Self-incompatibility in Cosmos atrosanauineus : a rare Mexican endemic species of Asteraceae
This work centres on Cosmos atrosanguineus, a rare Mexican endemic self-incompatible species of Asteraceae that is now believed to be extinct in the wild. The two known wild C. atrosanguineus collections, made in made in the 19th century, localise the species to the pine-oak mountain forest ecological region in two areas of central Mexico. Its disappearance from the natural environment is attributed to habitat destruction by the copper mining industry and subsequent urbanisation, so that C. atrosanguineus is now known only as a cultivated species. European and American C. atrosanguineus populations are probably the progeny of seed collected in the 1860s by Benedict Roezl and brought to Europe for plant verification. Sir William Jackson Hooker (1785-1865), Director of the Royal Botanic Gardens, Kew initially identified the Roezl specimen as Cosmos diversifolius var. atrosanguineus, but it was classified as the distinct species, Cosmos atrosanguineus by Andreas Voss in 1894. Cosmos atrosanguineus is much prized in horticulture for its deep-red "atrosanguinate" blooms, which are deliciously chocolate scented, giving rise to its common name, "the Hot Chocolate Plant". From 1885 to 1942, C. atrosanguineus was sold as seed in England by Thompson and Morgan Seed Merchants, under the name of C. diversifolius atrosanguineus, but disappeared from the Thompson and Morgan seed catalogue thereafter. Cosmos atrosanguineus exhibits strong sporophytic self-incompatibility (SSI), does not set seed, and must be propagated vegetatively, a factor that has contributed to the proliferation of closely related or identical genotypes. Microsporogenesis within the species appears normal, with no discernible differences between microsporogenesis in C. atrosanguineus and two seed-producing Cosmos species. The C. atrosanguineus karyotype and chromosome associations at meiosis identify the species as an allotetraploid that produces viable gametes with ~30% pollen viability. Self- and cross-pollinations of C. atrosanguineus are strongly incompatible with an average of zero pollen grains germinating per stigma. In contrast, cross and self-pollinations in C. bipinnatus and C. sulphureus produced three compatibility groups; incompatible (-), compatible (+) and semi-compatible (±), and four categories of pollen-stigma interaction that putatively identify a gametophytic-sporophytic (G-S) incompatibility system in Cosmos. Dominance interactions of S-alleles are prevalent in the genus and pseudo-self-compatibility was observed in C. bipinnatus but not C. atrosanguineus. The Cosmos stigma is dry, papillate and becomes receptive only after stigmas become reflexed. Prior to this period, the stigmas do not respond to pollination and selfed bud pollinations and crossed bud pollinations are incompatible. Degenerate S-specific primers identified 41 S-domain-encoding sequences from Cosmos and four other genera of Asteraceae. These 41 Asteraceae sequences encoded S-domains related to Brassica S-domain proteins, but none identified as a putative self-incompatible molecule, indicating the control of SI in Cosmos is probably different to that in Brassica. Genetic fingerprinting studies (AFLP) of various European sources of C. atrosanguineus showed no genetic variation. In conclusion, Cosmos atrosanguineus is a strongly self-incompatible species with a reduced genome. It does not set seed because the gene pool of cultivated individuals has narrowed to a level where S-allele numbers are too few to produce cross compatible genotypes. For this to have occurred, the genetic diversity of plants at the Royal Botanic Gardens Kew and in commercial cultivation must have been reduced to genets that share one, but more probably both S-alleles, thus preventing germination of pollen and formation of seed.