INDIRA GANDHI VISHVAVIDYALAYA,
RAIPUR (C.G.)
Assignment I
On
Incongruity, factors influencing incongruity and Methods to
overcome incongruity
Course No. GP-605
Advances in Plant Breeding Systems
Submitted by
Sandip Purushottam Patil
Ph.D. Ist Sementer
Department of Genetics and Plant Breeding
Incongruity, Factors Influencing Incongruity and Methods to
Overcome Incongruity
------------------------------------------------------------------------------------------Introduction
The relationship between pistil and pollen of flowering plants is an example of
an intimate relationship between partners, such as occurs in many different forms in
nature. It is the total of characters and mutual influences of pistil and pollen leading to
a successful fertilization. Much attention in research has been given to non-functioning
of the pistil-pollen relationship. Until recently this non-functioning has generally been
attributed to, or at least been associated with, one mechanism only: incompatibility.
This is an outbreeding system, which is widespread in the plant kingdom and based
on incompatibility genes or S genes. These S genes were supposed to govern
nonfunctioning of the pistil-pollen relationship, not only within but also between
populations (Anderson & de Winton 1931; Bateman 1943; Mather 1943; Ushkarnath
1953; Lewis 1954, 1955; McGuire & Rick 1954; Lewis & Crowe 1958; Garde 1959;
Rick i960; Martin 1961a, 1964, 1967, 1968; Pandey 1962, 1964, 1967&, 1968, 1969a,
6; Hardon 1962, 1967; Abdalla 1970; Gunther & Juttersonke 1971; Abdalla &
Hermsen 1972).
Interspecific incompatibility maintains species identity by discouraging the
formation of hybrids. In most cases the physiological and biochemical differences
between potential hybrid parents are so great that the necessary sequence of events
leading from pollen capture to fertilization cannot be completed, a circumstance
termed 'incongruity' (Hogenboom 1975).
It is the thesis of Hogenboom (1972a, 1972b, 1973, 1975) that inter-species
incompatibility, termed incongruity by this author, is a process completely distinct from
51 and foreign to the 5 locus. According to Hogenboom, the non-functioning of the
pistil-pollen relationship may be due to two separate mechanisms. One is
incompatibility, a mechanism that "prevents or disturbs the functioning of the
relationship" and regulates inbreeding and outbreeding at the intra-species level. The
second principle for non-functioning is incongruity, "the incompleteness of the
relationship". The partners, usually different species, do not fully fit together, and the
matching of the gene system is not complete. In contrast to incompatibility, which is an
evolutionary solution through a precise and specific reaction to the negative effects of
inbreeding, incongruity is considered by Hogenboom to be a by-product of
evolutionary divergence that may affect any part of the relationship between the pistil
and pollen. Thus, it can be of a very different nature in each case.
In species with other types of self-sterility, the term interspecific incompatibility
may be valid only in its widest sense of species incongruity (Hogenboom 1975).
Moreover, there are many cases of interspecific incompatibilty between species which
do not have an intraspecific mechanism. As would be expected, the block to
hybridisation can occur at any point in the pistil, and this has been reported in detail
for interspecific crosses of Populus (Gaget et al. 1984), Rhododendron (Williams et al.
1990) and Eucalyptus (Ellis et al. 1991). In some cases the block appears to be purely
physical, in that pollen of species with short styles are unable to grow to the base of
long styled species of the same genus (Potts and Cauvin 1988; Williams et al. 1990).
Thus, from the evidence avallable, it appears that there may be a relationship between
pre-zygotic incompatibility and interspecific incompatibility, and between post-zygotic
breakdown and incongruity.
TWO DISTINCT MECHANISMS FOR NON-FUNCTIONING:
INCOMPATIBILITY AND INCONGRUITY
The above picture of the pistil-pollen relationship clearly allows at least two
mechanisms for non-unctioning. One is incompatibility, a mechanism The above
picture of the pistil-pollen relationship clearly allows at least two prevents or disturbs
the functioning of the relationship. The pistil-pollen relationship is rendered nonfunctional, though the potential for functioning of both partners is complete. The
inhibition or disturbance acts as it were from the outside, that is, from outside the
matching genic system relative to normal progress of fertilization. The inhibiting or
disturbing incompatibility mechanism forms no part of this system. The other principle
for non-functioning is incompleteness of the relationship. The partners do not fully fit
together, the matching of the genic systems is not complete. For this phenomenon the
term incongruity has been proposed (Hogenboom 1973). Incompatibility (definition in
Rieger, Michaelis & Green 1968), a thoroughly studied outbreeding mechanism which
is widespread in plants (Esser 1967; Arasu 1968), is mostly based on multiple alleles
at one or two loci. If the products of S gene action in pistil and pollen have the same
specificity, an inhibiting principle results, which makes pollen tube growth impossible
or stops it (Sampson 1960; Lewis 1965; Ascher 1966; Pandey 1967a; Linskens 1968).
