Pattern of the floral variation was investigated in the hybrid zone of two distylous Primula species. The three taxa of the P. vulgaris x P. veris hybrid zone were identified on the basis of flower color and inflorescence structure. Hybrids were significantly different from the two parental species for all the floral characters examined: four out of the seven characters exhibited extreme values, and three characters were intermediate. The lack of great morphological variation and histograms of the characters exhibiting intermediate values in hybrids give the impression that there is no introgression in the studied hybrid zone. Stigma heights and, to a lesser degree, anther heights were in two discrete classes corresponding to short-styled and long-styled flowers in all the three taxa. Relative reciprocity ratios and examination of individual flowers revealed that there was strict reciprocity between anther and stigma heights in the two parental species, but reciprocity was strongly broken in the hybrids. Both stigma and anther heights showed considerable relationship with corolla tube length, and increasing corolla tube length resulted in deviation from the strict reciprocity in both floral morphs. Short-tubed long-styled and long-tubed short-styled flowers seem to be at a disadvantage in mating, and this might explain the extremely low variability of corolla tube length. Our results on the floral morphology indicate that there are no barriers against formation of the F1hybrids in the adjacent populations of the two parental species. A possible explanation for the lack of introgression is also discussed.
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