Instructor: C.F. Quiros

Asteraceae (Compositae): Lettuce

Genus Lactura







Origin, distribution and domestication.

Related species and interspecific crosses


Chromosome markers, genetic maps



Lactuca sativa
 Lettuce is diploid with 2n=2x=18 chromosomes, however in the genus contains species of n=5, 8 and 9, plus a series of natural polyploids with 2n= 32, 34, 36 and 48 chromosomes. It is economically the most important salad crop in the USA and in many other countries.

The most frequent chromosome numbers in the genus are n=8 and 9.

Lactuca is a Mediterranean genus, which includes more than 100 species, including annuals and perennials, although there are some derived polyploids species, which are endemic of North America. For example L. canadiensis and L. gaminifolia, both with n=17, are probable amphidiploids resulting from the hybridization of n=8 and n=9 diploid species.

  Lettuce was domesticated in Egypt 4500BC, based on evidence from tomb paintings. Probably it was used first as a medicinal plant and as a source of edible oil from the seeds. It was considered there an aphrodisiac, being featured in the festival of Min, the god of fertility and procreation. Lettuce contains latex, which is bitter, so most likely during domestication selection took place to lower the amount of this compound. Head lettuce is relatively recent, being reported in the 1500’s. It contains 4.2pg/2C DNA

There are four horticultural varieties:

var. capitata. heading lettuce, having a similar structure to cabbage. It includes “iceberg” type (cripshead) and butterhead, which has less compact head than crisphead, is flattened and had a more delicate taste.

var. longifolia.  Cos or romaine type. It has crispy and elongated leaves overlapping loosely with each other but without forming a head.

var. crispa, loose leaf forming a rosette

var asparagina. Celetuce or asparagus lettuce, nonheading grown for its thick, edible stems.

These different types can be distinguished by specific RFLP markers, indicating polyphyletic origin from different lineages (Kesseli et al. 1991).

Other classifications:

Latin type: Similarity to Romain and butterhead.

Bavaria: Softer, smaller Crisphead type grown in France.

Stem: Edible stalks, grown in china and Egypt.

Oilseed type. Large seeds pressed for oil, exotic type.

Cultivated lettuce is an annual, never found in wild stage, but L. serriola is the closest relative and probable ancestor. This wild species has been introduced to North America where it is now a serious weed. Sequence analysis of the intergenic spacer (ITS-1) of rDNA (Koopman et al. 1998) failed to disclose differences between the two species. Two other closely related species, L. drageana and L. altaica can be considered conspecific to L. serriola, according to the authors of the study. There is three other species forming part of the sativa gene pool, or what is called the L. sativa group:

L. serriola, small seeds, narrow toothed leaves with spines, pediceled flowers

L. saligna, similar to L. serriola, but with narrower leaves and sessile flowers.

L. virosa, morphologically distinct to two species above, flat seeds, strong rosette. It might have hybrid origin involving an L. serriola-like female parent and an unknown male parent contributing a unique character, winged black achenes. This species has a different karyotype to the others, having one instead of two chromosomes carrying nucleolar organizer regions.

L. serriola is the closest to lettuce, crosses freely in both directions. The most distant species in the group is L. saligna.

These species differ slightly in DNA content, being possible to separate them by flow cytometry (Koopman, 2000).



The last two species are more distant to the cultivated one but closer to each other. This relationship has been confirmed by Kesseli et al. (1991), using RFLP markers.

L. saligna, has been used as a source for resistance to cabbage looper and stemphylium

leafspot. Spontaneous chromosome doubling in some interspecific hybrids seems to be common. For example this has been observed in hybrids between L. saligna and

L. sativa.

All four are obligate self‑fertilizing species.

The related wild species contain bitter sesquiterpene lactones such as lactucin and lactupicrin, which have sedative properties. L. virosa is cultivated for pharmaceutical use to produce the drug lactucarium.

Chromosomal inversions distinguish most of these species. These have been detected in the following hybrids:

L. sativa x L. virosa

L. saligna x L. sativa

L. saligna x L. serriola

Variety Vanguard was developed from the interspecific cross of L. sativa x L. virosa.  Due to sterility, the chromosomes were doubled by colchicine and the resulting amphidiploid backcrossed to L. sativa. Tipburn and several unique quality traits were transferred, as well as bolting resistance, since L. virosa is biennial.

L. serriola can be used as a bridge to transfer easily traits from L. saligna to L. sativa.

L. serriola is a good source for downy mildew resistance. Embryo culture can be used to increase the success of hybrid recovery.

Recent review on challenges of using wild species in lettuce breeding (Lebeda et al 2009)

Floral Biology:

Flowers are protandrous, but autogamy is predominant due to elongation and growth of the pistil through the anther tube. They are organized in an inflorescence called capitulum, with 18 to 20 flowers. Single ligulated yellow petal, 5 stamens, 2 carpels, the fruits are achenes. Each floret produces a single seed. Each plant produces approximately 1500 seeds.

Several nuclear male steriles have been found, conditioned either by dominant or recessive genes. No cms has been reported for this crop.

Over 60 morphological markers have been reported in lettuce, including seedling markers. For example:

anthocyanin in the foliage due to single dominant allele  A

Crinckled leaves is dominant, determined by allele Cr

Wax coating, dull foliage is dominant over glossy  gl

Shape of leaf apex due to a single gene P. Cos type is dominant,

Head formation due to the action of at least at least 3 genes.

