Chromosome Numbers in Mammals

Abstract

Abstract 180 million years of mammalian radiation have shaped the genomes of approximately 4600 extant species and are witnessed by karyotypic diversity in extant species. Knowledge of chromosome numbers and morphology among mammalian species arrived during the period of classical cytogenetics in the 1960s and provided a first glimpse of the extent of karyotype reorganisation among various mammalian species. Further technical advances like banding techniques and the subsequent arrival of molecular tools like chromosome sorting, painting and deoxyribonucleic acid (DNA) probes of different complexity led to improved molecular tools that allowed the visualisation of homology at the DNA level directly on chromosomes leading to modern phylogenomics. This article will provide an overview of what is known about mammalian karyotypic diversity and the research in mechanisms of chromosome rearrangements that shaped the present‐day karyotypes, with a special emphasis on muntjac chromosome evolution. Key Concepts: The chromosome number and morphology is a species‐specific feature. Most diploid chromosome numbers of mammals lie within the range of 36–60. Comparative cytogenomics suggests a 2 n =44 karyotype for the mammalian ancestor. Comparative cytogenetic comparisons are only informative among species that display a similarly low rates of karyotypic change. Despite a wide range of chromosome numbers and morphologies there is remarkable conservation of the coding part of mammalian genomes. Chromosome rearrangements fixed during mammalian evolution are witness of benign karyotypic change. Polymorphisms in heterochromatin and copy number variation of repetitive DNA sequences are the source of significant karyotype and genome size variations in mammals and beyond. Cervidae evolution is associated with a rapid karyotypic turnover leading to chromosome numbers from 2 n =6 to 2 n =80. The extant muntjac karyotype displays the remnants of numerous chromosomal tandem fusions, an evolutionary rare aberration type. Illegitimate recombination events among telomeric and centromeric repetitive sequences in combination with a prevalence of inbreeding can be considered as driving force of muntjac karyotype evolution.