Incongruity stands for non-functioning of a relationship resulting from a lack of
genetic information in one partner about some relevant character of the other. In the
pollen some essential penetration or reaction gene or gene complex corresponding to
a certain barrier or promoter in the pistil may be lacking. The consequence will be th a
t at some moment between pollination and fertilization the pollen will not be able to
penetrate th a t barrier or to react to that promoter. Pollen tube growth will then stop.
All the evolution of incompatibility a positive selection pressure for genes inhibiting
self-fertilization has been the driving force. Incompatibility is thus an evolutionary
solution to a phenomenon with generally negative effects in nature: inbreeding.
Incongruity, on the other hand, is a by-product, namely of evolut io n a l divergence
(Dobzhansky 1947; Stebbins 1950; Grant 1971). In a population of freely interbreeding
plants no incongruity in the relationship between pistil and pollen occurs. But when a
sub-population meets a different environment, it may be differentiated. Through
natural selection a change may directly or indirectly occur in one of the partners. An
extra barrier may, for example, appear in the pistil. This results in incongruity within
this sub-population between pistil and pollen. In the pollen population genes
corresponding with the extra barrier may be present in a certain frequency as a
reserve of genetic information. The new incongruity puts these genes under selective
pressure, so that incongruity within this sub-population may be repaired. If this
process of differentiation continues, within the sub-population the relationship between
pistil and pollen may become essentially different from th a t of the original population.
Then an isolating mechanism has come into being between original population and
subpopulation: incongruity occurs between pistil of one and pollen of the other
population. The incongruity between these populations will at first be slight, but it may
increase as the divergence proceeds. (The origin of incongruity is described here as a
consequence of adaptation to a different environment, but the same process may
occur as a consequence of different ways of adaptation of populations to the same
habitat (Stebbins 1950).
Incompatibility results from a very specific reaction. In gametophytic
incompatibility systems each of a series of S alleles or S allele combinations governs
the synthesis of a specific protein. Identical proteins from pollen and pistil combine to
form a product which induces an inhibiting process for pollen tube growth (Lewis 1965;
Linskens 1968). In sporophytic systems the principle is probably the same: identical S
allele products in pollen wall and pistil surface are the basis of preventing germination
and/or penetration. This principle of non-functioning as a result of similarity of the
partners for S alleles is the same in a large number of species. By contrast,
incongruity is non-functioning as a consequence of non-matching of the partners for
the genetic information which regulates interaction and coordination and may
therefore in each case be of a very different nature. It may concern each part of the
relationship between pistil and pollen. The pollen may, for example, lack genetic
information on the pistil’s osmotic pressure, cutin or waxy layers on the epidermis,
length, and on one or more of its many other physiological, biochemical or structural
characters, depending on the direction and degree of the evolutionary divergence.
Therefore in the plant kingdom as a whole incongruity between pistil and pollen will be
the rule and congruity an exception, whereas incompatibility will be an exception and
compatibility the rule. It will be understood that the genetics of the two mechanisms for
non functioning will also be different. Arguments for this can, until now, nearly
exclusively be extracted from research on interspecific crosses. For, quite surprisingly,
studies on the inheritance of interspecific pollen tube growth inhibition were almost
exclusively carried out on interspecific hybrid material. In this way complicating
interspecific interactions are introduced. These complications will in many cases have
played a role and, consequently, the conclusions from this type of studies may be
unreliable. Grun & Aubertin (1966) had a better approach: within one species the
genetic difference was studied between plants accepting and plants not accepting
pollen from another species. (In fact this is the normal approach for a study on the
inheritance of any character, but here this was not always possible.) They found an
interspecific pollen tube growth inhibition based on two or more independent dominant
genes. The same approach was made possible in (Hogenboom 1972a). In this study it
was shown that the unilateral incongruity between two species was a complex of
separate processes, in the style as well as in the ovary (cf. Grun & Aubertin 1966),
which could be broken stepwise by inbreeding and artificial selection. Here, too, the
interspecific pollen tube growth inhibition was found to be based on a number of
independent dominant genes. Each of these probably governs one of the separate
processes of which the inhibition is built up. I t fits in with the hypothesis on the
evolution of incongruity that its complexity generally depends on the relatedness of the
populations (Sanz 1945; Bellartz 1956; Lewis & Crowe 1958; Grun 1961; Smith 1968).
