Zebrafish display degrees of masculinization in hot water

January 26, 2017 § 1 Comment

Domesticated zebrafish Credit: NICHD/ J. Swan

Domesticated zebrafish
Credit: NICHD/ J. Swan

Sex is determined in mammals, birds and a subset of fish, primarily by a pair of chromosomes known as the sex chromosomes. Wild-type zebrafish have sex chromosomes but their domesticated counterparts depend on polygenic sex determination, in which the responsible genetic factors for sex are distributed across the whole genome. Polygenic sex determination makes sexual differentiation more unstable because it permits environmental cues to play a greater role in sexual development. However, polygenic sex determination is less understood than sex-chromosomal determination.

In a paper published in Proceedings of the National Academies of Sciences on Jan. 23, researchers at the Temasek Life Sciences Laboratory in Singapore and Institute of Marine Sciences in Spain, have examined the transcriptomal changes that occur when domesticated female zebrafish transition into males in response to warm water. A transcriptome consists of the total mRNA in a cell that codes for proteins.

Timothy Karr, a developmental biologist at Arizona State University who was not involved in the study describes it “as one of the first studies of its kind.”

Zebrafish are native to the Indian subcontinent and, for more than 40 years, have been used as a model organism for biological research. While many fish display sexual plasticity due to environmental cues, domesticated zebrafish in this study are the first to be observed to retain female gonads while displaying male reproductive genes and proteins, rather than transitioning fully to something known as a neomale. “A neomale is an individual that’s genetically programmed to become a female, but as a result of the temperature treatment, becomes male,” says László Orbán at the Temasek Life Sciences Laboratory.

The instability of polygenic sex determination in zebrafish came across as an unintentional side-effect of cultivating distinct familial lines for research over the past four decades.

“Somehow, the sex chromosomes have been lost during the domestication process,” says Orbán. While there had been controversy as to whether zebrafish sex was more strongly determined by inherited chromosomes or polygenic cues, the split between wild and domestic families was confirmed about two years ago. Researchers examined zebrafish they had retrieved from northern India and found that the wild fish still displayed sex-chromosomal determination.

Within domesticated zebrafish, the family lines develop different ratios of males to females. Francesc Piferrer’s lab at the Institute of Marine Sciences, which had previously examined the effects of temperature on fish sex ratios and helped design the study, subjected a variety of zebrafish families to water at 36° C during a window of 18 and 32 days post-fertilization. The Orbán lab members then used microarrays to identify the differences in transcriptomes between the zebrafish males and females that been experienced control and heat-exposed conditions.

Examining the transcriptomes of these fish allowed the researchers to then identify which had become neomales or pseudofemales which have ovaries but a male-like transcriptome. The researchers found that the pseudofemales displayed gonadal transcriptomes that only differed from genuine male transcriptomes by a few thousand genes. “It looks like a reprogramming process that doesn’t complete,” says Orbán. “The details are not known to us so there’s a whole area of science opening up here.”

“If it can be replicated, the authors’ claim to have discovered ‘male-like’ transcriptomes in females with morphologically developed ovaries, would be an extraordinary finding, but perhaps not for the reasons the authors envision in this study,” Karr notes.  “It would be one of the most persuasive arguments” against the dominance of chromosome-based sex determination in developmental and evolutionary biology.

This post was written by John Arnst, ASBMB Today’s science writer. 

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