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Abstract

Key message: This work discusses several selected topics of plant genetics and breeding in relation to the 150th anniversary of the seminal work of Gregor Johann Mendel. In 2015, we celebrated the 150th anniversary of the presentation of the seminal work of Gregor Johann Mendel. While Darwin's theory of evolution was based on differential survival and differential reproductive success, Mendel's theory of heredity relies on equality and stability throughout all stages of the life cycle. Darwin's concepts were continuous variation and "soft" heredity; Mendel espoused discontinuous variation and "hard" heredity. Thus, the combination of Mendelian genetics with Darwin's theory of natural selection was the process that resulted in the modern synthesis of evolutionary biology. Although biology, genetics, and genomics have been revolutionized in recent years, modern genetics will forever rely on simple principles founded on pea breeding using seven single gene characters. Purposeful use of mutants to study gene function is one of the essential tools of modern genetics. Today, over 100 plant species genomes have been sequenced. Mapping populations and their use in segregation of molecular markers and marker-trait association to map and isolate genes, were developed on the basis of Mendel's work. Genome-wide or genomic selection is a recent approach for the development of improved breeding lines. The analysis of complex traits has been enhanced by high-throughput phenotyping and developments in statistical and modeling methods for the analysis of phenotypic data. Introgression of novel alleles from landraces and wild relatives widens genetic diversity and improves traits; transgenic methodologies allow for the introduction of novel genes from diverse sources, and gene editing approaches offer possibilities to manipulate gene in a precise manner.
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Theoretical and Applied Genetics
International Journal of Plant Breeding
Research
ISSN 0040-5752
Theor Appl Genet
DOI 10.1007/s00122-016-2803-2
From Mendel’s discovery on pea to today’s
plant genetics and breeding
Petr Smýkal, Rajeev K.Varshney,
Vikas K.Singh, Clarice J.Coyne, Claire
Domoney, Eduard Kejnovský & Thomas
Warkentin
1 23
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Theor Appl Genet
DOI 10.1007/s00122-016-2803-2
REVIEW
From Mendel’s discovery on pea to today’s plant genetics
and breeding
Commemorating the 150th anniversary of the reading of Mendel’s discovery
Petr Smýkal1 · Rajeev K. Varshney2 · Vikas K. Singh2 · Clarice J. Coyne3 ·
Claire Domoney4 · Eduard Kejnovský5 · Thomas Warkentin6
Received: 20 April 2016 / Accepted: 26 September 2016
© Springer-Verlag Berlin Heidelberg 2016
revolutionized in recent years, modern genetics will forever
rely on simple principles founded on pea breeding using
seven single gene characters. Purposeful use of mutants to
study gene function is one of the essential tools of mod-
ern genetics. Today, over 100 plant species genomes have
been sequenced. Mapping populations and their use in seg-
regation of molecular markers and marker–trait association
to map and isolate genes, were developed on the basis of
Mendel’s work. Genome-wide or genomic selection is a
recent approach for the development of improved breeding
lines. The analysis of complex traits has been enhanced by
high-throughput phenotyping and developments in statisti-
cal and modeling methods for the analysis of phenotypic
data. Introgression of novel alleles from landraces and wild
relatives widens genetic diversity and improves traits; trans-
genic methodologies allow for the introduction of novel
genes from diverse sources, and gene editing approaches
offer possibilities to manipulate gene in a precise manner.
Reflection on Mendel’s work on pea
In 2015, we celebrated 150 years since the presentation (8
February and 8 March 1865) of the seminal work of Gregor
Johann Mendel. Mendel’s 1865 work (published in Men-
del 1866) was at first largely ignored or not understood.
As documented by Olby (1979), Mendel’s plant hybridiza-
tion research was cited 11 times over the period of 30 years
beginning in 1865, but it was fully rediscovered and its
essence understood in 1900, 34 years after its publication
(Correns 1900; Tschermak 1900; de Vries 1900). From
then on, Mendel’s work has been widely discussed and
meticulously analyzed (Fisher 1936). Mendel’s insights
have been thoroughly tested and became the solid basis
of the new discipline of genetics (Weldon 1902; Bateson
Abstract
Key message This work discusses several selected topics
of plant genetics and breeding in relation to the 150th
anniversary of the seminal work of Gregor Johann
Mendel.
Abstract In 2015, we celebrated the 150th anniversary
of the presentation of the seminal work of Gregor Johann
Mendel. While Darwin’s theory of evolution was based
on differential survival and differential reproductive suc-
cess, Mendel’s theory of heredity relies on equality and
stability throughout all stages of the life cycle. Darwin’s
concepts were continuous variation and “soft” hered-
ity; Mendel espoused discontinuous variation and “hard”
heredity. Thus, the combination of Mendelian genetics
with Darwin’s theory of natural selection was the process
that resulted in the modern synthesis of evolutionary biol-
ogy. Although biology, genetics, and genomics have been
Communicated by H. Bürstmayr and J. Vollmann.
* Petr Smýkal
petr.smykal@upol.cz
1 Department of Botany, Faculty of Sciences, Palacký
University in Olomouc, Slechtitelu 27, Olomouc, Czech
Republic
2 International Crops Research Institute for the Semi-Arid
Tropics (ICRISAT), Patancheru, Hyderabad, India
3 USDA-ARS, Washington State University, Pullman, USA
4 John Innes Centre, Norwich Research Park, Norwich, UK
5 Department of Plant Developmental Genetics, Institute
of Biophysics, Czech Academy of Sciences, Brno, Czech
Republic
6 Crop Development Centre, University of Saskatchewan,
Saskatoon, SK, Canada
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Theor Appl Genet
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1902). Several reviews and commentaries have been pub-
lished related to Mendel’s achievements starting from the
controversy over his data (Fairbanks and Rytting 2001;
Hartl and Orel 1992; Daniel et al. 2007; Franklin et al.
