Abstract
Lizards represent unique model organisms in the study of sex determination and sex chromosome evolution. Among tetrapods, they are characterized by an unparalleled diversity of sex determination systems, including temperature-dependent sex determination (TSD) and genetic sex determination (GSD) under either male or female heterogamety. Sex chromosome systems are also extremely variable in lizards. They include simple (XY and ZW) and multiple (X1X2Y and Z1Z2W) sex chromosome systems and encompass all the different hypothesized stages of diversification of heterogametic chromosomes, from homomorphic to heteromorphic and completely heterochromatic sex chromosomes. The co-occurrence of TSD, GSD and different sex chromosome systems also characterizes different lizard taxa, which represent ideal models to study the emergence and the evolutionary drivers of sex reversal and sex chromosome turnover. In this review, we present a synthesis of general genome and karyotype features of non-snakes squamates and discuss the main theories and evidences on the evolution and diversification of their different sex determination and sex chromosome systems. We here provide a systematic assessment of the available data on lizard sex chromosome systems and an overview of the main cytogenetic and molecular methods used for their identification, using a qualitative and quantitative approach.
Highlights
IntroductionSex determination systems are important drivers of biological diversity and a better understanding of the mechanisms and pathways driving their evolution and the diversification of sex chromosome systems represents a fundamental objective of evolutionary biology
Sex determination systems are important drivers of biological diversity and a better understanding of the mechanisms and pathways driving their evolution and the diversification of sex chromosome systems represents a fundamental objective of evolutionary biology.Two main types of sex determination mechanisms (SDMs) occur in tetrapods: environmental sex determination (ESD), normally in the form of temperature-dependent sex determination (TSD), and genetic sex determination (GSD), which may be under female or male heterogamety [1].Unlike other amniotes such as crocodiles, mammals and birds, which show evolutionary stable mechanisms of sex determination, turtles and squamates are generally characterized by a remarkable evolutionary plasticity of SDMs (e.g., References [1,2,3,4])
We provide a synthesis on the main characteristics of the lizard genome and karyotype and discuss the main hypotheses and evidence on the evolution and diversification of their different sex determination and sex chromosome systems
Summary
Sex determination systems are important drivers of biological diversity and a better understanding of the mechanisms and pathways driving their evolution and the diversification of sex chromosome systems represents a fundamental objective of evolutionary biology. Two main types of sex determination mechanisms (SDMs) occur in tetrapods: environmental sex determination (ESD), normally in the form of temperature-dependent sex determination (TSD), and genetic sex determination (GSD), which may be under female or male heterogamety [1]. Lizards represent unique model organisms in the study of sex determination and sex chromosome evolution as their high phylogenetic and species diversity is matched by an unparalleled variability of different SDMs and sex chromosome systems. Shifts between different systems can occur in one generation or between different populations in some species [6,7], identifying lizards as promising model organisms to better understand the mechanisms and the evolutionary drivers of sex reversal and sex chromosome turnover. We provide an overview of the main molecular and cytogenetic methods commonly used in the characterization of sex chromosomes in lizards, highlighting their main strengths and limits in the identification of GSD and sex chromosome systems
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