Evolution of Colony Characteristics in The Harvester Ant Genus Pogonomyrmex Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universität Würzburg vorgelegt von Christoph Strehl Nürnberg Würzburg 2005 - 2 - - 3 - Eingereicht am: ......................................................................................................... Mitglieder der Prüfungskommission: Vorsitzender: ............................................................................................................. Gutachter : ................................................................................................................. Gutachter : ................................................................................................................. Tag des Promotionskolloquiums: .............................................................................. Doktorurkunde ausgehändigt am: ............................................................................. - 4 - - 5 - 1. Index 1. Index.................................................................................................................5 2. General Introduction and Thesis Outline.......................................................7 1.1 The characteristics of an ant colony......................................................8 1.2 Relatedness as a major component driving the evolution of colony characteristics.................................................................................................10 1.3 The evolution of polyandry: ultimate and proximate causes................11 1.4 Polygyny and slavery..........................................................................13 1.5 Morphological polymorphisms between workers and queens..............14 1.6 Introduction to the study genus Pogonomyrmex..................................16 1.7 Outline of the thesis............................................................................26 1.8 Publications........................................................................................28 1.9 Literature............................................................................................28 3. Determinants of Intracolonial Relatedness in Pogonomyrmex rugosus (Hymenoptera; Formicidae): Mating Frequency and Brood Raids....................41 3.1 Abstract..............................................................................................41 3.2 Introduction........................................................................................41 3.3 Materials and methods........................................................................43 3.4 Results................................................................................................46 3.5 Discussion..........................................................................................48 3.6 Acknowledgements.............................................................................53 3.7 Tables.................................................................................................53 3.8 Figures................................................................................................57 3.9 References..........................................................................................58 4. Extremely High Mating Frequency in the Florida Harvester Ant (Pogonomyrmex badius).........................................................................................62 4.1 Abstract..............................................................................................62 4.2 Introduction........................................................................................62 4.3 Methods..............................................................................................66 4.4 Results................................................................................................70 4.5 Discussion..........................................................................................72 4.6 Acknowledgements.............................................................................76 4.7 Appendix............................................................................................77 4.8 References..........................................................................................88 5. A Genetic Component in the Determination of Worker Polymorphism in the Florida Harvester ant Pogonomyrmex badius................................................92 5.1 Summary............................................................................................92 5.2 Introduction........................................................................................92 5.3 Methods..............................................................................................94 5.4 Results................................................................................................95 5.5 Discussion..........................................................................................95 5.6 Acknowledgments..............................................................................97 5.7 References..........................................................................................97 5.8 Tables.................................................................................................99 6. Cladistic Analysis of Paleo-Island Populations of the Florida Harvester Ant (Hymenoptera: Formicidae) Based Upon Divergence of Mitochondrial DNA Sequences....................................................................................................103 6.1 Abstract............................................................................................103 6.2 Resumen...........................................................................................103 - 6 - 6.3 Introduction......................................................................................104 6.4 Materials and Methods......................................................................105 6.5 Results..............................................................................................109 6.6 Discussion........................................................................................113 6.7 Acknowledgments............................................................................114 6.8 References cited................................................................................115 7. Colony structure and morphometrics in the queen dimorphic harvester ant, Pogonomyrmex pima.....................................................................................118 7.1 Summary..........................................................................................118 7.2 Introduction......................................................................................118 7.3 Methods............................................................................................120 