title: Cytotype diversity in the Sorbus complex (Rosaceae) in Britain: sorting out the puzzle.
abstract: OBJECTIVE
  Large-scale ploidy surveys using flow cytometry have become an essential tool to study plant genome dynamics and to gain insight into the mechanisms and genetic barriers framing ploidy diversity. As an ideal complement to traditional techniques such as chromosome counting, the analysis of cytotype diversity in plant systems such as Sorbus provides primary investigation into the potential patterns and evolutionary implications of hybrid speciation.
  
  
  METHODS
  Ploidy was assessed by means of relative nuclear DNA content using propidium iodide flow cytometry in 474 Sorbus samples collected from 65 populations in southern Wales and South-West England. Statistical tests were applied to evaluate the utility of this technique to confidently discriminate ploidy in the genus.
  
  
  RESULTS
  Flow cytometric profiles revealed the presence of four cytotypes (2x, 3x, 4x and 5x), confirming in many cases chromosome counts previously reported and demonstrating cytotype heterogeneity within specific Sorbus aggregates. Diploid cytotypes were restricted to the potential parental species and homoploid hybrids. Most of the samples processed were polyploid. The occurrence of the pentaploid cytotype had previously only been reported from a single specimen; it is now confirmed for two taxa occurring at different sites.
  
  
  CONCLUSIONS
  Flow cytometry results obtained have proved useful in shedding light on the taxonomy of several controversial taxa and in confirming the presence of cytoypes which occur at very low frequencies. Notably, the coexistence of several cytotypes in Sorbus populations has probably been facilitated by the overlapping distribution of many of the species studied, which might also explain the high incidence of potential hybrid apomictic polyploids. These results will provide a solid baseline for molecular research aiming to better understand the genetic pathways controlling the formation and establishment of polyploid Sorbus.
authors.affiliation: Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK. j.pellicer@kew.org
authors.forename: Jaume
authors.initials: J
authors.lastname: Pellicer
authors.forename: Sandra
authors.initials: S
authors.lastname: Clermont
authors.forename: Libby
authors.initials: L
authors.lastname: Houston
authors.forename: Timothy C G
authors.initials: TC
authors.lastname: Rich
authors.forename: Michael F
authors.initials: MF
authors.lastname: Fay
mesh_terms: D005075:Biological Evolution; D002678:Chimera / Q000235:genetics; D032461:Chromosomes, Plant / Q000235:genetics; D018744:DNA, Plant / Q000032:analysis; D004171:Diploidy; D004739:England; D005434:Flow Cytometry / Q000379:methods*; D049810:Genetic Speciation; D014644:Genetic Variation; D018745:Genome, Plant / Q000235:genetics*; D059785:Karyotype; D011003:Ploidies; D011123:Polyploidy; D011419:Propidium; D031994:Sorbus / Q000235:genetics*; D014852:Wales
chemical_list: D018744:DNA, Plant; D011419:Propidium