Abstract
BackgroundWhen 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.ResultsBy 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.ConclusionOur 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.
Highlights
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
Maternal inheritance of chloroplast DNA in Sonneratia and Bruguiera We used DAPI (4',6-diamidino-2-phenylindole)-staining fluorescence microscopy to determine if chloroplast DNA is maternally inherited in a certain plant
No punctated fluorescence corresponding to plastid DNA aggregates was observed in mature pollen grains of Sonneratia alba and Bruguiera gymnorrhiza (Fig. 1), indicating the mode of nFDoiAgrruPhIri-zesata1ined epifluorescence microphotographs showing squashed pollen grains of (A) Sonneratia alba and (B) Bruguiera gymDAPI-stained epifluorescence microphotographs showing squashed pollen grains of (A) Sonneratia alba and (B) Bruguiera gymnorrhiza
Summary
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 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. Patterns of interspecific hybridization are of great interest to many biologists, because they may provide insights into the mechanisms and evolution of reproductive barriers. Some cytological, physiological and ecological mechanisms, such as ploidy level [4], breeding system [7] and unidirectional nuclear-cytoplasmic incompatibility [2,5], may account for the asymmetrical hybridization in specific cases. We shall consider general hypotheses that i) do not invoke the specific reproductive biology of the plants in question, and ii) can account for hybridization patterns that depend on the relative species abundance in the hybrid zone
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