Apparently native to coastal areas from northern Peru to central Chile, 0-500 m, although populations are found to central coastal Ecuador (but see below). Solanum pimpinellifolium grows in wet places and on the edges of cultivated fields throughout its native range and has apparently escaped from cultivation in the Galápagos (see Darwin et al. 2003).
Solanum pimpinellifolium is a member of the Potato clade (sensu Weese & Bohs, 2007); within the tomatoes and wild relatives it is a member of the “Lycopersicon group” and is a member of section Lycopersicon.
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Solanum pimpinellifolium is very closely related to S. lycopersicum and hybridizes with it freely. It differs from cultivated plants of S. lycopersicum in a variety of genetic characteristics, and we feel maintenance of S. pimpinellifolium at the specific rank will better serve both nomenclatural stability and the plant breeding community, who have long used this species in studies of tomato genetics (Frary et al. 2000; van der Knaap & Tanksley 2001). Since the two taxa are so similar, and identification of individual specimens can be problematic owing to putative introgression we use a suite of characters to identify S. pimpinellifolium. Our rule of thumb for identifying individual specimens (especially those that we suspect are of hybrid origin) was to assign to S. pimpinellifolium any specimen that possessed a majority of the characters below.
1. Corollas stellate, deeply (>3/4) divided, the lobes narrowly lanceolate
2. The sterile appendage of the staminal tube longer than the pollen bearing portion, making buds long and thin in comparison to those of S. lycopersicon.
3. Elongate inflorescence (greater than 3 cm) and many (>20) flowers
4. Very small fruits (<1 cm in diameter)
5. Few interjected leaflets
6. Leaflets entire or only slightly crenate in proximal 1/3
7. Terminal leaflet base cordate
In addition, identification can be facilitated by observation of the terminal leaflets, broadly elliptical in S. pimpinellifolium rather than narrowly ovate (S. lycopersicum) and the overall velvety pubescence of very short trichomes in S. pimpinellifolium, without long trichomes (except in putatively introgressed populations). Populations in the northern part of the range of S. pimpinellifolium (coastal Ecuador) and occasionally in higher elevation valleys in northern Peru apparently are the result of introgression with S. lycopersicum and consequently have deeper crenate leaflets and often have a few stout, patent trichomes on the stems and inflorescences. There is great variation and difficulty in identification owing to introgression wherever the two species are in contact.
In the Galápagos Islands, S. pimpinellifolium has been introduced or has only arrived recently, and forms hybrids with the endemic Galápagos tomato S. cheesmaniae where the two species occur in sympatry (Darwin et al. 2003; S.C. Darwin, unpubl.). Some plants identified by Nuez et al. (2004) as “Lycopersicon esculentum var. cerasiforme” we assign to S. pimpinellifolium.
Autogamous populations of S. pimpinellifolium have smaller flowers than outcrossing populations (Georgiady & Lord 2002), caused by heterochronic growth differences in the flower. A single QTL is responsible for four of six flower traits associated with autogamy (Georgiady et al. 2002), but much variation in all these traits exists in both sets of populations. Autogamy is associated with the margins of the range of S. pimpinellifolium (Rick et al. 1977), and studies of variation in nuclear gene sequences (Vac – fruit vacuolar invertase) in northern Peruvian populations (Caicedo & Schaal 2004a) show levels of outcrossing decreasing towards the south (towards the center of the species range). Caicedo & Schaal (2004a, 2004b) showed that genetic variation is high, and that there is no congruence between geographical distribution and genealogy in these populations. They suggest their data shows a northern Peruvian origin for S. pimpinellifolium, but they did not sample across the entire species range (into Chile). The more northerly populations in both their and Rick et al.’s (1977) study may have introgressed with S. lycopersicum.
Solanum pimpinellifolium has been of great importance to tomato breeding for the introduction of important agronomic traits such as disease resistance, and for the investigation of the genetic control of traits such as fruit shape and size (e.g. van der Knaap et al. 2004). Crosses between S. lycopersicum and the TGRC accession S. pimpinellifolium accession LA1589 (an autogamous population from the lower Virú Valley in the Department of La Libertad, Peru) have been the mainstay of studies in fruit development in the tomato (see Morphology above) and LA1589 has recently been used in metabolic profiling (Schauer et al. 2005). Solanum pimpinellifolium had the lowest fruit protein content of any of the species tested, but its leaf metabolite content is very close to that of S. lycopersicum, further underlining their close relationship (Schauer et al. 2005).
One TGRC accession of S. pimpinellifolium (LA1589) has been used to produce backcross recombinant inbred lines with the cultivated tomato. The 126 inbred backcross lines contain introgressed segments of S. pimpinellifolium that covers most of the genome. These lines have been important to produce genetic maps and analysis of quantitative traits (QTL) that are available from the Solanaceae Genome Network (www.sgn.cornell.edu).
The type specimen of S. pimpinellifolium in the Linnaean herbarium in London (see Knapp & Jarvis 1990) is a close match for collections from around Lima (e.g., Mexia 8328) in central Peru. These populations are comprised of glabrous plants with entire leaflets with cordate bases, elongate inflorescences, and very narrow petals. Seed of the type specimen of S. peruvianum was sent to Linnaeus by Bernard de Jussieu (Knapp & Jarvis 1990), whose brother Joseph was in the Lima area (see discussion under S. peruvianum). Linnaeus indicated no provenance for S. pimpinellifolium, but perhaps seeds also came to him via Jussieu along with S. peruvianum. Miller (1768) mentioned that he received seeds of both species from B. de Jussieu.
Luckwill (1943a) misunderstood Lycopersicon pissisi and equated it with S. peruvianum, possibly due to its habitat in coastal deserts. The protologue clearly states that the species has red fruit and no inflorescence bracts. The type specimen also has 3 leaves per sympodial unit, making it unmistakably a member of the “Lycopersicon” species group.
Luckwill (1943a: 24) described his L. esculentum subsp. intermedium citing a “well-defined and pure breeding form … which has been under cultivation for scientific purposes at the John Innes Horticultural Institute, Merton, for a number of years and has been widely employed for research work under the name of L. racemigerum Lange.” From his description of this taxon and from several UK cultivated specimens collected during that time (but none from John Innes found), we believe this is a hybrid between S. lycopersicum and S. pimpinellifolium, as is the type of L. racemigerum itself. These hybrids apparently form easily in cultivation, the reason that Brezhnev (1958) recognized S. pimpinellifolium only at the subspecific rank (see synonymy above).
Lycopersicon pimpinellifolium (L.) Mill. subsp. intermedium Luckwill is a misprint in Index Kewensis for L. esculentum Mill. subsp. intermedium Luckwill, but the subspecific epithet was indeed transferred to L. pimpinellifolium later (see synonymy above) by Prokhanov (1950) in the treatment of Lycopersicon for the Flora of the USSR (see also discussion under S. lycopersicum).