Systematic Studies in Hemizonia (Asteraceae: Madiinae): Hybridization of H. fasciculata with H. clementina and H. minthornii

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Abstract
The annual Hemizonia fasciculata (sect. Madiomeris) was artificially crossed with two perennials, H. clementina and H. minthornii (sect. Zonamra). Morphological analyses of these respective hybrids were shown to be intermediate in most floral characters, while vegetative characters resembled on parent more than the other. Meiotic analyses of the hybrids suggested strongly that the genome of H. clementina is more closely related to that of H. fasciculata than the genome of H. minthornii is to that of H. fasciculata. Hybrid sterility (pollen stainability less than 5%) is presumed to be the result of chromosomal and genic differences between the respective genomes. Morphological and cytogenetical analyses support the contention that these two perennial species are independently derived from annual taxa. Lack of natural hybridization and low fertility of artificial hybrids suggest strong external and internal reproductive isolating barriers occur among these three species.