Fast flowering is dominant, F, flowers in 45‑55 after germination.

The normal life cycle of lettuce is 85‑185 days

Pollen grain color, white is dominant over orange.

Flower color, blue is dominant over yellow.

Disease resistance

At least 25 disease resistance genes for seven diseases have been identified in lettuce .McHale et al (2009) expanded this number by identifing 702 candidate genes involved in pathogen recognition resistance, resistant signal transduction and disease susceptibility and mapping 294 of these genes.

Downy mildew caused by Bremia lactucae is the most important disease to this crop. This is a highly variable pathogen with many races. There are more than 15 dominant genes (Dm genes) conferring resistance, and these are organized in gene families. The resistance is dominant and race specific, but it is easily overcome by development of new races. The Dm genes have the typical structure of NBS-LRR resistance genes.

Lettuce mosaic virus. It is transmitted by aphids. This disease is controlled by using virus free seed. This is done by testing seed for absence of virus (seed indexing). Resistance is recessive, determined by the gene mo1. Molecular markers have been reported linked at 8cM from this gene (Irwin et al. 1999).

Powdery mildew resistance is dominant, Pm.

Turnip mosaic resistance, is dominant, Tm.

Corky root. It is a bacterial disease. The resistance is recessive and determined by allele cor.

Lettuce dieback (tomato bushy stunt virus)

Biochemical and molecular markers:

Close to 20 segregating isozyme markers have been reported in lettuce. RFLP (Landry et al. 1987), RAPD and AFLP makers are also available in this crop (Hill et al. 1996). SNPs recently developed in this crop. A SNP markers has bee nreported for corky root rot resistance (Moreno et al 2003)

A series of monogenic traits have been mapped or linked to molecular markers (Waycott, 1999). An extensive linkage map based on the cross Calmar x Kordaat is now available for potential use in marker assisted selection, especially for downy mildew resistance (Paran et al. 1991). The Dm3 gene has been cloned by positional cloning.

Syed et al (2006) produced an extensive map containing close to 500 markers in sativa x serriola F2 population based on  AFLP, NBS profiling (based on disease resistance gene consensus seq) and retrotransposon long terminal (LTR) repeats SSAP markers.  50% the NBS markers matched to disease resistance analog sequences. Truco et al 2007 have produced a high density map consisting of 2744 markers (AFLP, RFLP, SSR and RAPD. Simko (2009) reported 61SSR developed from L. sativa and L serriola unigenes.

The Dm gene clusters:

There are three clusters and most likely they have originated by gene duplication. The largest cluster, called  locus  RGC2 (resistance gene candidate) has 3MBp (but <01cM) and contains at least 24 genes (depending on accession tested) and includes the following Dm genes:  Dm1, Dm2, Dm3, Dm6, Dm14, Dm15, Dm16, Dm18. It includes also a virus and an aphid resistance gene.A series of makers have been developed for this cluster allowing screening of different haplotypes. Gene copy number variation in different varieties and related species is quite common. These genes evolve by birth (duplication) and death (deletion) where unequal crossing over and gene conversion are prominent events (Michelmore and Meyers 1998; Kuang et al. 2004). RGC2 varies greatly in copy number , from 10 to >30 copies and it is very diverse in natural populations of the lettuce progenitor species L serriola. Diversity seems to be the in response to biotic and abiotic stresses.

Dm3 has been cloned (Chin et al (2000). Absent in L. serriola populations due to deletion and gene conversion (Kuang et al 2006). Silencing experiments with RNAi of Dm3 gene also reduced mRNA levels of 7 other related genes in the RGC2 cluster (Wroblewski et al 2007).

  Other uses of the map includes QTL mapping of root structural traits. A cross between lettuce (shallow root) and L. serriola (long taproot) detected 13 QTLs. One of these loci was associated to efficiency of water extraction (Johnson et al. 2000). QTL for non-host resistance to downy mildew in lactuce L. saligna introgression lines (Jeuken et al 2008) and shelf live, which associated to leaf properties (strength, elasticity, size, weight and others) (Zhang et al 2007).  Gene tagging has been accomplishedin lettuce  with the Tnt1 retrotransposon from tobacco (Mazier et al 2007).

Compositae Genome Project. Comparative genomics of lettuce, sunflower and Arabidopsis. You will find the latest genomic databases, ESTs and linkage maps on this web site.

Biotechnology: Lettuce can be regenerated from many different organs such as axillary and apical buds, cotyledons and hypocotyls. It can be also regenerated from protoplasts and is amenable to suspension culture.

Somatic hybrids by protoplast fusion were obtained by fusing lettuce protoplasts two those of two remotely related species, L. tatarica and L. perennis. The complete sterility of the hybrids did not allowed further breeding progress.

Lettuce is susceptible to transformation by Agrobacterium tumefaciens (Michelmore et al 1987). Transformation has also been reported by electroporation of protoplasts (Chupeau et al. 1989).

Zhang et al 2010 obtained nematode resistant lettuce by introducing the tomato resistant gene Mi. Big vein virus resistant lettuce produced experimentally.


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Last update: May 11th, 2010

Prepared by  C.F. Quiros, 2001