2008; Radick 2015), to references to his personality and
work (Zirkle 1951; Gasking 1959; Orel 1984, 1996; Weil-
ing 1991; Sandler 2000; Ellis et al. 2011; Reid and Ross
2011; Klein and Klein 2013; Gliboff 2013). Although biol-
ogy, genetics and mainly genomics have been revolution-
ized in recent years, modern genetics will forever rely on
principles of heredity founded on pea using seven single
gene characters.
Mendel was not the first to choose pea as an experimen-
tal model (Smýkal 2014 and references herein); however,
he was the first to apply calculus of ratios to a biological sit-
uation (Monaghan and Corcos 1990). In fact, it seems that
Mendel had the theory in mind (Dunn 1965). He formu-
lated the hypothesis first and then based on the comparison
of observed numbers and expected ratios, he tested them
with larger sets (Fisher 1936; Klein and Klein 2013). This
is the result of Mendel’s training as he was not just a bota-
nist and plant breeder, but also well trained in physical sci-
ences such as meteorology (Klein and Klein 2013), where
precise records were always essential and used to predict
future situations. One of Mendel’s innovations was to look
at the inheritance of traits as random events and analyze the
results based on expectations. This may have been one rea-
son why his paper was ignored. Random events, statistics
and probabilities were more common of the language used
by nineteenth century physicists and mathematicians than
nineteenth century biologists (Sheynin 1980). His genius is
that he discusses the laws of combination in relation to the
formation of zygotes. Careful in observations, he denoted
manifested traits as dominant, while those “hidden” as
recessive. This classification and letter code we use still
today. Mendel was also very lucky to have chosen unlinked
traits/genes (Reid and Ross 2011). The single case in which
he might have detected linkage (depending on whether he
studied the v or p gene), and, if he did study v, which is
linked to le, there are indications in a letter to Nägeli (Men-
del 1950) that he studied the alleles in repulsion conforma-
tion, and thus would not have been likely to detect linkage
as readily as had the recessive alleles been in coupling con-
formation. Mendel has given new meaning to the word of
hybrid, as not a simple mix of parents but a contribution of
parents to their progeny.
Besides pea, Mendel tested several other plant species,
popular among hybridist scientists at that time, namely
Hieracium, Cirsium and Geum (Orel 2003; Nogler 2006).
These species reflected a key question asked at that time,
i.e., the transmission of traits after species hybridization, to
shed light on the origin of species. There was a common
belief in the fixity of species. It seems likely that Mendel
himself did not ask questions on the origin of species but
rather was looking for laws governing the inheritance of
particular characters that did not change over time, reject-
ing the popular theory of blending of characters and spe-
cies essence. Thus, whereas Darwin held that species var-
ied over time, Mendel believed that species characteristics
remained constant (Wynn 2007). At that time, the existence
of constant hybrids was of great interest, as these hybrids
attain the status of new species (Mendel 1866; Bishop
1986). However, as pea experimental material did not ful-
fil the species criteria considered necessary by theoretical
biologists of the time (Gasking 1959), Mendel tested 26 dif-
ferent genera over the years (Mendel 1870, 1950, letters to
Nägeli in Orel 2003). Some of his results agreed with those
he obtained with pea, some however, did not, in particular
with different colored beans where he found a great range
of colors in hybrids as a result of quantitative inheritance
and, with Hieracium, where hybrids remained constant as a
consequence of apomixis. He suggested first that the com-
mon bean phenomenon might be explicable if flower colors
were determined not by one, but by two or more pairs of
factors. Actually, Mendel discussed the matter of quantita-
tive traits especially with respect to the pea attribute tall/
dwarf, which actually was “length of stem”. Luckily, Men-
del stayed with pea, as with Hieracium he would not have
been able to make any plausible explanation at that time,
due to the existence of apomixis, while several traits eas-
ily observable in common bean are encoded by quantitative
trait loci as we know today.
Until now, we have molecular evidence for four out of
seven (possibly eight, which includes purple pods, not used
in Mendel’s thesis) traits he used (Hellens et al. 2010; Ellis
et al. 2011; Reid and Ross 2011; Smýkal 2014). However,
for some of the characters (as Mendel called them “ele-
ments”) we are unsure which loci were responsible. More-
over, there will likely forever remain uncertainty over the
mutations he used. One of the most impressive aspects of
Mendel’s thinking lies in the notation that he developed
to represent his data: a capital and a lowercase letter (Aa)
for the hybrid genotype actually represented what we now
know as the two alleles of one gene. Mendel deliberately
chose specific characters. He wanted to demonstrate sta-
sis, formulate a theory, and then extrapolate to all other
modes of inheritance. Mendel did not deliberately use plant
mutants, although some of the alleles he used in pea are
considered as mutants today (Bhattacharyya et al. 1990,
Hellens et al. 2010). The purposeful use of mutants to
study gene function is one of the essential tools of modern
genetics. This is expected to proliferate even more in the
near future, as we understand more the process of directed
mutagenesis (Osakabe and Osakabe 2015).
Since the nineteenth and twentieth centuries, when tradi-
tional plant science was subdivided into discrete, classical
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disciplines, including anatomy, morphology, physiology,
biochemistry and genetics, our knowledge has expanded
greatly. In addition to the emergence of new research fields
and disciplines, including genomics and bioinformatics, we
are now combining distant disciplines to uncover complex
biological situations. Fundamental plant science is increas-
ingly becoming a collaborative domain, with research pro-
jects including aspects of physics, mathematics and chem-
istry. This is largely fueled by the last decade’s technical
advances, allowing for more discoveries, but also creating
new challenges, such as data storage, analysis and predic-
tions. Interdisciplinary collaborations are often the solu-
tion to these challenges. This led to the establishment of
systems biology, as an integrative approach to understand
complex networks that characterize the phenotypes in the
cell. When molecular biology emerged, plants were not the
organism of choice for experimentation. As a genetic model
for plants, pea was gradually superseded by other species,
such as Nicotiana tabacum and Antirrhinum majus, but it is
Arabidopsis thaliana that became the prominent model and
which has a smaller physical size, much smaller genome
and a shorter reproductive cycle (Meyerowitz 2001; Somer-
ville and Koornneef 2002; Koornneef and Meinke 2010).
