Navigation Links
Two papers unraveled the mystery of sex determination and benthic adaptation of the flatfish

February 2, 2014, Shenzhen, China - Researchers from Yellow Sea Fisheries Research Institute, Chinese Academy of Fisheries Sciences, BGI-Shenzhen and other institutes have successfully decoded the first genome of a flatfish - half-smooth tongue sole (Cynoglossus semilaevis), providing insights into ZW sex chromosome evolution and adaptation to a benthic lifestyle. The data generated in this project also lay foundation on the genetic breeding of tongue sole. The latest study has been published online today in Nature Genetics. In another study published in Genome Research at the same time, the researchers also uncovered the epigenetic mechanism underlying the temperature-dependent sexual reversal as well as the trans-generational inheritance of such sexual reversal phenomena in tongue sole.

The genetic mechanisms underlying sex determination as well as the origin and evolution of sex chromosomes have fascinated biologists for decades. Half-smooth tongue sole is an important cultured marine flatfish along China coast, whose sex is primarily determined by the inheritance of sex chromosomes (genetic sex determination, GSD) but also affected by environmental temperature (environmental sex determination, ESD). In normal condition, the sex of tongue sole is determined by the ZW/ZZ sex determination system with female containing a heteromorphic W chromosome. However, if juvenile fish are reared in high temperature, the genetic females (ZW) can develop into phenotypic males (so-called pseudo-males). Interestingly, these sex-reversed pseudo-males are fertile and can mate with normal females to produce viable offspring. Astonishingly, most of their genetic female offspring (ZW) develop into pseudo-males, but without temperature induction. Thus, with its complex sex determination system governed by the interaction between genetic determination and environmental factors, tongue sole is an excellent model to understand the molecular mechanism of sex determination in fishes and the interplay of genome and environment.

In this study, the researchers sequenced and assembled the genomes of one male (ZZ) and one female (ZW) tongue sole, respectively. Based on the difference of sequencing depth of Z/W linked-scaffolds between female and male, together with the high-resolution genetic map constructed by SSR and SNP, they assembled the Z and W chromosome of tongue sole at high quality. Using the Z-W homologous genes, they estimated that the age of the tongue sole sex chromosome pair is relatively young (about 30 million years), which contrasts with the age of hundreds of millions of years for the mammalian and avian sex chromosomes. Interestingly, researchers found that the sex chromosomes of tongue sole are derived from the same ancestral vertebrate protochromosome as the avian W and Z chromosomes. Furthermore, the same gene on the Z chromosome, dmrt1, which is the male-determining gene in birds, showed convergent evolution of features that are compatible with a similar function in tongue sole.

To understanding the regulatory mechanisms involved in the temperature-dependent sex reversal as well as the inheritance of sex reversal in tongue sole, researchers then analyzed the gonadal DNA methylomes and transcriptomes of normal males (ZZ), pseudo-males (ZW), normal females (ZW), and the offspring of a pseudo-male crossed with a female. They found that, after phenotypic sexual reversal, the genome-wide methylation patterns of genetic females have been accurately shaped to the patterns of normal males. Furthermore, the methylation changes after sex reversal were enriched in genes in the sex determination pathway. By comparing parental and offspring samples, researchers also found that the methylation changes between parental pseudo-males and females were inherited by offspring pseudo-males, probably explaining why the offspring genetic females of pseudo-males can undergo sexual reversal without temperature induction. For organisms with sex chromosomes, a challenge for the sex-reversed individuals is the unequal gene dosage on sex chromosomes when compared with normal individuals. For example, pseudo-males (ZW) of tongue sole lack one Z chromosome compared with normal males (ZZ), and contain one extra W chromosome with female-specific genes. Thus, how to revolve gene dosage inequality on sex chromosomes is a fundamental question for sex reversal. Researchers found that, dosage compensation only occurs in a restricted, methylated cytosine enriched Z chromosomal region in pseudo-male testes, achieving equal expression level in normal male testes. Some spermatogenesis-related genes were found in this region. For the W chromosome, they observed that many W-linked genes are still actively expressed in pseudo-males. The researchers speculate that the expression of these W-linked genes may compensate the dosage of their counterparts on the Z chromosomes. However, female-specific genes, such as figla, were suppressed in pseudo-males by methylation regulation. In conclusion, the researchers propose that epigenetic regulation plays multiple crucial roles in sexual reversal of tongue sole fish.

