Evolutionary morphology of sperm in pholcid spiders (Pholcidae, Synspermiata)

Abstract

Background Pholcidae represent one of the largest and most diverse spider families and have been subject to various studies regarding behavior and reproductive biology. In contrast to the solid knowledge on phylogeny and general reproductive morphology, the primary male reproductive system is strongly understudied, as it has been addressed only for few species. Those studies however suggested a high diversity of sperm and seminal secretions across the family. To address this disparity and reconstruct the evolution of sperm traits, we investigate the primary male reproductive system of pholcid spiders by means of light, X-ray, and transmission electron microscopy using a comprehensive taxon sampling with 46 species from 33 genera, representing all five subfamilies. Results Our data show a high disparity of sperm morphology and seminal secretions within pholcids. We document several sperm characters that are unique for pholcids, such as a helical band (Pholcinae) or a lamellate posterior centriolar adjunct material (Modisiminae). Character mapping revealed several putative synapomorphies for individual taxa. With regard to sperm transfer forms, we found that synspermia occur only in the subfamily Ninetinae, whereas the other subfamilies have cleistospermia. In several species with cleistospermia, we demonstrate that spermatids remain fused until late stages of spermiogenesis before ultimately separating shortly before the coiling process. Additionally, we explored the previously hypothesized correlation between sperm size and minimum diameter of the spermophor in the male palpal organ. We show that synspermia differ strongly in size whereas cleistospermia are rather uniform, but neither transfer form is positively correlated with the diameter of the spermophor. Conclusions Our data revealed a dynamic evolution of sperm characters, with convergences across all subfamilies and a high level of homoplasy. The present diversity can be related to subfamily level and allows for assignments of specific subtypes of spermatozoa. Our observations support the idea that Ninetinae are an ancestral clade within Pholcidae that have retained synspermia and that synspermia represent the ancestral sperm transfer form of Pholcidae.

Short-legged daddy-long-leg spiders in North America: the genera Pholcophora and Tolteca (Araneae, Pholcidae)

“…Huber et al 2018). The monophyly of this northern clade suggests that cleistospermia have evolved at least twice in Pholcidae: once in the ancestor of Pholcophora and Tolteca (Dederichs et al 2022), and once in all Pholcidae except Ninetinae. The sperm transfer form of Papiamenta is unknown but our results generate the prediction that Papiamenta males also transfer cleistospermia.…”

“…The most comprehensive phylogeny of Pholcidae (Eberle et al 2018) recognized a "North and Central American and Caribbean" clade of Ninetinae, but the species sample was small (four species, of which two could not confidently be assigned to a genus) and lacked the genus Tolteca. A recent comparative study of pholcid sperm ultrastructure (Dederichs et al 2022) found that Pholcophora and Tolteca share cleistospermia while all other studied Ninetinae have synspermia; since synspermia are thought to be plesiomorphic for Pholcidae, this was a first indication that Pholcophora and Tolteca might be closely related.…”

Copulatory behaviour and genital mechanics suggest sperm allocation by a non-intromittent sclerite in a pholcid spider

“…Three of these have been subject to detailed studies regarding copulatory mechanics so far: Pholcinae [ 35 , 36 , 38 , 39 ], Smeringopinae [ 34 , 35 ] and Modisiminae [ 10 , 37 ], leaving Arteminae and Ninetinae understudied in this regard. Ninetines are particular as previous studies on the primary male reproductive system reveal the presence of synspermia (aggregation of fused spermatozoa; [ 41 , 42 ]) instead of cleistosperm as in other pholcids, and a remarkable variation in the female genital system [ 29 ], with, however, very little to no available data on their reproductive biology so far. Furthermore, Ninetinae has been suggested to occupy a ‘basal' position in the pholcid phylogeny [ 31 ], which makes them highly suitable for studies on the evolution of pholcid spiders.…”

Andean giants: Priscula spiders from Ecuador, with notes on species groups and egg-sac troglomorphism (Araneae: Pholcidae)

“…Molecular data later suggested instead an affinity either with Modisiminae (Bruvo-Mađarić et al 2005;Eberle et al 2018) or with Modisiminae + Arteminae (Eberle et al 2018). Recent data on sperm ultrastructure also found similarities between Priscula and certain Modisiminae (Dederichs et al 2022). Such an affinity makes sense geographically, as both Priscula and Modisiminae are restricted to the New World.…”