The values of mutant analysis and genetic transformation
for plant physiology and biochemistry were demonstrated
using A. thaliana. Ten years after the publication of the
Arabidopsis genome sequence, it remains the standard ref-
erence for plant biology (Koornneef and Meinke 2010).
Today, we do not need to rely only on such simplified
models, but can also use more complex crop species (such
as maize, rice or soybean and common bean in the case
of grain legumes), as well as long-lived trees (poplar) to
understand different evolutionary and life strategies.
Mendelizing continuous variation: quantitative
trait loci
Immediately after the rediscovery of Mendel’s laws, biolo-
gists addressed the issue of continuous variation. Castle
(1903) remarked “Bateson makes the pregnant suggestion
that even cases of continuous variation may possibly prove
conformable with Mendelian principles” and gave the
example of intermediate height of pea from a short × tall
cross. East (1916) discussed the “general proof of the
cumulation effect of genes” found in maize (Hayes and
East 1915) and “most Mendelizing characters have been
shown to be due to several traceable factors.” East (1916)
presented quantitative data analysis on corolla length in
Nicotiana as evidence, and then summarized the addi-
tional evidence of the authors of numerous studies (citing
Belling, Castle, Davenport, East, Emerson, Hayes, Heri-
bert-Nilsson, Kajanus, MacDowell, Nilsson-Ehle, Pearl,
Phillips, Punnett, Shull, Tammes and Tschermak) in sup-
port of “plural segregating factors.” East and others noted
the effect of environment on quantitative trait expression
and further proposed eight requirements to test the multi-
ple factor hypothesis. Sax (1923) tested the hypothesis on
seed size and seed coat color in common bean and used
Castle’s (1921) data to estimate the number of factors.
He stated that “various assumptions necessary in estimat-
ing the number of size factors, based on F2 distribution,
make the results obtained of little value”, as also pointed
out by Shull (1921). Sax proposed an elegant explanation
for bean seed size and coloration segregation ratios by link-
age, i.e., genetic factors on the same or different linkage
groups and significantly suggested that “the size factors in
different chromosomes may not be equal in their effect.
Indeed, earlier, Shull (1921) discussed that all factors are
not necessarily additive, or equal in effect, while some
factors act in a negative direction and some in a positive
direction. Linked and unlinked were common terms used
in the literature since Sturtevant in 1913 (Frost 1921), but
Sax (1923) is credited with the first report of quantitative
trait linkage using a marker (seed coat pigmentation) to
classify chromosomes and detect linkage between major
genes and quantitative genes (seed size) in common bean.
Detection of genes controlling quantitative traits using
segregating marker genes and analyzing quantitative vari-
ation took a significant step forward with Thoday’s publi-
cation in 1961. Thoday challenged the assumption that the
determination of quantitative inheritance for a trait is the
end point and presented the basic thesis of using markers
in segregating populations to detect genes controlling quan-
titative traits. Already, breeders had found positive mor-
phological marker–quantitative trait associations (Everson
and Schaller 1955). Thoday (1961) suggested that with the
first demonstrated quantitative variation (biometrical genet-
ics) by Johannsen (in German, 1909) by progeny testing,
and with Sax’s 1923 experiments, the theoretical ground-
work for mapping quantitative trait loci was complete
and was an apparent next research objective for quantita-
tive geneticists. Understatedly, he noted the main limita-
tion as being the availability of markers for the detection
of the polygenes. The term quantitative trait locus and the
abbreviation (QTL) first appeared in the literature in 1975
by Geldermann studying animal genetics, who also noted
the paucity of available markers and added the importance
of precise phenotypes for QTL detection. The deployment
of co-dominant isozyme markers (Rick and Fobes 1975)
in the 1980s improved the detection of possible QTL by
increasing the coverage of the genome, while avoiding the
dominant/recessive effects of morphological markers but
with still too few markers to detect epistasis (Tanksley et al.
1982; Stuber et al. 1982; Edwards et al. 1987). The advent
of recombinant DNA techniques ushered in true genetic
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maps in humans of “DNA marker loci”, based initially on
restriction fragment length polymorphisms (RFLPs), also
as co-dominant markers but much more plentiful than ear-
lier ones (Botstein et al. 1980). Such maps were found to
have broad applications in plant and animal improvement
programs for marker-assisted introgressions, especially
of QTLs (Beckmann and Soller 1983). The next land-
mark came with the publication of “Mapping Mendelian
factors underlying quantitative traits using RFLP link-
age maps” with the improvement of QTL detection using
interval mapping and LOD score analysis providing the
genetic location and phenotypic effect of the QTL (Lander
and Botstein 1989), along with the companion publica-
tion (Paterson et al. 1988). RFLPs allowed for the scan-
ning of all the chromosomes (70 markers, 14.3 cM aver-
age spacing in the case of tomato) and launched waves of
QTL mapping in plants and animals for a wide range of
phenotypic traits. Advances in DNA sequencing technolo-
gies (Sanger and Coulson 1975) and the breakthrough of
the polymerase chain reaction (PCR) (Mullis et al. 1986)
steadily increased the DNA marker density of maps, allow-
ing for fine mapping QTL, both innovations resulting in
Nobel prizes in chemistry. These discoveries allowed for
the eventual cloning of the first causative gene underlying
a QTL, fruit size in tomato (Frary et al. 2000), completing
the assertion of Bateson that a quantitative trait could be
converted to single Mendelian factors. The limitations of
many linkage-based QTL studies still included the paucity
of high-density genetic maps and the limits of biparental
or pedigree-based mapping populations, with resolution
to large genetic regions rather than gene(s) due to insuf-
ficient recombination events. A new technique was pro-
posed for mapping complex trait loci, named association
mapping, based on using collections of genotypes to cap-
ture historic meiotic events (Risch and Merikangas 1996).
