<b><i>Hox</i></b> Genes in Reptile Development, Epigenetic Regulation, and Teratogenesis

Abstract

Reptiles are ancestral organisms presenting a variety of shapes, from the elongated vertebral column of the snake to the turtle dorsalized ribs or retractile neck. Body plans are specified by a conserved group of homeobox-containing genes (Hox genes), which encode transcription factors important in cell fate and vertebral architecture along the anteroposterior axis during embryonic development; thus, dysregulation of these genes may cause congenital malformations, from mild-sublethal to embryonic-lethal. The genetic pool, maternal transfer, and environmental conditions during egg incubation affect development; environmental factors such as temperature, moisture, oxygen, and pollution may alter gene expression by epigenetic mechanisms. Thus, in this review, we present information regarding Hox genes and development in reptiles, including sex determination and teratogenesis. We also present some evidence of epigenetic regulation of Hox genes and the role of the environment in epigenetic modulation of gene expression. So far, the evidence suggests that the molecular instructions encoded by Hox genes to build a snake, a lizard, or a turtle represent the interplay between genome and epigenome after years of evolution, with occasional environmentally induced molecular mistakes leading to abnormal body shapes.

Embryonic Temperature Programs Phenotype in Reptiles

“…Epigenetic mechanisms have been shown to play a role in environmental regulation of several traits in animals. In recent years, attempts have been made to survey epigenetic mechanisms to understand how temperature programs phenotype including studies of DNA methylation and histone modifications in several reptiles with TSD (Navarro-Martin et al, 2011;Matsumoto et al, 2013Matsumoto et al, , 2016Czerwinski et al, 2016;Yatsu et al, 2016;Fan et al, 2017;Radhakrishnan et al, 2017Radhakrishnan et al, , 2018Ge et al, 2018;Martín-del-Campo et al, 2019).…”

RNA-sequence reveals differentially expressed genes affecting the crested trait of Wumeng crested chicken

“…Homeobox genes are clusters of highly conserved gene sequences in the Hox gene family ( Robert et al., 1989 ). Hox genes can regulate the normal development of individual morphology during the development of animal embryos ( Morgan, 1997 ), including effects on the morphology of head ( Krumlauf, 2016 ), body ( Morin-Kensicki et al., 2002 ), and vertebrae ( Martín-Del-Campo et al., 2019 ) among other organs. The Hox gene family consists of 39 genes divided into 4 clusters ( HOXA, HOXB, HOXC , and HOXD ) ( Cillo et al., 2001 ) located on 4 different chromosomes; the HOXC6 and HOXC10 genes are members of the HOXC cluster.…”

Congenital Malformations in Sea Turtles: Puzzling Interplay between Genes and Environment

“…Congenital malformations originate from multifactorial aetiologies, involving a complex interplay between genetic, epigenetic, and environmental factors influencing develop-mental programmes, affecting transcriptional networks and protein function and resulting in aberrant phenotypes [122]. Depending on the time and duration of exposure, even subtle genetic or epigenetic modifications may promote dramatic alterations of developmental processes, resulting in physical and/or physiological malformations, usually incompatible with life [15,122].…”

“…The teratogenic effect of extreme temperatures depends on timing, intensity, duration, and actual temperature involved [123]. The incidence of congenital malformations depends on the species and the embryological stage at the time of exposure [121,122]. In most mammalian species, hyperthermia is reached between 1.5 and 2.5 • C above the normal body temperature; in fact, 1.5 • C above normal temperature in the pre-implantation embryo may result in a high mortality rate, whereas after implantation it may cause congenital malformations [126].…”