EVOLUTION OF THE DAZ GENE FAMILY SUGGESTS THAT Y-LINKED DAZ PLAYS LITTLE, OR A LIMITED, ROLE IN SPERMATOGENESIS BUT UNDERLINES A RECENT AFRICAN ORIGIN FOR HUMAN-POPULATIONS

The recent transposition to the Y chromosome of the autosomal DAZL1 ge ne, potentially involved in germ cell development, created a unique op portunity to study the rate of Y chromosome evolution and assess the s elective forces that may act upon such genes, and provided a new estim ate of the male-to female mutation rate (a,), Two different Y-located DAZ sequences were observed in all Old World monkeys, apes and humans, Different DAZ copies originate from independent amplification events in each primate lineage. A comparison of autosomal DAZL1 and Y-linked DAZ intron sequences gave a new figure for male-to-female mutation rat es of alpha(m) = 4. It was found that human DAZ exons and introns are evolving at the same rate, implying neutral genetic drift and the abse nce of any functional selective pressures, We therefore hypothesize th at Y-linked DAZ plays little, or a limited, role in human spermatogene sis. The two copies of DAZ in man appear to be due to a relatively rec ent duplication event (55 000-200 000 years). A worldwide survey of 67 men from five continents representing 10 distinct populations showed that most males have both DAZ variants. This implies a common origin f or the Y chromosome consistent with a recent 'out of Africa' origin of the human race.