However, it took a breakthrough in statistical genetics to
reduce the rate of false positives in association mapping
studies with Bayesian-based statistics to identify previ-
ously cryptic underlying population structure of the assem-
bled genotypes (Pritchard and Rosenberg 1999; Pritchard
et al. 2000). Immediately, Thornsberry et al. (2001) applied
this approach to flowering time in maize and identified a
deletion in the Dwarf8 gene as the causal allele. Risch and
Merikangas (1996) also noted that the limitations for asso-
ciation studies were the paucity of polymorphisms across
the human genome. However, this limitation was solved
with the advent of sequencing of whole genomes, notably
the first plant species Arabidopsis thaliana (The Arabidop-
sis Genome Project 2000). Application of these advances
in agricultural crops has been complicated by problems
caused by low predictive power in current models and of
resource allocation between phenotyping and genotyp-
ing (Heslot et al. 2015). New models under development
are expected to improve prediction. Today, over 100 plant
species’ genomes have been sequenced (Michael and
VanBuren 2015), assisted by the implementation of next-
generation sequencing (NGS) technologies and reduced
costs (Balasubramanian et al. 2004; Bentley et al. 2008).
The advent of widespread whole-genome sequencing has
opened a new era of Mendelizing QTL, where a paucity
of genetic markers is no longer an issue (Hori et al. 2016).
However, new challenges are now revealed. In a recent
review of maize genetics summarizing progress with iden-
tifying candidate genes, it was stated that most quantitative
traits are controlled by a large number of small effect genes
“locked away in low-recombination regions”, presenting
challenges in (even) sequenced and highly genotyped asso-
ciation mapping panels (Wallace et al. 2014).
The advent of plant genomics
Beginning in the early twentieth century, advances in
microscopy, chromosome banding, DNA labeling, in situ
hybridization, flow cytometry, micromanipulation and
chromosome-imaging systems transformed classical
cytogenetics, paving the way for present-day molecular
cytogenetics. Cytogenetics contributed to the early stages
of genome mapping projects in diverse organisms, first
by mapping specific repetitive DNA, and later by map-
ping entire genes using fluorescence in situ hybridization
(FISH), or by distinguishing between parental genomes
in hybrids using genomic in situ hybridization (GISH)
(Kato et al. 2005). Further development of cytogenetic
approaches has led to chromosome painting in plants
(Lysak et al. 2001). Identification of chromosome territo-
ries occupied by specific chromosomes within interphase
nuclei using in vivo fluorescent labeling systems, in combi-
nation with other methods (e.g., fluorescence recovery after
photobleaching, FRAP), have increased our understanding
of chromatin dynamics. These methods, which allow for
the examination of sequence localization in 3D nuclei, will
be soon applied to plant genomes, much as analyses of 4D
chromosome dynamics in cycling cells were used in mam-
mals (Strickfaden et al. 2011).
Similarly, genetic mapping using molecular methods,
either RFLP based or later PCR based, led to the advent
of comparative genetics. The RFLP technique was first
applied to plants in the mid-1980s with the aim of pro-
ducing a new generation of markers for breeders. This
method resulted in reports of synteny across genomes, for
example, between tomato and potato (Bonierbale et al.
1988). It was not clear at that time that intergenomic syn-
teny holds mostly to genes. Alternative marker systems
based on PCR have complemented RFLP since the 1990s.
The first consensus grass map, which aligned the genomes
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of seven grass species, revealed extensive conservation
of gene order, despite many differences in organization
among the genomes observed (Moore et al. 1995; Gale
and Devos 1998). A multi-species approach allowed plant
genomics to evolve into a powerful and routine tool, espe-
cially when plant genomes first began to be studied within
genome sequencing projects. A. thaliana was the first com-
pletely sequenced plant species. Rice became the second
sequenced plant, not only because of its economic impor-
tance, but also due to its small genome, reasonable transfor-
mation competence and a detailed genetic map. The list of
sequenced plant species grew quickly thereafter (Michael
and Jackson 2013). Genomes of model species share rea-
sonable genetic synteny with key crop plants, which facili-
tates the discovery of genes and the association of genes
with phenotypes. During the last decade, large genomic
centers have generated massive data sets beneficial for
plant biologists. The current decade will bring essentially
completed sequences for multiple branches of virtually all
angiosperm clades that include major crop and botanical
models (Paterson et al. 2010). The acceleration of genome
projects was made possible by the invention and wide-
spread use of next-generation sequencing. “Progress in sci-
ence depends on new techniques, new discoveries, and new
ideas, probably in that order,” said Sydney Brenner in 2002.
