The Sexes in Fish – a Complicated Story

Tilapia ruweti, an African cichlid caring for its clutch. © LIB, Geiger

 

“It’s complicated,” smiles LIB researcher Astrid Böhne. While in mammals like us, the same sex chromosomes are responsible for determining sex, this can differ among other animal species, even among closely related sister species like in certain cichlid fishes. A recently published study now lays the groundwork for unraveling the true role of genetic conflict in the formation of sex chromosomes.

The mechanisms of sex determination are diverse: environmental factors such as the incubation temperature of the egg, genetic systems, or complex combinations of different signals are known. In some animal groups, the pathway is conserved, for example all mammals have the same sex chromosomes (XY). In other groups, even the most closely related species have different systems. Dr. Astrid Böhne, head of the Comparative Genomics section at the Museum Koenig Bonn, has been focusing on genetic sex determination for years and researching sex chromosomes in African cichlids. She has already shown that cichlids from Lake Tanganyika exhibit an unusually high number of chromosome switches that determine sex.

In the comprehensive literature review of the past four years, now published by Böhne’s team, together with their PhD candidates Sophie Smith and Kevin Hsiung, they demonstrate that the number of discoveries of sex chromosomes based on DNA sequencing and newly created reference genomes has dramatically increased. However, according to the team, most studies fail to provide explanations of how these sex chromosomes originated or persist within a population. “On the one hand, there must be mechanisms that counteract the switching of sex chromosomes. We focused on summarizing the available data that sheds light on whether sexual conflict is a driving force in the emergence and fixation of new sex chromosomes,” says Astrid Böhne.

Such sexual conflicts are well known, especially in fish, where males and females often differ significantly in colour or size. A conflict arises when the same genes are responsible for these traits in both sexes. However, Böhne’s team found that many articles discuss sex antagonism as a general factor in the formation of sex chromosomes, but direct evidence for this is lacking.

So far, there have been very few studies that actually present experimental evidence for sexual antagonism and their role in the evolution of sex chromosomes. Now is the time to explore the treasure trove of newly discovered sex chromosomes in a variety of species, supplement it with functional and molecular analyses, and place the results in a broader context to understand the dynamics of sex chromosome evolution.

Astrid Böhne now has the opportunity, with the support of the German Research Foundation, to investigate these questions with her team at LIB. They are studying cichlids, pipefish, and seahorses.

Original publication:

Evaluating the role of sexual antagonism in the evolution of sex chromosomes: new data from fish. by Sophie Smith, Kevin Hsiung, Astrid Böhne (2023) Curr Opin Genet Dev, Volume 81, August 2023, 102078. Available online 26 June 2023, Version of Record 26 June 2023.(impact factor: 4), PMID: 37379742 doi: 10.1016/j.gde.2023.102078

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