Causes and consequences of cuticular hydrocarbon divergence in parabiotic ants
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Abstract
Species interactions such as competition, antagonism and mutualism are thought to promote diversification in phenotypic traits and thus significantly contribute to species diversity on Earth. Cuticular hydrocarbons (CHCs) are the major components of the waxy layer covering basically all terrestrial arthropods. Most importantly they protect insects from desiccation and act as agents of chemical communication. The communication functions of CHCs are especially important in social insects, such as ants, bees, wasps or termites. Amongst other functions, ants use CHC profiles to identify mutualistic partner species. A special form of such a mutualism is parabiosis, i.e. two ant species mutualistically sharing the same nest. In this thesis, I investigate the causes and consequences for the divergence of CHC profiles in the parabiotic ant species Crematogaster levior and Camponotus femoratus from the South American rainforest.
In Chapter 1, I elucidate the species status of chemically diverged morphs of the ants of both genera mentioned above and discuss which role CHCs could have mediating speciation. I conclusively demonstrate that both, Cr. levior and Ca. femoratus, in fact consist of two cryptic species that, despite only slight morphological differences, strongly differ in their CHC profiles and their genetic background. By in detail investigating CHC differences within and between the cryptic species, I identify several ultimate causes for variation of the CHC profiles in Chapter 2. Especially the parabiotic lifestyle led to strong changes such as elongations of the carbon backbone of the CHCs, in both cryptic species of Ca. femoratus, but only one of Cr. levior. Although the cryptic species are closely related, they show vastly different CHC profiles, which is why I further investigated if gene expression differences at the site of CHC biosynthesis might explain this in Chapter 3. In this chapter, I identify several candidate genes and their expression patterns as proximate causes for the CHC variation. In many cases I am able to show that the gene expression differences between the cryptic species are mirrored in the differences observed in their CHC profiles.
The strong differences in CHC profiles are likely to have consequences for nestmate recognition and aggression behavior, which is why I investigated recognition within and between the cryptic species of Cr. levior in Chapter 4, trying to identify which substances or structural CHC classes are involved. In line with the hypothesis that elongations of the carbon backbone of CHCs make the molecules harder to perceive, I found that CHC extracts of the shorter-chained Cr. levior species were probably more perceivable and thus treated more aggressively. In Chapter 5, I examine if the species divergence was accompanied by trophic niche partitioning a) between the mutualistic partners and b) between the cryptic species of Cr. levior and Ca. femoratus. Here, the results imply that competition between the mutualists is mediated by a discovery- dominance trade-off and differences in the trophic niche. The differences between the cryptic species, however, were very subtle suggesting that there is either niche differentiation in dimensions I did not investigate so far or that these cryptic species might be an example for ‘neutral species’.
Finally, in Chapter 6, I provide an overview on the levels and magnitude of variation in CHC profiles in ants, but also other insects. By reviewing up-to-date literature, I provide detailed insights into sources of fixed and plastic variation on the levels of individuals, social insect colonies, populations and species. Furthermore, I discuss which factors may lead to adaptive CHC changes and how these could be constraint by biosynthetical and biophysical mechanisms.
In conclusion, this thesis unravels the existence of cryptic species using integrative taxonomy, provides important insights into the complexity of selection pressures shaping the evolution of CHC profiles and identifies several candidate genes that could be involved in divergence of such profiles. The divergence in CHC profiles could play an important role in mediating speciation. However, it is yet unclear if chemical differences mediated mate choice and led to prezygotic reproductive isolation or if the CHC profiles diverged through reinforcement after speciation. While CHC divergence enabling assortative mating would allow speciation even in sympatry, allopatric populations might diverge through isolation-by-distance, genetic drift or local adaptation reinforcing CHC differences. Furthermore, this thesis identifies the cryptic species of parabiotic ants as an interesting model system to examine how ecologically similar species might avoid competitive exclusion and potentially for the investigation of ‘neutral processes’ mediating species coexistence in tropical ecosystems.