Mostly, if not always, incongruity will be genetically more complex than incompatibility.
On the other hand, the way in which incongruity is expressed does not in itself say
anything about its complexity. In connexion with the relatedness it should be noted th
a t the occurrence of incongruity does not necessarily run parallel with the taxonomical
classification. Incongruity is known to be absent between certain species. On the other
hand it may occur between populations within a species (Martin 19616,1963). I t may
therefore be more convenient here not to use the concept of the species but th a t of
the population. Apart from the above, arguments were obtained from many other
studies in different genera for the distinctness of incompatibility and incongruity. In a
species the stop of pollen tube growth caused by the incompatibility reaction and tha t
which occurs after an interspecific cross are mostly different (Bateman 1943; McGuire
& Rick 1954; Lewis & Crowe 1958; Rick i960; Ascher & Peloquin 1968; Pandey 1968,
1969; Hogenboom 1972 d). Arguments for physiological came from studies by Ascher
& Peloquin (1968), Newton et al. (1970). All these studies show th a t incompatibility
and incongruity are entirely different phenomena. The only similarity seems to be th a t
the final effect is the same: inhibition of pollen tube growth and failing fertilization. The
above reasoning on co-evolution, intimate relationships and incongruity may also be
applied to the genomes that come together after the fusion of nuclei. Between these
partners also, for a normal development of the new individual an accurate coordination
is necessary. If one of the genomes, as a result of a divergent development, does not
meet this requirement (e.g. by deviating gene location or gene interactions, different
rate of processes in replication, etc.) certain processes at a certain moment may fail to
occur or have disadvantageous effects. Possible consequences may be, for example,
embryo abortion, hybrid weakness or hybrid sterility. I t will be difficult to study this
relationship, as the partner genomes cannot easily be separated. But in this case also
the same mechanism is active: the genetic information of two partners in a common
process is not fully matching and coordinated. This incongruity may also concern
different developmental stages and very different aspects of development.
Genetic Basis of Incongruity
Hogenboom describes the barrier capacity (b) as the sum total of the pistil
characters that are relevant to pollen-tube growth and fertilization. In the same
manner, the sum total of information in the pollen used for normal functioning in the
pistil is called penetration capacity (p). The barrier capacity is considered to be
governed by dominant genes, while the corresponding penetration genes may be
dominant or recessive. In his model, Hogenboom designates the b-p complex of
genes by letters from A to Z. For example (b:AA) refers to the barrier capacity genes
AA to ZZ and (b:BB) designates a case where AA is lacking; similarly, (p:AA) indicates
the presence of all penetrating genes, while (p:AACC) means a penetration capacity
from AA to ZZ where BB genes are lacking. Two plants with notations such as (b:AA,
p:AA) and (b:AA, p:AA), which match one another, are reciprocally "congruous".
Because only pollen bearing all the penetration genes that correspond to the barrier
capacity in the pistil can function, the pistil of a plant (b:AA, p:AA) will reject the pollen
of a plant (b:BB; p:BB). There will be no incongruity, however, in the reciprocal cross,
because all pistil barrier genes in (b:BB, p:BB) are matched by the penetration genes
in the pollen (Table 1). It is clear that a model of this kind explains all possible types of
cross-incompatibility, whether reciprocal or unilateral, between 5C and/or 51 species.
At the limit, the model could also account for 51 itself, because any plant that, through
the action of repressors, gene splicing or of certain mutations, acts as (b:AA, p:BB) will
reject its own pollen.
