The Systematics and Evolution of Circaea (Onagraceae)

Boufford, David

Circaea (Onagraceae) is a well delimited genus and the sole member of the tribe Circaeeae. Unlike other Onagraceae, Circaea is restricted to the mixed mesophytic and boreal forests of the northern hemisphere and reaches its greatest diversity in eastern Asia, where 12 of the 14 taxa occur. The very closely related C. lutetiana subspp. canadensis and quadrisulcata exhibit the frequent pattern of disjunction between eastern North America and eastern Asia. Hybridization in the genus is common and widespread and has long been recognized in Europe and North America. In Asia, hybridization is equally widespread and more complex because of the greater number of species involved. In all cases, hybrids are highly sterile and, in crosses involving C. alpina, they are often completely sterile. Very little or no backcrossing and introgression occur in Circaea. Hybrids often form large colonies and reproduce vigorously by vegetative means. They often behave as fertile species in the genus in being wide-ranging, in occupying habitats distinct from those of the parents, in being morphologically similar from population to population and in often occupying ranges partially outside the range of one or both parents. All species are diploid with n = 11, the original basic chromosome number for the family. The structure of the nectaries, the leaves, and the retention of the basic chromosome number, n = 11, are generalized characters shared with Fuchsia, which is one of the most primitive genera of Onagraceae. Fuchsia is bird-pollinated and primarily tropical in distribution while Circaea, whose epizoochoric fruits are known from the Oligocene, is pollinated chiefly by syrphid flies and small bees and is temperate in distribution. It is likely that both genera were derived from a common ancestor. The ancestor of Circaea apparently reached North America from the probable place of origin for the family in South America. The high diversity of taxa, including the most primitive in the genus, in eastern Asia seems to be related to the favorable climatic conditions prevailing there since Circaea reached this region, a well-known center of survival for many Arcto-Tertiary genera, and not to an Asian origin for Circaea. The genus has evolved in two directions since its origin: 1) towards more efficient outcrossing through modification of the nectary so that it is more conveniently positioned for visits by short-tongued insects, primarily syrphid flies, and 2) towards self-pollination by having the anthers appressed to the stigma and dehiscing before opening of the buds, and in reduction of the locules in the ovaries and fruits from two to one. Circaea repens of the Himalayan region occupies an intermediate position between the two groups in the presence of a trace of a second locule. Circaea is treated in accordance with other recent work in the Onagraceae. Seven species are recognized. Circaea alpina, the most wide-ranging of the species, comprises six subspecies, each occupying a distinct ecological and/or geographical area. Circaea lutetiana includes three subspecies, one each in Asia, Europe, and North America. The remaining species are C. cordata, C. erubescens, C. glabrescens, C. mollis, and C, repens, all confined to Asia. Three new hybrids, C. x skvortsovii, C. x decipiens, and C. x mentiens, are described, one new combination, C. x ovata, is made, and two taxa are changed in status. Full descriptions are provided for the eight known hybrids in addition to those for the species and subspecies.

The Systematics and Evolution of Circaea (Onagraceae)

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