Trichostema L., a North American genus in the Lamiaceae family comprised of annual and perennial species, is found growing across the continental United States and extending north into Canada and south into Mexico (Lewis, 1960). Of the species, the perennial T. lanatum, also known as romero or woolly blue curls, is probably the most recognizable species, as it is commercially available and is used in California landscapes (Clausen and Ekstrom, 1989; Crane 2007; Evans and Bohn, 2001; Wilson 2007). Trichostema spp. grows vigorously during the summer and early autumn dry season, making it an excellent plant for xeriscaping (Dalby, 2004; Heisey and Delwiche, 1984). The flowers are also reported to have a pleasant, aromatic fragrance, and are a favorite of hummingbirds and butterflies (Crane, 2007; Dalby, 2004). Not all species are of ornamental value, as a number of annual species, including T. brachiatum L., T. dichotomum L., and T. setaceum Houtt., are reported as weedy (Lewis, 1945).
The exact number of species has been unclear in the literature, although 16 species was reported as late as 1978 (Lewis and Rzedowski, 1978). Epling (1940) had suggested that T. mexicanum Epling could be separate from T. arizonicum based on reduced hairiness, flower size, and branching inflorescences. Lewis (1945) considered T. mexicanum to fall within the range of T. arizonicum and thus to be synonymous with T. arizonicum. Henrickson (1982) agreed with Epling that T. mexicanum is a distinct species based on corolla size and color, anther length, inflorescence habit, growth habitat, and distribution. Lewis (personal communication) agreed that T. mexicanum should be considered a separate species. He recently described another species, T. ruygtii H. Lewis, and notes its close relationship to T. lanceolatum Benth. (Lewis, 2006). This brings the total to 18 species.
Originally, Lewis had proposed five sections (Chromocephalum, Paniculatum, Rhodanthum, Orthopodium Benth., and Trichostema Benth.) based on flower morphology, chromosome counts, and growth habit (Lewis, 1945). Trichostema purpusii (formerly the monotypic genera Eplingia L.O. Williams) was originally placed in sect. Rhodanthum, as Lewis (1945) notes only remote similarities with any of the other species. However, upon germination of some collected T. purpusii seed, Lewis and Rzedowski (1978) later reported that T. purpusii and T. arizonicum were more closely related to each other than any other species in the genus; therefore, he included T. purpusii with T. arizonicum in sect. Paniculatum. Huang (2002) agreed with Lewis's original decision to separate the species into their individual sections by reporting ndhF data showing sect. Paniculatum and Rhodanthum as being sisters to one another, and thus T. purpusii will be considered in sect. Rhodanthum.
Sections Chromocephalum and Paniculatum were reported to have n = 10 chromosomes, whereas other chromosome numbers within the genus vary among the species and even sections, ranging from n = 7 to n = 19 (Lewis, 1960). Two shrubby perennial California species are included in sect. Chromocephalum: T. lanatum and T. parishii Vasey.
Section Trichostema species observed in the wild generally grow in isolated populations; however, Lewis (1945, 2006) did report that some species can be found growing together where boundaries overlap, as in the case of T. parishii and T. lanatum populations, as well as T. ruygtii and T. lanceolatum. No intermediate hybrids have been reported for sect. Trichostema from field observations, even though interspecific hybridization within Lamiaceae is reported to be common and has been seen in Salvia L., Mentha L., Monarda L., Dicerandra Benth., and Hyptis Jacq. (Huck, 1992). Lewis (personal communication) attempted crosses between T. lanatum and T. parishii, both sect. Chromocephalum, with no success.
Thus, little is known of the breeding compatibility among the species. Artificial hybridization yields information on crossability, hybrid fertility, and the genetic contributions to a phenotype, all of which give insight into the relationship among species (Kruckeberg, 1962). The main objective of this work was to assess the genetic relationship through interspecific hybridization between three ornamental perennial Trichostema species: T. lanatum (sect. Chromocephalum), T. purpusii (sect. Rhodanthum), and T. arizonicum (sect. Paniculatum).
Brewbaker, J.L. & Kwack, B.Y. 1963 The essential role of calcium ion in pollen germination and pollen tube growth Amer. J. Bot. 50 859 865
Crane, K. 2007 Yerba Buena Nursery (Trichostema lanatum) 13 May 2007 http://www.yerbabuenanursery.com/online_album/0810.htm.
Evans, M. & Bohn, J. 2001 Trichostema lanatum, Woolly Bluecurls 13 May 2007 <http://www.treeoflifenursery.com/main/PDFs/Plant-Profiles/Trich_lana.pdf>
Flematti, G.R., Ghisalberti, E.L., Dixon, K.W. & Trengove, R.D. 2004 A compound from smoke that promotes seed germination Science 305 977
Huck, R.B. 1992 Overview of pollination biology in the Lamiaceae 167 181 Harley R.M. & Reynolds T. Advances in Labiate science Royal Botanic Gardens Kew, UK
Lindstrom, J.T. & Pelto, M.C. 2003 Micropropagation of Virginia sweetspire (Itea virginica ‘Henry's Garnet’) J. Environ. Hort. 21 206 208
Huang, H. 2002 Systematics of Trichostema L. (Lamiaceae) and phylogenetic relationships with its disjunct taxa in Asia Ohio State University Columbus, OH PhD dissertation.
Qureshi, S.J., Awan, A.G., Khan, M.A. & Bano, S. 2002 Study of pollen fertility of the genus Launaea from Pakistan Asian J. Plant Sci. 1 73 74
Sarkissian, T.S. & Harder, L.D. 2001 Direct and indirect responses to selection on pollen size in Brassica rapa L J. Evol. Biol. 14 456 468
Sanford, J.C. 1983 Ploidy manipulations 100 123 Moore J.N. & Janick J. Methods in fruit breeding Purdue University Press West Lafayette, IN
Spira, T.P. 1978 Floral parameters associated with breeding system and pollinator type in Trichostema (Labiatae) California State University Chico, CA MA thesis.
Ubera-Jiménez, J.L. & Hidalgo-Fernández, P.J. 1992 Temporal gynodiocy in Rosmarinus officinalis 281 289 Harley R.M. & Reynolds T. Advances in Labiate science Royal Botanic Gardens Kew, UK