SoxE gene duplication and development of the lamprey branchial skeleton: Insights into development and evolution of the neural crest

Abstract

SoxE genes are multifunctional transcriptional regulators that play key roles in specification and differentiation of neural crest. Three members (Sox8, Sox9, Sox10) are expressed in the neural crest and are thought to modulate the expression and activity of each other. In addition to regulating the expression of other early neural crest marker genes, SoxE genes are required for development of cartilage. Here we investigated the role of SoxE genes in development of the neural crest-derived branchial skeleton in the sea lamprey. Using a morpholino knockdown approach, we show that all three SoxE genes described in lamprey are required for branchial basket development. Our results suggest that SoxE1 and SoxE2 are required for specification of the chondrogenic neural crest. SoxE3 plays a morphogenetic role in patterning of the branchial basket and may be required for the development of mucocartilage, a tissue unique to larval lampreys. While the lamprey branchial basket develops primarily from an elastin-like major extracellular matrix protein that is specific to lampreys, fibrillar collagen is also expressed in developing branchial cartilage and may be regulated by the lamprey SoxE genes. Our data suggest that the regulation of Type II collagen by Sox9 might have been co-opted by the neural crest in development of the branchial skeleton following the divergence of agnathan and gnathostome vertebrates. Finally, our results also have implications for understanding the independent evolution of duplicated SoxE genes among agnathan and gnathostome vertebrates.

Roles for FGF in lamprey pharyngeal pouch formation and skeletogenesis highlight ancestral functions in the vertebrate head

“…Whereas lamprey does not express Runx or Barx in NCC-derived prechondrocytes (Cattell et al, 2011), the lamprey Sox9 homolog SoxE1 is strongly expressed in pharyngeal NCCs and is required for their development into cellular cartilage Lakiza et al, 2011;McCauley and Bronner-Fraser, 2006). Consistent with a role for FGFs in specifying chondrogenic fate, we observed coexpression of SoxE1 and FGFRs in lamprey pharyngeal NCCs starting shortly after their migration into the pharynx.…”

“…Furthermore, mucocartilage is apparently lost during metamorphosis and replaced with typical cellular cartilage in adult lampreys (Armstrong et al, 1987). These differences have led to speculation that mucocartilage might represent a form of connective tissue that is unrelated to cellular cartilage, that it is a larval cartilage type unique to lamprey, or is an evolutionary precursor to cellular cartilage (Armstrong et al, 1987;Johnels, 1948;Wright and Youson, 1982 (Lakiza et al, 2011;Neidert et al, 2001;Sauka-Spengler et al, 2007). The present study extends these results, showing that mucocartilage expresses FGFRs and requires some FGF signaling to fully develop, although it is less sensitive to FGF inhibition than cellular cartilage.…”

“…In contrast to the rigid, jointed cartilage elements of gnathostomes, the pharyngeal component of the lamprey head skeleton consists of thin rods of cellular cartilage that secrete an Roles for FGF in lamprey pharyngeal pouch formation and skeletogenesis highlight ancestral functions in the vertebrate head extracellular matrix (ECM) made mainly of elastins and elastin-like proteins, with little fibrillar collagen, the main protein component of gnathostome cellular cartilage ECM (Martin et al, 2009;McBurney and Wright, 1996;McCauley, 2008;Robson et al, 1993;Yao et al, 2011). At the genetic level, although the lamprey head skeleton requires SoxE function to develop (Lakiza et al, 2011;McCauley and Bronner-Fraser, 2006), lamprey cellular cartilage does not express Runx or Barx, which are essential for gnathostome chondrogenesis (Cattell et al, 2011). Finally, while the lamprey head skeleton includes cellular cartilage in the pharynx, the oral region and ventral pharynx is supported by a soft mesenchymal skeletal tissue called mucocartilage (Johnels, 1948).…”

CRISPR/Cas9-mediated mutagenesis in the sea lamprey, Petromyzon marinus: a powerful tool for understanding ancestral gene functions in vertebrates

“…This difficulty is pronounced in lampreys given their relatively long developmental time compared with other model vertebrates (∼4 days until gastrulation, ∼6 days until neural crest migration). Furthermore, the effectiveness of different morpholinos can vary dramatically in lamprey, with a large proportion producing no effect or early embryonic death (Lakiza et al, 2011;Nikitina et al, 2011;Sauka-Spengler et al, 2007) …”

Cartilage of Marine Vertebrates

“…Meanwhile, "the subchordal, parachordal, and trabecular cartilages form as aggregate condensations of polygonal cells. The subchordal and parachordal cartilage condensations form anchor points that tether the skeletal rods to the notochord," (Lakiza et al 2011 ). This rod-like midline structure is of mesodermal origin and serves as a primitive axial skeleton.…”