Sequence heterochrony led to a gain of functionality in an immature stage of the central complex: A fly–beetle insight

Abstract

Animal behavior is guided by the brain. Therefore, adaptations of brain structure and function are essential for animal survival, and each species differs in such adaptations. The brain of one individual may even differ between life stages, for instance, as adaptation to the divergent needs of larval and adult life of holometabolous insects. All such differences emerge during development, but the cellular mechanisms behind the diversification of brains between taxa and life stages remain enigmatic. In this study, we investigated holometabolous insects in which larvae differ dramatically from the adult in both behavior and morphology. As a consequence, the central complex, mainly responsible for spatial orientation, is conserved between species at the adult stage but differs between larvae and adults of one species as well as between larvae of different taxa. We used genome editing and established transgenic lines to visualize cells expressing the conserved transcription factor retinal homeobox, thereby marking homologous genetic neural lineages in both the fly Drosophila melanogaster and the beetle Tribolium castaneum. This approach allowed us for the first time to compare the development of homologous neural cells between taxa from embryo to the adult. We found complex heterochronic changes including shifts of developmental events between embryonic and pupal stages. Further, we provide, to our knowledge, the first example of sequence heterochrony in brain development, where certain developmental steps changed their position within the ontogenetic progression. We show that through this sequence heterochrony, an immature developmental stage of the central complex gains functionality in Tribolium larvae.

Shaking hands is a putative terminal selector and controls axon outgrowth of central complex neurons in the insect modelTribolium

“…We observed a similar arrangement in the Tribolium embryonic brain (Figure S 3 A-A’’). Recently, we have shown that a subset of embryonically-born columnar neurons express the Retinal homeobox protein (Rx) (Farnworth et al, 2019). Double-staining with anti-GFP and anti-Rx revealed a partial overlap of Rx- and G10011-GFP expressing neurons (Figure S 3 B-B’’’).…”

Shaking hands is a homeodomain transcription factor that controls axon outgrowth of central complex neurons in the insect model Tribolium

“…We asked whether these cells are of embryonic origin and establish the FB primordium (prFB) of the embryonic brain. The prFB is formed by four contra-laterally projecting fiber tracts which emanate from each brain hemisphere and constitute a part of the early commissural system (Farnworth et al, 2020) (schematic illustration in Fig. 3E'').…”

“…At late stage NS15 these fibers show the characteristic pattern of defasciculation which initiates the development of the columnar architecture of the FB (Fig. 3G', schematic illustration: G'' and (Farnworth et al, 2020). In Drosophila it has been shown that the contralaterally projecting fibers which constitute the prFB pass through a channel formed by glial membranes (Andrade et al, 2019).…”

“…Although the overall architecture of the CX is well conserved across different insect species, the size and shape of its neuropils vary, reflecting evolutionary adaptations ( Loesel et al, 2002 ; Strausfeld, 1999 ; Immonen et al, 2017 ). We study the regulatory mechanisms that underlie CX development in the red flour beetle Tribolium castaneum ( He et al, 2019 ; Farnworth et al, 2020 ). Tribolium is an insect model well suited to the study of gene regulatory pathways: its genome is fully sequenced ( Herndon et al, 2020 ) and Tribolium is amenable to genetic manipulation, including enhancer trapping ( Trauner et al, 2009 ).…”

Large portion of essential genes is missed by screening either fly or beetle indicating unexpected diversity of insect gene function

“…16,500). Further, a number of developmental processes are represented in a more insect-typical way in T. castaneum like for instance segmentation (Tautz et al, 1994), head (Posnien et al, 2010) and leg development, brain development (Farnworth et al, 2019), extraembryonic tissue movements (Panfilio, 2008) and mode of metamorphosis (Snodgrass, 1954). In most cases, the situation in the fly is simplified and streamlined for faster development.…”