The roles of growth regulation and appendage patterning genes in the morphogenesis of treehopper pronota

Abstract

Treehoppers of the insect family Membracidae have evolved enlarged and elaborate pronotal structures, which is hypothesized to involve co-opted expression of genes that are shared with the wings. Here, we investigate the similarity between the pronotum and wings in relation to growth. Our study reveals that the ontogenetic allometry of the pronotum is similar to that of wings in Membracidae, but not the outgroup. Using transcriptomics, we identify genes related to translation and protein synthesis, which are mutually upregulated. These genes are implicated in the eIF2, eIF4/p70S6K and mTOR pathways, and have known roles in regulating cell growth and proliferation. We find that species-specific differential growth patterning of the pronotum begins as early as the third instar, which suggests that expression of appendage patterning genes occurs long before the metamorphic molt. We propose that a network related to growth and size determination is the more likely mechanism shared with wings. However, regulators upstream of the shared genes in pronotum and wings need to be elucidated to substantiate whether co-option has occurred. Finally, we believe it will be helpful to distinguish the mechanisms leading to pronotal size from those regulating pronotal shape as we make sense of this spectacular evolutionary innovation.

The hierarchical basis of serial homology and evolutionary novelty

“…These hypotheses illustrate how simple pairwise comparisons of structures will often be inadequate to resolve serial homology relationships or determine whether the concept of serial homology is applicable. For example, to determine whether treehopper helmets are novel or serially homologous to wings (Kudla et al, 2022;Prud'homme et al, 2011), these structures must be compared at the appropriate organizational level and, if they are homologous, assessed for whether their homology is embedded in an evolutionary character tree or paramorph hierarchy. It is possible that helmets are serially homologous to wings because they belong to an exite-derived character family, or the character family of some other basal F I G U R E 4 A basal wingless insect from the suborder Monura (order Archaeognatha), illustrating proposed source tissues for wing origins.…”

Ontogenetic trajectories and early shape differentiation of treehopper pronota (Hemiptera: Membracidae)

“…This focus stems from the dramatic change in size that occurs between these two stages when the pronotum takes on the adult morphology. We found, however, that shapes characteristics of the adult form are already visible in earlier instars (Figure 2) (Kudla et al, 2022). This indicates that patterning of many aspects of adult pronotal morphology develops gradually and progressively during the nymphal instars.…”

“…The Hippo pathway is linked to the Insulin/mTOR signaling by way of the transcription factor Yorkie (McKenna et al, 2019;Straßburger et al, 2012). In our previous work we showed that the fifth instar pronotum overexpressed many genes related to protein synthesis, cell growth, and cell proliferation linked to the eIF2 pathway and the mTOR and eIF4/ p70S6K pathways (Kudla et al, 2022), The spatial positioning of signaling molecules in the anterior region could allow for greater cell proliferation along the anterior median carina than anywhere else, leading to a raised bump in the subsequent nymphal stage that eventually becomes an anterior horn. Each nymphal instar represents a distinct developmental timepoint with quantifiable changes in shape, and the mechanisms of species-specific spatial patterning of differential growth could be uncovered by comparative molecular studies of early instars.…”

“…Wings and genitalia, by contrast, are functional only in the adult but develop gradually during the nymphal stages (Truman & Riddiford, 1999). The unusual pronotum, likewise, acquires its elaborate adult shape gradually during nymphal development (Kudla et al, 2022). Almost all the studies on membracid pronotal development have concentrated on the fifth (last) nymphal instar because it is the last stage that leads to the metamorphic transition to the adult (Adachi et al, 2020; Fisher et al, 2020; Stegmann, 1998).…”