers). Queens may therefore be controlling male-male competition in order to maximise genetic diversity within the colony and improve task allocation.
new project-the evolution of genetic kin recognition
A prominent unsolved problem in evolutionary biology concerns the evolution of genes that are used by organisms to identify kin. Many organisms behave more favourably towards kin than non-kin, e. g. by being altruistic towards their relatives or aggressive towards no-relatives, whom they differentiate on the basis of heritable recognition cues such as odours or facial similarity. However, such genetic kin recognition should not be stable over evolutionary time, because individuals with common recognition alleles should have higher fitness than those with rare cues, leading to a loss of the genetic diversity required for kin recognition. Sexual selection provides one possible resolution to the paradox. If individuals preferentially mate with others that have different cues in order to avoid inbreeding, rare recognition alleles may have a sexually-selected advantage. We used an individual-based simulation to rigorously examine this hypothesis for the first time. The model shows that sex may resolve the paradox of kin recognition, and a systematic review showed that this mechanism may be widespread in all taxa, from microbes to humans.
impact of the project
The project is expected to result in a minimum of 14 peer-reviewed publications in international journals, 7 of which have been published so far. The discovery of the first ant queen pheromone is very significant, especially to social insect researchers; the paper has so far received 11 citations in 8 months. Queen pheromones are of great interest because they are effectively the "on/off" switch for sociality that determines whether workers remain sterile and help their queen, or attempt to reproduce individually. We expect the paper on the paradox of kin recognition to be similarly influential, as it provides explicit predictions and recommendations for empirical work that will go a long way towards resolving this long-standing problem. Over the project, Luke Holman collaborated extensively with other research groups in Helsinki, Groningen, Leuven, Pennsylvania and Copenhagen, forging new intra-European links and maintaining the status of the Centre for Social Evolution as a world-leader for interdisciplinary collaborative research. Luke Holman was trained in many new empirical, theoretical and complementary research skills, and started a new position at Australian National University working with renowned theoretical biologist Prof. Hanna Kokko.
- To identify the first ant queen pheromone regulating worker reproduction, and investigate its proximate functions and ultimate consequences
To investigate the mechanisms and consequences of sexual selection in ants
Primary results
Identification and functional tests of the first ant queen pheromone
We first narrowed down the list of candidate queen pheromones by investigating which of the many chemicals produced by queens are associated with fertility, using the black garden ant Lasius niger as a model. By manipulating the amount of brood present in incipient ant colonies, we were able to non-invasively increase and decrease queen fecundity, and identify hydrocarbons whose expression was changed correspondingly. We also compared the hydrocarbons of queens at different stages of reproductive maturity to identify those that increase as queens begin to produce eggs. Lastly, we compared the hydrocarbons of queens that were killed or spared by the workers in colonies with supernumerary queens. The hydrocarbon 3-methylhentriacontane, or 3-MeC31, was a strong predictor of all three traits (fertility, maturity and surviving the cull by workers).
we next used samples of pure 3-MeC31, synthesized by chemist collaborators at the University of Copenhagen, to experimentally investigate whether this chemical is a queen pheromone. Workers were less aggressive to glass models treated with 3-MeC31 compared to controls, demonstrating that 3-MeC31 affects worker behaviour. Moreover, workers in the control groups developed their ovaries (as is normal when their queen is not present), while those supplied with 3-MeC31 did not. 3-MeC31 is responsible for the inhibitory effect of queens on worker reproduction, making it the first "primer pheromone" (one that affects physiology) to be identified in any social insect other than the well-studied honey bee and one species of termite.
we further showed that 3-MeC31 is present on ant brood (eggs, larvae and pupae), and experimentally demonstrated that 3-MeC31 lowers the reproductive output of queens just as it does in workers. This suggests that 3-MeC31 has a multi-faceted role in colony organisation. As well as inhibiting worker reproduction and aggression and facilitating "election" of the most fertile queen by workers, the pheromone may allow queens to assess the amount of brood in the colony and thereby regulate their own productivity by negative feedback.
