title: Testing a hypothesis of unidirectional hybridization in plants: observations on Sonneratia, Bruguiera and Ligularia.
abstract: BACKGROUND
  When natural hybridization occurs at sites where the hybridizing species differ in abundance, the pollen load delivered to the rare species should be predominantly from the common species. Previous authors have therefore proposed a hypothesis on the direction of hybridization: interspecific hybrids are more likely to have the female parent from the rare species and the male parent from the common species. We wish to test this hypothesis using data of plant hybridizations both from our own experimentation and from the literature.
  
  
  RESULTS
  By examining the maternally inherited chloroplast DNA of 6 cases of F1 hybridization from four genera of plants, we infer unidirectional hybridization in most cases. In all 5 cases where the relative abundance of the parental species deviates from parity, however, the direction is predominantly in the direction opposite of the prediction based strictly on numerical abundance.
  
  
  CONCLUSIONS
  Our results show that the observed direction of hybridization is almost always opposite of the predicted direction based on the relative abundance of the hybridizing species. Several alternative hypotheses, including unidirectional postmating isolation and reinforcement of premating isolation, were discussed.
authors.affiliation: State Key Laboratory of Biocontrol and Key Laboratory of Gene Engineering of Ministry of Education, Sun Yat-Sen University, Guangzhou 510275, PR China. zhrench@mail.sysu.edu.cn
authors.forename: Renchao
authors.initials: R
authors.lastname: Zhou
authors.forename: Xun
authors.initials: X
authors.lastname: Gong
authors.forename: David
authors.initials: D
authors.lastname: Boufford
authors.forename: Chung-I
authors.initials: CI
authors.lastname: Wu
authors.forename: Suhua
authors.initials: S
authors.lastname: Shi
mesh_terms: D019659:Asteraceae / Q000235:genetics*; D018742:DNA, Chloroplast / Q000235:genetics*; D018745:Genome, Plant; D006824:Hybridization, Genetic / Q000235:genetics*; D040582:Inheritance Patterns; D029561:Lythraceae / Q000235:genetics*; D008957:Models, Genetic; D043982:Rhizophoraceae / Q000235:genetics*; D017422:Sequence Analysis, DNA; D013045:Species Specificity
chemical_list: D018742:DNA, Chloroplast