Mendel, 150 years on T.H. Noel Ellis1, Julie M.I. Hofer1, Gail M. Timmerman-Vaughan2, Clarice J. Coyne3
and Roger P. Hellens4
1 Institute of Biological, Environmental & Rural Sciences, Aberystwyth University, Gogerddan Campus, Aberystwyth, Ceredigion, SY23 3EB, UK 2 The New Zealand Institute for Plant & Food Research Ltd, Christchurch 8140, New Zealand 3 USDA-ARS Western Regional Plant Introduction Station, Washington State University, Pullman, Washington, USA 4 The New Zealand Institute for Plant & Food Research Ltd, Auckland, New Zealand
Mendel’s paper ‘Versuche über Pflanzen-Hybriden’ is the best known in a series of studies published in the late 18th and 19th centuries that built our understanding of the mechanism of inheritance. Mendel investigated the seg- regation of seven gene characters of pea (Pisum sativum), of which four have been identified. Here, we review what is known about the molecular nature of these genes, which encode enzymes (R and Le), a biochemical regula- tor (I) and a transcription factor (A). The mutations are: a transposon insertion (r), an amino acid insertion (i), a splice variant (a) and a missense mutation (le-1). The nature of the three remaining uncharacterized characters (green versus yellow pods, inflated versus constricted pods, and axial versus terminal flowers) is discussed.
Mendel’s studies: species, traits and genes Mendel’s paper ‘Versuche ü ber Pflanzen-Hybriden’  is the best known in a series of studies published in the late 18th and 19th centuries [2–4] that built our understanding of the mechanism of inheritance . The title of Mendel’s paper is usually mistranslated in English as ‘Experiments in Plant Hybridisation’ rather than ‘Experiments on Plant Hybrids’, reflecting the impact of his work on the science of genetics rather than Mendel’s own concern with the nature of hybrids and their implications for the ‘Umwandlung einer Art in eine andere’ – transformation of one species into another. There is also a misconception, as a result of R.A. Fisher’s attack on Mendelism , that Mendel’s results and experimentation were in some way suspect. These defamatory criticisms include imputations on the scope of his experimental work, his understanding of what he wrote and statistical interpretations of his results; although they have been roundly debunked [7,8], they remain embedded in common opinion.
In his paper, Mendel described eight single gene char- acters of pea, of which he investigated the segregation of seven. The eighth is the ‘purple podded’ character deter- mined by the gene Pur on linkage group I. He also dis- cussed the segregation of three traits (tall versus short, green versus yellow pods and inflated versus constricted pods) in common bean (Phaseolus vulgaris) that are likely orthologues of the corresponding characters he studied in pea. For both species Mendel used additional species names (such as Phaseolus nanus or Pisum saccharatum).
Corresponding author: Hellens, R.P. (email@example.com).
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These names are no longer used and we would consider these types as variants – Mendel commented that there is no ‘sharp line between the hybrids of species and varieties as between species and varieties themselves’.
From a biological perspective Mendel’s genes appear to be an unrelated set of genes that are uninformative about a single process; but they did elucidate the process of genetic inheritance itself. They are therefore important from an historical perspective and they illustrate a diversity of gene functions and types of mutation. Uncovering the molecular basis of these mutations solves a longstanding mystery in genetics.
This review focuses on the identification of four of Men- del’s genes (R/r, round versus wrinkled seed; I/i, yellow versus green cotyledons; A/a, coloured versus unpigmented seed coats and flowers; and Le/le, long versus short internode length). In addition, the possible natures of three other characters studied by Mendel (Gp/gp, green versus yellow pods; P/p or V/v, inflated versus constricted pods; and Fa/fa or Fas/fas, axial versus terminal flowers) are discussed.
Linkage A major conclusion from Mendel’s work was that the factors determining individual traits segregated independently of one another. We now know that this is not always the case. The associated segregation of parental allelic combinations, known as genetic linkage, is well established. Fortunately Mendel studied segregation at multiple unlinked loci. This meant his results were not confounded by linkage, which would have been much more difficult to interpret. The issue of linkage is sometimes egregiously combined with the criticism of the quality of Mendel’s data to imply falsely that he somehow suppressed inconvenient data . Unfor- tunately these discussions suffered from confusion in the literature regarding chromosome numbers, linkage data and their combination . Our current view of the position of the genetic loci Mendel studied is presented in Figure 1. As discussed below, there is some uncertainty about the identity of the genes for the fasciated (terminal) flowers (Fa or Fas) or the constricted pod phenotypes (P or V); therefore, the map locations of all are indicated. From this distribution of genetic loci it is clear that there are two possible cases where linkage could have confounded Mendel’s results: these are R–Gp and Le–V.
