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Evolutionary Morphology Laboratory July 9, Press Release The origin of the turtle shell: mystery solved April 29, Press Release Turtle genome analysis sheds light on turtle ancestry and shell evolution. Zoological Letters Editor-in-chief. Page in Japanese.

On basis of the correlation between genomic control and phenotypic changes noted above, the present study of morphological variation of the cervical vertebrae served, in general, as a Hox gene expression pattern proxy. Results In all turtles analyzed, the morphometric analysis permitted discrimination of vertebrae in four or ive diferent cervical regions. Vertebral morphology of turtles visualized using computer tomography. Cervical vertebra 7 of P. Vertebra 6 of this species has a doubled convex f and cervical 8 has a simple convex articular process g note the typical cryptodiran anatomy of the posterior zygapophyses in this last cervical vertebra.

A simple procoelous articular process is shown for vertebra 7 of Phrynops geofroanus h. For details on data sources see Werneburg et al. Morphological modularity of the neck in stem turtles.

Whereas, in the neck of both the Late Triassic turtle Proganochelys quenstedti and the Early Cretaceous turtle Naomichelys speciosa Fig. Morphological modularity of the neck in Pleurodira. Both pleurodiran turtles in the sample display a four-unit pattern in the cervical vertebral column Fig. Morphological modularity of the neck in Cryptodira.

Kinosternon scorpioides and Testudo hermanni are unique among the whole turtle sample because the morphology of CV2 is not as distinct from CV3. Both vertebrae form one unit in these taxa. Discussion Turtles display great morphological diversity within their cervical vertebral column8. Hox gene inventory. Overview of the Hox gene inventory of the mouse Mus musculus and of the turtle Pelodiscus sinensis information based on Liang et al.

Previous reports on the serial varia- tion in the type of central articulation between the cervical vertebrae already documented a broad morphological regionalization of the turtle neck8, 36— However, early turtles, such as Proganochelys quenstedti, do not show signs of clear morphological regionalization since all cervical vertebrae have amphicoelous biconcave central articulations Hox gene inventory in turtles.

And the eight Hox genes of the PG 6, 7, and 8 map to the dorsal trunk vertebral region marking the cervicodorsal transition In particular, HoxC-6 is expressed at the irst dorsal vertebra in amniotes with dif- ferent cervical count Although the complete Hox gene inventory is known for Pelodiscus sinensis32, we only know the spatial expression of few Hox genes yet HoxA-5, B-5, C-6, A-7, and C-8 Nevertheless, we predict that the equivalent number of Hox genes from PG 4 to 8 is also expressed in the vertebral column of turtles.

A comparative survey identiied only variation in the presence of the HoxC-3 gene among amniotes Whereas squamates represented by anole lizard, snake, gecko, and blind skink possess the HoxC-3 gene similar to osteoichthyian ishes42, 43 and amphibians, all other studied amniotes lack it Misexpression experiments have elucidated the direct role of Hox genes in determining proper vertebral mor- phology44, Hox gene expression is anteriorly distinct with mainly gradual posterior boundaries and negatively regulated by Hox genes posterior to them posterior prevalence Hox mutants usually show anomalies restricted to their anterior expression domain afecting the cervical vertebral or dorsal trunk vertebral phenotype.

Targeted disrup- tion of Hox 4 and Hox 5 genes results in transformations of the cervical vertebrae, and mutants of posterior Hox genes show anomalies in the anatomy of dorsal vertebrae47— Morphological modularity and vertebral Hox code in the neck. As in lizards, crocodiles, chickens and mice, the irst cervical vertebra of turtles develops from somites 5 and 6, the second cervical vertebra from somites 6 and , 14, 52, From anterior to posterior along the body axis of living archosaurs, the irst Hox gene that is expressed in the cervical vertebral column is HoxB, 13 Fig.

Its expression starts at CV2 in croco- diles, chicken11, 13, and mouse Morphological modularity in turtles. Result of the geometric morphometric analysis of a the Late Triassic turtle Proganochelys quenstedti and b the Pleistocene turtle Meiolania platyceps Supplementary Fig.

