Hmx gene conservation identifies the origin of vertebrate cranial ganglia

Abstract

The evolutionary origin of vertebrates included innovations in sensory processing associated with the acquisition of a predatory lifestyle 1 . Vertebrates perceive external stimuli through sensory systems serviced by cranial sensory ganglia (CSG) which develop from cranial placodes; however understanding the evolutionary origin of placodes and CSGs is hampered by the gulf between living lineages and difficulty in assigning homology between cell types and structures. Here we use the Hmx gene family to address this question. We show Hmx is a constitutive component of vertebrate CSG development and that Hmx in the tunicate Ciona is able to drive the differentiation program of Bipolar Tail Neurons (BTNs), cells previously thought neural crest homologs 2,3 . Using Ciona and lamprey transgenesis we demonstrate that a unique, tandemly duplicated enhancer pair regulated Hmx in the stem-vertebrate lineage. Strikingly, we also show robust vertebrate Hmx enhancer function in Ciona, demonstrating that deep conservation of the upstream regulatory network spans the evolutionary origin of vertebrates. These experiments demonstrate regulatory and functional conservation between Ciona and vertebrate Hmx, and confirm BTNs as CSG homologs. Our analysis also identifies derived evolutionary changes, including a genetic basis for secondary simplicity in Ciona and unique regulatory complexity in vertebrates.CSG, including the trigeminal, vestibuloacoustic and epibranchial ganglia, relay information from sensory cells to the brain. CSG neurons derive from two sources: cranial placodes provide neurons that delaminate from the cranial ectoderm, and cranial neural crest cells migrate into ganglia providing all of the glia and some neurons of the trigeminal ganglia. The evolution of neural crest, placodes and CSG form part of the influential 'New Head Hypothesis' which posits that these and other innovations underlie the transformation of an ancestral chordate filter feeder into the ancestral vertebrate-type predator 1 . Our molecular and genetic understanding of this transformation has been limited, however, by the substantial anatomical gulf between vertebrates and their nearest living relatives, amphioxus and tunicates, which lack most or all of these characters (Fig. 1A) 4 .To address this gap we focused on the Hmx gene family, which encodes homeodomain transcription factors. We previously used transcriptomics to find markers for placode-derived CSG neurons, identifying Hmx3 as one such gene 5 . Jawed vertebrates have 4 Hmx family genes named Hmx1, Hmx2, Hmx3 and SOHo 6 , with expression in mouse, chicken, Xenopus

Specification of distinct cell types in a sensory-adhesive organ for metamorphosis in theCionalarva

“…Our understanding of the evolutionary origins of structures long presented as vertebrate novelties has benefited from an increasing number of comparative studies with tunicates. Several discrete populations of peripheral sensory cells originating from distinct ectodermal regions in tunicates have respectively been linked to neural crest and cranial placodes, among them the sensory adhesive papillae (Abitua et al, 2015;Abitua et al, 2012;Horie et al, 2018;Papadogiannis et al, 2022).…”

Stress Granule-Related Genes during Embryogenesis of an Invertebrate Chordate

“…C. robusta is an invertebrate chordate belonging to the subphylum Tunicata, widely used in embryogenetic studies since it allows to obtain hundreds of embryos, after a single artificial fertilization event, which develop up to the larval stage within a day, ready to adhere to a solid substrate and metamorphose into adults (Passamaneck and Di Gregorio, 2005; Hotta et al, 2007; Hotta et al, 2020). In addition, Ciona embryos can be easily electroporated with specific constructs for mutagenic experiments (Stolfi et al, 2014; Fujiwara and Cañestro, 2018; Zeller, 2018; Papadogiannis et al, 2022). Ciona represents an excellent model organism also for its annotated compact genome, easily accessible in the ANISEED database, with relatively short intergenic regions which facilitate the identification of cis -regulatory elements for the control of gene expression (Dardaillon et al, 2020).…”

Multiple Forms of Neural Cell Death in the Cyclical Brain Degeneration of A Colonial Chordate

“…Tunicates manifest apoptosis, necrosis and autophagy throughout their lifespan [ 9 , 10 , 11 , 12 , 13 , 14 , 15 ], which includes a mobile larva phase. It exhibits most of the chordate features that tunicates share with vertebrates: a notochord in the tail, a dorsal neural tube giving rise to a tripartite brain [ 16 ], a ventral endoderm and a bilateral striated musculature. These traits are lost during metamorphosis, a process during which the tail is completely resorbed through apoptosis [ 17 , 18 , 19 , 20 ].…”