The wrinkled seed character that Mendel studied was R versus r on linkage group V . The character ‘green versus
oi:10.1016/j.tplants.2011.06.006 Trends in Plant Science, November 2011, Vol. 16, No. 11
AA vs aa
II vs ii
LeLe vs lele
VV vs vv
FaFa vs fafa
RR vs rr
GpGp vs gpgp
PP vs pp
FasFas vs fasfas
TRENDS in Plant Science
Figure 1. Genetic location of Mendel’s seven characters on pea linkage groups. Yellow versus green cotyledons II/ii on linkage group (I); seed coat (and flower) colour AA/aa
on linkage group (II); tall versus dwarf plants (LeLe/lele) on linkage group (III); difference in the form of the ripe pods (PP/pp or VV/vv) on linkage groups (III) and (VI),
respectively; difference in the position of the flower (FasFas/fasfas or FaFa/fafa) on linkage groups (III) or (IV), respectively; round versus wrinkled (RR/rr) on linkage group
(V); and colour of unripe pod (GpGp/gpgp) on linkage group (V).
Review Trends in Plant Science November 2011, Vol. 16, No. 11
yellow pod’ is unambiguously Gp versus gp, also on linkage group V. Linkage between these two loci can be detected . In Mendel’s study of two- and three-factor crosses he used approximately 600 F2 individuals. He did not present data on the combination of RR GpGp crossed with rr gpgp, but some F2 plants derived from this cross were probably grown as implied by the text, ‘further experiments were made with a smaller number of experimental plants in which the remaining characters by twos and threes were united as hybrids’ . In one recombinant inbred population derived from the cross between the inbred John Innes Germplasm lines JI15 and JI399 , the recombination fraction between the R locus (genotyped using a molecular marker assay) and Gp is 36%, resulting in an expected segregation ratio of 9.6:2.4:2.4:1.6 rather than 9:3:3:1. Men- del would have needed about 200 plants in the ‘smaller number’ to have a 5% statistically significant deviation from independent assortment. Furthermore, linkage group V in pea, most likely corresponding to chromosome 3, behaves unusually in this cross because the number of chiasmata is never greater than one ; usually two or three occur. The recombination fraction calculated above is therefore the smallest that Mendel could have encountered, so it is un- likely that genetic linkage would have been discernable in any of the crosses that Mendel examined.
Genes and their mutant alleles Round versus wrinkled (R versus r) The wrinkled phenotype is striking because plants that appear completely normal bear seeds that are irregular in shape (Figure 1). The immature seeds do not appear
unusual, but by maturity there are many differences be- tween the wild-type and mutant seeds. These include diverse features such as subcellular arrangement of orga- nelles, the ratio of the two major types of storage protein, the shape of starch granules, the amylose to amylopectin ratio of the starch polymers and sugar content . There are several genes in pea that confer a wrinkled (rugosus) phenotype and all are lesions in enzymes involved in starch biosynthesis [14–17]. However, only the r mutant is known to have been available to Mendel .
A biochemical approach was taken to identify the gene encoded by R . It was known that rr lines were distin- guished from wild-type by their reaction to an antibody raised against the starch branching enzyme, so this anti- body was used to identify cDNA clones. These cDNAs provided the route to isolating the structural gene encod- ing a starch branching enzyme (EC 18.104.22.168). Subsequent analysis showed that this gene co-segregated with the R locus and that wrinkled (r) mutants were disrupted in this gene by the insertion of a non-autonomous type II trans- poson (called Ips-r) related to the Ac/Ds family  (Figure 2). Thus the first of Mendel’s mutants to be char- acterized corresponded to a mutation in a gene encoding a biosynthetic enzyme and it was potentially associated with an active transposon. No systematic search for other alleles at the R locus has been undertaken and the active and autonomous form of the transposon has not been identified.
Yellow versus green cotyledons (I versus i) Ripe wild-type II seeds are yellow because the chlorophyll is lost as the seeds mature, whereas ii seeds remain green
R vs r I vs i A vs a Le vs le
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