Plots of the irst two Relative Warp RW axes. In Naomichelys, cervical ribs are not preserved, but they are assumed because of the presence of diaphyses and parapophyses. Although the present study detected a similar irst unit comprising the axis, this morphological diferentiation is not mediated by HoxB-5 because, in turtles, the anterior expression limit of HoxB-5 is shited anteriorly by one vertebra14 Fig.

And the genetic pattern may further suggest that the morphology of CV2 is not as distinct from the posterior cervical vertebrae as in other amniotes because they appear to share the same Hox code in the turtle. However, this interpretation is limited since we lack information on the expression pattern of the Hox genes of PG 4 Fig. Yet, it is striking to note that the present analysis revealed a irst morphological unit comprising CV2 and CV3 in at least two cryptodiran species, Kinosternon scorpioides and Testudo hermanni Supplementary Fig.

Evolutionary Origin of the Turtle Shell

Further genetic analyses are required to provide direct evidence of the Hox code in a pleurodiran turtle in which, as a side note, the atlas is not as derived in its morphology when compared to cryptodiran turtles. Disruption experiments suggested that HoxB-5 is involved in specifying the position of the limbs along the primary body axis because HoxB-5 mutants revealed an anterior shit of the shoulder girdle It is striking to observe that the expression of HoxB-5 is shited anteriorly in turtles with their eight cervical vertebrae similar to the mouse, but in contrast to archosaurs with longer necks crocodilians: nine CV, chicken: 14 CV Fig.

In this regard, the developmental shit of the shoulder girdle in turtles is worth mentioning.

It moves anterior and is laterally covered by the anterior trunk ribs later on Evolutionary this results in the unique position of the turtle shoulder within the body wall whereas it is situated outside in all other amniotes57, From anterior to posterior along the body axis of living archosaurs, the subsequent Hox genes that are expressed in the cervical vertebral column are of PG —13 Fig.

In the mouse, the expression pattern of the Hox 4 genes is diferent from living archosaurs12, 59 Fig. In lizard and turtle, there is no information available to date. It correlates with the last morphological unit posterior module in the neck of archosaurs In the lizard, the expression of HoxC-5 is shited posteriorly and starts at the last cervical vertebra CV6 However, the present morphological analyses showed that all turtle species have a posterior morphological unit comprising solely the last cervical vertebra CV8 Fig.

How tortoises turn right-side up

In the mouse, the anterior expression limit of HoxA-5 is shited anteriorly to CV In the turtle, the expression of HoxA-5 starts in the anterior region of the cervical vertebral column at CV Fig. Neck ribs where reduced in crown turtles, which is likely associated with the requirement to enable a greater mobility between vertebrae and to pack the neck into the body wall during neck retraction3, 5—7, 9. Single mutation experiments in the mouse revealed that disruption of HoxA-5 results in homeotic transfor- mations of the axial skeleton conined between CV3 and DV2 and one of the most frequent morphological mod- iications is the emergence of a pair of ribs on CV, 48, In the bird, the expression of HoxA-5 corresponds with an additional morphological unit midposterior detected in the cervical vertebral column and was interpreted to be a consequence of the long neck in the chicken 14 CV However, this hypothesis remains untested until further genetic studies will be performed.

Compared to pleurodires, cryptodires show a vertical orientation of the retracted neck Fig. To rotate the whole neck into the body, a highly modiied anatomy of CV8 is necessary with highly elongated and curved posterior zygapophyses Fig. In order to enable even more angular change below the convex carapace and hence to further retract the neck into the shell, CV7 could show a particular variation in some taxa as the raw mobility between macerated vertebrae suggests3. Diferences in the degree and mode of cryptodiran retraction may explain the great variation found in the modular pattern of their neck vertebrae, including the one additional module in two of them.

Summary of the somitic Hox code in amniotes. Data for turtle a are based on Ohya et al. Anterior limits of expression are taken from determinations at relatively late stages. Posterior boundaries are not clearly deined.

Morphological Evolution of the Turtle Shell and its Mechanical Implications, Part 2: Theoretical

In turtles, the anterior expression limit of only HoxC-6 is known and it is at DV1 marking the cervicodorsal transition Considering the rib-promoting role of HoxA-6 it is likely that its expression is conined to the dorsal vertebral col- umn in turtles as seen in archosaurs. However, it has to be noted that cervical ribs develop in turtles as mesenchy- mal condensations and their chondriication is repressed later in development7, Interestingly, in mouse HoxB-6 mutants, the shoulder girdle is shited anteriorly51, but we can only speculate if the expression of HoxB-6 in turtles starts anteriorly in the neck as seen in lizards or posteriorly in the trunk as seen in archosaurs and mammals.

