Chromosome numbers in Hypericum and related genera

Abstract

New or confirmatory chromosome counts for 16 taxa ofHypericum in Britain, one of northwest Africa, and 24 of North and Central America are reported. First records for any member of the genusVisma (n = 10) are also included. The counts are discussed with respect to those previously reported forHypericum and related genera. Some of this chromosomal information is incorporated in a diagram showing suggested evolutionary trends withinHypericum. There appears to be a descending series of basic numbers (x = 12, 10, 9, 8, 7), of which all but the last have been recorded in polyploid, as well as diploid, form. Observations of chromosome morphology suggest thatHypericum is cytologically relatively unspecialized. Studies of chromosome morphology, therefore, are not likely to yield much information about the evolutionary history of the genus. It is suggested that the basic number forAllanblackia andPentadesma is 7 or 14, whereas inCalophyllum andMesua it is probably 8 or 16. Known gametic chromosome numbers inMammea (n = 16, 18) andGarcinia (n = 24, ca. 27, ca. 29, ca. 38, ca. 40, and 48) do not indicate an obvious basic number for these genera, although x = 8, 9, and 16 might be involved. Ring-formation of theOenothera-type, possibly indicative of structural hybridity, is reported for the first time inHypericum mitchellianum Rydb., a close relative ofHypericum punctatum Lam., which was previously shown to possess this anomalous chromo some condition.