An extra chromosome behind the mystery of sex determination in the Pachón blind cavefish

An international collaboration coordinated by Yann Guiguen from INRAE Rennes and Sylvie Rétaux from the Paris-Saclay Institute of Neuroscience shows for the first time in a vertebrate that a non-essential chromosome, called B chromosome, determines the sex of Pachón cavefish, a species that serves as a study model for underground evolution. Their results, published September 7 in the journal Current Biology, describe a new sex determination system in fish, a major contribution to evolutionary biology.

A supernumerary “B-sex” chromosome drives male sex determination in the Pachón cavefish, Astyanax mexicanus. Boudjema Imarazene, Kang Du, Séverine Beille, Elodie Jouanno, Romain Feron, Qiaowei Pan, JorgeTorres-Paz, Céline Lopez-Roques, Adrien Castinel, Lisa Gil, Claire Kuchly, Cécile Donnadieu, Hugues Parrinello, Laurent Journot, Cédric Cabau, Margot Zahm, Christophe Klopp, Tomáš Pavlica, Ahmed Al-Rikabi, Thomas Liehr, Sergey A.Simanovsky, Joerg Bohlen, Alexandre Sember, JuliePerez, Frédéric Veyrunes, Thomas D.Mueller, John H. Postlethwait, Manfred Schartl, Amaury Herpin, Sylvie Rétaux, Yann Guiguen.

By analyzing the chromosomes of the cells of these fish, we found that there is a B chromosome present in all males and absent (or extremely rare) in females of this fish population.

Each cell that constitutes a living being contains genetic information within pairs of type A chromosomes, which must be present in an integral and unaltered manner to ensure the survival of the individuals of a species. For example, the genetic information of the human species is contained in a set of 23 pairs of A chromosomes. But there is also another type of chromosomes, called type B chromosomes, in many species. Unlike type A chromosomes, they are not systematically present in all individuals of a species, are not transmitted in a Mendelian fashion and have often been called accessory or non-essential chromosomes because they are not necessary for the survival of the species. They have long been considered genomic parasites, and their biology remains quite enigmatic.

The importance of the A … and B chromosomes

In most vertebrates, it is also genetic information that will determine the sex of an individual. In these species, there are sex chromosomes, derived from A chromosomes, which control the development of the male or female sex. In mammals, for example, the development of the female sex is driven by the presence of an XX chromosome pair and the male sex by the XY pair. In a few species of fish with B chromosomes, their presence has sometimes been found predominantly in one sex. However, their real implication in the genetic determinism of sex had never been studied.

The role of the B chromosome in determining the sex of cavefish in the Pachón cave, in Mexico

We studied the B chromosomes of a species of small, blind cavefish from the Pachón cave in Mexico (Astyanax mexicanus). A species that serves as a study model for adaptation to underground life in the absence of light.

By analyzing the chromosomes of the cells of these fish, we found that there is a B chromosome present in all males and absent (or extremely rare) in females of this fish population. We then sequenced the genome of male fish and studied a gene present in 2 supernumerary copies in the sequence of the B chromosome, the gdf6b gene. During the development of fish, the sexual organs are initially undifferentiated between male and female. The gdf6b gene is expressed during sexual differentiation to male organs, but not during differentiation to female sex organs. Finally, by inactivating this gene in male fish, we observed that they reversed into females. This shows that this gene carried by the B chromosome has an essential role in the sexual differentiation of Pachón cavefish.

This study shows for the first time in a vertebrate that a type B chromosome determines the sex of individuals. This ” B sex ” chromosome behaves much like a Y chromosome in mammals by possessing a major sex determining gene, which will induce the development of a male sex and which is transmitted only from father to son. Many questions persist about the origin of these “B sex ” chromosomes because they are not present in all populations of Astyanax mexicanus and research continues to better understand the evolution of sex determinism in this species.

Article published in Current BiologyAccess to the manuscript