insights into queen pheromone evolution
Our recent experiments and a comparative analysis suggest that 3-methylalkanes are queen pheromones throughout the ant genus Lasius, suggesting that queen pheromones evolve slowly, and therefore that there is little queen-worker conflict over queen pheromones. We are currently expanding the search for queen pheromones to distantly-related taxa such as wasps (in collaboration with the group of Dr. Tom Wenseleers, in Leuven).
the evolutionary conservation of the queen pheromone suggests that it is somehow uniquely linked to egg production, so we investigated the links between fertility and egg production using quantitative genetics. 3-MeC31 is positively genetically correlated with egg production, in contrast to all other hydrocarbons. This suggests that genes that cause increased production of 3-MeC31 also tend to increase fertility and vice versa, suggesting that fertility and queen pheromone production cannot evolve independently.
we also manipulated egg production by treating queens with insect "juvenile hormone". The hormone inhibited queens'fertility but stimulated their pheromone production. The study demonstrates that queen pheromone production is traded-off against another important trait because of a shared, antagonistic dependence on juvenile hormone. These results shed light on the evolutionary mechanisms that keep animal signals honest, and the fellow is currently engaged in reconciling the "handicap" model of honest signalling with one of its main rivals, the index hypothesis.
mechanisms and consequences of sexual selection in ants
Using a novel form of genetic analysis (competitive PCR, which allows quantification of the relative contributions of different individuals to a mixed sample of DNA), we investigated the mechanics of sperm competition in the leafcutter ant Atta colombica. We showed that the relative paternity of different males mating with the same queen was directly related to the relative number of sperm stored by the queen from each male (e. g. if 30 % of the sperm belonged to one male, he sired around 30 % of the offspring). This suggests the absence of overt sperm competition between males after the sperm have been stored, and shows that all sperm in storage have an equal chance of being used in fertilisation. Sexual selection also had important sociogenetic consequences: some males'offspring are more likely to develop into one worker morph over another (either giant "soldiers" or tiny "farmer" workers). Queens may therefore be controlling male-male competition in order to maximise genetic diversity within the colony and improve task allocation.
new project-the evolution of genetic kin recognition
A prominent unsolved problem in evolutionary biology concerns the evolution of genes that are used by organisms to identify kin. Many organisms behave more favourably towards kin than non-kin, e. g. by being altruistic towards their relatives or aggressive towards no-relatives, whom they differentiate on the basis of heritable recognition cues such as odours or facial similarity. However, such genetic kin recognition should not be stable over evolutionary time, because individuals with common recognition alleles should have higher fitness than those with rare cues, leading to a loss of the genetic diversity required for kin recognition. Sexual selection provides one possible resolution to the paradox. If individuals preferentially mate with others that have different cues in order to avoid inbreeding, rare recognition alleles may have a sexually-selected advantage. We used an individual-based simulation to rigorously examine this hypothesis for the first time. The model shows that sex may resolve the paradox of kin recognition, and a systematic review showed that this mechanism may be widespread in all taxa, from microbes to humans.
impact of the project
The project is expected to result in a minimum of 14 peer-reviewed publications in international journals, 7 of which have been published so far. The discovery of the first ant queen pheromone is very significant, especially to social insect researchers; the paper has so far received 11 citations in 8 months. Queen pheromones are of great interest because they are effectively the "on/off" switch for sociality that determines whether workers remain sterile and help their queen, or attempt to reproduce individually. We expect the paper on the paradox of kin recognition to be similarly influential, as it provides explicit predictions and recommendations for empirical work that will go a long way towards resolving this long-standing problem. Over the project, Luke Holman collaborated extensively with other research groups in Helsinki, Groningen, Leuven, Pennsylvania and Copenhagen, forging new intra-European links and maintaining the status of the Centre for Social Evolution as a world-leader for interdisciplinary collaborative research. Luke Holman was trained in many new empirical, theoretical and complementary research skills, and started a new position at Australian National University working with renowned theoretical biologist Prof. Hanna Kokko.