Before the East African Radiation: Sex Chromosome Systems in Basal Haplotilapiine Cichlids

Area of Science:

• Evolutionary biology
• Genomics
• Ichthyology

Background:

• East African cichlids exhibit remarkable diversification and high rates of sex chromosome turnover.
• Studying cichlid sex chromosome evolution offers insights into speciation and adaptation.
• Previous research suggested specific genes (figla, banf2) for certain sex determination systems.

Purpose of the Study:

• Investigate the evolution of sex chromosomes in several cichlid tribes (Tilapiini, Coptodonini, Heterotilapiini, Gobiocichlini, Pelmatolapiini, Oreochromini).
• Identify sex chromosomes and their evolutionary patterns across these lineages.
• Evaluate candidate genes and propose new mechanisms for sex determination.

Main Methods:

• Assembled chromosome-scale genomes for Pelmatotilapia mariae (male and female).
• Mapped pooled sequencing reads from multiple species to identify sex chromosomes.
• Analyzed phylogenetic distribution of sex determination systems.

Main Results:

• Identified ZW systems on LG3 in P. mariae and shared XY systems on LG3 in other species.
• Discovered XY systems on LG1 in several species and on LG14 and LG20 in others.
• Phylogenetic analysis suggests slower turnover in some lineages compared to lacustrine cichlids.
• Data contradicts figla and banf2 as candidate genes for LG1XY and LG3ZW systems.

Conclusions:

• Sex chromosome turnover in cichlids is complex, with multiple independent origins and varying rates.
• Ubiquitination is proposed as a potential mechanism for XY systems on LG3.
• Further research is needed to fully understand the genetic basis of cichlid sex determination.