Evolution of SAMD9 and SAMD9L genes in primates: a complex history of gene loss, gene duplication and pseudogenization.

The Sterile Alpha Motif Domain-containing 9 (SAMD9) and SAMD9-like (SAMD9L) are two paralogous genes responsible for important antiviral and antitumoral functions. While their functions and mechanisms have become well defined throughout years of research, research into their evolution has seemingly stagnated. In this work, we analysed the evolution of these two genes in primates. We have found that the evolutionary history of these two genes in primates is remarkably variable, with genome deletions, gene duplications, pseudogenes and translocations being identified, with roughly half of all analysed primate species having lost functional copies of one of these genes. Microcebus murinus lacks functional copies of both genes, having undergone a deletion of SAMD9L and the pseudogenization of SAMD9, making it the only currently known mammal to have no functional copy of either SAMD9 or SAMD9L. Hominidae and Cercopithecinae were found to be the only two primate families to have maintained both SAMD9 and SAMD9L in their functional forms, while the remaining primate families and wider clades preserved only one gene in its functional form: Colobinae have one functional SAMD9L and a pseudo-SAMD9; Platyrrhini have SAMD9L and lost SAMD9 and Strepsirrhini have lost SAMD9L, while preserving their SAMD9. Interestingly, SAMD9 underwent deletions and pseudogenizations multiple times in Haplorhini. We hypothesise that the frequent loss of primate SAMD9s may be related to their proviral effects in certain primate viruses, such as HIV-1, although functional studies must be performed to test our hypothesis.