Table 1. Examples of congruous and incongruous relationships (reciprocal or nonreciprocal) in the incongruity hypothesis of Hogenboom (1973). A pollen grain is
congruous if it displays the penetration capacities corresponding to the barrier
capacities manifested in the foreign pistil, or if the pistil fails to express a barrier
capacity corresponding to the penetration capacity lacking in the pollen. For instance,
two species lacking the same barrier capacity in the pistil (BCP) and lacking the same
penetration capacity in the pollen (PCP) and that both express, as in the table, AA CC DD, are reciprocally congruous (Rc). Two species with different penetration and
barrier capacities (for instance, AA BB - DD in one species and AA - CC DD in the
second species) are reciprocally incongruous (Ri). Other combinations lead to a nonreciprocal relationship (NRc or NRi)
Congruity
Incongruity
RC
NRc
Ri
NRi
BCP
PCP
BCP
PCP
PCP
PCP
BCP
PCP
AA
AA
AA
AA
AA
AA
AA
AA
-
-
-
BB
BB
-
BB
-
CC
CC
CC
CC
-
CC
CC
-
DD
DD
DD
DD
DD
DD
-
-
Pre-zygotic incompatibility and incongruity barriers are defined here as any of
the relationships (or absence of relationships) between the pollen and pistil that
impede the formation of hybrid zygotes between two fertile species. They occur
through the failure of the pollen to germinate or failure of the pollen tubes to reach the
ovules and discharge sperm nuclei able to fuse with the egg cells. The barriers
prevent gene flow among species and [in opposition to self-incompatibility (So, which
restricts inbreeding) establish upper limits to outbreeding and panmixis. They
contribute to the isolation of species and consequently favor speciation (or its
reinforcement) and the gradual increase of polymorphism within the genus and the
family. The inter-species incompatibility, which is a starting point of divergence,
operates between closely related species belonging to a same family or to a cluster of
families hosting the same SI system. The barrier is usually unilateral, among SI
species as pistillate parents and among SC species as staminate partners. In certain
species of the genus Nicotiana, to involve the intervention of pistil proteins encoded by
the S gene active in the pistillate parent. Incongruity, on the contrary, is the
consequence of divergence. It does not appear to require S-gene activity and often
occurs between species that have been isolated for a long time and have
differentiated considerably from one another.
Species hybrids and later generations
1. The presence or absence of functioning S-alleles and, if present, the type of
incompatibility system in one or both parents are important. Intact S-alleles lead
to incompatibility by interaction only with identical S-alleles in an adequate
background.
2. The number of genes in which the penetration capacities of the parents differ,
may be one or more. It is one of the factors that determine the percentage of
pollen in the F1 hybrids with the penetration capacity of the parent with the
highest barrier capacity. The greater the difference, the smaller the percentage
of this pollen.
3. One parent may be heterozygous or homozygous for the barrier gene(s) for
which in the second parent the corresponding penetration gene(s) are lacking.
Therefore F1 hybrids may or may not occur with a barrier capacity matching the
penetration capacity of the second parent.
4. The degree of heterozygosity of barrier capacities in the parents determines
segregation of different barrier capacities in the hybrids.
5. The number of genes governing barrier and penetration capacities probably
being large, linkage of one or more of them with the S-locus is likely to occur. A
certain degree of linkage may for example occur in one parent between the Slocus and the barrier gene(s) for which the corresponding penetration gene(s)
in the other parent are lacking.
6. In interspecific hybrids recombination is often restricted. For instance
crossingover between the S-locus and the locus (loci) for the penetration
gene(s) lacking in one parent may be slight or absent. A possible consequence
is that in hybrids of the cross elf-compatible X self-incompatible no Sc-pollen
develops that has the complete penetration capacity of the hybrids.
7. In interspecific hybrids reduced fertility is often found.
8. In F2 and later generations the situation in regard to crossing-over etc. may
change and become very complicated by segregation of modifyers and
disturbed segregations from different causes, such as interaction between
genes and cytoplasm.
