CGN lettuce collection

History

The lettuce collection originates from the former Institute for Horticultural Plant Breeding (IVT) where it was used as a working collection for breeding purposes, particularly as a source for resistances to downy mildew and aphids. The collection was adopted by CGN in 1985, after which the taxonomic status of the accessions was verified through analysis of seed characteristics, the accession names of varieties were checked for synonyms and the passport data were complemented as much as possible (Rulkens 1987).

Subsequent collection management focussed on improvement of collection composition in order to develop a collection representing the total genetic variation in cultivated lettuce (Lactuca sativa) and its wild relatives as wide as possible, and on assessment of accession characteristics through characterisation and evaluation trials and other research activities (Boukema et al. 1990). The collection was regularly improved by uptake of a selection of the most important cultivars released during a five-year time period in The Netherlands and abroad (De Groot and Boukema 1992; Van Treuren et al. 2008). The collection has also been extended through IPGRI funded expeditions to Egypt, collecting missions to Turkey, Russia (Daghestan), Armenia and Georgia in the framework of the German-Dutch cooperation in Plant Genetic Resources and collecting expeditions to Uzbekistan in cooperation with the Uzbekistan Research Institute of Plant Industry and the Vavilov Institute in Russia (Van Soest et al. 1998). The collection was also extended with wild Lactuca samples from colleague genebanks and botanical gardens and from collecting activities in Israel (Eenink and Roelofsen 1977). During 2008-2011 the collection was rationalised by removing redundant germplasm and non-authentic varieties based on analysis of passport, molecular and morphological data. More recently, collecting expeditions for wild relatives were organised in Armenia and Azerbaijan (2013), Jordan (2017) and Uzbekistan (2019).

Composition

Currently (August 2024), the collection comprises about 1200 cultivars, 180 landraces, 30 accessions of research material, 1050 samples of wild species and 110 accessions of unknown population type. The accessions of cultivated lettuce cover the seven generally recognised horticultural types, i.e. butterhead, cos, crisp, cutting, latin, stalk and oilseed lettuce.

In general, accessions of cultivated lettuce mainly originate from Europe, with the exception of crisp lettuce (USA), stalk lettuce (China) and oilseed lettuce (Egypt). Accessions of wild species are predominated by samples of L. saligna, L. virosa and particularly L. serriola. Accessions of wild species mainly originate from Europe and Asia. In 2008 the collection of cultivated lettuce was investigated for the presence of redundancies using passport, molecular and morphological data, after which nearly 200 accessions were removed (Van Treuren et al. 2010). Based on molecular data and a morphological field trial in 2010, 38 accessions of cultivated lettuce were discarded because of non-authenticity (Van de Wouw et al. 2011). In 2011, a redundancy analysis using passport and molecular data was carried out for the collection of wild species, resulting in the removal of 74 accessions. Improvement of collection composition will be continued with the periodical uptake of novel cultivars and with the organisation of expeditions to thus far unexplored distribution regions of wild species that are considered useful in lettuce improvement. In this respect, expeditions have focussed on the collecting of L. serriola and L. georgica in the Trans Caucasus (2013), L. aculeata in Jordan (2017) and L. altaica and L. serriola in Uzbekistan (2019).

Seed samples from the two latter expeditions are not yet available for distribution.

Characterisation

The majority of the lettuce collection has been morphologically and molecularly characterised, while the accessions are also well photo-documented. Accessions of cultivated material are described for over 20 morphological features, including seed traits and plant characteristics recorded during field trials that are also used for identity verification with the help of crop experts. Accessions of wild species are described for nine morphological characters during greenhouse regeneration.

All characterisation data are made available on-line and in downloadable files. Within the framework of the European Union project ‘Molecular markers for genebanks’, CGN’s entire lettuce collection, which at the time of tissue sampling in 1997 consisted of 2323 accessions, was characterised with microsatellites and AFLPs (Van Hintum 2003; Jansen et al. 2006). The obtained molecular data have been used for various purposes, including studies on core collection strategies (Jansen and Van Hintum 2007), acquisition protocol improvement for modern varieties (Van Treuren et al. 2008), molecular biogeography of L. serriola (Van de Wiel et al. 2010), collection redundancy reduction (Van Treuren et al. 2010) and cultivar authenticity (Van de Wouw et al. 2011). In addition to microsatellites and AFLPs, molecular data were also generated with SSAP, SRAP, TRAP and NBS-DP markers for a selected set of 80 accessions in order to compare the ability of the marker technologies to describe genetic diversity in the collection (Van Treuren and Van Hintum 2009). More recently, plants from 150 accessions were described for over 2,000 secondary metabolites (Van Treuren et al. 2018). DNA resequencing data are currently being generated for the entire collection in cooperation with BGI (Shenzhen, China).

Evaluation

As part of the standard material transfer agreement (sMTA), users of CGN material are requested to donate their evaluation data to CGN after finalisation of the experiments. These data are processed by CGN, included in their information system and made publically available on-line or in downloadable files. In addition, evaluation programs are carried out in close collaboration with seven lettuce breeding companies affiliated to the Dutch association for the plant reproduction material sector (Plantum NL).

Data from such joint experiments can be exploited exclusively by the involved parties for an agreed time period (usually 5 years) before they are made publically available by CGN (Van Treuren et al., 2011). The majority of available evaluation data are related to resistances to pest and diseases, such as downy mildew (Bremia lactucae), powdery mildew (Erysiphe cichoracearum), leaf aphids (Macrosiphum euphorbiae, Nasonovia ribisnigri and Uroleucon sonchi), root aphids (Pemphigus bursarius), lettuce drop (Sclerotinia sclerotiorum) and tomato spotted wilt virus (TSWV). Data are also available on chemical compounds, such as nitrate, iron and ß-carotene.

Maintenance

To check for maintenance of seed viability, the germination ability of lettuce accessions is monitored according to ISTA procedures at regular intervals (Van Treuren et al., 2013). Accessions are regenerated when seed viability drops below 80% for cultivated material or below 60% for accessions of wild species. Regeneration is also carried out when germination has decreased with at least 15% during a monitoring interval or when the remaining seed quantity has dropped below 1.7 g.

The majority of lettuce accessions are regenerated under controlled greenhouse conditions. Since cultivated lettuce and the majority of its wild relatives are self-fertilising, no protective measures are used to avoid cross-pollination, with the exception of a few outcrossing species. Regenerations are carried out in close cooperation with seven lettuce breeding companies affiliated to the Dutch association for the plant reproduction material sector (Plantum NL). More detailed information about regeneration procedures can be accessed through the link provided in the right margin of this page.

Utilisation

Samples of 50-100 seeds are distributed to users for the purpose of food and agriculture under the terms of the standard material transfer agreement (sMTA). Plant breeding companies and research institutes are amongst the most frequent users of CGN’s lettuce collection.

Between 1988 and January 2020, a total number of 31,038 Lactuca seed samples have been distributed for external use, corresponding to an average of nearly 1,000 accessions per year and to 12 times the current collection size. Until June 2016, the use of CGN accessions was reported in 144 published papers, mostly related to plant breeding or genetic resources management. The collection is currently being used for transcriptomics research and phenotypic analyses in the NWO-TTW research program “LettuceKnow, Science-Based Improvement of Salad”, which started in 2019.