Crossing Relationships among North American and Eastern Asian Populations of Agastache sect. Agastache (Labiatae)

Abstract

Agastache sect. Agastache consists of seven species occurring in North America and one in eastern Asia. Intra- and interspecific crosses were made within sect. Agastache, with special emphasis given to crossing relationships between Asian and North American groups. Interspecific hybrids were obtained between all species-pairs of sect. Agastache; percentages of pollen stainability for these hybrids ranged from 0.0 to 99.5%. Crosses between Asian A. rugosa and North American species from which it presumably has been separated for over 12 million years produce vigorous but infertile offspring. Cytogenetic analyses indicate that chromosomal rearrangements may in part be responsible for the reduced pollen fertility observed in these intercontinental hybrids. However, meiosis in these hybrids appears to be otherwise normal. In addition, intersectional hybrids were obtained between sects. Agastache and Brittonastrum; intersectional percentages of hybrid pollen stainability ranged from 0.0 to 49.1%. Since many in- tersectional hybrids are more fertile than many intrasectional hybrids in sect. Agastache, pollen stainability data do not support current sectional delimitation of the genus.