title: Evidence on the molecular basis of the Ac/ac adaptive cyanogenesis polymorphism in white clover (Trifolium repens L).
abstract: White clover is polymorphic for cyanogenesis, with both cyanogenic and acyanogenic plants occurring in nature. This chemical defense polymorphism is one of the longest-studied and best-documented examples of an adaptive polymorphism in plants. It is controlled by two independently segregating genes: Ac/ac controls the presence/absence of cyanogenic glucosides; and Li/li controls the presence/absence of their hydrolyzing enzyme, linamarase. Whereas Li is well characterized at the molecular level, Ac has remained unidentified. Here we report evidence that Ac corresponds to a gene encoding a cytochrome P450 of the CYP79D protein subfamily (CYP79D15), and we describe the apparent molecular basis of the Ac/ac polymorphism. CYP79D orthologs catalyze the first step in cyanogenic glucoside biosynthesis in other cyanogenic plant species. In white clover, Southern hybridizations indicate that CYP79D15 occurs as a single-copy gene in cyanogenic plants but is absent from the genomes of ac plants. Gene-expression analyses by RT-PCR corroborate this finding. This apparent molecular basis of the Ac/ac polymorphism parallels our previous findings for the Li/li polymorphism, which also arises through the presence/absence of a single-copy gene. The nature of these polymorphisms may reflect white clover's evolutionary origin as an allotetraploid derived from cyanogenic and acyanogenic diploid progenitors.
authors.affiliation: Department of Biology, Washington University, St. Louis, MO 63130-4899, USA. kolsen@wustl.edu
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Olsen
authors.forename: Shih-Chung
authors.initials: SC
authors.lastname: Hsu
authors.forename: Linda L
authors.initials: LL
authors.lastname: Small
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics*; D001483:Base Sequence; D015139:Blotting, Southern; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics* / Q000378:metabolism; D017931:DNA Primers / Q000235:genetics; D018628:Gene Dosage; D005960:Glucosides / Q000096:biosynthesis / Q000235:genetics; D006856:Hydrogen Cyanide / Q000378:metabolism*; D008969:Molecular Sequence Data; D011110:Polymorphism, Genetic / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D029921:Trifolium / Q000235:genetics* / Q000378:metabolism
chemical_list: D017931:DNA Primers; D005960:Glucosides; D006856:Hydrogen Cyanide; D003577:Cytochrome P-450 Enzyme System