Highly dynamic sex chromosomes in cichlid fishes

Date:
September 2, 2021
Source:
University of Basel
Summary:
The cichlids of Lake Tanganyika in Africa are highly diverse --
including with regard to sex chromosomes. These have changed
extremely frequently in the course of the evolution of these fish
and, depending on the species, can be of the type XY or ZW.



FULL STORY ========================================================================== Based on the biology lessons back in our schooldays, we know that XX
means female, and XY means male. In mammals, these constellations of
sex chromosomes determine whether an individual becomes a female or
a male. It is different in birds, where the principle of ZW = female,
ZZ = male applies. In many reptiles, the temperature at which the eggs
develop determines the sex. The way in which genetic sex is determined can therefore vary from species to species, with different sex-determining
systems having occurred during evolution. In fish, the most biodiverse
group of vertebrates, we find different genetic systems (including the XY
and ZW types of mammals and birds, respectively) as well as environmental systems (as in many reptiles).


==========================================================================
The evolution of sex chromosomes Sex chromosomes are derived from " normal
" chromosomes -- the autosomes - - after acquiring a new, sex-determining mutation. The resulting new chromosome pair takes separate paths in the
course of evolution, ensuring that there is no exchange of sex-determining genes between the two sex chromosomes.

In mammals, this process took place around 165 million years ago, and
resulted in the XY system with a male-specific Y chromosome. In birds,
the W is a female-specific chromosome. In fish, on the other hand, it has
been known for some time that there is no general sex chromosome system.

With the aid of next-generation genome sequencing methods that allow
for a comparison between the genomes of both sexes, a new world of
sex chromosomes has been identified in fish: During their evolution,
different chromosomes have developed independently of each other in
different fish lineages as sex chromosomes. Yet, the repeated appearance
of similar or functionally related sex-determining genes suggests that
there is a pool of chromosome candidates that are particularly suitable
for sex determination.

Cichlids are especially diverse A research team at the Department of Environmental Sciences at the University of Basel led by Dr. Astrid
Bo"hne (now at the Research Museum Koenig in Bonn) has now investigated
the evolution of sex chromosomes in a highly diverse group of fish, the cichlids from Lake Tanganyika in Africa. A particular feature of these
fish is that they are a prime example of adaptive radiation, a process
in which a large number of species emerge in a relatively short time
through rapid adaptation to various habitats.

The researchers analyzed extensive genome and transcriptome data
of around 240 cichlid species for differences between females and
males, which enabled them to identify sex chromosomes in over 70
species. Interestingly, substantial differences between the species
became apparent with regard to the chromosomes responsible for sex determination. The course of the evolution of cichlids in Lake Tanganyika
also saw several changes from sex chromosomes of type XY to ZW and
vice versa.

"This means that these cichlids hold the record for transitioning between
sex chromosomes in vertebrates," says Astrid Bo"hne. At the same time,
the Basel zoologists found out that certain chromosomes become sex
chromosomes particularly often. This supports the hypothesis that there
is a group of genes or even whole chromosomes that are particularly
suitable for determining genetic sex.

========================================================================== Story Source: Materials provided by University_of_Basel. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Athimed El Taher, Fabrizia Ronco, Michael Matschiner, Walter
Salzburger
and Astrid Bo"hne. Dynamics of sex chromosome evolution in a rapid
radiation of cichlid fishes. Science Advances, 2021 DOI: 10.1126/
sciadv.abe8215 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210902124956.htm

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