A closer look at the cellular expression of HoxC-6 has shown that it corresponds with the lack of ventral ribs in turtles In contrast to other amniotes, HoxC-6 is expressed in the sclerotome prevertebra and the dermis in the epaxial domain future carapacial dermis , but not in the somatopleure embryonic region in which the ventral ribs are missing in turtles It is assumed that the ventral ribs in turtles are likely to have been modiied at least partly into the plastron64, And there is evidence indicating that the plastron derivates from a late-emigrating population of trunk neural crest cells, which appears to be unique to turtles Although trunk neural crest cells are generally thought to lack skeletogenic potential in contrast to cranial neural crest cells , these late-emigrating cells additionally contribute to the sclerotome-derived vertebral and rib cartilages in turtles In general, the expression of the Hox 7 genes is restricted to the dorsal vertebral column in amniotes Fig.

Supporting Information

In the turtle, however, the anterior expression limit of HoxA-7 is in the neck at CV Although possi- bly turtle-speciic, the expression pattern of HoxA-7 does not appear to be associated with the unique turtle body plan according to Ohya et al. It is striking to note that mouse HoxA-7 mutants are healthy and display no abnormalities in the formation of the axial skeleton Nevertheless, in combination with a mutation in HoxB-7 double mutant the functional role of HoxA-7 in the patterning of the anterior dorsal vertebral region is evident In pleurodires, the extent of lateral neck lexion remarkably increased compared to stem turtles — particularly in CV As mentioned above, cryp- todires evolved highly modiied posterior cervical vertebrae to enable their highly derived retraction mode.

In the turtle, the expression of HoxC-8 starts at DV An originally crocodile-like Hox gene expression pattern in amniotes may have been modiied in birds and mammals, which have undergone signii- cant modiications in their axial skeleton Correspondingly, the expression of HoxC-8 may be associated with the specialized body plan of turtles In summary, in contrast to archosaurs, at least one trunk-related Hox gene - HoxA-7 - is involved in the forma- tion of the cervical vertebral column in turtles.

In this regard, the correlation between morphological modularity and Hox code in the neck of turtles difers from all other analyzed amniotes.

Galapagos Tortoises and Evolution | AMNH

Stem turtles, morphological modularity, and cervical ribs during turtle evolution. Functional analyses, however, revealed that its short cervical vertebrae71 permit limited degrees of movement And this may correspond to the relatively uniform modular pattern in the neck as indi- cated by the present analysis. Cervical ribs appear to be plesiomorphically present in most basal turtles In living turtles, cervical ribs are highly rudimentary and evident only in embryos7, Although the cervical ribs are not preserved, the presence of free ribs on the cervical vertebrae of the Early Cretaceous stem turtle N.

Numeric constraint in the cervical vertebral column in turtles. In general, the forelimb of tetrapods deviates from somitic cells of the cervical and dorsal thoracic vertebrae. An anterior or posterior transposition of the forelimb in relation to the axial skeleton can result in change of cervical count. However, this is postulated to be associated with adaptive costs and possibly afecting related anatomical structures Buchholtz et al.

It induces the fan- shape arrangement of the trunk ribs contributing to the development of the carapace. Asterisk indicates modiied vertebral morphology. An anterior or posterior transposition of the forelimb are thought to generate a deicient diaphragm, forelimb, or both Buchholtz et al. Numerical constraint in the neck of turtles. Mammals are highly constrained in the number of cervical vertebrae almost exclusively seven CV and the neck kinematics rely on interspeciic variation in vertebral mor- phology.

Despite signiicant morphofunctional diferences, the pattern of shape change within the neck appears to be consistent among diverse mammalian taxa22, 23, 25, 26, However, in contrast to mammals, in which the morphological regionalization within the neck appears to be similar across the species analyzed to date, turtles difer in the modular pattern of the cervical vertebral column. Correspondingly, the present study suggests that the underlying Hox code may show some variation.