Somite number and vertebrate evolution

Development ◽  
1998 ◽  
Vol 125 (2) ◽  
pp. 151-160 ◽  
Author(s):  
M.K. Richardson ◽  
S.P. Allen ◽  
G.M. Wright ◽  
A. Raynaud ◽  
J. Hanken
Keyword(s):  
Expression Patterns ◽  
Vertebrate Evolution ◽  
Pharyngeal Arches ◽  
Evolution And Development ◽  
Evolutionary Changes ◽  
Phylotypic Stage ◽  
Tailbud Stage ◽  
Hourglass Model ◽  
Somite Number ◽  
Developmental Hourglass

Variation in segment number is an important but neglected feature of vertebrate evolution. Some vertebrates have as few as six trunk vertebrae, while others have hundreds. We examine this phenomenon in relation to recent models of evolution and development. Surprisingly, differences in vertebral number are foreshadowed by different somite counts at the tailbud stage, thought to be a highly conserved (phylotypic) stage. Somite number therefore violates the ‘developmental hourglass’ model. We argue that this is because somitogenesis shows uncoupling or dissociation from the conserved positional field encoded by genes of the zootype. Several other systems show this kind of dissociation, including limbs and feathers. Bmp-7 expression patterns demonstrate dissociation in the chick pharyngeal arches. This makes it difficult to recognise a common stage of pharyngeal development or ‘pharyngula’ in all species. Rhombomere number is more stable during evolution than somite number, possibly because segmentation and positional specification in the hindbrain are relatively interdependent. Although developmental mechanisms are strongly conserved, dissociation allows at least some major evolutionary changes to be generated in phylotypic stages.

scholarly journals Bichir external gills arise via heterochronic shift that accelerates hyoid arch development

eLife ◽  
2019 ◽  
Vol 8 ◽  
Author(s):  
Jan Stundl ◽  
Anna Pospisilova ◽  
David Jandzik ◽  
Peter Fabian ◽  
Barbora Dobiasova ◽  
...  
Keyword(s):  
Pharyngeal Arch ◽  
Developmental Sequence ◽  
Free Living ◽  
Pharyngeal Arches ◽  
Hyoid Arch ◽  
Early Appearance ◽  
Evolutionary Changes ◽  
Insight Into ◽  
Living Group ◽  
Early Larvae

In most vertebrates, pharyngeal arches form in a stereotypic anterior-to-posterior progression. To gain insight into the mechanisms underlying evolutionary changes in pharyngeal arch development, here we investigate embryos and larvae of bichirs. Bichirs represent the earliest diverged living group of ray-finned fishes, and possess intriguing traits otherwise typical for lobe-finned fishes such as ventral paired lungs and larval external gills. In bichir embryos, we find that the anteroposterior way of formation of cranial segments is modified by the unique acceleration of the entire hyoid arch segment, with earlier and orchestrated development of the endodermal, mesodermal, and neural crest tissues. This major heterochronic shift in the anteroposterior developmental sequence enables early appearance of the external gills that represent key breathing organs of bichir free-living embryos and early larvae. Bichirs thus stay as unique models for understanding developmental mechanisms facilitating increased breathing capacity.

scholarly journals Selection against expression noise explains the origin of the hourglass pattern of Evo-Devo

2019 ◽  
Author(s):  
Jialin Liu ◽  
Michael Frochaux ◽  
Vincent Gardeux ◽  
Bart Deplancke ◽  
Marc Robinson-Rechavi
Keyword(s):  
Regulatory Elements ◽  
Mutational Robustness ◽  
Expression Variability ◽  
Expression Noise ◽  
Environmental Perturbations ◽  
Phylotypic Stage ◽  
Robustness To Noise ◽  
Selection For ◽  
Hourglass Model ◽  
Evo Devo

The evolution of embryological development has long been characterized by deep conservation. Both morphological and transcriptomic surveys have proposed a “hourglass” model of Evo-Devo1,2. A stage in mid-embryonic development, the phylotypic stage, is highly conserved among species within the same phylum3–7. However, the reason for this phylotypic stage is still elusive. Here we hypothesize that the phylotypic stage might be characterized by selection for robustness to noise and environmental perturbations. This could lead to mutational robustness, thus evolutionary conservation of expression and the hourglass pattern. To test this, we quantified expression variability of single embryo transcriptomes throughout fly Drosophila melanogaster embryogenesis. We found that indeed expression variability is lower at extended germband, the phylotypic stage. We explain this pattern by stronger histone modification mediated transcriptional noise control at this stage. In addition, we find evidence that histone modifications can also contribute to mutational robustness in regulatory elements. Thus, the robustness to noise does indeed contributes to robustness of gene expression to genetic variations, and to the conserved phylotypic stage.

scholarly journals 16p12.1 deletion orthologs are expressed in motile neural crest cells and are important for regulating craniofacial development in Xenopus laevis

