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Species are the basic units of evolution and biodiversity, and the process of speciation has been one of the most important questions in biology. The evolution of species with common descent is considered to be mainly driven by natural and sexual selection. The material basis and mechanical cause of organismic evolution were recognized during the formation of the modern synthesis of the evolutionary theory in the early 20th century, providing the framework for speciation studies. During this period, the biological species concept was developed in the frame of population genetics, putting emphasis on the reproductive isolation between populations. The phylogenetic species concept developed in the 1980s, on the other hand, does not make any particular assumption about evolutionary or speciation processes. It defines species via their unique combination of character states which are compatible with phylogenetic practices. However, the aforementioned two species concepts are difficult to apply in alpha-taxonomy, where newly discovered species are largely described by the morphological (typological) species concept for practical reasons. Nevertheless, the description of morphological species provides the basis for further assessments of species delimitation via other species concepts and approaches. One of the tools for assisting the identification and discovery of animal species is DNA barcoding, which uses a standard region of mitochondrial DNA sequence as a universal DNA barcode. However, its assumption of intraspecific genetic distances being smaller than interspecific genetic distances does not always hold. Species-level poly-/paraphyly is prevalent due to the discrepancy between the phylogenies of mitochondrial DNA and species. This suggests that the application of DNA barcodes must be combined with an integrative taxonomic approach. Beside the application as a tool for assisting species identification, the information from mitochondrial DNA sequences opens up a window for looking into the complex history of species.
Sexual selection is a potential mechanism driving the evolution of species. It favors traits that increase mating probability and mating success. It can result from intrasexual competition, female preference or sexual conflict. However, previous comparative studies using the degree of sexual dimorphism as a proxy for the strength of sexual selection have yielded inconsistent results as to the relationship between sexual selection and species richness. A possible cause of the inferred low association are factors other than sexual selection, which can also lead to the evolution of sexual dimorphism, such as selection for increased female fecundity. In order to assess the effect of sexual selection on speciation, the lability and evolvability of traits need to be studied that are clearly under sexual selection.
The aim of this thesis is to improve the knowledge about dwarf spider (Erigoninae, Linyphiidae) diversity and taxonomy, and to assess the evolutionary patterns of dimorphic traits that are under sexual selection. I focused on the abundant and diverse male prosomal modifications in dwarf spiders that are linked to the transfer of secretions from the male to the female during courtship and mating (gustatory courtship). This approach explores the process of speciation and the role of sexual selection on species diversification. I described new erigonine species and revised the classification of known species based on phylogenetic analyses. I also applied X-ray micro-computed tomography (micro-CT) to investigate the distribution and evolutionary pattern of the gustatory glands to tease apart the evolution of prosomal shape and glandular equipment.
This cumulative thesis consists of three publications:
Publication 1: This publication aimed at contributing to the knowledge of erigonine diversity. The genus Shaanxinus previously contained only two species from China. I collected dwarf spiders from multiple locations in Taiwan from above-ground vegetations with a seldom applied collecting method. Inspection of the collected material resulted in the discovery of 13 Shaanxinus species. An additional species from Vietnam was described from a museum collection. I provided a revision of the genus Shaanxinus. A phylogenetic analysis using morphological characters was conducted for determining the possible generic synamomorphies. I also reconstructed the glandular distribution associated with male prosomal modifications, as well as the detailed structure of a male secondary sexual organ (pedipalp) by micro-CT. Furthermore, I conducted phylogenetic analyses based on sequences from two mitochondrial and one nuclear loci, and assessed the efficacy of different criteria in species identification using DNA barcoding. Distinction of morphologically similar species have been assisted by molecular data. The species level poly-/paraphyly found in mitochondrial DNA sequences caused the low efficacy of many distance- and tree-based species identification methods, while the nearest neighbor method showed high identification success. The non-monophyly is likely caused by instances of interspecific hybridization and recent parapatric speciation. The genus Shaanxinus thus lend itself as an ideal group for congeneric phylogeographic studies addressing the interactions between closely related species. Published in: Lin, S.-W., Lopardo, L., Haase, M. & Uhl, G. 2019. Taxonomic revision of the dwarf spider genus Shaanxinus Tanasevitch, 2006 (Araneae, Linyphiidae, Erigoninae), with new species from Taiwan and Vietnam. Organism Diversity & Evolution, 19, 211-276.
Publication 2: Sexually dimorphic prosomal modifications that are related to gustatory courtship occur in many dwarf spider species. These features evolved in the context of sexual selection, which has a potential effect on species diversification. In contrast to many
erigonine genera which present little variability in male prosomal traits, the genus Oedothorax presents higher diversity in male prosomal structures among species not only in the position and shapes of the modifications, but also in the degree of modification, ranging from absent to highly elaborated. This genus thus lends itself as a suitable target group for studying the effect of gustatory-courtship-related traits on species diversification. I conducted a revision of the 82 species previously belonging to this genus. Based on the result of a phylogenetic analysis, this genus was re-delimited with 10 species as Oedothorax sensu stricto, while taxonomic decisions were made for other species including synonymization with species from other genera and transferring species to other existing and newly defined genera. 25 species were deemed as “Oedothorax” incertae sedis. The reconstruction of character state evolution suggested multiple origins of specific prosomal modification types. Convergent evolution of these traits among different lineages suggests that sexual selection has played an important role in the species diversification of dwarf spiders. Published in: Lin, S.-W., Lopardo, L. & Uhl, G. 2021. Evolution of nuptial-gift-related male prosomal structures: taxonomic revision and cladistic analysis of the genus Oedothorax (Araneae: Linyphiidae: Erigoninae). Zoological Journal of the Linnean Society, XX, 1-168.
Publication 3: Although sexually dimorphic traits have inspired the concept of sexual selection as the driving force of their evolution, they might also have evolved due to other ecological factors. These factors include the sexual signal adaptation to the environment as well as sexual differences in ecological relations and parental investment. In contrast, the gustatory courtship in dwarf spiders is associated with sexually dimorphic male prosomal modifications, which have clearly evolved in the context of sexual selection. Multiple origins of various external prosomal modifications have been shown in erigonine phylogeny, but the evolutionary pattern of the associated glands has not been investigated. Our phylogenetic analysis incorporated the characters related to the glandular distribution in the male prosoma as well as the external shapes yielded from X-ray micro-computed-tomography showed a single origin of gland among the investigated erigonine taxa. The internal anatomy revealed previously undetected trait lability in attachments of muscles to the cuticular structures, as well as the presence/absence and differences in glandular distribution even in species without external modification. Our finding further supports that erigonine male prosomal traits are under divergent selection, and corroborates the argument that erigonines are a suitable group for investigating the effect of sexual selection on speciation. Published in: Lin, S.-W., Lopardo, L. & Uhl, G. 2021. Diversification through gustatory courtship: an X‑ray micro‑computed tomography study on dwarf spiders. Frontiers in Zoology, 18: 51.
The results of this thesis corroborate the importance of applying phylogenetic methods and an integrative approach in the description of new species, as well as in revising taxa which might not be monophyletic. Overall, the studies contributed to a more comprehensive knowledge about erigonine species diversity, phylogeny and the possible diversifying effect of sexual selection on male traits associated with gustatory courtship.