Taking these conditions into account, the available results of inheritance studies on
the behaviour of plants in various interpopulational crosses and in further generations
can all be reinterpreted and explained on the same basis with the present model. The
results of Grun & Auberttn (1966) also fully agree with it. Their non-acceptor genes in
one parent are barrier genes for which in the other parent the corresponding
penetration genes are lacking. The results, found in different genera, are all
confirmations of the model. They show that in interspecific crosses two mechanisms
are active: incongruity as the most important one and, if present and secondary,
incompatibility. In interspecific hybrid plants the two mechanisms may occur together:
part of the pollen does not function after self-pollination because of lack of penetration
genes (self-incongruity), an other part does not do so because of action of S-alleles
(self-incompatibility). The two may be difficult to distinguish.
S-gene polymorphism
S-gene polymorphism in the control of interspecific crosses has been assumed
to account for the difference, as found in Nicotiana, between self-incompatible plants
accepting and such plants rejecting the pollen of an other population. The hypothesis
distinguishes S-alleles with quite different actions in relation to Sc- and S-alleles from
other populations.
As Mather (1943) stated, 'this is a complicating assumption for which there is
no external evidence, and is unlikely to be true because it postulates a sharp
difference between two allelomorphs, which otherwise show qualitatively similar
behaviour, in their reaction to an allelomorph from a different species'. The results
leading to the hypothesis of S-gene polymorphism can be reinterpreted with the model
for incongruity, by supposing that the S-locus and one or more loci for barrier genes
are close together (cf. Simmonds, 1966), so that linkage occurs between certain Sallele(s) and certain barrier gene(s) for which the corresponding penetration gene(s) in
the pollen parent are lacking. E.g. if in the chromosome segment —A—S—B— (A and
B barrier genes) no crossing-over occurs, this allows, by mutations in A and/or B, at
least four different S-allele/barrier gene combinations. With more genes more
possibilities occur. In interpopulational crosses these may each give their own reaction
pattern, depending on the penetration capacity of the pollen parent. This pattern is
thus mainly determined by barrier and penetration capacities. As a result of linkage it
only seems as if S-alleles play this role. Certain results (Pandey, 1969a) point to some
crossing over between S-locus and some barrier gene. In this way and taking into
account the earlier mentioned conditions in interspecific crosses, all results can be
explained. The model for incongruity also explains how this 'polymorphism' may as
well occur in populations of self-compatible plants, at least in the first period of selfcompatibility. This has been found in Nicotiana and Lycopersicon (Chmielewski, 1966,
1968; Hogenboom, 1972a, b). This phenomenon is at the same time an indication of
an other mechanism than incompatibility. Some results obtained with Nicotiana
(Pandey 1967b, 1968, 1969) can be used for the test only after a correction. Pandey
considers a cross of $ with long style with short style compatible if the pollen tubes
grow the length of the short style. However, this is just one of the possible expressions
of incongruity: by adaptation of the penetration capacity to the barrier capacity, which
may in certain cases be adaptation to a short style, the pollen is not informed to grow
far enough in long styles of other species. This adaptation to style length is also
referred to in the work of Swaminathan & Murty (1957). It is, however, certainly not a
common phenomenon
Complex patterns of crossability
Stepwise unilateral relations between populations, resulting in complex
crossability patterns, have been found in Lycopersicon (Chmtelewski, 1962, 1968;),
Nicotiana (Pandey, 1968, 1969) and Petunia (Stout, 1952). To reject pollen of other
populations a primary specificity element per population is necessary. These different
primary specificity elements are thought to have evolved through duplication and
differentiation and spread and be maintained by selection pressure based on the
presence of foreign pollen. Isolation of the population would lead to S-gene erosion.
To explain complex patterns Pandey supposes an inverse relationship between stylar
and pollen compatibilities, resulting from complex interactions, which are different for
pollen and style, between the elements for primary and secondary specificity in the Sgene complex. This hypothesis on the evolution, the structure and the action of the Sgene to explain interspecific relationships, which is an extension of that of Lewis &
Crowe (1958), is a complex of rather speculative and disputable suppositions. With
the model for incongruity between populations the complex crossability patterns are
easily explained. Each population has its own pistil-pollen relationship, and its own
barrier and penetration capacities. Actually, according to the present model, such
complex patterns are likely to be the normal situation in taxa with a certain degree of
differentiation.
Also for this test with the Nicotiana results (Pandey, 1968, 1969b) it should be
remembered that if pollen tubes do not grow into the ovules as a consequence of style
length, there is incongruity. The conclusion from this and the previous section is that
the S-gene is not a supergene. Different properties ascribed to it, which do not agree
with the S-gene model of Lewis (1965).
