Shared Enhancer Activity in the Limbs and Phallus and Functional Divergence of a Limb-Genital cis-Regulatory Element in Snakes

Abstract

Summary The amniote phallus and limbs differ dramatically in their morphologies but share patterns of signaling and gene expression in early development. Thus far, the extent to which genital and limb transcriptional networks also share cis-regulatory elements has remained unexplored. We show that many limb enhancers are retained in snake genomes, suggesting that these elements may function in non-limb tissues. Consistent with this, our analysis of cis-regulatory activity in mice and Anolis lizards reveals that patterns of enhancer activity in embryonic limbs and genitalia overlap heavily. In mice, deletion of HLEB, an enhancer of Tbx4, produces defects in hindlimbs and genitalia, establishing the importance of this limb-genital enhancer for development of these different appendages. Further analyses demonstrate the HLEB of snakes has lost hindlimb enhancer function while retaining genital activity. Our findings identify roles for Tbx4 in genital development and highlight deep similarities in cis-regulatory activity between limbs and genitalia.

When the Snake Lost Its Limbs, What Did the Mouse and Lizard Say?

“…In sum, Infante et al (2015) add to the growing evidence for dynamic regulatory activity in vertebrate evolution (Vierstra et al, 2014), where the frequency and mechanisms dictating repurposing of tissue-specific regulatory regions are an increasingly exciting area of investigation.…”

“…Many limb-specific enhancers have been identified in mammals (Cotney et al, 2013). Here, Infante et al (2015) use limbless snakes to show that the sequences of many of these enhancers can be retained, even when the anatomical structures where they are active have vanished. These results further highlight the plasticity of amniote gene-regulatory activities and demonstrate that shared regulatory architectures can contribute to the development of limbs and external genitalia.…”

“…Commonalities in the development of these anatomical structures include shared signaling pathways and gene expression similarities (Tschopp et al, 2014), suggesting an evolutionary link between the two appendages. In this issue of Developmental Cell, Infante et al (2015) provide compelling evidence for shared cis-regulatory activities between developing limbs and genitalia. Many limb-specific enhancers have been identified in mammals (Cotney et al, 2013).…”

“…In this issue of Developmental Cell, Infante et al (2015) compare regulatory DNA sequences in mice, lizards, and limbless snakes to reveal widespread sharing of enhancer activity in developing limbs and genitalia. Genetic deletion of a limb-genital enhancer demonstrates that common regulatory elements affect development of both appendages.…”

“…The analysis by Infante et al (2015) raises a number of questions for follow-up investigation. The interesting observations on the transgenic activity of snake HLB sequences suggest that limb enhancers can be repurposed in species Developmental Cell 35, October 12, 2015 ª2015 Elsevier Inc. 3 Developmental Cell that have evolved limb loss, and future work should analyze the activity of these sequences in snakes (or other limbless vertebrates) to determine how often this repurposing occurs in the GT as opposed to other tissues.…”