Abstract
Sea turtles are wild animals whose embryonic development largely depends on environmental factors. The shell of sea turtles is a novelty among vertebrates and follows a Hox code and axial formula for the group. The hoxb9 gene is a transcription factor involved in the regulation of embryonic development, mainly during neural tube development and plays an important role in shaping the thoracic-lumbar transition of sea turtles. Although it is known that Hox genes are regulated by chromatin modulating proteins, DNA methylation in CpG islands may also contribute to gene regulation. Previous studies have shown a high prevalence of congenital malformations in the olive ridley sea turtle (Lepidochelys olivacea), including Schistosomus Reflexus (SR) syndrome. Here we studied the methylation profiles in the 5’ flanking region of hoxb9 comparing normal and abnormal sea turtle embryos. Our results indicate that methylation of the putative promoter region of hoxb9 in sea turtle embryos occurred, however methylated cytosines showed individual patterns, not directly related to the development of congenital malformations.
Highlights
Homeotic genes act as master regulators for the specification of body plans in development, regulating other genes and programming certain developmental pathways [1]
Primers were designed according to the coding region of the hoxb9 gene (HOXB9-201 ENSPSIT00000007772.1) reported for the Chinese soft-shelled turtle (Pelodiscus sinensis) identified on ENSEMBL database, and the first exon in the draft genome of the green sea turtle (Chelonia mydas) identified in the National Center for Biotechnology Information (NCBI) database; the sequences of both species were compared and primers were designed to amplify a fragment of about 948 bp (Table 1); the reverse primer was designed within the coding region of the hoxb9 gene, whereas the forward primer was designed in the putative promoter region
There was a consistent pattern in which sites +128 and +136, located in the 5’ UTR region Figure 1 and 3 were methylated regardless of whether the embryos presented developmental defects, site +128 was methylated in all embryos analyzed, which could suggest an important role of locus-specific methylation in the 5’ UTR of the hoxb9 gene related to embryonic development, but not related to any specific malformation
Summary
Homeotic genes act as master regulators for the specification of body plans in development, regulating other genes and programming certain developmental pathways [1]. Turtles have the most unusual body plan among amniotes, with a dorsal shell that fuses with the ribs [8]; for this group, as well as for other tetrapods, including mammals, birds, and amphibians, a total of 39 Hox genes have been reported, divided into four paralog groups (A, B, C and D) [7]. According to Böhmer & Werneburg [9], the Hox code in turtles is characterized by changes in spatial expression in the vertebral column compared to the closest phylogenetic group, the archosaurs (which includes crocodiles and birds), to give rise to the appearance of the unique shell of the turtle [8,9]. Deletions in the chromosomal region encompassing the HOXB1-HOXB9 cluster are associated with developmental
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