The first DNA sequencing techniques were developed in the
1970s by Sanger and colleagues (Sanger et al. 1977) and by
Maxam and Gilbert (1977). Sanger sequencing (considered
first-generation sequencing) became the prevailing DNA
sequencing method for the next 30 years and enabled sci-
entists to complete the genomes of Arabidopsis, rice and
many other plant species. The emergence of NGS in 2005,
first developed by 454 Life Sciences (now Roche), entails
massive parallel sequencing (based on an older pyrose-
quencing method, Ronaghi et al. 1996) and was a great leap
forward toward faster, high-throughput and cheaper DNA
sequencing. NGS techniques now include several different
platforms (454, Illumina, SOLiD, Ion Torrent, PacBio and
others; for review see van Dijk et al. 2014) and allow sci-
entists to get billions of sequencing reads corresponding to
terabases (Tb) per run. The application of NGS in plant sci-
ence not only made feasible the whole-genome assembly of
many species but also facilitated other studies—e.g., gene
expression, DNA–protein interactions, the relationship
between genomic variation and phenotype—that covered
a wide range of related disciplines from molecular biology
via developmental biology to agrigenomics (Varshney et al.
2009). The recent advent of a third-generation technology,
represented by nanopore sequencing (Oxford Nanopore
Technologies), allows for single-molecule sequencing with-
out the need for library preparation or sequencing reagents.
Such technology has established single-cell genomics that
has recently been utilized in animals, and its application in
plants is only a question of time (Thudi et al. 2012).
Genomes: from C values to whole‑genome structure
and evolution
The amount of DNA in plant nuclei was estimated for the
first time 66 years ago, when the genetic role of DNA was
already known but before the double helix structure of
DNA was discovered in 1953. The haploid nuclear com-
plement was defined as the 1C value (Swift 1950). The C
values estimated in 2802 plant species (representing 1 %
of all angiosperm species and about 30 % of angiosperm
families) were obtained by 1997, and the C values of
approximately 1700 other species were estimated by 2003
(Bennet and Leitch 2005). Several methods have been used
to measure plant DNA C values, including Feulgen micro-
densitometry, flow cytometry or computer-based image
analysis (Greilhuber 2008). Since plant C values were first
estimated, it has become evident that there is no correlation
between the complexity of an organism and the size of its
genome. Closely related species often differ significantly in
their nuclear content. These enigmatic differences have led
to the term “C value paradox”. Originally, when the mosaic
structure of genes was discovered, differences in genome
size were attributed to the introns. Later, when genomes
were studied in more detail, it became clear that repetitive
DNA sequences, in combination with polyploidization,
provide the main keys to resolving the “C value paradox”.
Genome sizes vary by >2000-fold among the angiosperms,
from fewer than 107 base pairs (1C = 0.065 pg–63.4 Mbp)
in Genlisea margaretae, Lentibulariaceae (Greilhuber et al.
2006), to more than 1011 (1C = 152.23 pg–150 Gbp) in
Paris japonica, Melanthiaceae (Pellicer et al. 2010).
Genomes evolve by duplication of genes, chromosome
or whole genomes, by various rearrangements, insertions of
organellar, bacterial or viral DNA that are part of horizon-
tal gene transfer (HGT), (micro)satellite expansions, trans-
posable element insertions and other processes. Although
the first comparative studies suggested that plants have a
“one-way ticket to genomic obesity” (Bennetzen and Kel-
log 1997), later phylogenetic evidence showed that the
processes leading to the elimination of DNA, which often
involve repetitive elements, are also present and result in
genome downsizing (Petrov 2001; Petrov et al. 2003).
Genome sequencing projects led to the discovery of
the genome structure of many plants. A major part of the
nuclear genome of most plants is represented by different
repetitive DNA elements (Kubis et al. 1998); these elements
contribute to the higher evolutionary dynamics of genomes,
while genes represent slowly evolving (conservative)
genetic units. A high turnover of repetitive DNA (compared
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to genes) results in a fast divergence of these genome com-
ponents and leads, e.g., to an infeasibility of GISH map-
ping when more distantly related species are studied (Lim
et al. 2007; Koukalova et al. 2010), as well as causing prob-
lems with chromosome painting in plants (Schubert et al.
2001). Perhaps, the most distinctive feature of angiosperm
genomes is the large amount of genome duplication, i.e.,
polyploidization. It has long been suspected that many
angiosperms were paleopolyploids (Stebbins 1966), but
recent analyses of genome sequences suggest that virtually
all angiosperms are paleopolyploids (Bowers et al. 2003;
Paterson et al. 2004). According to a speculative hypothesis
(Chapman et al. 2006), genome duplications are not epi-
sodic but rather cyclic, providing various fitness advantages
that erode over time, which favors new polyploidizations.
Higher repetitive DNA turnover, repeated polyploidizations
and subsequent gene losses lead to much more rapid struc-
tural changes of plant genomes when compared with verte-
brates, where gene order conservation is evident even after
hundreds of millions of years of divergence (Kejnovsky
et al. 2009).
Repetitive DNA: from junk DNA to a major
evolutionary force
Repetitive DNA elements can be divided into two major
groups, distinguished by their genomic organization: trans-
posable elements (TEs) that are dispersed throughout a
genome and satellites arranged in tandem (Schmidt and
Heslop-Harrison 1998). Intermediate forms can also exist,
e.g., TEs can contribute to the origin and/or amplification
of satellite DNA. Satellite DNA, whose name was inspired
by the “satellite” band produced during density gradient
centrifugation, is subdivided according to monomer length
into microsatellites, minisatellites and satellites. Satellites
often constitute long arrays in genomes and could be a sub-
ject to concerted evolution (Elder and Turner 1995). Copy
numbers of individual repetitive DNA motifs can vary from
several hundreds to hundreds of thousands, and the tandem
arrangement of their multiple copies have not only non-
genic sequences, but also ribosomal genes. The balance
between homogenization and mutations results in a specific
range of satellite variability. Microsatellites go through the
phases of birth, expansion and regression (Ellegren 2004;
Kelkar et al. 2011). The discovery of transposable elements
by Barbara McClintock (1950) represented a major mile-
stone in genetics, but the greatest importance of her discov-
ery was, much as in the case of Mendel, recognized several
decades later when McClintock was awarded the Nobel
Prize in 1983. The recessive allele locus rugosus, the cause
of one of the traits (wrinkled seeds) studied by Mendel, is
caused by a DNA transposon insertion into a gene encod-
ing a starch-branching enzyme (Bhattacharyya et al. 1990).
