Toll Genes Have an Ancestral Role in Axis Elongation

Abstract

One of the key morphogenetic processes used during development is the controlled intercalation of cells between their neighbors. This process has been co-opted into a range of developmental events, and it also underlies an event that occurs in each major group of bilaterians: elongation of the embryo along the anterior-posterior axis [1]. In Drosophila, a novel component of this process was recently discovered by Paré et al., who showed that three Toll genes function together to drive cell intercalation during germband extension [2]. This finding raises the question of whether this role of Toll genes is an evolutionary novelty of flies or a general mechanism of embryonic morphogenesis. Here we show that the Toll gene function in axis elongation is, in fact, widely conserved among arthropods. First, we functionally demonstrate that two Toll genes are required for cell intercalation in the beetle Tribolium castaneum. We then show that these genes belong to a previously undescribed Toll subfamily and that members of this subfamily exhibit striped expression (as seen in Tribolium and previously reported in Drosophila [3-5]) in embryos of six other arthropod species spanning the entire phylum. Last, we show that two of these Toll genes are required for normal morphogenesis during anterior-posterior embryo elongation in the spider Parasteatoda tepidariorum, a member of the most basally branching arthropod lineage. From our findings, we hypothesize that Toll genes had a morphogenetic function in embryo elongation in the last common ancestor of all arthropods, which existed over 550 million years ago.

Embryonic expression of a Long Toll (Loto) gene in the onychophorans Euperipatoides kanangrensis and Cephalofovea clandestina

“…Onychophora comprises the likely sister group (or at least a very close outgroup) to Arthropoda (Campbell et al 2011). This current study is thus the continuation of the work started by Paré et al (2014) and Benton et al (2016) to investigate the evolutionary origin of the role that Toll/Loto genes play in CE. It also addresses the question of whether CE is involved in germ band formation in onychophorans.…”

“…Asterisks mark genes with incomplete sequence information. Note that Loto and Toll genes of Drosophila and Tribolium (based on the analysis in Benton et al (2016)) have been used to calculate the tree, except for Parasteatoda and Strigamia for which only confirmed Loto genes were used to construct the tree. See text for further information.…”

“…In their study, at least one Toll gene has been identified that has a likely (based on conserved gene expression) or proven (by means of RNAi interference experiments) function in CE in each investigated species (Benton et al 2016). The Benton et al (2016) study shows that Toll genes that are likely involved in CE are “longer” and possess a higher number of LRRs than Toll genes that fall outside this group. Therefore, and because of the likely conserved role in CE, these Toll genes were named “Long Toll” genes (abbreviated as “Loto” genes) (Benton et al 2016).…”

“…The Benton et al (2016) study shows that Toll genes that are likely involved in CE are “longer” and possess a higher number of LRRs than Toll genes that fall outside this group. Therefore, and because of the likely conserved role in CE, these Toll genes were named “Long Toll” genes (abbreviated as “Loto” genes) (Benton et al 2016). …”

“…We also investigated Loto class genes in a myriapod, the millipede Glomeris marginata , representing the second main branch of Myriapoda (the study by Benton et al (2016) investigated Loto gene expression in the centipede Strigamia maritima ).…”