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Background

The common house spider Parasteatoda tepidariorum represents an emerging new model organism of arthropod evolutionary and developmental (EvoDevo) studies. Recent technical advances have resulted in the first single-cell sequencing (SCS) data on this species allowing deeper insights to be gained into its early development, but mid-to-late stage embryos were not included in these pioneering studies.

Results

Therefore, we performed SCS on mid-to-late stage embryos of Parasteatoda and characterized resulting cell clusters by means of in-silico analysis (comparison of key markers of each cluster with previously published information on these genes). In-silico prediction of the nature of each cluster was then tested/verified by means of additional in-situ hybridization experiments with additional markers of each cluster.

Conclusions

Our data show that SCS data reliably group cells with similar genetic fingerprints into more or less distinct clusters, and thus allows identification of developing cell types on a broader level, such as the distinction of ectodermal, mesodermal and endodermal cell lineages, as well as the identification of distinct developing tissues such as subtypes of nervous tissue cells, the developing heart, or the ventral sulcus (VS). In comparison with recent other SCS studies on the same species, our data represent later developmental stages, and thus provide insights into different stages of developing cell types and tissues such as differentiating neurons and the VS that are only present at these later stages.

Spiders evolved different types of eyes, a pair of primary eyes that are usually forward pointing, and three pairs of secondary eyes that are typically situated more posterior and lateral on the spider's head. The best understanding of arthropod eye development comes from the vinegar fly Drosophila melanogaster, the main arthropod model organism that also evolved different types of eyes, the larval eyes and the ocelli and facet eyes of the imago. The gene regulatory networks (GRNs) that underlay eye development in this species are well investigated revealing a conserved core network, but also show a number of differences between the different types of eyes. Recent candidate gene approaches identified a number of conserved genes in arthropod eye development, but also revealed crucial differences including the apparent lack of some key factors in some groups of arthropods, including spiders. Here we use single cell sequencing (SCS) to overcome the bias of candidate gene approaches with the aim to identifying new genes involved in spider eye development, and thus to gain deeper insight into spider eye development, and arthropod eye development in general. Our main finding is that Hedgehog (Hh)-signalling is involved in eye development in the spider Parasteatoda tepidariorum, and by investigating embryonic gene expression in representatives of all main groups of spiders, we demonstrate that this involvement is conserved in spiders as a whole. 

Background: In the vinegar fly Drosophila melanogaster, the homeodomain containing transcription factor Teashirt (Tsh) appears to specify trunk identity in concert with the function of the Hox genes. While in Drosophila there is a second gene closely related to tsh, called tiptop (tio), in other arthropods species only one copy exists (called tio/tsh). The expression of tsh and tio/tsh, respectively, is surprisingly similar among arthropods suggesting that its function as trunk selector gene may be conserved. Other research, for example on the beetle Tribolium castaneum, questions even conservation of Tsh function among insects. The zinc-finger transcription factor Spalt (Sal) is involved in the regulation of Drosophila tsh, but this regulatory interaction does not appear to be conserved in Tribolium either. Whether the function and interaction of tsh and sal as potential trunk-specifiers, however, is conserved is still unclear because comparative studies on sal expression (except for Tribolium) are lacking, and functional data are (if at all existing) restricted to Insecta.

Results: Here, we provide additional data on arthropod tsh expression, show the first data on onychophoran tio/tsh expression, and provide a comprehensive investigation on sal expression patterns in arthropods and an onychophoran.

Conclusions: Our data support the idea that tio/tsh genes are involved in the development of "trunk" segments by regulating limb development. Our data suggest further that the function of Sal is indeed unlikely to be conserved in trunk vs head development like in Drosophila, but early expression of sal is in line with a potential homeotic function, at least in Arthropoda.