The evolution of selector gene function: Expression dynamics and regulatory interactions of tiptop/teashirt across Arthropoda

Abstract

The transcription factors spineless (ss) and tiptop/teashirt (tio/tsh) have been shown to be selectors of distal appendage identity in an insect, but it is unknown how they regulate one another. Here, we examined the regulatory relationships between these two determinants in the milkweed bug Oncopeltus faciatus, using maternal RNA interference (RNAi). We show that Ofas-ss RNAi embryos bear distally transformed antennal buds with heterogeneous Ofas-tio/tsh expression domains comparable to wild type legs. In the reciprocal experiment, Ofas-tio/tsh RNAi embryos bear distally transformed walking limb buds with ectopic expression of Ofas-ss in the distal leg primordia. These data suggest that Ofas-ss is required for the maintenance of Ofas-tio/tsh expression in the distal antenna, whereas Ofas-tio/tsh represses Ofas-ss in the leg primordia. To assess whether expression boundaries of tio/tsh are associated with the trunk region more generally, we surveyed the expression of one myriapod and two chelicerate tio/tsh homologs. Our expression survey suggests that tio/tsh could play a role in specifying distal appendage identity across Arthropoda, but Hox regulation of tio/tsh homologs has been evolutionarily labile.

Panarthropod tiptop/teashirt and spalt orthologs and their potential role as “trunk”-selector genes

“…In all hitherto investigated panarthropods, there is only one tiptop ( tio )/ teashirt ( tsh )-type gene, except for Drosophila that possesses two paralogs, tsh and tio [ 24 , 41 , 45 , 55 , 68 ]. Tio- and Tsh-type proteins can be identified by the presence of a unique set of five zinc fingers (ZFs) (Fig.…”

“…2 E–G). Expression of the single tio/tsh gene of the spider Parasteatoda has partially been described by March et al [ 45 ]. However, in their work they only describe late developmental stages, and some aspects of expression at these stages have not been recognized or described.…”

“…Together, they repress head-identity in the trunk [ 2 , 16 , 61 , 62 , 71 ]. One conserved function of tio/tsh genes appears to be the discrimination between highly modified anterior “head” segments, and more posterior (and less modified) “trunk” segments, as recently highlighted by March et al [ 45 ].…”

“…We believe however, that the study of March et al [ 45 ] suffers from some technical limitations that may have led the authors to somewhat misinterpret and over-simplify their data. The centipede expression data provided in March et al [ 45 ] are rather unclear, but the statement that tio/tsh is in all limb buds except for the mandibles, is not supported by our millipede data. The statement that expression of tio/tsh in insects is restricted to the thoracic segments is an over-simplification.…”

“…In all other previously investigated arthropods, however, there is only one ortholog of tio and tsh , called tiptop/teashirt ( tio/tsh ) (e.g. [ 45 ]). Investigation of the tio/tsh gene expression in other arthropods than Drosophila revealed widely conserved expression suggesting that the overall function of tio/tsh as a trunk-regulator gene may be conserved in arthropods as a whole [ 24 , 45 , 55 , 68 ].…”