2020 ◽  
Author(s):  
Micaela Lasser ◽  
Jessica Bolduc ◽  
Luke Murphy ◽  
Caroline O'Brien ◽  
Sangmook Lee ◽  
...  
Keyword(s):  
Xenopus Laevis ◽  
Neural Crest ◽  
Expression Patterns ◽  
Copy Number Variants ◽  
Neural Crest Cell ◽  
Underlying Mechanism ◽  
Craniofacial Development ◽  
Pharyngeal Arches ◽  
Individual Gene ◽  
Vertebrate Model

Copy number variants (CNVs) associated with neurodevelopmental disorders are characterized by extensive phenotypic heterogeneity. In particular, one CNV was identified in a subset of children clinically diagnosed with intellectual disabilities (ID) that results in a hemizygous deletion of multiple genes at chromosome 16p12.1. In addition to ID, individuals with this deletion display a variety of symptoms including microcephaly, seizures, cardiac defects, and growth retardation. Moreover, patients also manifest severe craniofacial abnormalities, such as micrognathia, cartilage malformation of the ears and nose, and facial asymmetries; however, the function of the genes within the 16p12.1 region have not been studied in the context of vertebrate craniofacial development. The craniofacial tissues affected in patients with this deletion all derive from the same embryonic precursor, the cranial neural crest, leading to the hypothesis that one or more of the 16p12.1 genes may be involved in regulating neural crest cell (NCC)-related processes. To examine this, we characterized the developmental role of the 16p12.1-affected gene orthologs, polr3e, mosmo, uqcrc2, and cdr2, during craniofacial morphogenesis in the vertebrate model system, Xenopus laevis. While the currently-known cellular functions of these genes are diverse, we find that they share similar expression patterns along the neural tube, pharyngeal arches, and later craniofacial structures. As these genes show co-expression in the pharyngeal arches where NCCs reside, we sought to elucidate the effect of individual gene depletion on craniofacial development and NCC migration. We find that reduction of several 16p12.1 genes significantly disrupts craniofacial and cartilage formation, pharyngeal arch migration, as well as NCC specification and motility. Thus, we have determined that some of these genes play an essential role during vertebrate craniofacial patterning by regulating specific processes during NCC development, which may be an underlying mechanism contributing to the craniofacial defects associated with the 16p12.1 deletion.

Functional equivalence of the transcription factors Pax2 and Pax5 in mouse development

Development ◽  
2000 ◽  
Vol 127 (17) ◽  
pp. 3703-3713 ◽  
Author(s):  
M. Bouchard ◽  
P. Pfeffer ◽  
M. Busslinger
Keyword(s):  
Transcription Factors ◽  
Expression Patterns ◽  
Brain Regions ◽  
Vertebrate Evolution ◽  
Developmental Expression ◽  
Temporal Expression ◽  
Mouse Development ◽  
Highly Sensitive ◽  
Organizing Center ◽  
Cerebellum Development

Pax2 and Pax5 arose by gene duplication at the onset of vertebrate evolution and have since diverged in their developmental expression patterns. They are expressed in different organs of the mouse embryo except for their coexpression at the midbrain-hindbrain boundary (MHB), which functions as an organizing center to control midbrain and cerebellum development. During MHB development, Pax2 expression is initiated prior to Pax5 transcription, and Pax2(−/−) embryos fail to generate the posterior midbrain and cerebellum, whereas Pax5(−/−) mice exhibit only minor patterning defects in the same brain regions. To investigate whether these contrasting phenotypes are caused by differences in the temporal expression or biochemical activity of these two transcription factors, we have generated a knock-in (ki) mouse, which expresses a Pax5 minigene under the control of the Pax2 locus. Midbrain and cerebellum development was entirely rescued in Pax2(5ki/5ki) embryos. Pax5 could furthermore completely substitute for the Pax2 function during morphogenesis of the inner ear and genital tracts, despite the fact that the Pax5 transcript of the Pax2(5ki)allele was expressed only at a fivefold lower level than the wild-type Pax2 mRNA. As a consequence, the Pax2(5ki)allele was able to rescue most but not all Pax2 mutant defects in the developing eye and kidney, both of which are known to be highly sensitive to Pax2 protein dosage. Together these data demonstrate that the transcription factors Pax2 and Pax5 have maintained equivalent biochemical functions since their divergence early in vertebrate evolution.

sucker encodes a zebrafish Endothelin-1 required for ventral pharyngeal arch development

Development ◽  
2000 ◽  
Vol 127 (17) ◽  
pp. 3815-3828 ◽  
Author(s):  
C.T. Miller ◽  
T.F. Schilling ◽  
K. Lee ◽  
J. Parker ◽  
C.B. Kimmel
Keyword(s):  
Neural Crest ◽  
Expression Patterns ◽  
Neural Crest Cells ◽  
Paraxial Mesoderm ◽  
Dorsoventral Patterning ◽  
Surface Ectoderm ◽  
Pharyngeal Arches ◽  
Endothelin 1 ◽  
Lower Jaw ◽  
Arch Neural

