Understanding the Evolution of Reptile Chromosomes through Applications of Combined Cytogenetics and Genomics Approaches

Abstract

Studies of reptile (nonavian reptiles) chromosomes began well over a century ago (1897) with the initial report on the description of sand lizard (Lacerta agilis) chromosomes. Since then, chromosome analysis in reptiles has contributed significantly to understanding chromosome evolution in vertebrates. Reptile karyotypes are also unique, as being the only vertebrate group where the majority of the species possess variable numbers of macro- and microchromosomes, which was first reported for iguanids and teiids in 1921. In addition, many reptiles have microchromosomes as sex chromosomes, highlighting their evolutionary significance, yet very little is known about their evolutionary origin and significance in shaping amniote genomes. Advances in genomic technologies in recent years have accelerated our capacity to understand how sequences are arranged within a genome. However, genomic and cytogenetic analyses have been combined for only 3 species to provide a deeper understanding of reptile chromosome evolution and sequence organization. In this review, we highlight how a combined approach of cytogenetic analysis and sequence analysis in reptiles can help us answer fundamental questions of chromosome evolution in reptiles, including evolution of microchromosomes and sex chromosomes.

Consequence of Paradigm Shift with Repeat Landscapes in Reptiles: Powerful Facilitators of Chromosomal Rearrangements for Diversity and Evolution

“…It is difficult to trace the evolutionary history of reptile microchromosomes. Microchromosomes were first recorded in iguanid and teiid lizards [202][203][204]. Microchromosomes are considered to have originated from fragments of ancestral macrochromosomes [205].…”

Chromosome Painting Does Not Support a Sex Chromosome Turnover in Lacerta agilis Linnaeus, 1758

“…For example, the XY chromosomes of A. carolinensis correspond to chicken chromosome 15 (GGA15) [Alföldi et al, 2011]. This "genomic identity" is known for many reptile sex chromosome systems and can be uncovered by different molecular genetic, genomic, and cytogenetic approaches [Deakin and Ezaz, 2019].…”

Identification of Y chromosome markers in the eastern three-lined skink (Bassiana duperreyi) using in silico whole genome subtraction

“…Unlike mammals, squamates show a remarkable diversity in sex chromosome structure, representing various degrees of differentiation in sex homologs [9][10][11][12][13]. Such heterogeneity is brought about by variation in the evolutionary age of lineages with independently evolved sex chromosomes [11,14].…”