Breeding systems of northeastern North American Spiranthes (Orchidaceae)

Abstract

Breeding systems in northeastern North American Spiranthes orchids were studied to provide background information for taxonomic and evolutionary interpretations. Pollination experiments revealed that Spiranthes casei var. casei, S. cernua var. cernua, S. magnicamporum (southwestern Ontario), and S. ochroleuca (southern Nova Scotia) are agamospermic, while S. lacera var. lacera, S. lacera var. gracilis, S. laciniata, S. magnicamporum (Illinois), S. ochroleuca (Pennsylvania, New York), S. romanzoffiana, S. tuberosa, and S. vernalis are sexual. In the sexual taxa, cross-pollination is followed by earlier dehiscence of capsules, a higher percentage of ovaries with seeds, and a higher percentage of seed coats with well-developed embryos than either self-pollination or geitonogamous pollination. This may be explained by partial self-incompatibility involving restricted pollen-tube growth and (or) inbreeding depression. In Spiranthes romanzoffiana and races of S. ochroleuca requiring pollination for seed development, the two or three layers of cells comprising the inner integument degenerate after fertilization. Seed produced by species requiring pollination (i.e., produced sexually) is monoembryonic with the embryo located centrally. Examination of stained serial sections in successive developmental stages indicated that agamospermy in northeastern taxa of Spiranthes is by adventitious embryony, the embryos (one or more) being formed by proliferation of the micropylar inner integument beginning at the time of the first division of the megasporocyte. Direct anatomical evidence of adventitious embryony was obtained for S. casei var. casei, S. casei var. novaescotiae, S. cernua var. cernua, and S. magnicamporum (Ontario). Adventitious embryony in northeastern Spiranthes taxa is autonomous but not necessarily obligate and can be readily detected because of its association with polyembryonic seed, extruded embryos, free embryos, and ruptured embryo sacs. Three northeastern taxa are characterized by autonomous agamospermy throughout their areas of distribution, while another three are agamospermic only at their northeastern range limits. These six comprise a taxonomically complex group. In contrast, correlative seed morphology indicates that eight taxonomically distinctive northeastern taxa are uniformly sexual, some clearly favouring outbreeding. Adventitious embryony in northeastern North American Spiranthes is associated with glaciated territory, northeastern range limits, and more or less isolated bioclimatic zones.