The evolution of caste-specific morphology and colony structure in ants
An organism can adapt to its environment physiologically, behaviorally, and morphologically, but biological functions may conflict with each other and therefore constrain their evolution. Eusocial organisms live in colonies that possess morphologically distinct castes that divide biological roles among individuals. Males and reproductive females fill the reproductive role of the colony which includes mating, dispersal and egg laying. The typically sterile worker caste performs other tasks for the colony such as food collection, defense, and brood care. Workers are therefore released from the morphological constraints of reproduction and can physically adapt to other roles. In addition to exhibiting large degrees of morphological variation, eusocial insect colonies can also vary in colony structure. For example, some species have a single reproductive queen while others can vary in queen number that in turn influences the genetic relatedness of colony members. Furthermore, colonies can be centralized in one nest, or dispersed over several spatially separated nests. Variation in morphology and colony structure within and among species reflects a combination of adaptation to ecological pressures and historical constraints. My research goals were to investigate intra and interspecific variation in caste specific morphology and colony structure in a comparative framework to understand how traits evolve relative to species’ ecology. In chapter 2, I studied the species rich ant genus Pheidole to understand how worker morphology can adapt to environmental factors. Workers of Pheidole are almost always dimorphic, comprising a small minor subcaste and a major or soldier subcaste with a distinctively large head. Pheidole species vary in their diet, but many specialize on seeds that are milled by majors using large jaws powered by mandible closer muscles that fill the head cavity. I hypothesized that seed harvesting species will have majors with larger heads compared to non-seed-harvesting species to accommodate more powerful mandibular muscles required to mill seeds. I found that majors of seed-associated Pheidole do not have larger heads than majors of non-seed-harvesting species, but there is a positive relationship between seed-harvesting and minor head length. Additionally, there was a difference in the size of minors relative to majors within species; seed-harvesting Pheidole species have smaller minors and larger majors than their non-seed-harvesting congeners. This finding suggests that head size of major and minor subcastes can evolve for diet specialization, but they do so relative to the morphology of other worker castes. In chapter 3, I used the ant genus Linepithema to examine intra and interspecific variation in morphology and colony structure. Despite our general understanding of caste differentiation, there are few studies quantifying morphological variation within and among reproductive females, workers, and males. I hypothesized males and queens would vary less than workers that are not constrained by dispersal and reproduction. Workers and reproductive females were more similar to each other than they were to males, which had smaller antennal scapes and heads. Reproductive females were larger overall and males exhibited the largest amount of variation among species. Males of some species also possessed unique morphological features that may reflect adaptations for reproductive strategies suggesting the genus Linepithema deserves further research to understand what factors are driving male specific variation in morphology. Finally, in chapter 4 I examined the evolution of colony structure in the genus Linepithema. I estimated within and among population variation in nest number and dispersion (polydomy), and queen number for colonies of eight species in the genus from Argentina, Ecuador and Brazil. These were compared to published data for native and introduced populations of L. humile, an invasive species. My research revealed significant variation in nest number among the nine species of Linepithema, however, L. humile was the only species to have colonies with nests dispersed over 250 meters. Polydomy occurs at larger distances in a stepwise fashion within the ‘humile’ clade, suggesting species have incrementally increased the spatial extent of their colonies sequentially rather than via an abrupt change in colony structure in L. humile. Queen number estimates from microsatellite genotyping reveal a similar pattern; species within the ‘humile’ clade typically exhibit multiple queens but vary from an average of just over one per nest for L. gallardoi to over nine per nest in L. oblongum and none possess as many egg-laying queens as seen in L. humile colonies. Together, these findings suggest the evolution of unicoloniality in L. humile involved a shift from limited polydomy and polygyny throughout the genus, to extreme polydomy and polygyny in L. humile via a gradual change in the ‘humile’ species group. Additionally, I compared the data on colony structure in Linepithema to the frequency in which they have been detected in quarantine at ports of entry into the United States (a measure analogous to propagule pressure). Six species of Lineptihema have been intercepted in commerce arriving to the United States, yet only L. humile has become established outdoors. Of the remaining intercepted species, none exhibit large degrees of polygyny and polydomy suggesting that opportunity alone is not sufficient for establishment. We did not find records of the two species that exhibit high degrees of polygyny and polydomy L. oblongum and L. micans, suggesting they may pose invasion risks based on their biology, but may not have been commonly transported. Collectively, these results highlight how both species-level characters and opportunity influence species’ ability to invade new habitats.
biological invasion, Linepithema, Pheidole, caste-specific morphology, colony structure
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign