Pea (Pisum Sativum L.) in Biology Prior and After Mendel's Discovery

Czech J. Genet. Plant Breed., 50, 2014 (2): 52–64 Review Pea (Pisum sativum L.) in Biology prior and after Mendel’s Discovery Petr SMÝKAL Department of Botany, Faculty of Science, Palacký University Olomouc, Olomouc, Czech Republic Abstract Smýkal P. (2014): Pea (Pisum sativum L.) in biology prior and after Mendel’s discovery. Czech. J. Genet. Plant Breed., 50: 52–64. Pea (Pisum sativum L.) has been extensively used in early hybridization studies and it was the model organ- ism of choice for Mendel’s discovery of the laws of inheritance, making pea part of the foundation of modern genetics. Pea has also been used as model for experimental morphology and physiology. However, subsequent progress in pea genomics has lagged behind many other plant species, largely as a consequence of its genome size and low economic significance. The availability of the genome sequences of five legume species (Medicago truncatula, Lotus japonicus, Glycine max, Cajanus cajan and Cicer aerietinum) offers opportunities for genome wide comparison. The combination of a candidate gene and synteny approach has allowed the identification of genes underlying agronomically important traits such as virus resistances and plant architecture. Useful genomic resources already exist and include several types of molecular marker sets as well as both transcriptome and proteome datasets. The advent of greater computational power and access to diverse germplasm collections en- able the use of association mapping to identify genetic variation related to desirable agronomic traits. Current genomic knowledge and technologies can facilitate the allele mining for novel traits and their incorporation from wild Pisum sp. into elite domestic backgrounds. Fast neutron and targeting-induced local lesions in genomes (TILLING) pea mutant populations are available for reverse genetics approaches, BAC libraries for positional gene cloning as well as transgenic and in vitro regeneration for proof of function through gene silencing or over-expression. Finally, recently formed International Pea Genome Sequencing Consortium, holds promise to provide the pea genome sequence by 2015, a year of 150 anniversary of Mendel’s work. Keywords: heredity; hybridization; legume; Mendel; pea Pea as model for hybridization experiments Kölreuter (1733–1806) and by Carl Friedrich von Gärtner (1772–1850). However, the first report, of us- Pea has been an object of experimental work well ing pea variation to gain insight into the transmission before Mendel’s genetic discoveries. This might be of traits among generations is from Thomas Andrew attributed likely to the appearance and availability Knight (1759–1838). Although Knight’s interest was of large number of varieties with distinct traits, in fruit trees improvement, he soon realized that an such as seed, pod and flower colours, seed shape, annual plant with good trait variation is needed to plant height etc. There were other plants with even address his questions. He wisely chose pea. Notably, higher variation like the cabbage family, but these Knight’s introductory statement is a curious reminder were either biannual plants or displayed outcrossing in point of form of Mendel’s own introduction nearly pollination and incompatibility. half a century later (Knight 1799; Hellens et al. Experiments in plant hybridization can be traced 2010). Knight determined that, in crossing a pea with back to 1694 when Rudolph Jacob Camerer (1665– grey (e.g. pigmented) upon one with white (we say 1721) started systematic crossing of plants considered transparent today) seed coat, the resulting hybrid to be different varieties and species (Camerer 1694). seeds were uniformly grey seeded, as well as having Camerer’s work was continued by Joseph Gottlieb purple-coloured flowers of the male parent. Knight 52 Review Czech J. Genet. Plant Breed., 50, 2014 (2): 52–64 further discovered that by crossing plants grown the green-seeded P. sativum viride. In the first case from these (heterozygous) grey seeds, with pollen the hybrid seeds were all pure yellow; in the second from what he called a “permanent” white variety, case twelve seeds were produced in four hybrid pods; plants of two types appeared, one bearing grey and and these were all a greenish-yellow colour, although the other white seeds. No numbers were reported, the greenish tinge disappeared from some of them on so that a scientific foundation based on ratios was drying. Another yellow-seeded pea (P. sativum nanum not laid (Roberts 1929). Twenty-five years earlier, repens) fertilized with the pollen of the green-seeded Knight undertook experiments with plants to test (P. sativum viride) gave five hybrid pods with seeds, of the theory of “superfetation”, e.g. the possibility of which one contained five “dirty green” seeds, a second two males combining in the fecundation of a female. had five seeds which were “not distinctly yellow, but At the time, the behaviour of the fertilizing cells