Transposable elements are ubiquitous mobile genetic ele-
ments spread through genomes either by a copy and paste
mechanism via an RNA intermediate, used by retrotranspo-
sons, or by a cut and paste mode used by DNA transposons.
These two main classes of TEs are further subdivided into
several orders and many families and subfamilies (Wicker
et al. 2007). TEs can together constitute up to 80 % of an
individual genome, and a single TE family may represent
up to 38 % of a whole genome (Neumann et al. 2006). The
function of repetitive DNA has not been completely elu-
cidated, despite the many debates ongoing since the dis-
covery of repetitive DNA. Repetitive DNA was originally
considered to be “junk DNA” (Doolittle and Sapienza
1980; Orgel and Crick 1980), but the last decades have
shown that it represents an important evolutionary force
and may even function as a driver and facilitator of evo-
lution. Repetitive DNA, especially transposable elements,
can affect genome diversity and plasticity, induce epige-
netic changes, influence gene expression or build cellular
regulatory networks (Kazazian 2004; Oliver and Green
2009; Biémont and Vieira 2006; Feschotte 2008). Differ-
ences in repetitive DNA are the major factors responsible
for genome size variation, not only between species, but
also within a species. Some TEs are used for important cel-
lular functions in a process called domestication or exapta-
tion (Volff 2006; Kokosar and Kordis 2013). For example,
an integral part of the immune systems of vertebrates, V(D)
J recombination, evolved from Transib DNA transposons
(Kapitonov and Jurka 2005). Similarly, telomeres of Dros-
ophila melanogaster are formed by HeT-A and TART retro-
transposons (Abad et al. 2004; Biessmann et al. 1992), and
the centromere-binding protein CENP-B evolved from the
transposase of DNA transposons (Kipling and Warburton
1997). Present genomics views genomes as ecosystems of
various elements (genes, various repeats) interconnected by
a plethora of interactions, from symbiosis via competition
to parasitism. The character of these relationships between
elements can change over time, and originally parasitic ele-
ments can evolve into cellular functions, simply increase
individual variability or induce genome reshuffling, thereby
increasing the evolutionary potential of a species.
Impact of Mendelian genetics on plant breeding
and food security
From the dawn of agriculture until today, farmers have
acted as plant breeders, working almost exclusively through
mass selection, that is, by ensuring that some individual
plants made a proportionately greater genetic contribution
to the following generation than did others. Natural out-
crossing was frequent enough, even in self-pollinating spe-
cies, to generate useful genetic recombinants. Early plant
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Theor Appl Genet
1 3
breeders worked without the benefits of progeny testing or
replication, both of which can enhance gain from selection,
but they had two other important factors working in their
favor: time and ecosystems.
In the twentieth century, many plant breeding techniques
were developed on the basis of Mendelian principles of
inheritance (Kingsbury 2009). These include pedigree,
mass selection, and backcrossing approaches. Mutagenesis
techniques have allowed for the identification of many use-
ful new variants. In some crops hybrid approaches are prac-
tical and allow for the exploitation of heterosis to achieve
substantial gains in crop yield (Bernardo, this issue). Inter-
specific hybridization methods are being used to introgress
alleles for important traits such as disease resistance and to
broaden genetic diversity in crops.
Rate of gain in plant breeding has also been enhanced
in the past century by several improvements in methodol-
ogy. Contra-season nurseries allow for the production of
more than one generation in a year. Well-managed glass-
house and phytotron chambers also allow for off-season
advances. Improved small plot machinery has given rise
to major increases in the scale of breeding programs.
Improved agronomic practices for disease, weed, and insect
control have increased productivity in breeding. Improved
sample handling techniques such as bar-coding allow for
major improvements in the efficiency of plant breeding.
Improved experimental designs and statistical packages
have improved the efficiency of selection and made best
use of limited resources.
Until the nineteenth century, crop improvement and its
production were mainly in the hands of farmers and gener-
ally based upon the expansion of the cultivated area to pro-
duce the required food grains. The understanding of crop
improvement science based on Mendel’s genetic principles
laid a firm foundation to science-based agriculture. Under-
standing of trait genetics in the light of Mendel’s princi-
ples of heredity, Norman Borlaug led the development
of high-yielding semi-dwarf varieties of rice and wheat,
which revolutionized wheat and rice production in Asia
in the mid-1960s. This breakthrough came to be known as
the Green Revolution and symbolized the process of using
agricultural science to develop modern techniques for the
benefit of developing countries. More precisely, these vari-
eties transferred many nations such as India, Pakistan, and
the Philippines from “mouth-to-ship” situation. Presently,
science-based crop improvement, which owes its founda-
tion to Mendelian principles, contributes 2784 million tons
(FAO 2015) of cereal grains to the world food basket to
nourish the planet.
Methods of crop breeding have undergone major
changes, and a range of technologies is improving the
rate and success of crop improvement in some breeding
programs, but these are yet to be widely adopted. Contribu-
tions are being made through new selection strategies that
are informed by sophisticated genetics, the use of comput-
ers to track and manage field trials, and biometric meth-
ods for field trial design and assessment of interactions
between genotype, environment, and management. Hetero-
sis (hybrid vigor) for inbreeding species can offer 20–50 %
yield increases. Strategies for using heterosis more widely
to increase yields in inbreeding crops center on finding
ways of reducing the cost and increasing the efficiency of
producing hybrid seed (Kingsbury 2009). These include
identifying new sources of male sterility for hybrid creation
and using transgenic approaches to engineer sterility and
restore fertility. Another potential future mechanism for
producing hybrid seeds involves the use of apomixis, where
plants produce seeds without the need for fertilization.
