Retinoic Acid Is Involved in the Metamorphosis of the Anal Fin into an Intromittent Organ, the Gonopodium, in the Green Swordtail (Xiphophorus hellerii)

Abstract

In poeciliid fish the male anal fin has been transformed into a gonopodium, an intromittent organ required for internal fertilization. Elevated testosterone levels induce metamorphosis of a subset of anal fin rays to grow and form the specialized terminal structures of the gonopodium. The molecular mechanisms underlying these processes are largely unknown. Here, we investigated whether retinoic acid (RA) signaling is involved in gonopodium development in the swordtail Xiphophorus hellerii. We showed that aldh1a2, a RA synthesizing enzyme, and the RA receptors, rar-ga and rar-gb, are expressed in anal fins during metamorphosis. aldh1a2 expression is regulated by testosterone in a concentration-dependent manner and is up-regulated in both hormone-induced and naturally developing gonopodia. Androgen receptor (ar), a putative regulator of gonopodial development, is co-expressed with aldh1a2 and the RA receptors in gonopodial rays. Importantly, experimental increase of RA signaling promoted growth of the gonopodium and increased the number of new segments. Based on gene expression analyses and pharmacological manipulation of gonopodium development, we show that the RA signaling pathway is activated in response to androgen signaling and promotes fin ray growth and development during the metamorphosis of the anal fin into the gonopodium.

Transcriptomics of two evolutionary novelties: how to make a sperm‐transfer organ out of an anal fin and a sexually selected “sword” out of a caudal fin

“…The latter is more likely as our previous study using in situ hybridization (Offen et al. ) detected the expression of androgen receptor β ( arβ ) in the untreated control anal fin. This hypothesis is further supported by our transcriptome data demonstrating that differential expression of androgen receptors between the anal fin (CG) and the ventral caudal fin (CV) in untreated control fish (Table S10).…”

“…; Offen et al. , ), and even another poeciliid fish, Gambusia affinis (Turner ; Angus et al. ; Ogino et al.…”

“…Testosterone increases outgrowth of the gonopodium in a concentration‐dependent manner in X. hellerii (Offen et al. ). Exogenous testosterone can also induce swords or sword‐like protrusions from ventral caudal fins in several swordtail and platyfish species (Dzwillo ; Zauner et al.…”

“…All these pieces of evidence imply that androgen signaling is involved in the hormone‐induced sword and gonopodial development (Offen et al. ). However, the signaling pathways have not been investigated at the molecular level, and our previous work aimed to find other molecular mechanisms potentially involved in the development of those two male‐specific traits.…”

“…Despite numerous phylogenetic and behavioral studies that aimed to study the evolutionary history and behavioral roles in determining mating success of the sword and gonopodium, much less is known about the genetic basis underlying the development of these traits. Interestingly, the induction of the development of the gonopodium by exogenous testosterone has been demonstrated in juveniles, adult females (Grobstein 1947(Grobstein , 1948Zauner et al 2003;Offen et al 2009Offen et al , 2013, and even another poeciliid fish, Gambusia affinis (Turner 1947;Angus et al 2001;Ogino et al 2004). Testosterone increases outgrowth of the gonopodium in a concentration-dependent manner in X. hellerii (Offen et al 2013).…”