Molecular mechanisms underlying nuchal hump formation in dolphin cichlid, Cyrtocara moorii

Laurène Alicia Lecaudey,Ehsan Pashay Ahi,Pooja Singh,Christian Sturmbauer

doi:10.1038/s41598-019-56771-7

Laurène Alicia Lecaudey, Ehsan Pashay Ahi + Show 2 more

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https://doi.org/10.1038/s41598-019-56771-7

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Abstract

East African cichlid fishes represent a model to tackle adaptive changes and their connection to rapid speciation and ecological distinction. In comparison to bony craniofacial tissues, adaptive morphogenesis of soft tissues has been rarely addressed, particularly at the molecular level. The nuchal hump in cichlids fishes is one such soft-tissue and exaggerated trait that is hypothesized to play an innovative role in the adaptive radiation of cichlids fishes. It has also evolved in parallel across lakes in East Africa and Central America. Using gene expression profiling, we identified and validated a set of genes involved in nuchal hump formation in the Lake Malawi dolphin cichlid, Cyrtocara moorii. In particular, we found genes differentially expressed in the nuchal hump, which are involved in controlling cell proliferation (btg3, fosl1a and pdgfrb), cell growth (dlk1), craniofacial morphogenesis (dlx5a, mycn and tcf12), as well as regulators of growth-related signals (dpt, pappa and socs2). This is the first study to identify the set of genes associated with nuchal hump formation in cichlids. Given that the hump is a trait that evolved repeatedly in several African and American cichlid lineages, it would be interesting to see if the molecular pathways and genes triggering hump formation follow a common genetic track or if the trait evolved in parallel, with distinct mechanisms, in other cichlid adaptive radiations and even in other teleost fishes.

Highlights

East African cichlid fishes represent a model to tackle adaptive changes and their connection to rapid speciation and ecological distinction
In the Central American cichlid species, Amphilophus citrinellus, which is well-known for its large nuchal hump, the hump tissue seems to develop in response to hormonal stimulus and contain modified nuchal hypodermis and high amount of fat stored in the same tissue[10]
It is highly probable that most of the overlapping differentially expressed (DE) genes are involved in the formation of nuchal hump phenotype in adult males of C. moorii

Summary

Introduction

East African cichlid fishes represent a model to tackle adaptive changes and their connection to rapid speciation and ecological distinction. The aim of this study was to identify genes with potential role in the formation of nuchal hump soft tissues in Cyrtocara moorii, to better understand its evolution This species is a member of Haplochromine cichlid tribe, endemic to Lake Malawi in East Africa, which develops conspicuous hump-headed morphology in adult males[17,24,25] (Fig. 1A). To exclude genes which are differentially expressed during later phases in the adult ontogeny and to have a profile of interspecies comparison, we used soft tissues from the nuchal region of adult males of Copadichromis sp Mbenji, a closely related Haplochromine species without hump-headed morphology[26] (Fig. 1A) Both C. moorii and C. sp Mbenji belong to two closely related genera within non-Mbuna group of Haplochromine cichlids in Lake Malawi[27,28]. This study provides the first transcriptional profile of the nuchal hump in fish, and the list of identified differentially expressed genes in the nuchal hump tissues allowed us to hypothesize potential molecular mechanisms controlling the formation of this craniofacial structure reported in different groups of teleost fish

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