Derived X chromosome in the turtle genus Staurotypus

Area of Science:

• * Cytogenetics
• * Evolutionary Biology
• * Herpetology

Background:

• * Sex determination in vertebrates typically involves sex chromosomes (X, Y or Z, W).
• * In most vertebrates, the homogametic sex (XX or ZZ) is considered ancestral, and the heterogametic sex (XY or ZW) is derived.
• * The turtle Staurotypus salvinii presents an opportunity to study alternative sex chromosome evolution pathways.

Purpose of the Study:

• * To investigate the sex chromosome system of the turtle Staurotypus salvinii.
• * To compare the sex chromosome evolution in S. salvinii with general models in vertebrates.
• * To understand the evolutionary relationships between sex chromosomes and ancestral conditions.

Main Methods:

• * Chromosomal analysis using C-banding, G-banding, and silver (Ag-AS) staining techniques.
• * Comparative cytogenetic analysis of Staurotypus salvinii sex chromosomes.
• * Evaluation of sex chromosome morphology and homology.

Main Results:

• * Distinctive sex chromosome system identified in Staurotypus salvinii.
• * The X chromosome in S. salvinii is derived, making the homogametic sex (female) more derived.
• * The male (heterogametic sex) exhibits an intermediate condition between the female and the ancestral state observed in other turtles.

Conclusions:

• * Staurotypus salvinii exhibits an atypical sex chromosome system that inverts the typical evolutionary pattern seen in most vertebrates.
• * The female sex in S. salvinii is evolutionarily more derived than the male.
• * This species does not conform to the general model of sex chromosome evolution for diploid dioecious organisms, highlighting diversity in evolutionary pathways.