Evolution of nuptial-gift-related male prosomal structures: taxonomic revision and cladistic analysis of the genus Oedothorax (Araneae: Linyphiidae: Erigoninae)

Sexual selection has been shown to drive speciation. In dwarf spiders (erigonines), males possess diverse, sexually selected prosomal structures with nuptial-gift-producing glands. The genus Oedothorax is suitable for investigating the evolution of these features due to high structural variation. We have re-delimited this genus based on a phylogenetic analysis. Ten species are Oedothorax s.s.; five are transferred back to their original generic placement; 25 remain unplaced as ‘Oedothorax’. Four junior synonymies are proposed: Callitrichia simplex to Ca. holmi comb. nov.; Gongylidioides kougianensis to G. insulanus comb. nov.; Ummeliata ziaowutai to U. esyunini comb. nov.; Oe. kathmandu to Mitrager unicolor comb. nov. Oedothorax seminolus is a junior synonym of Soulgas corticarius and the transfer of Oe. alascensis to Halorates is confirmed. The replacement name Ca. hirsuta is proposed for Ca. pilosa. The male of Callitrichia longiducta comb. nov. and the female of ‘Oedothorax’ nazareti are newly described. Thirty-eight Oedothorax species are transferred to other genera. Callitrichia spinosa is transferred to Holmelgonia. Three genera are erected: Cornitibia, Emertongone and Jilinus. Ophrynia and Toschia are synonymized with Callitrichia. Character optimization suggests multiple origins of different prosomal modification types. Convergent evolution in these traits suggests that sexual selection has played an important role in erigonine diversification.

Diversification through gustatory courtship: an X-ray micro-computed tomography study on dwarf spiders

BACKGROUND

Sexual selection has been considered to promote diversification and speciation. Sexually dimorphic species have been used to explore the supposed effect, however, with mixed results. In dwarf spiders (Erigoninae), many species are sexually dimorphic-males possess marked prosomal modifications. These male traits vary from moderate elevations to bizarre shapes in various prosomal regions. Previous studies established that male dwarf spiders produce substances in these prosomal modifications that are taken up by the females. These substances can act as nuptial gifts, which increase the mating probability of males and the oviposition rate in females. Therefore, these dimorphic traits have evolved in the context of sexual selection. Here, we explore the evolutionary lability of this gustatory trait complex with the aim of assessing the role of this trait complex in species divergence by investigating (1) if erigonine modified prosomata are inherently linked to nuptial-gift-producing glands, (2) if the evolution of the glands evolution preceded that of the modified prosomal shapes, and by assessing (3) the occurrence of convergent/divergent evolution and cryptic differentiation.

RESULTS

We reconstructed the position and extent of the glandular tissue along with the muscular anatomy in the anterior part of the prosoma of 76 erigonine spiders and three outgroup species using X-ray micro-computed tomography. In all but one case, modified prosomata are associated with gustatory glands. We incorporated the location of glands and muscles into an existing matrix of somatic and genitalic morphological traits of these taxa and reanalyzed their phylogenetic relationship. Our analysis supports that the possession of glandular equipment is the ancestral state and that the manifold modifications of the prosomal shape have evolved convergently multiple times. We found differences in gland position between species with both modified and unmodified prosomata, and reported on seven cases of gland loss.

CONCLUSIONS

Our findings suggest that the occurrence of gustatory glands in sexually monomorphic ancestors has set the stage for the evolution of diverse dimorphic external modifications in dwarf spiders. Differences among congeners suggest that the gland position is highly susceptible to evolutionary changes. The multiple incidences might reflect costs of glandular tissue maintenance and nuptial feeding. Our results indicate divergent evolutionary patterns of gustatory-courtship-related traits, and thus a likely facilitating effect of sexual selection on speciation.

Testing mitochondrial marker efficacy for DNA barcoding in spiders: a test case using the dwarf spider genus Oedothorax (Araneae : Linyphiidae : Erigoninae)

The present study focusses on comparatively assessing the efficacy for DNA barcoding of the two most commonly used mitochondrial markers (cox1 and 16S) in a genus of erigonine spiders. In total, 53 specimens representing five species, including four multi-sampled species, were sampled from several European localities. Initial evaluation of species monophyly was performed through parsimony and Bayesian phylogenetic analyses. Efficacy of mitochondrial markers was tested using operational (including distance-, tree-based measures and Barcode Gap) and evolutionary criteria (using the General Mixed Yule-coalescent Model) for species delimitation. We propose that the cox1 marker can potentially overestimate analyses of biodiversity and thus might not be the preferred marker for DNA species identification and delimitation methods in Oedothorax. Instead, our results suggest that the 16S marker appears to be a promising candidate for such endeavour. Evaluating the contribution and suitability of markers to the re-identification of species, measured by their recovery of well established morphological species, is critical for future studies and for reliable results in species identification in spiders.

Comparative study of the femoral organ in Zodarion spiders (Araneae: Zodariidae)

The femoral organ of Zodarion spiders has not been investigated in detail yet. In this study we describe the external and internal structure of this organ. The organ is situated at the distal tip of the femora of all the legs during all developmental stages. The size of the organ (expressed as the number of hairs) increased with the ontogenetic development of Zodarion species. The organ was confirmed to occur in both sexes of all 47 Zodarion species examined. It is possibly present in all species of this genus. The size of the organ increased with the size of the species. A comparative anatomical study was performed in juveniles and adults of both sexes of Zodarion rubidum, and females of both Z. cyrenaicum and Z. jozefienae. The femoral organ represents an exocrine gland composed of a group of secretory cells located below the epidermis. Each gland cell is connected with the leg surface by a single duct. The ducts run in intercellular spaces and specialised canal cells are lacking. The structure of the secretory cells, namely the abundance of smooth endoplasmic reticulum, suggests that the gland produces a volatile compound(s). The composition and the role of the secretion, however, remains unknown.

Sexual selection research on spiders: progress and biases

The renaissance of interest in sexual selection during the last decades has fuelled an extraordinary increase of scientific papers on the subject in spiders. Research has focused both on the process of sexual selection itself, for example on the signals and various modalities involved, and on the patterns, that is the outcome of mate choice and competition depending on certain parameters. Sexual selection has most clearly been demonstrated in cases involving visual and acoustical signals but most spiders are myopic and mute, relying rather on vibrations, chemical and tactile stimuli. This review argues that research has been biased towards modalities that are relatively easily accessible to the human observer. Circumstantial and comparative evidence indicates that sexual selection working via substrate-borne vibrations and tactile as well as chemical stimuli may be common and widespread in spiders. Pattern-oriented research has focused on several phenomena for which spiders offer excellent model objects, like sexual size dimorphism, nuptial feeding, sexual cannibalism, and sperm competition. The accumulating evidence argues for a highly complex set of explanations for seemingly uniform patterns like size dimorphism and sexual cannibalism. Sexual selection appears involved as well as natural selection and mechanisms that are adaptive in other contexts only. Sperm competition has resulted in a plethora of morphological and behavioural adaptations, and simplistic models like those linking reproductive morphology with behaviour and sperm priority patterns in a straightforward way are being replaced by complex models involving an array of parameters. Male mating costs are increasingly being documented in spiders, and sexual selection by male mate choice is discussed as a potential result. Research on sexual selection in spiders has come a long way since Darwin, whose spider examples are reanalysed in the context of contemporary knowledge, but the same biases and methodological constraints have persisted almost unchanged through the current boom of research.