SCIENCE: Reconstruction of the first mammalian genome reveals 38 chromosomes

Sci­en­tists don’t real­ly know what the first mam­mals looked like, but they lived 180 to 250 mil­lion years ago, from the blue whale to the platy­pus. We know that every mam­mal on the plan­et is descend­ed from them.

But new research is begin­ning to tell us what that genome looked like.

An inter­na­tion­al team of sci­en­tists has recon­struct­ed this genome, which is thought to be the com­mon ances­tor of mam­mals, from 32 exist­ing genomes on a computer.

A wide range of species was ana­lyzed for the three species, includ­ing vivip­a­rous mam­mals such as nar­whals, bats, pan­golins and humans, and mar­su­pi­als such as Tas­man­ian dev­ils, wom­bats and platypuses.

Chick­ens and Chi­nese croc­o­diles were used as non-mam­malian comparisons.

The research team traced the com­mon ances­tor of mam­mals and recon­struct­ed the com­plete set of chro­mo­somes at 16 nodes (a node rep­re­sents the last com­mon ances­tor between two dif­fer­ent genet­ic lin­eages, and the phy­lo­ge­net­ic tree can be divid­ed into sev­er­al branch­es) is the branch­ing point).

They con­clud­ed that the species at the top of the mam­malian tree prob­a­bly has 38 chromosomes.

It shares the com­mon ances­tor of mam­mals, birds and rep­tiles with the nine small­est chro­mo­somes, and is at the bot­tom of the tree.

Lead author and evo­lu­tion­ary biol­o­gist Har­ris Lewin said, “This strik­ing dis­cov­ery demon­strates that the order and ori­en­ta­tion of genes on chro­mo­somes is sta­ble over a long evo­lu­tion­ary peri­od of more than 320 mil­lion years. It shows that there is such sta­bil­i­ty,” he said.

Many of these high­ly con­served regions con­tain genes asso­ci­at­ed with devel­op­men­tal function.

The researchers exam­ined how chro­mo­somes have split, joined, delet­ed, repeat­ed and moved over time.

Regions affect­ed by chro­mo­some rearrange­ments are called “break­points,” which are rich in genet­ic vari­a­tion and play a role in the sep­a­ra­tion of species dur­ing evolution.

The high­est like­li­hood of diver­gence occurred when the “ther­a­peu­tics,” which car­ry liv­ing off­spring in mar­su­pi­als and pla­cen­tal mam­mals, diverged from egg-lay­ing monotremes.

“These find­ings have impor­tant impli­ca­tions for under­stand­ing mam­malian evo­lu­tion and for con­ser­va­tion,” Lewin says.

Ear­ly mam­mals prob­a­bly resem­bled small mouse-like mam­mals called mor­ganu­codons, which lived and laid eggs about 200 mil­lion years ago. A fos­sil dis­cov­ered in 1949 in a lime­stone fis­sure in Wales, England.

Although the genus is close­ly relat­ed to mod­ern mam­mals, it is not con­sid­ered a com­mon ances­tor and is a sis­ter group in the mam­malian lineage.

There is also a sis­ter clade to the mam­mals, the rodent-like genus Trityrodon.

Fos­sils found in Africa and North Amer­i­ca are too spe­cial­ized for there to be a com­mon ances­tor for all mam­mals, but they must have lived at the same time as the old­est mam­malian species.