was yellowish green”. The others were not yellow like the absolutely unknown, as was the fact that but one mother, but “dirty yellow” (Gärtner 1849). It is clear sperm cell was required to fertilize the egg. In that that the “greenness” of P. sativum viride did in these report, Knight first recorded colour-dominance (grey, cases affect the colour of the seeds, when its pollen pigmented colour of testa) and possibly also heterosis was used to fertilize plants of yellow-seeded variety, in peas. Beside A. Knight, there was John Goss, which though it is difficult to judge exactly how great the in 1820 pollinated flowers of variety Blue Prussian effect (Roberts 1929). with pollen of a dwarf pea known as Dwarf Spanish, In general, these 18th- and 19th-century researchers obtaining, three pods of hybrid seeds. In the follow- dealt with questions regarding the variation or fixity ing spring, when he opened pods, he was surprised of natural forms and the physiological process by to find that the colour of the seeds (i.e. cotyledons), which either variety or homogeneity was transferred instead of being a deep blue like those of the female from one generation to the next (Wynn 2007). parent, was yellowish-white like that of the male. He Gregor Johann Mendel (1822–1884) chose pea for observed a case of segregation in the next genera- his work after preliminary experiments with several tion, as well as recording evidence of dominance and plant species and an examination of botanical litera- segregation, however he did not recognize them as ture on plant hybridization, particularly of Gärtner such (Goss 1824). Goss either did not sow the seeds (1849). For his hybridization experiments, Men- of different plants separately, or did not make counts del selected 22 pea varieties that he had confirmed as Mendel later (Roberts 1929). At the meeting of through two years of testing to be true-breeding. He the Horticultural Society on 20th of August 1822, a reported data from hybridization experiments on communication on the same subject was presented seven traits that differed among the varieties (Orel by Alexander Anderson-Seton (1769–1850), sec- 1971). He does not discuss the question whether retary of Horticultural Society. He pollinated the all his variants belong to one “species” or not, but flowers of the Dwarf Imperial, a green-seeded pea, describes the result of crossing any two of them as with the pollen of a tall white-seeded variety. One “hybrid.” The fact that Mendel used the same species, pod with four peas was produced, all of which were Pisum sativum, fuelled many of the later criticisms green, possibly demonstrating the dominance of (Orel 1971). However the use of pea was in fact green cotyledon colour over its absence (white). The essential in order to dissect inheritance of char- plants growing from the four peas (F1 seeds) were acters. Mendel paid special attention to seven sets intermediate in size between the two parents (Seton of characters, with regard to each of which it was 1824). Finally, in 1872, Thomas Laxton (1830–1893) possible to separate into two categories. Thus the published the results of hybridization experiments, shape of the seeds might be round, with only slight which have several points of interest: first, the fact and shallow wrinkles on the surface, or irregular and of dominance in colour and form of the seeds was deeply wrinkled. The cotyledons of the seeds might brought out; secondly, a statement of numerical re- be yellow or green in colour, and so on. The pairs of sults was attempted (Laxton 1866, 1872). Notably, characters, recognised in this way for each organ or Laxton corresponded on these findings with Charles set of organs studied, are distinguished, according Darwin. In 1849 Gärtner described the result of to their power of affecting hybrid offspring, into crossing the yellow-seeded variety Pariser Wach- dominant and recessive, as we call them today. He serbse which he called Pisum sativum luteum, ferti- precisely recorded the numbers of plants with such lized either with pollen of P. sativum macrospermum traits in any studied generation and was thus able, which had greenish-yellow seeds, or with pollen of all what was necessary and sufficient to deduce his 53 Czech J. Genet. Plant Breed., 50, 2014 (2): 52–64 Review three “laws”. The high number of studied plants striking exception to the law of dominance is that enabled Mendel to see the underlying nature of the observed by Carl Correns (1864–1933). The “grüne ratios between the different sets of experiments that späte Erfurter Folger-Erbse” has a nearly colourless would not have been so discernable with smaller plant seed-coat; the “Purpurviolettschottige Kneifel-Erbse” numbers. The only qualifications Mendel offers, in and the “Pahl-Erbse mit purpurrothen Hülsen” have applying his general statements to these very varied the seed-coat uniformly orange, becoming brown characters, are (1) that the violet dots on the seed- with age.

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