title: Dearth of polymorphism associated with a sustained response to selection for flowering time in maize.
abstract: BACKGROUND
  Long term selection experiments bring unique insights on the genetic architecture of quantitative traits and their evolvability. Indeed, they are utilized to (i) monitor changes in allele frequencies and assess the effects of genomic regions involved traits determinism; (ii) evaluate the role of standing variation versus new mutations during adaptation; (iii) investigate the contribution of non allelic interactions. Here we describe genetic and phenotypic evolution of two independent Divergent Selection Experiments (DSEs) for flowering time conducted during 16 years from two early maize inbred lines.
  
  
  RESULTS
  Our experimental design uses selfing as the mating system and small population sizes, so that two independent families evolved within each population, Late and Early. Observed patterns are strikingly similar between the two DSEs. We observed a significant response to selection in both directions during the first 7 generations of selection. Within Early families, the response is linear through 16 generations, consistent with the maintenance of genetic variance. Within Late families and despite maintenance of significant genetic variation across 17 generations, the response to selection reached a plateau after 7 generations. This plateau is likely caused by physiological limits. Residual heterozygosity in the initial inbreds can partly explain the observed responses as evidenced by 42 markers derived from both Methyl-Sensitive Amplification- and Amplified Fragment Length- Polymorphisms. Among the 42, a subset of 13 markers most of which are in high linkage disequilibrium, display a strong association with flowering time variation. Their fast fixation throughout DSEs' pedigrees results in strong genetic differentiation between populations and families.
  
  
  CONCLUSIONS
  Our results reveal a paradox between the sustainability of the response to selection and the associated dearth of polymorphisms. Among other hypotheses, we discuss the maintenance of heritable variation by few mutations with strong epistatic interactions whose effects are modified by continuous changes of the genetic background through time.
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. eleonore.durand@univ-lille1.fr.|CNRS, Ferme du Moulon, Gif sur Yvette, 91190, France. eleonore.durand@univ-lille1.fr.
authors.forename: Eleonore
authors.initials: E
authors.lastname: Durand
authors.affiliation: CNRS, Ferme du Moulon, Gif sur Yvette, 91190, France. tenaillon@moulon.inra.fr.
authors.forename: Maud I
authors.initials: MI
authors.lastname: Tenaillon
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. raffoux@moulon.inra.fr.
authors.forename: Xavier
authors.initials: X
authors.lastname: Raffoux
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. stephanie.thepot@gmail.com.
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Thépot
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. falque@moulon.inra.fr.
authors.forename: Matthieu
authors.initials: M
authors.lastname: Falque
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. jamin@moulon.inra.fr.
authors.forename: Philippe
authors.initials: P
authors.lastname: Jamin
authors.affiliation: CNRS, Ferme du Moulon, Gif sur Yvette, 91190, France. bourgais@moulon.inra.fr.
authors.forename: Aurélie
authors.initials: A
authors.lastname: Bourgais
authors.affiliation: INRA, Ferme du Moulon, Gif sur Yvette, 91190, France. ressayre@moulon.inra.fr.
authors.forename: Adrienne
authors.initials: A
authors.lastname: Ressayre
authors.affiliation: Univ Paris-Sud, Ferme du Moulon, Gif sur Yvette, 91190, France. dillmann@moulon.inra.fr.
authors.forename: Christine
authors.initials: C
authors.lastname: Dillmann
mesh_terms: D005075:Biological Evolution; D044127:Epigenesis, Genetic; D035264:Flowers; D005787:Gene Frequency; D015810:Linkage Disequilibrium; D009154:Mutation; D011110:Polymorphism, Genetic; D012641:Selection, Genetic; D003313:Zea mays / Q000235:genetics / Q000502:physiology*