7.4 Results..............................................................................................122 7.5 Discussion........................................................................................125 7.6 Acknowledgments............................................................................129 7.7 References........................................................................................130 7.8 Table.................................................................................................133 7.9 Figures..............................................................................................135 8. Colony Structure and Sociogenetics of the Queen Dimorphic Harvester Ant, Pogonomyrmex (Ephebomyrmex) pima.......................................................137 8.1 Abstract............................................................................................137 8.2 Introduction......................................................................................137 8.3 Materials and Methods......................................................................140 8.4 Results..............................................................................................144 8.5 Discussion........................................................................................147 8.6 Acknowledgements...........................................................................153 8.7 Tables...............................................................................................154 8.8 Figures..............................................................................................158 8.9 References........................................................................................159 9. Phylogeny of the Harvester Ant Genus Pogonomyrmex (Hymenoptera: Formicidae) in North-America and its relation to South-American Pogonomyrmex species........................................................................................165 9.1 Abstract............................................................................................165 9.2 Introduction......................................................................................165 9.3 Material and Methods.......................................................................168 9.4 Results..............................................................................................170 9.5 Discussion........................................................................................174 9.6 Acknowledgments............................................................................177 9.7 Figures..............................................................................................178 9.8 Tables...............................................................................................185 9.9 Literature..........................................................................................191 10. General Discussion......................................................................................196 11. Summary......................................................................................................207 12. Zusammenfassung.......................................................................................209 13. Curriculum Vitae.........................................................................................212 14. Publikationsliste...........................................................................................214 15. Danksagung.................................................................................................216 16. Ehrenwörtliche Erklärung..........................................................................217 - 7 - 2. General Introduction and Thesis Outline “For such is the way of it: to find and lose, as it seems to those whose boat is on the running stream” (Tolkien 1991, p.399) The selective forces of evolution created a manifold life with impressive forms in shaping not only the morphology of species, but also their behavior. An extraordinary example for the evolution of behavioral characters is the clumping of individuals into one single group that collectively performs social tasks. Its apex is an altruistic self restraint, as found in impressive ways among colonies of ants or bees (eusocial Hymenoptera): their eusocial colonies are not only characterized by overlapping generations and cooperative brood care by the adults, but also by a reproductive division of labor implementing the presence of non-reproductive helpers as “workers” (Michener 1969; Wilson 1971; Alexander 1974; Michener 1974; Hölldobler and Wilson 1990; Ratnieks and Anderson 1999). The altruistic behavior of eusocial Hymenoptera was for a long time explained by the perception of natural selection not only acting at the level of single organisms, but also at the family-level (Darwin 1859). Later, this was mathematically formulated by Hamilton in his theory of kin selection (Hamilton 1964), which is now the key concept for explaining the evolution of group living behavior. The Hymenopteran sex determination system hits the underlying genetic assumption of Hamilton’s formula (br- c>0; b = benefits connected to behavior, r = relatedness of actor to recipient, c = costs involved for actor), because unfertilized eggs become males and fertilized eggs normally females (relatedness “asymmetry”) (Hamilton 1964; Trivers and Hare 1976). By this the cost to benefit ratio for ‘altruistic’ behaviors decreases for females, favoring their accumulation in eusocial groups (colonies). Group living is quite divers, and might even be lost again if no longer favorable, depending on internal and external needs (Heinsohn and Legge 1999; Gadagkar 2002; Krieger and Ross 2002; Roux and Korb 2004). Basic rules generating these social - 8 - phenotypes are pertinent across taxa, and they produce ubiquitous patterns of social organization, like mass action responses, division of labor and social hierarchies (Fewell 2003). The tremendous variety of group living among Hymenoptera (bees, ants, wasps) has its greatest diversity among ants (Formicidae), making them especially suitable in studying the characteristics of social colonies (Hölldobler and Wilson 1990). In the present work analyses on the evolution of colony characteristic in the harvester ant genus Pogonomyrmex (Hymenoptera: Formicidae) will be presented and discussed. In the following chapters of this introduction in a first step the general characteristics of an ant colony will be described, relatedness as a major component driving the evolution of colony characteristics is introduced, and the phenomena of polyandry, polygyny and morphological polymorphism are emphasized. Subsequently the studied genus Pogonomyrmex and the aims of this study are introduced. 