Spiny and soft-rayed fin domains in acanthomorph fish are established through a BMP-gremlin-shhsignaling network

Abstract

With over 18,000 species, the Acanthomorpha, or spiny-rayed fishes, form the largest and arguably most diverse radiation of vertebrates. One of the key novelties that contributed to their evolutionary success are the spiny rays in their fins that serve as a defense mechanism. We investigated the patterning mechanisms underlying the differentiation of median fin Anlagen into discrete spiny and soft-rayed domains during the ontogeny of the direct-developing cichlid fish Astatotilapia burtoni. Distinct transcription factor signatures characterize these two fin domains, whereby mutually exclusive expression of hoxa13a/b with alx4a/b and tbx2b marks the spine to soft-ray boundary. The soft-ray domain is established by BMP inhibition via gremlin1b, which synergizes in the posterior fin with shh secreted from a zone of polarizing activity. Modulation of BMP signaling by chemical inhibition or gremlin1b CRISPR/Cas9 knockout induces homeotic transformations of spines into soft rays and vice versa. The expression of spine and soft-ray genes in nonacanthomorph fins indicates that a combination of exaptation and posterior expansion of an ancestral developmental program for the anterior fin margin allowed the evolution of robustly individuated spiny and soft-rayed domains. We propose that a repeated exaptation of such pattern might underly the convergent evolution of anterior spiny-fin elements across fishes.

Developmental independence of median fins from the larval fin fold revises their evolutionary origin

“…In some fish, such as sharks and cichlids, it has been shown that FGF signaling is involved in dorsal fin development 19 , 37 . In larval zebrafish, it has also been shown that FGF signaling plays a role in the early stages of LMFF development 10 .…”

“…One influential idea assumes that paired fins arose by co-opting of the genetic patterning modules established during median fin evolution 17 , 52 , 53 . Indeed, gene expression studies in paired and median fins have identified a similar pattern in the expression of developmental genes, such as the nested expression of Hox genes 19 , 37 , 52 – 54 . We found that prdm16 is expressed in dorsal fin mesenchyme by observations of EGFP expressions in the gt1116A line, and this gene is also expressed in pectoral fin mesenchyme at the early embryonic stage 35 , 55 , 56 .…”

Oca2targeting using CRISPR/Cas9 in the Malawi cichlidAstatotilapia calliptera

“…other cichlids [13,[29][30][31][32][33][34][35]. HDR on the other hand uses a DNA template to guide the repair, thus by providing a single-stranded DNA (ssDNA) template, one can either insert a sequence of interest (e.g.…”

oca2targeting using CRISPR/Cas9 in the Malawi cichlidAstatotilapia calliptera

“…NHEJ is an imprecise mechanism that generates small insertions or deletions that result in a frameshift. This method has been widely used to induce frameshift mutations in coding protein sequences leading to loss of function alleles in many model systems including other cichlids (Fang et al, 2018;Höch et al, 2021;Kratochwil et al, 2018;Li et al, 2021Li et al, , 2014Livraghi et al, 2021;Rasys et al, 2019;Wang et al, 2021). HDR on the other hand utilises a DNA template to guide the repair, thus by providing a single-stranded DNA (ssDNA) template, one can either insert a sequence of interest (e.g.…”