Mendel’s principles in the era of genomics
Present-day genomics research has developed on three
milestone discoveries of biology, namely, Mendelian prin-
ciples of heredity, evolutionary principles of Darwin, and
the discovery of the DNA structure. Mapping populations,
their use in segregation of molecular markers and marker–
trait association to map and isolate genes, were developed
on the basis of Mendelism. With the advent of NGS-based
technologies and the rapid decline in per sample cost,
many sequencing-based approaches have been proposed.
SHOREmap (Schneeberger et al. 2009), next-generation
mapping (NGM) (Austin et al. 2011), MutMap (Abe et al.
2012), isogenic mapping by sequencing (Hartwig et al.
2012), SNP-ratio mapping (SRM) (Lindner et al. 2012),
MutMap+ (Fekih et al. 2013), MutMap-Gap (Takagi et al.
2013), and Seq-BSA (Singh et al. 2015a, b) are some of the
important approaches for trait mapping. These approaches
are not only fast and reliable, but more cost-effective in
comparison to the conventional approach of trait mapping
and deployment.
In addition to classical and modern plant breeding, Men-
del’s work laid the foundation for today’s molecular breed-
ing and genetic engineering. Mendel’s laws were helpful
for selection of stable and promising plants/events based on
segregation ratios. Globally, using Mendelian genetics in
terms of foreground selection (selection of plants possess-
ing allele(s) of interest in the segregating generation though
linked markers), with and without background selection
(selection of plants with a higher proportion of the recur-
rent parent genome using genome-wide markers), many
cultivars have been developed using molecular breeding
(especially, through marker-assisted backcross breeding)
approaches. This approach is useful for precise and rapid
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Theor Appl Genet
1 3
development of improved breeding lines for the target traits
such as disease resistance, nutritional quality, drought tol-
erance and submergence tolerance across different crops.
Although markers can be used at any stage during a
typical plant breeding program, marker-assisted selection
(MAS) is a great advantage in early generations, because
plants with undesirable gene combinations can be elimi-
nated. This allows breeders to focus attention on a lesser
number of high-priority lines in subsequent, more expen-
sive, field generations. Although DNA markers were first
developed in the 1980s, more user-friendly PCR-based
markers such as SSRs were not developed until the mid- to
late 1990s, and SNPs in the past decade. The cost of using
MAS compared with conventional phenotypic selection
may vary considerably.
Genome-wide or genomic selection (GS) is a recent
approach for the development of improved breeding lines
(Meuwissen et al. 2001). GS also relies on MAS and is
under evaluation for the feasibility of incorporating desir-
able alleles at many loci that have small genetic effects
when used individually. In this approach, breeding values
can be predicted for individual lines in a “training popu-
lation” based on phenotyping and whole-genome marker
genotyping. These values can then be applied to progeny in
a breeding population based on marker data only, without
the need for phenotypic evaluation. Successful examples of
the application of GS have been reported in several crops
(Heffner et al. 2011; Asoro et al. 2011; Lorenz et al. 2012;
Crossa et al. 2014; Spindel et al. 2015). Complex trait dis-
section using high-throughput technologies have recently
been developed to determine the phenotypic components
of complex traits, for example, robotic greenhouse sys-
tems with nondestructive imaging to monitor growth rates.
These phenomic techniques yielding precise digital data in
combination with the recent throughput and cost-efficiency
in genomics techniques offer the prospect of powerful asso-
ciative analysis being established to link genotype to phe-
notype. Increasing genetic diversity requires an expansion
of the germplasm base in breeding programs, but this is
dependent on enhancing techniques for assessing the value
and use of individual accessions from germplasm collec-
tions. Improvements in phenotyping and genotyping will
help remove this limitation by facilitating the identification
and characterization of key adaptive QTLs. Introgression of
novel alleles from landraces and wild relatives is often slow
and tedious, but options are now being developed for accel-
erating introgression using molecular approaches (Zamir
2001). The wider deployment of genetically modified (GM)
approaches will be needed for the introduction of novel
genes and alleles from diverse sources, and particularly for
traits that are absent in plant genomes (for example, Bacil-
lus thuringiensis toxin from soil bacteria), or where there
is insufficient variation for practical utility (for example,
vitamin A accumulation in rice endosperm) (Tester and
Langridge 2010). The slow advances in GM crops besides
political decisions can be attributed to the “inefficiencies
of conventional random mutagenesis and transgenesis”
(Shukla et al. 2009) and the lack of target genes of impor-
tance to crop production hampered by these inefficien-
cies (Townsend et al. 2009). Early success in more precise
gene editing in plants was reported by Shukla et al. (2009)
in maize and by Townsend et al. (2009) in tobacco using
engineered Zn finger nucleases (ZNFs). The resulting effi-
ciencies demonstrated in engineering herbicide resistance
in tobacco and maize represent a huge step forward fol-
lowed by Li et al. (2012) using transcription activator-like
effector nucleases (TALEN)-based gene editing to produce
disease resistance in rice. The breakthrough of the decade
was publication of gene editing with the clustered regularly
interspaced short palindromic repeats (CRISPR)/CRISPR-
associated (Cas) system for RNA-programmable genome
editing (Jinek et al. 2012) followed quickly by multiplexed
genome engineering using the CRISPR/Cas system (Cong
et al. 2013). The first CRISPR/Cas system gene editing was
demonstrated in model plants (Arabidopsis and tobacco) by
Li et al. (2013). While not a panacea (Fu et al. 2013), this
is an important progress in precision gene editing. Details
of the three gene editing systems are presented in a review
article by Gaj et al. (2013).
