title: Extreme selection in humans against homeotic transformations of cervical vertebrae.
abstract: Why do all mammals, except for sloths and manatees, have exactly seven cervical vertebrae? In other vertebrates and other regions, the vertebral number varies considerably. We investigated whether natural selection constrains the number of cervical vertebrae in humans. To this end, we determined the incidence of cervical ribs and other homeotic vertebral changes in radiographs of deceased human fetuses and infants, and analyzed several existing datasets on the incidence in infants and adults. Our data show that homeotic transformations that change the number of cervical vertebrae are extremely common in humans, but are strongly selected against: almost all individuals die before reproduction. Selection is most probably indirect, caused by a strong coupling of such changes with major congenital abnormalities. Changes in the number of thoracic vertebrae appear to be subject to weaker selection, in good correspondence with the weaker evolutionary constraint on these numbers. Our analysis highlights the role of prenatal selection in the conservation of our common body plan.
authors.affiliation: Institute of Biology, Leiden University, P.O. Box 9516, 2300 RA Leiden, The Netherlands. f.galis@biology.leidenuniv.nl
authors.forename: Frietson
authors.initials: F
authors.lastname: Galis
authors.forename: Tom J M
authors.initials: TJ
authors.lastname: Van Dooren
authors.forename: Johan D
authors.initials: JD
authors.lastname: Feuth
authors.forename: Johan A J
authors.initials: JA
authors.lastname: Metz
authors.forename: Andrea
authors.initials: A
authors.lastname: Witkam
authors.forename: Sebastiaan
authors.initials: S
authors.lastname: Ruinard
authors.forename: Marc J
authors.initials: MJ
authors.lastname: Steigenga
authors.forename: Liliane C D
authors.initials: LC
authors.lastname: Wijnaendts
mesh_terms: D000328:Adult; D002574:Cervical Vertebrae / Q000002:abnormalities* / Q000000981:diagnostic imaging; D005260:Female; D005333:Fetus / Q000002:abnormalities / Q000000981:diagnostic imaging; D005801:Genes, Homeobox; D006801:Humans; D007223:Infant; D008297:Male; D009139:Musculoskeletal Abnormalities / Q000453:epidemiology; D015995:Prevalence; D011859:Radiography; D012641:Selection, Genetic; D013904:Thoracic Vertebrae / Q000002:abnormalities* / Q000000981:diagnostic imaging