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Meet the world's largest bony fish

For the first time, the genome of the ocean sunfish (Mola mola), the world's largest bony fish, has been sequenced. Researchers involved in the Genome 10K (G10K) project want to collect 10,000 nonmammalian vertebrate genomes for comparative analyses. The ocean sunfish genome has now revealed several altered genes that may explain its' fast growth, large size and unusual shape.


The ocean sunfish belongs to a unique group of fish whose skeletons are primarily made up of bone, as opposed to cartilage. But, unlike other bony fish, it's skeleton is largely made up of cartilage — not bone. Looking for clues as to why, researchers analysed genes that are known to be involved in bone formation. Found in tropical and temperate seas such as the Mediterranean and the Atlantic, the sunfish or Mola mola, can grow up to a length of 2.7m (8.85827 feet) and weigh 2.3 tons (4,600 pounds).

Researchers include the Nobel Laureate Sydney Brenner. Their results appear in the open access journal GigaScience.

Even though its diet is mostly jellyfish and nutritionally poor, the ocean sunfish grows unusually fast at almost one kilogram per day (2.20462 pounds) — other fish grow at 0.02 to 0.5 kilogram per day.

The Mola mola's growth rate isn't its only extreme feature, females can produce more eggs than any other known vertebrate — up to 300 million at once. Also, without a tail, it has the appearance of being a head without a body, stunning many who have never seen this giant oddity before.


"We found changes in genes encoding cartilage formation. This may contribute to the development of predominantly cartilaginous skeleton in this gigantic fish."

Guojie Zhang, Associate Director, China National Genbank and co-leader of the project.


Researchers believed the ocean sunfish's unusual appearance might be due to a lack of HOX genes to control its' body plan. Hox genes organize the body axis which governs head, thorax and abdomen segment formation. But, they were surprised to find out this isn't true. The ocean sunfish has HOX genes organized in clusters similar to that of a pufferfish.

Focusing then on genes involved in the ocean sunfish's fast growth and unusual body shape, they discovered several that signal growth hormones to evolve much faster than in those in other bony fish. This may explain the Mola mola's large size.

Identifying these genomic changes in the ocean sunfish's unusual body shape, size and skeleton could help future study of genetic differences in other fish — some used to model human embyo development.


"Vertebrates exhibit a wide diversity in their morphology, physiology and behaviour. Understanding the genetic basis of this diversity is a major goal of evolutionary biology. We still have a lot to learn from the assembly of the ocean sunfish genome."

Byrappa Venkatesh PhD, A*STAR researcher and co-leader of the project.

"This is one of the 30 fish genomes that have been sequenced for the G10K project. The availability of high throughput DNA sequencing technology makes it possible to sequence whole genomes of a wide range of 'non-model' species, allowing us to uncover the genetic basis of phenotypic diversity and their adaptations."

Guojie Zhang PhD, State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, China; China National Genebank, BGI-Shenzhen Centre for Social Evolution, Department of Biology, University of Copenhagen, Denmark.


Abstract Background
The ocean sunfish (Mola mola), which can grow up to a length of 2.7 m and weigh 2.3 tons, is the world’s largest bony fish. It has an extremely fast growth rate and its endoskeleton is mainly composed of cartilage. Another unique feature of the sunfish is its lack of a caudal fin, which is replaced by a broad and stiff lobe that results in the characteristic truncated appearance of the fish.

Results
To gain insights into the genomic basis of these phenotypic traits, we sequenced the sunfish genome and performed a comparative analysis with other teleost genomes. Several sunfish genes involved in the growth hormone and insulin-like growth factor 1 (GH/IGF1) axis signalling pathway were found to be under positive selection or accelerated evolution, which might explain its fast growth rate and large body size. A number of genes associated with the extracellular matrix, some of which are involved in the regulation of bone and cartilage development, have also undergone positive selection or accelerated evolution. A comparison of the sunfish genome with that of the pufferfish (fugu), which has a caudal fin, revealed that the sunfish contains more homeobox (Hox) genes although both genomes contain seven Hox clusters. Thus, caudal fin loss in sunfish is not associated with the loss of a specific Hox gene.

Conclusions
Our analyses provide insights into the molecular basis of the fast growth rate and large size of the ocean sunfish. The high-quality genome assembly generated in this study should facilitate further studies of this ‘natural mutant’.

Keywords
Ocean sunfish Mola mola Growth rate Body size Cartilaginous skeleton Positive selection

Research article: The genome of the largest bony fish, ocean sunfish (Mola mola), provides insights into its fast growth rate
Hailin Pan, Hao Yu, Vydianathan Ravi, Cai Li, Alison P. Lee, Michelle M. Lian, Boon-Hui Tay, Sydney Brenner, Jian Wang, Huanming Yang, Guojie Zhang and Byrappa Venkatesh
GigaScience 2016
DOI: 10.1186/s13742-016-0144-3

GigaScience aims to revolutionize reproducibility of analyses, data dissemination, organization, understanding, and use. As an open access and open-data journal, we publish all research objects (data, software tools and workflows) from 'big data' studies across the entire spectrum of life and biomedical sciences.

BioMed Central is an STM (Science, Technology and Medicine) publisher which has pioneered the open access publishing model. All peer-reviewed research articles published by BioMed Central are made immediately and freely accessible online, and are licensed to allow redistribution and reuse. BioMed Central is part of Springer Nature, a major new force in scientific, scholarly, professional and educational publishing, created in May 2015 through the combination of Nature Publishing Group, Palgrave Macmillan, Macmillan Education and Springer Science+Business Media. http://www.biomedcentral.com
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Sep 30, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   



Sun fish (Mola mola) appear to be giant heads without bodies having a large, flattened disk-like
body and rudder-like fins. They can weigh 2.3 tons, and generate 300 million eggs in one ovulation.
Image Credit:
Dive Photo Guide


 

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