Conclusions
Despite tremendous progress made over the past 150 years,
genetics will forever rely on basic principles discovered
and formulated by G.J. Mendel in 1865 on garden pea. As
a genetical model for plants, pea was gradually superseded
by Arabidopsis thaliana. Today, however, we do not need
to rely only on such simplified models, but we can use
more complex crop species with often large genomes. Men-
del’s experiments were based on qualitative traits; however
with the use of statistical analysis the issue of continuous
variation, quantitative variation, was made accessible. QTL
provide another demonstration that quantitative traits are
governed by the same principles as single qualitative genes.
During the last 150 year period, key discoveries of heredi-
tary principles were made, among others the relationship
between genes and proteins, the double helical structure of
the DNA molecule and, based on these, currently flourish-
ing disciplines of molecular biology and genomics. Today,
over 100 plant species’ genomes have been sequenced,
assisted by the implementation of NGS technologies. There
is an emergence of new research fields and disciplines,
including genomics and bioinformatics, and we are now
combining distant disciplines to uncover complex biologi-
cal situations. In addition to classical and modern plant
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Theor Appl Genet
1 3
breeding, Mendel’s work laid the foundation for today’s
molecular breeding and genetic engineering. This is largely
fueled by the last decade’s technical advances, allowing for
more discoveries but also creating new challenges, such
as data storage, analysis and prediction. Genetics has had
a tremendous impact on agriculture through crop breeding
and similarly genomic knowledge is gradually being trans-
lated to molecular breeding and genome-wide or genomic
selection for the development of improved breeding lines.
Author contribution statement P.S. coordinated the
manuscript layout, assembly and wrote the Reflection on
Mendel’s work on pea; C.C. and C.D. wrote the section
on quantitative traits and gene editing; E.K. wrote the sec-
tion on cytogenetics and repetitive DNA; R.K.V., V.K.S.
and T.W. wrote the section on translational genomics and
breeding. All authors edited the manuscript and read and
approved the article.
Acknowledgments This research was supported by the Czech Sci-
ence Foundation (Grant 15-02891S to E.K. and 14-11782S to P.S.)
and Palacký University Grant IGA 2015_1 and IGA 2016_1 to P.S.
Various colleagues are acknowledged for their fruitful discussions on
the earlier versions of the manuscript.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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... Despite the tremendous progress in genetics and modern plant breeding in recent years, it will forever rely on the basic principles formulated by Mendel on the garden pea. Genetics has a great role in crop breeding and similarly genomic knowledge is gradually being translated to molecular breeding and genome-wide or genomic selection for the development of improved breeding lines (Smýkal et al. 2016). ...
Chapter
Garden pea (Pisum sativum L.), a member of the Fabaceae family, is one of the most important self-pollinating legume crops. Globally, the pea is an economic crop, utilized as food, feed and industrial uses. Garden pea is an annual winter-season crop grown around the world from winter to early summer depending on the country. Gene banks have conserved a large genetic resource collection of pea germplasm. Pisum harbors significant diversity based on biological status, geographical regions and morpho-agronomic traits. Introgression of novel alleles through crossing between various pea genetic resources, e.g. modern varieties with locally adapted varieties, enhances genetic diversity and preselection for traits of interest, which is required to ensure meaningful natural variation at the phenotypic level. Improving pea for biotic and abiotic stress tolerance traits, quality traits and yield attributes are the main objectives of breeders and geneticists. These can be achieved with genomics tools to augment traditional breeding programs. In this chapter, we will provide an overview of the origin of the pea, distribution, genetic resources, conservation, cultivation practices, recent developments in biotechnology and molecular genetics to improve traditional breeding methods.
... Luckily, Mendel had chosen monogenic, independent traits, rather than more complex or linked characteristics, so he was able to distinct manifested dominant, and "hidden" recessive traits. Mendel explained laws of recombination in respect of progeny formation, accepting hybrids as the contribution of parents to offspring, rather than the progeny would be a simple mix of parents (Smýkal et al., 2016). By the irony, Mendel's work was not valued in his time. ...
... Since the origins of plant breeding as a systematic science, after the rediscovery of Mendel's laws just over a century ago, plant breeders have successfully developed new improved varieties by crossing and selection (Smýkal et al., 2016). In recent decades, integration of new molecular breeding and biotechnological techniques into the process of crop improvement has contributed further to enhanced genetic gain and breeding has increased the productivity and sustainability of major crops (Voss-Fels et al., 2019). ...
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This book contains 29 chapters focusing on wheat, maize and sorghum molecular breeding. It aims to contribute the latest understandings of the molecular and genetic bases of abiotic stress tolerance, yield and quality improvement of wheat, maize and sorghum to develop strategies for improving abiotic stress tolerance that will lead to enhance productivity and better utilization of natural resources to ensure food security through modern breeding.
... Mendel is a famous genetic scientist through his law of heredity, First law: Law of Dominance; Second law: Law of Segregation; and Third law: Law of Independent Assortment (Yihoop et al., 2009;Gautam, 2018). There were many researchers conducted experiment to trace Mendel's law such Smykal et al. (2016) cited that in early 1900s de Vries, Correns, and Tschermak had published each research paper to confirm Mendel's second law (3:1). Even though there have been many papers published so far, there are not any paper that had yet been rejected Mendel's law of heredity. ...
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... Without our modern understanding of genetics, these farmers propagated plants with desirable characteristics that arose from mutations or hybridization (Bennett 2010). In the late 19th and early 20th centuries, plant breeders rediscovered Gregor Mendel's work on genetics and applied studies with replicated field trials, controlled crossings, statistical analyses, and formal experimental designs (Smýkal et al. 2016;Wallace et al. 2018). In modern agriculture systems, most of these breeding technologies migrated to high-throughput genotyping and agricultural biotechnology (Wallace et al. 2018). ...
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