Male Head Secretion Triggers Copulation in the Dwarf SpiderDiplocephalus permixtus

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.

Cephalic secretion release in the male dwarf spider Oedothorax retusus (Linyphiidae: Erigoninae): an ultrastructural analysis

Secondary sexual traits in males can extend to glandular structures that play a role during courtship and mating. In dwarf spiders (Linyphiidae, Erigoninae), glandular secondary sexual traits are particularly common. Males are characterized by cephalic modifications which produce secretions that females contact with their mouthparts during courtship and/or copulation. We used the dwarf spider Oedothorax retusus as a model species to investigate if and when the contents of the glands are released during a mating sequence and if so, if the gland reservoirs are refilled after mating. To this aim, we quantitatively compared the glandular tissue on the ultrastructural level between a) inexperienced males, b) males that performed courtship, c) males immediately after copulation, and d) males three days after mating. We assessed whether the treatment groups differed in the filling state of the conducting canals and receiving canals (reservoir regions) of the glandular units. Our study shows that courting males as well as males three days after mating did not differ significantly from control (inexperienced) males in the presence of secretions. However, males exhibited significantly less secretion immediately after mating. This strongly suggests that the main function of the secretions is gustatorial courtship and not the emission of volatile pheromones for mate attraction as was previously assumed.