Diversification in Exocrine Systems

Scientific Background: With more than 10.000 species described, oribatid mites produce a stunning diversity of natural exocrine components that are expelled from large opisthosomal oil glands. These secretions functionally allomones and pheromones - represent a perfectly suited model to study, understand and reconstruct the evolutionary history of secretion diversification in an exocrine system (phylogenetic chemosystematics). Current knowledge: Apart from a chemical trait in so-called Astigmata-compounds-bearing oribatids (a lineage from lower oribatids to astigmatid mites, characterized by a set of terpenes and aromatics), very little is known about the chemistry of oil gland secretions. However, cursory investigations revealed that particularly the chemistry of higher Oribatida (= the vast majority of oribatid mites) represents an enormously rich source of novel compounds, indicating a network of chemical traits across Oribatida and many compounds new to science. Hypotheses & aims: We hypothesize that many more chemically distinct traits in Oribatida exist, besides Astigmata compounds from lower Oribatida. A network of different biosynthetic pathways underlying novel chemical traits is expected. In addition, chemical traits are considered to have diversified during evolution, in total leading to the richness of compounds/compound classes in the secretions of extant oribatid species. This oribatid chemosystematic picture, i.e. the evolutionary history of different chemical traits in the oil gland secretions of Oribatida, is aimed to be resolved and logically reconstructed. New scientific ground: Our major goal is to present a first comprehensive model for the evolutionary history of secretion chemistry in an exocrine system from its ancient beginnings over diversification periods to its extant diversity. Methods include: i) extraction and chemical analysis of oil gland secretions of a representative selection of oribatid taxa, using modern mass spectrometric and nuclear magnetic resonance spectroscopic methods. ii) Evaluation of the phylogeny of Oribatida, using a novel transcriptomic approach, and iii) tracing the evolutionary history of oil gland compounds all across Oribatida by techniques of Ancestral Character State Reconstruction (ASR). Scientists involved: Günther Raspotnig (PI, zoology, chemical ecology) & Christoph Hahn (co- applicant; evolutionary biologist specialized in transcriptomics) in collaboration with Roy A. Norton (acarology), Hans-Jörg Leis & Olaf Kunert (analytical chemistry) and Michael Heethoff (ecology).

Evolution of exocrine systems: How oil glands evolved Exocrine gland systems represent key evolutionary innovations, yet their long-term evolution across entire animal taxa remains poorly understood. This project used oribatid mites (Oribatida) as an exceptional model to reconstruct the evolutionary history of an ancient homologous exocrine system within a highly diverse arthropod group. Oribatida comprises more than 12,000 described species, most of which possess paired oil glands (opisthonotal glands) located in the opisthosoma. These glands are absent only in the most basal lineages and are thought to have evolved more than 400 million years ago. Over evolutionary time, their secretions diversified into a rich array of taxon- and species-specific chemical blends, providing a unique opportunity to trace the evolution of exocrine chemistry from its origins to present-day diversity. To unravel this chemosystematic history, the project was based on the most comprehensive taxon sampling conducted to date. It included all glandulate oribatid cohorts (Parhyposomata, Mixonomata, Desmonomata, Brachypylina), primitive non-glandulate lineages (Palaeosomata, Enarthronota), and astigmatid mites, which also possess oil glands and have long been suspected to derive from Oribatida. Using a unique transcriptomic approach, we reconstructed a robust phylogenetic framework for Oribatida that served as the basis for mapping chemical traits. The results revealed several key evolutionary patterns. First, transcriptome-based phylogenies unequivocally demonstrated that Astigmata originated within Mixonomata, finally resolving a long-standing debate in acarology. Second, chemical trait mapping showed that most secretion compounds derive from three ancient biosynthetic pathways that early gave rise to hydrocarbons, terpenes, and aromatic compounds. These pathways were differentially modified during evolution, being reduced or extended in lower and middle-derivative Oribatida, including Astigmata. Consequently, hydrocarbons, aromatics, and terpenes characterize middle-derivative Oribatida and Astigmata. Striking, lineage-specific extensions of the terpene pathway evolved in several groups, including Nothridae, Euphthiracaroidea, and parts of Brachypylina such as Hermannioidea. Similarly, pronounced extensions of the aromatic pathway characterize early-derivative Oribatida, including Parhyposomata and (mixonomatan) Epilohmannioidea. In higher Brachypylina, entirely novel biosynthetic routes emerged. These include the production of multi-ringed "hermanniellines" in Hermanniellidae and, in more derived lineages such as Oripodoidea, nitrogen-containing compounds including cyanogenic substances and diverse alkaloids. Additional evolutionary trends identified include the emergence of chemo-metamorphosis (juvenile-adult chemical polymorphism) and a progressive reduction of oil glands in some highly derived Oribatida. Overall, this project provides the first comprehensive chemo-phylogenetic model for the evolution of a gland system across an entire animal taxon. By integrating transcriptomics with comparative chemistry, it establishes a new framework for understanding the evolution of exocrine systems and resolves fundamental questions on the origin of Astigmata within Oribatida. So far, the project led to 3 peer-reviewed publications, but 9 additional manuscripts are currently submitted or in preparation.