Unilateral incongruity (UI)
Hogenboom coined the term 'incongruity' to encompass passive reproductive
barriers which were caused by isolation between taxa (Hogenboom 1973, 1975). He
postuated that incongruity occurs from a lack of or miscommunication of signales)
between the male gametophyte (pollen tube) and female sporophyte (pistil) necessary
for reproductive success. Thus, the results of incongruous crosses can include such
phenomena as poor pollen tube formation and growth, lack of fertilization, poor
embryo development or embryo failure, decreased seed viability or germinability and
lack of vigor or fertility of hybrid plants.
The term 'unilateral incongruity' (UI) refers to a specific incongruity response
where a cross between two species produces viable seed, but the reciprocal cross
either fails or is considerably more difficult. The basic mechanism underlying UI is
unknown. Since the cross can be successfully made in one direction, gross genomic
imbalance or chromosomal abnormality of the hybrid are precluded as the cause of
the incongruity in the reciprocal cross. UI has been observed in a number of genera.
This listing is restricted to cases in which there is sufficient data to indicate the
presence of a unilateral barrier (not necessarily UI), and is limited to cases using
diploid species which have the same chromosome 'x' number, since it is difficult to
determine whether true UI or another factor is involved in inability to set seed on
crosses between species having different chromosome numbers or ploidy levels.
There is a strikingly regular pattern of crossability and barriers in the crosses within
the Lycopersicon species. (Williams Elizabeth G et.al 1994)
The term unilateral incongruity (UI) describes an interspecific barrier, which
does not block the success of a cross in one direction, but, renders the reciprocal
cross either unsuccessful or less likely to succeed (Hogenboom 1975, 1986). In a
number of crop species, UI requires the use of the domesticated species as the
female parent in crosses with related wild species for success (Hermsen 1979;
Hogenboom 1975, 1979, 1986; Ascher 1986; Knox 1986). Interspecific barriers can
prec1ude the production of the F, or affect later generations. The barrier 'hybrid
breakdown' can result in non-viable seeds in the F, or later generations, in plants that
are non-vigorous, male sterile, or non-fecund and in assortment to parental species
type in the F2 or subsequent generations that is more rapid than expected (Bateman
1943; Chmielewski 1966; Rick 1969; Grun 1970; Hogenboom 1972c)
The accessions within the Chamaya-Cuyca race are notable in that they fail to
cross successfully with any other L. peruvianum accessions except those in the
Maraiian canyon races, and the F, hybrids resulting from the latter cross are highly
pollen sterile despite normal chromosome pairing. Thus, part of the system
determining cross ability within the peruvianum-complex may be similar to that
affecting inter-crossability within the esculentum-complex. Little is known regarding the
genetic mechanism controlling UI. Data available demonstrate that segregation for UI
exists, therefore nuclear gene(s) are involved in control of the trait. Mutschler et al.
used sexual crosses and embryo rescue to create an iso-cytoplasmic line, LeLp, in
which the nucleus is from L. esculentum line 'New Yorker' (Le) and the cytoplasm is
from L. pennellii LA 716 (Lp). Southern analysis of all of the plants in the pedigrees of
the iso-cytoplasmic line demonstrated that these plant contained the chloroplast and
mitochondrial DNA of L. pennellii and that LeLp had only the genomic DNA of L.
esculentum for the more than sixty RFLP probes used as markers (Mutschler et al.,
submitted). LeLp is male and female fertile, readily pollinated by L. esculentum, and
self fertile. LeLp shows typieal VI in crosses to L. pennellii; it accepts L. pennellii
pollen, but can not successfully fertilize L. pennellii. Therefore, there is no indication
that cytoplasm affects UI.
No provisions appear to have been made by Hogenboom to take into account
the incompleteness of relationships that may occur between species or populations at
the post-zygotic level. Nevertheless, it seems obvious that incongruity barriers, the
existence of which no one denies (how else would one explain pre-zygotic isolation
between Prunus and Triticum or between the field poppy and a cauliflower), are not
restricted to the inhibition of pollen germination or pollen-tube growth. All reproductive
barriers between sympatric species, including embryo abortion and hybrid sterility,
may be indications of incongruity (Hermsen and Sawicka 1979). