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A Brief Overview of the Compositae, Lettuce and Sunflower

• Overview
• Members
• Lettuce
• Sunflower
• Domestication
• Bibliography

     The Compositae (Asteraceae) is one of the largest and most diverse families of flowering plants, comprising one-tenth of all known angiosperm species. It is characterized by the compound inflorescence that has the appearance of a single "composite" flower from which it derives its name. The Compositae is divided into two major subfamilies and one minor subfamily with 12 to 18 tribes, 1,100 to 2,000 genera and 20,000 or more species (Stebbins, 1953, 1974; Cronquist, 1981; Jansen et al., 1991). Lettuce and sunflower are representatives of each of the two major subfamilies. The base chromosome number for the family is probably nine. All Compositae species analyzed to date have genomes larger than 1.1 pg/1C; unfortunately none is known to have a small genome (reviewed in Kesseli and Michelmore, 1997). The family most likely originated 30 to 100 MYA in South America (Raven and Axelrod, 1974) and underwent rapid and extensive diversification producing a cosmopolitan array of taxa. They are predominantly herbaceous, although woody species also exist. Compositae are found in diverse habitats; anaerophytic, xerophytic, and halophytic specialists thrive in some of the more inhospitable habitats (vertisols, deserts, and salt marshes). Lactuca and Helianthus are both drought tolerant taxa. The little known oilseed, noug (Guizotia abysinnica), is an anaerophyte that grows on water-logged vertisols in Ethiopia and India, where few other species and no other oilseeds can be grown. The size and adaptive success of the Compositae have stimulated considerable research into its systematics and evolution.

     The Compositae contains over 40 economically important species including food (lettuce, Jerusalem artichoke), oil (sunflower, safflower), medicinal (chamomile) and many ornamental (Chrysanthemum, dahlia, zinnia, marigold) crops (Table). The high quality edible oils of sunflower, safflower, and noug are low in saturated and high in mono- and di-unsaturated fatty acids. Additional novel industrial and edible safflower and sunflower oils have been developed (e.g., high oleic sunflower oil). The Compositae are renowned for the production of a variety of novel secondary chemicals. Several novel industrial fatty acids are found in the seed oils of composites, e.g. conjugated dienolic fatty acids in Dimorphotheca, acetylenic fatty acids in Crepis, and epoxy fatty acids in Vernonia and Stokesia (Smith, 1985). The family is a rich source of powerful insecticides and industrial chemicals, e.g., pyrethrum (Chrysanthemum) and rubber (guayule) (Heywood et al., 1977). Several species are grown as medicinal and culinary herbs (Table). Echinacea and others may be sources of biologically active compounds with medical or nutritional benefits. The Compositae also includes several detrimental weeds (dandelion, ragweed, thistle). Lettuce and sunflower are the best genetically characterized members of this family.

Economically Significant Members of the Compositae Family

Lettuce

     Lettuce (Lactuca sativa L.) is a diploid (2n = 18) species within the Lactuceae subfamily of the Compositae (Asteraceae) and has a genome size of ~2.3 pg (Koopman & De Jong, 1996). There are four well-established species within the subsection Lactuca, the cultivated L. sativa and three wild species, L. serriola, L. saligna, and L. virosa (in order of decreasing sexual compatibility with L. sativa). L. serriola is probably the progenitor of L. sativa and may be conspecific (Kesseli et al., 1991; de Vries, 1997). Wild species, particularly L. serriola, have been sources of several disease resistance genes (Crute, 1988); however, they remain a rich source of variation for a range of traits that has not been accessed systematically, particularly for horticultural traits.

     Lettuce is an important crop species with an annual farm-gate value in the US of over $1.1 billion (Anon., 1995; Davis et al., 1997). Genetic improvement programs are focused on morphology, horticultural performance, and physiological disorders as well as disease resistance and post-harvest problems (Ryder, 1986).

     Lettuce is amenable to classical and molecular genetic analyses. Most cultivars of L. sativa are highly inbred and therefore exhibit extensive genetic homozygosity. Crosses can be made readily, although outcrossing cannot be guaranteed. Therefore, most genetic analysis has relied on F₂ rather than backcross or test-cross populations. The generation time is usually three to five months depending on the genotype and environment, allowing multiple generations each year. A few genotypes have generation times of less than two months. Each plant produces large numbers (1,000s) of seeds allowing rare recombinants and mutants to be detected. A detailed genetic map has been constructed as described (link to www address).

Sunflower

     Cultivated sunflower (Helianthus annuus L.) is a diploid (n = 17) species belonging to subtribe Helianthinae in the second major subfamily Asteroideae within the Compositae (Seiler and Rieseberg, 1997). The genus Helianthus is comprised of 12 annual and 36 perennial species and belongs to the subtribe Heliantheae. Sunflower and Jerusalem artichoke (H. tuberosus; n = 34, a perennial tetraploid) are the two significant food sources in the genus. The centers of diversity for both species are North America (Heiser and Smith, 1955; Heiser 1976). Sunflower is one of the few major food crops of the world that is native to temperate North America.

     Sunflower is one of the most important sources of edible oil in the world. Twenty million hectares of oilseed sunflower were produced worldwide in 1997 (USDA Statistical Reporting Service) with a yield of 24.3 million metric tons of oil and an estimated farm-gate gross value of $37.4 billion. Confectionery and ornamental sunflowers generate additional revenues. Cultivated sunflower is primarily grown from single-cross hybrid seed. Eighty million kg of hybrid seed were produced and sold in 1997 with gross seed sales of $640 million. Sunflower ranked second only to maize in hybrid seed sales in 1997.

     Wild Helianthus species have been important sources of genes for disease resistance, cytoplasmic male-sterility (CMS), drought tolerance, and other traits (Jan, 1997). The development of CMS sunflower from interspecific crosses (e.g., H. annuus x H. petiolaris) and the fertility restorer genes from H. annuus opened the way to hybrid seed production and the hybrid seed industry of sunflower (Leclerq, 1969; Kinman, 1970; Fick and Zimmer, 1976).

     Sunflower has several characteristics that facilitate genetic analyses. Although naturally allogamous and self-incompatible, self-compatible and strongly self-pollinated germplasm has been developed and is the core for hybrid breeding and genetic analysis. Sunflower is a widely adapted summer annual, can be crossed without difficulty, is a prolific seed producer (1,000⁺ seeds/plant), and has a moderately short seed-to-seed generation time (70 to 100 days).

Domestication

     Domestication of both lettuce and sunflower has resulted in dramatic changes in morphology, development, and physiology. In lettuce, the vegetative rosette has become exaggerated with a reduction in branching and a delay in flowering and bolting. There has also been a reduction in spininess and bitter compounds. A single recessive locus determines non-shattering of the capitulum and consequently retention of the seed. Both seed size and seedling vigor are greater in the cultivated species. In sunflower, a single compound flower is enlarged and branching is eliminated. Seed size and seedling vigor are increased and secondary metabolism has been modified to accentuate the accumulation of oil (Dedio 1982; Leon et al., 1996) in the seeds. Self-incompatibility has been lost which enhances high seed yields and shattering has been eliminated to ensure the retention of seeds for harvesting. In ancient times, lettuce was also cultivated as an oil seed crop but this is not a current use.

Bibliography

Anonymous (1995).
Marketing lettuce from Salinas, Watsonville, other Central California districts and Colorado in 1994. Federal State Market News Service.

Cronquist, A. (1977).
The Compositae revisited. Brittonia 29:137-153.

Crute, I.R. (1988).
The impact of breeding on pest and disease control in lettuce. Aspects Appl. Biol. 17:305-312.

Davis, R. M., Subbarao, K.V., Raid, R.N., and Kurtz, E.A. (1997).
Compendium of lettuce diseases. APS Press, St. Paul, MN.

Dedio, W. (1982).
Variability in hull content, kernel oil content, and whole seed oil content of sunflower hybrids and parental lines. Can. J. Plant Sci. 62: 51-54.

Fick, G.N. and Zimmer, D.E. (1976).
Yield stability of sunflower hybrids and open pollinated varieties. In "Proc. 7^TH Int. Sunflower Conf.", Krasnadar, Russia, June 27-July 3, Int. Sunflower Assoc., Paris, France, pp 253-258.

Heiser, C.B. (1976).
Sunflowers. In: Evolution of Crop Plants. Ed. N.W. Simmonds. pp36-38.

Heiser, C.B. and Smith, D.M. (1955).
New chromosome numbers in Helianthus and related genera. Proc. Ind. Acad. Sci. 64: 250-253.

Heywood V.H., Harbourne J.B., Turner B.L. (1977).
An overture to the Compositae. In: Heywood VH, Harbourne JB, Turner BL Eds. The Biology and Chemistry of the Compositae. Acad. Press.. pp1-20.

Jansen R.K., Michaels H.J., Palmer J.D. (1991).
Phylogeny and character evolution in the Asteraceae based on chloroplast DNA restriction site mapping. Syst. Bot. 16:98-115.

Kesseli, R.V., Ochoa, O., and Michelmore, R.W. (1991).
Variation at RFLP loci in Lactuca spp. and origin of cultivated lettuce. Genome 34:430-436.

Kesseli, R.V. and Michelmore, R.W. (1997).
The Compositae: systematically fascinating but specifically neglected. In: Genome Mapping in Plants, A.H. Paterson ed. R.G. Landes Co. Georgetown, TX. pp179 - 191.

Kinman, M.L. (1970)
New developments in the USDA and state experimental station breeding programmes. In "Proc. 4^TH Int. Sunflower Conf.", Int. Sunflower Assoc., Toowoomba, Qld, Australia, pp181-183.

Koopman, W.J.M. and De Jong, J.H. (1996).
A numerical analysis of karyotypes and DNA amounts in lettuce cultivars and species (Lactuca subsect. Lactuca, Compositae. Acta Bot. Neerl. 45: 211-222.

Leclerq, P. (1969)
Une sterilite male cytoplasmique chez le tournesol. Ann. Amerlior Plantes 19: 99-106.

Leon, A., Lee, M. Rufener, G.K. Berry, S.T., and Mowers, R.P. (1996).
Genetic mapping of a locus (hyp) affecting seed hypodermis color in sunflower. Crop Sci. 36: 1666-1668.

Raven P.H., Axelrod D.I. (1974).
Angiosperm biogeography and past continental movements. Annals Missouri Bot. Garden 61:539-673.

Ryder, E.J. (1986).
Lettuce breeding. In Breeding Vegetable Crops. AVI Pub., Westport, CT. pp 433-474.

Seiler, G.J. and Rieseberg, L.H. (1997).
Systematics, origin and germplasm resources of the wild and domesticated sunflower. In Sunflower technology and production, Schneiter, A.A. (ed.). Amer. Soc. Agronomy, Madison, WI. pp 21-66.

Smith, C.R. (1985).
Unusual seed oils and their fatty acids. pp. 29-47. In: E.H. Pryde (eds.) Fatty acids. Amer. Oil Chemists Soc., Champaign-Urbana, IL.

Stebbins G.L. (1953).
A new classification of the tribe Cichorieae, family Compositae. Madrono 12:65-81.

Stebbins G.L. (1974).
Flowering Plants: Evolution Above the Species Level. Second ed. Cambridge MA: Belknap Press Harvard.

Vries, I.M. de (1997).
Origin and domestication of Lactuca sativa L. Genetic resources and Crop Evolution 44: 165-174.