1.1 The characteristics of an ant colony Several characters are prominent in constituting ant-colonies (e.g.: Alexander 1974; Gordon 1984b; Hölldobler and Wilson 1990; Gordon 1995; Ratnieks and Anderson 1999; Sanders and Gordon 2000; Anderson and McShea 2001; Fewell 2003): (1) number of individuals (colony size); (2) cohesion (nest, odor, age); (3) nest architecture; (4) organization of colony members (e.g. reproductive division of labor, task allocation, information transfer); (5) robustness (e.g. defense, disease and parasite transmissibility, territoriality in space and time); (6) efficiency (e.g. resource use, reproductive output); (7) relatedness among colony members. These single colony characteristics are influenced by various factors. For illustration, task performance and division of labor, which constitute a wide field of recent research, are effected among other things by colony size (Murakami et al. 2000; Gautrais et al. 2002), circadian rhythmic (Gordon 2002; Wuellner and Saunders 2003), information transfer (Gordon 1989; Tofts and Franks 1992; Franks et al. 2001; Fewell 2003), presence or absence of queens (Vienne et al. 1998; Backen et al. 2000; Heinze and Keller 2000; Kikuchi and Higashi 2001), or genetic variability (Stuart and Page 1991; Myerscough and Oldroyd 2004). Analogous several other factors exist, which influence group living behavior in ants. In the main, colony characteristics are influenced by extrinsic environmental and intrinsic nest factors (e.g.: Alexander 1974; Fewell 2003). For example changes of the - 9 - external environment are influencing the colonies efficiency and robustness, and can be managed via a flexible response of the whole colony, e.g. by information transfer in the “social environment” between colony members (Fewell 2003). Territoriality on the other hand, i.e. the ability of colonies to obtain and maintain a certain foraging area within their population, is strongly shaped by several intrinsic factors like behavior, aggressiveness, or growth rate of colonies (Hölldobler 1974; Hölldobler 1979; Cole and Wiernasz 1999; Cole et al. 2001). Additionally single colony characteristics can be reciprocally influenced by others. For example, pathogen resistance as important intrinsic factor is shaped by the colonies efficiency and organization, because it is most importantly influenced by the cleanliness of the whole colony (maybe improved by task specialists), or by the recognition of medically parasitized individuals and their exclusion / elimination or their cure of parasitation (Moore 1995; Julian and Cahan 1999; Bot et al. 2001; Hart and Ratnieks 2001; Moore 2002). The benefits of group living are various and general items for the benefits of group living, which might also be applicable to a social colony might be (1) a lowered susceptibility to predation, e.g. via an aggressive group defense or using the group as cover, and (2) reaching a profitable utilization of food sources, e.g. if these are scattered (Alexander 1974). Therefore, the robustness and efficiency of ant colonies might be regarded as being predominantly in the focus of natural selection. Particularly, the efficient mechanisms of division of labor, “in particular series-parallel operation and transfer of information among group members” are enabling ant colonies to interact with an unpredictable environment (Krieger et al. 2000). These direct benefits are also mirrored in non-Hymenopteran societies, and even robotic systems can be enhanced by implementing such “ant-inspired algorithms” (Krieger et al. 2000). However, “there is no automatic or universal benefit from group living” (Alexander 1974) and there are also costs involved to living in a colony (Heinsohn and Legge 1999). For example social parasitism evolved several times in ants to exploit existing colonies (Hölldobler and Wilson 1990; Parker and Rissing 2002; Savolainen and Vepsalainen 2003). Accordingly, the spread of medical parasites and pathogens might be favored in eusocial groups, especially under the condition of high genetic uniformity due to a high relatedness among group members, like in ants or bees. Additionally, colony characteristics by themselves can have negative feedback (Fewell 2003; Johnson - 10 - 2003). For instance, there is a relationship between group size and efficiency, such that the benefits of group living decrease in larger groups, most probably because of interference during foraging (Krieger et al. 2000). 1.2 Relatedness as a major component driving the evolution of colony characteristics Although it is hard to determine characters which are applicable in explaining the connections that occur between any two individuals within a colony, and much harder the connections across the whole colonies society (Fewell 2003), at least the relatedness among its members is representing such a key factor. The relatedness among its members is influencing the various characteristics of a colony. For instance, in Camponotus yamaokai a decrease in relatedness is affecting the cooperation of group members, because the frequency of food exchange between workers (trophallaxis) drops and the number of foraging workers rises (Sanada et al. 1999). Or, there might be drawn a direct relation between colony kin structure and size (Sundström 1995; Bourke 1999), e.g. in fungus-growing ants (Attini) where a positive correlation of mating frequency with colony size is reported, and "some" correlation between mating frequency and caste complexity (Murakami et al. 2000). Relatedness is, beside costs and benefits of group living behavior, the major component driving the evolution of colony characteristics among colony members, according to Hamilton’s formula for kin selection (Hamilton 1964). Moreover, nowadays relatedness measurements are easily made accessible due to the rapidly developing molecular methods based on PCR (Saiki et al. 1985; Sambrook et al. 1989; Simon et al. 1994; Loxdale and Lushai 1998; Hwang et al. 2001), making it a leading tool in social insect research. Moreover, molecular markers are helping to determine skews in the kin structure of colonies, e.g. if paternity is unequally distributed between the mates of a queen (Boomsma and Ratnieks 1996). Relatedness among colony members is determined by the colonies number of matrilines, i.e. number of queens, and the number of patrilines, i.e. number of matings per queen(s), and altered via a change in the number of matrilines and / or patrilines. Consequently, polygyny (several queens) and multiple mating of queens (polyandry) are main sources for genetic heterogeneity of colonies of social insects (Bourke and Franks 1995; Crozier and Pamilo 1996; Gadau et al. 1998; Heinze and Keller 2000). However, in ants low genetic heterogeneity is caused for the most part by polygyny, whereas