Divergent palate morphology in turtles and birds correlates with differences in proliferation and BMP2 expression during embryonic development

Abstract

During embryonic development, amniotes typically form outgrowths from the medial sides of the maxillary prominences called palatal shelves or palatine processes. In mammals the shelves fuse in the midline and form a bony hard palate that completely separates the nasal and oral cavities. In birds and lizards, palatine processes develop but remain unfused, leaving a natural cleft. Adult turtles do not possess palatine processes and unlike other amniotes, the internal nares open into the oral cavity. Here we investigate craniofacial ontogeny in the turtle, Emydura subglobosa to determine whether vestigial palatine processes develop and subsequently regress, or whether development fails entirely. We found that the primary palate in turtles develops similarly to other amniotes, but secondary palate ontogeny diverges. Using histology, cellular dynamics and in situ hybridization we found no evidence of palatine process development at any time during ontogeny of the face in the turtle. Furthermore, detailed comparisons with chicken embryos (the model organism most closely related to turtles from a molecular phylogeny perspective), we identified differences in proliferation and gene expression patterns that correlate with the differences in palate morphology. We propose that, in turtles, palatine process outgrowth is never initiated due to a lack of mesenchymal bone morphogenetic protein 2 (BMP2) expression in the maxillary mesenchyme, which in turn fails to induce the relatively higher cellular proliferation required for medial tissue outgrowth. It is likely that these differences between turtles and birds arose after the divergence of the lineage leading to modern turtles.

Recent insights into the morphological diversity in the amniote primary and secondary palates

“…Proliferation drops at the base of the nasal pit while relatively higher proliferation is maintained in the surrounding facial prominences (Minkoff and Kuntz, 1977; Abramyan et al, 2015). The maxillary prominences also become enlarged during this period through differential proliferation relative to the head mesenchyme, as shown in chicken (Minkoff and Kuntz, 1978; Bailey et al, 1988; Abramyan et al, 2014) and turtle embryos (Abramyan et al, 2014). We have previously also shown this pattern of cellular proliferation in the bearded dragon lizard ( Pogona vitticeps ) (Abramyan et al, 2015), indicating that these processes are likely similar across all amniotes.…”

“…After diversification, adaptations such as teeth and beaks evolved in order to facilitate more efficient and specialized feeding mechanisms (Davit-Beal et al, 2009). The diversity in adult morphology initiates during embryonic development, where subtle, interspecific differences in molecular signaling lead to tissue restructuring in a lineage-specific manner (Abzhanov et al, 2004; Tokita et al, 2013; Abramyan et al, 2014; Bhullar et al, 2015; Lainoff et al, 2015). …”

“…The posterior-most portion of the secondary palate in both mammals and crocodilians is comprised of a muscular structure called the soft palate in mammals (Bush and Jiang, 2012) or the palatal valve (alternatively called the gular valve or basihyal valve) (Putterill and Soley, 2006; Jankowski, 2013; Abramyan et al, 2014) in crocodilians. These structures are utilized for separating the oral or nasal cavities from the nasopharynx during processes such as breathing, swallowing or vocalization (Putterill and Soley, 2006; Lane and Kaartinen, 2014; Kummer et al, 2015).…”

Avian Facial Morphogenesis Is Regulated by c-Jun N-terminal Kinase/Planar Cell Polarity (JNK/PCP) Wingless-related (WNT) Signaling

“…Cell proliferation is normally repressed by WNT11. Indeed, there is a good correlation between the area of high WNT11 expression under the eye (35) with the relatively lower proliferation index (78). The function of WNT11 may be to create the gradient of proliferation in the maxillar prominence that is necessary to promote contact and fusion with the frontonasal mass.…”

Common mechanisms in development and disease: BMP signaling in craniofacial development

“…Bmp2 appears to be the primarily responsible factor regulating cell proliferation [96]. Differences in Bmp2 expression correlate with differences in proliferation also in turtles and birds [97]. Deletion of Bmpr1a in the craniofacial primordium using Nestin-Cre results in cleft palate [30], whereas a neural crest-specific deletion using Wnt1-Cre causes anterior clefting only [98].…”