I first encountered the aptly named Great Lettuce this summer, whilst strolling in Leith. I thought it was Lactuca serriola the Prickly Lettuce but after almost a month of email exchanges with friendly botanists I became convinced it was Lactuca virosa, the Great Lettuce.
The two species are quite rare in Scotland. They are similar in appearance but Lactuca virosa is taller (often exceeding 2 m, whilst L. serriola is usually no taller than 1.5 m). L. virosa has achenes that are ‘maroon to blackish’ as opposed to ‘olive-grey’. Leaves of L. virosa are often maroon or brown-tinged, but leaves are an unreliable guide; one must examine carefully the achenes to be sure. A useful comparison table of characters is given by Prince and Carter: http://archive.bsbi.org.uk/Wats11p331.pdf
All lettuces produce white latex or ‘milk’ (hence the genus name, Lactuca). However, the white latex of L. virosa becomes orange after 10 minutes or so. Some say that bruised L. virosa smells like opium (I wouldn’t like to say).
It turned out that I was not the only one to find L. virosa. Three Edinburgh colleagues found it before me, in somewhat different locations. It had ‘suddenly popped up’ In the street where I found it; it was so conspicuous that I first suspected someone had sown its seeds. I saw mature plants, but also juveniles, lots of them. Local inhabitants seemed blissfully unaware of the rarity in their midst, and the local Council had abandoned their weed control measures in this Covid year. Unfettered, it flowered and towered majestically for a while.
Later, I corresponded with the Recorder for North Lanarkshire, Michael Philip. He had found it too. He has also recorded many instances of L serriola.
A search on the Web of Science for L. virosa was not especially fruitful. Most of the published papers relate to the chemistry, and especially the nature of the latex. The plant contains several compounds with pharmaceutical interest, and in the past some poor people have used it as an opium substitute or a sedative. It is sometimes called ‘lettuce opium’.
The lettuces found in the British Isles are the Prickly, the Garden, the Great, the Least and the Blue (respectively, serriola, sativa, virosa, saligna and tatarica). Great and Least are native and Prickly is considered by most authorities to be an archaeophyte (Preston et al 2004). With the possible exception of tatarica, all these lettuces have 18 chromosomes (2n=18), and the karyotypes are quite similar, except for L. virosa having (strangely) much longer and fatter chromosomes (Matoba et al. 2007).
The ‘salad lettuce’ in your bacon, lettuce and tomato sandwich seems to be more closely related to L. serriola than to L. virosa. The cultivated form was probably selectively bred by the Ancient Egyptians as early as 2680 BC. Domestication would have involved selection of wild relatives (thought to be serriola) for leaves that had a reduction in leaf spines, latex content and bitter flavour, and for plants with rapid growth and delayed bolting (Lededa et al 2009).
The relatedness of the species has been revealed through cytogenetic studies. Matoba et al (2007) used fluorescence in situ hybridisation (FISH) to mark same-sequence regions of the chromosomes in Lactuca. Their result is intriguing, demonstrating how translocation of pieces of DNA from one chromosome to another may account for the differentiation of the species. Evolution does not always proceed through small random mutations, as neo-Darwinism states, but sometimes the genome is reshuffled by translocations during meiosis, producing ‘hopeful monsters’ that usually die but may produce new species (Theißen 2006). Something like this seems to have happened in Lactuca.
Unsurprisingly (in view of the chromosome similarity), hybrids within Lactuca do occur. L. sativa x L. serriola produces fertile progeny, but progeny of the virosa x serriola cross are of limited fertility (de Vries 1990). Hybrids have not been reported in the wild, but the demonstration that hybridisation can occur suggests scope for breeding disease resistant lettuces and novel crops (perhaps for pharmaceutical purposes). However, Lededa et al.(2009) stress that progress in lettuce breeding is hampered by mistaken taxonomy. It was noted that one-third of the 49 accessions of 24 Lactuca species received from world genebanks had been mis-identified.
These lettuces are predominantly warmth-loving southern plants. Prince’s study of the distribution of serriola on the M5 motorway (Exeter to Birmingham) showed how sensitive that species is to small variations in temperature, both inter-annual and altitudinal (Prince et al. 1985). The lettuces seem to have crept northwards with the isotherms. The mean July temperatures of the distributions of virosa and serriola in Britain are 16.0 and 16.1 oC respectively (Hill et al. 2004), slightly higher than what they might encounter in Scotland: long term July averages in Edinburgh and Glasgow are 15.3 and 15.8 oC. Aberdeen is 14.6 oC. This migration is consistent with what we might expect from climate warming, and similar to the behaviour of several other species.
As I was finishing this article I rather idly glanced through an old local flora (Martin 1934), and found to my surprise that L. virosa was already in this part of Scotland when she was preparing that publication; moreover it is also in Plant Life of Edinburgh and the Lothians (Smith et al, 2002). So it is not a new arrival, albeit both publications stress its rarity.
de Vries IM (1990) Crossing experiments of lettuce cultivars and species (Lactuca sect. Lactuca, Compositae). Plant Systematics and Evolution, 171, 233-248. https://www.jstor.org/stable/23674278
Hill M et al (2004) PLANTATT http://nora.nerc.ac.uk/id/eprint/9535/
Prince SD et al (1985) The geographical distribution of prickly lettuce (Lactuca serriola) ii. characteristics of populations near its distribution limit in Britain. Journal of Ecology (1985), 73, 39-48.
Matoba H (2007) Chromosomal study of lettuce and its allied species (Lactuca spp.,Asteraceae) by means of karyotype analysis and fluorescence in situhybridization. Hereditas 144, 235-243. https://pubmed.ncbi.nlm.nih.gov/18215246/
Theißen, G. (2006) The proper place of hopeful monsters in evolutionary biology. Theory Biosci. 124, 349–369. https://doi.org/10.1016/j.thbio.2005.11.002
Lebeda, A., Doležalová, I., Křístková, E. et al. Wild Lactuca germplasm for lettuce breeding: current status, gaps and challenges. Euphytica 170, 15 (2009). https://doi.org/10.1007/s10681-009-9914-7
Martin IH (ed.) (1934) The Field-club Flora of the Lothians. William Blackwood, Edinburgh
Preston CD et al (2004) Archaeophytes in Britain. Botanical Journal of the Linnean Society, 145, 257–294.
Prince et al (1985) https://www.jstor.org/stable/2259766
Smith PM, Dixon ROD & Cochrane MP (2002) Plant Life of Edinburgh and the Lothians. Edinburgh University Press.