Flatfish are characterized by a transition from pelagic to benthic habitats when they metamorphose from free swimming larvae to bottom-dwelling fish, accompanied by a series of biological and phenotypic changes, such as eye migration, displacement of internal organs, the difference of pigmentation and the sensitivity of light. By comparing the transcriptomes of pelagic and benthic fish, the researchers revealed that the differentially expressed genes between these two developmental stages are enriched in genes potentially involved in adaptation to a benthic lifestyle. They also identified 15 positively selected genes, which have putative roles in metamorphosis. In addition, they found that the expression levels of rod pigment (rh1) and cone pigment (lws1) genes, which are responsible for scotopic vision and long-wavelength-sensitivity, respectively, were significantly up-regulated in benthic stages, whereas the expression of the middle-wavelength-sensitive pigment gene (rh2) was significantly up-regulated in pelagic stages. Moreover, they also found that several crystallin genes were lost or became pseudogenes. They speculate that these changes reveal the adaptation of the tongue sole visual system after shifting to the benthic environment with weak light conditions. As compensation, tongue sole have developed a strong lateral line sense organ and especially, the papillae-a specific mechanoreceptorsense organ during evolution.

Dr. Guojie Zhang, who is the leading author of both papers from China National Genebank of BGI, said: "The evolution of sex is one of the major transitions in evolution, that has significantly enhanced the adaptation ability of the species. However, our knowledge of sex determination mechanisms is still very limited. Vertebrates have various sex-determination mechanisms. The sex determination process of many vertebrate species is regulated by environmental cues, like temperature. Investigation of the molecular mechanisms underlying this process will shed new lights on the understanding of the interaction between functional gene networks and their regulations by environmental factors, and increase our knowledge on how genome evolution drives species diversification and diversity."


Contact: Jia Liu
BGI Shenzhen

Related biology news :

1. Assessing dangerous climate change and call for climate change response papers
2. Managing waters shared across national boundaries: Treasury of papers helps capture 20 years of lessons
3. GSA Bulletin starts 2013 with 13 new papers published online ahead of print
4. A new type of data papers designed to publish online interactive keys identifying biodiversity
5. Some factors that impact islet transplantation explored in Cell Transplantation papers
6. Some factors that impact islet transplantation explored in Cell Transplantation papers
7. Stomata development in plants unraveled -- a valuable discovery for environmental research
8. The science of baby-making still a mystery for many women
9. Scientists solve 40-year mystery of how sodium controls opioid brain signaling
10. Discovery spotlights key role of mystery RNA modification in cells
11. Cloud mystery solved: Global temperatures to rise at least 4C by 2100
Post Your Comments:
(Date:11/17/2015)... , November 17, 2015 Paris ...   --> Paris from 17 th ... DERMALOG, the biometrics innovation leader, has invented the first combined ... on the same scanning surface. Until now two different scanners ... one scanner can capture both on the same surface. ...
(Date:11/12/2015)... Nov. 12, 2015  A golden retriever that stayed ... dystrophy (DMD) has provided a new lead for treating ... the Broad Institute of MIT and Harvard and the ... . Cell, pinpoints a protective ... the disease,s effects. The Boston Children,s lab of ...
(Date:11/10/2015)... 10, 2015 About signature ... helps to identify and verify the identity of ... as the secure and accurate method of authentication ... particular individual because each individual,s signature is highly ... when dynamic signature of an individual is compared ...
Breaking Biology News(10 mins):
(Date:11/25/2015)... Mass. , Nov. 25, 2015 Harvard ... biotechnology company developing bioengineered organ implants for life-threatening conditions, ... present at the LD Micro "Main Event" investor conference ... The presentation will be webcast live and posted for ... be available at the conference for one-on-one meetings on ...
(Date:11/25/2015)... , Nov. 25, 2015 /PRNewswire/ - Aeterna Zentaris ... that its business and prospects remain fundamentally strong ... Zoptrex™ (zoptarelin doxorubicin) recently received DSMB recommendation to ... completion following review of the final interim efficacy ... 2 Primary Endpoint in men with heavily pretreated ...
(Date:11/25/2015)... , Nov. 25, 2015 Orexigen® Therapeutics, ... will participate in a fireside chat discussion at the ... New York . The discussion is scheduled for ... .  A replay will be available ... Contact:McDavid Stilwell  , Julie NormartVP, Corporate Communications and ...
(Date:11/25/2015)... (PRWEB) , ... November 25, 2015 , ... ... company uBiome, were featured on AngelList early in their initial angel funding process. ... AngelList syndicate for individuals looking to make early stage investments in the microbiome ...
Breaking Biology Technology: