PNAS: The genome of the common ancestor of all mammals

Every modern mammal, from platypuses to blue whales, is the descendant
of a common ancestor who lived 180 million years ago.

"Our findings have important implications for understanding mammalian evolution and conservation efforts," said
Harris Lewin, Distinguished Professor of Evolution and Ecology at the University of California, Davis and senior author of the paper.

The researchers extracted high-quality genomic sequences from 32 extant species, representing 23 of the 26 known mammalian
orders.

Joanna Damas, a postdoctoral researcher at the University of California's Davis Genomics Center and a first author of the study, said the reconstructions show that mammalian ancestors had 19 autosomals that control the inheritance of features of the organism outside the control of sex-linked chromosomes, (these chromosomes are paired in most cells, 38 for a total) plus two sex chromosomes
.

The chromosomes are stable for more than 300 million years

The researchers found 9 complete chromosomes or chromosomal fragments in mammalian ancestors that were genetically sequenced in the same order as
those of modern birds.

"This remarkable discovery shows the evolutionary stability
of the order and direction of genes on chromosomes over an evolutionary time frame of more than 320 million years," Lewin said.

In contrast, the regions between these conserved blocks contain more repeating sequences and are more prone to breaks, rearrangements, and sequence duplications, which are the main drivers
of genomic evolution.

"The reconstruction of the ancestral genome is crucial
to explaining where and why the selection pressures on the genome are and why they change.

The researchers were able to trace ancestral chromosomes forward along time from a common ancestor
.

The authors say the results will help understand the genes behind the adaptations
of mammals to thrive on a changing planet over the past 180 million years.

essay

Evolution of the ancestral mammalian karyotype and syntenic regions