Mutation of sucker (suc) disrupts development of the lower jaw and other ventral cartilages in pharyngeal segments of the zebrafish head. Our sequencing, cosegregation and rescue results indicate that suc encodes an Endothelin-1 (Et-1). Like mouse and chick Et-1, suc/et-1 is expressed in a central core of arch paraxial mesoderm and in arch epithelia, both surface ectoderm and pharyngeal endoderm, but not in skeletogenic neural crest. Long before chondrogenesis, suc/et-1 mutant embryos have severe defects in ventral arch neural crest expression of dHAND, dlx2, msxE, gsc, dlx3 and EphA3 in the anterior arches. Dorsal expression patterns are unaffected. Later in development, suc/et-1 mutant embryos display defects in mesodermal and endodermal tissues of the pharynx. Ventral premyogenic condensations fail to express myoD, which correlates with a ventral muscle defect. Further, expression of shh in endoderm of the first pharyngeal pouch fails to extend as far laterally as in wild types. We use mosaic analyses to show that suc/et-1 functions nonautonomously in neural crest cells, and is thus required in the environment of postmigratory neural crest cells to specify ventral arch fates. Our mosaic analyses further show that suc/et-1 nonautonomously functions in mesendoderm for ventral arch muscle formation. Collectively our results support a model for dorsoventral patterning of the gnathostome pharyngeal arches in which Et-1 in the environment of the postmigratory cranial neural crest specifies the lower jaw and other ventral arch fates.

scholarly journals Gene Expression Does Not Support the Developmental Hourglass Model in Three Animals with Spiralian Development

2019 ◽  
Vol 36 (7) ◽  
pp. 1373-1383 ◽  
Author(s):  
Longjun Wu ◽  
Kailey E Ferger ◽  
J David Lambert
Keyword(s):  
Gene Expression ◽  
Large Fraction ◽  
Molecular Data ◽  
Development Stage ◽  
Data Sets ◽  
Rna Seq ◽  
Developmental Evolution ◽  
Phylotypic Stage ◽  
Hourglass Model ◽  
Almost All

Abstract It has been proposed that animals have a pattern of developmental evolution resembling an hourglass because the most conserved development stage—often called the phylotypic stage—is always in midembryonic development. Although the topic has been debated for decades, recent studies using molecular data such as RNA-seq gene expression data sets have largely supported the existence of periods of relative evolutionary conservation in middevelopment, consistent with the phylotypic stage and the hourglass concepts. However, so far this approach has only been applied to a limited number of taxa across the tree of life. Here, using established phylotranscriptomic approaches, we found a surprising reverse hourglass pattern in two molluscs and a polychaete annelid, representatives of the Spiralia, an understudied group that contains a large fraction of metazoan body plan diversity. These results suggest that spiralians have a divergent midembryonic stage, with more conserved early and late development, which is the inverse of the pattern seen in almost all other organisms where these phylotranscriptomic approaches have been reported. We discuss our findings in light of proposed reasons for the phylotypic stage and hourglass model in other systems.

scholarly journals The microtubule skeleton and the evolution of neuronal complexity in vertebrates

2019 ◽  
Vol 400 (9) ◽  
pp. 1163-1179 ◽  
Author(s):  
Nataliya I. Trushina ◽  
Armen Y. Mulkidjanian ◽  
Roland Brandt
Keyword(s):  
Alternative Splicing ◽  
Binding Proteins ◽  
Neuronal Development ◽  
Bioinformatics Analysis ◽  
Vertebrate Evolution ◽  
Evolution Of Complexity ◽  
Evolutionary Changes ◽  
The Individual ◽  
Clear Increase ◽  
Tubulin Structure

Abstract The evolution of a highly developed nervous system is mirrored by the ability of individual neurons to develop increased morphological complexity. As microtubules (MTs) are crucially involved in neuronal development, we tested the hypothesis that the evolution of complexity is driven by an increasing capacity of the MT system for regulated molecular interactions as it may be implemented by a higher number of molecular players and a greater ability of the individual molecules to interact. We performed bioinformatics analysis on different classes of components of the vertebrate neuronal MT cytoskeleton. We show that the number of orthologs of tubulin structure proteins, MT-binding proteins and tubulin-sequestering proteins expanded during vertebrate evolution. We observed that protein diversity of MT-binding and tubulin-sequestering proteins increased by alternative splicing. In addition, we found that regions of the MT-binding protein tau and MAP6 displayed a clear increase in disorder extent during evolution. The data provide evidence that vertebrate evolution is paralleled by gene expansions, changes in alternative splicing and evolution of coding sequences of components of the MT system. The results suggest that in particular evolutionary changes in tubulin-structure proteins, MT-binding proteins and tubulin-sequestering proteins were prominent drivers for the development of increased neuronal complexity.

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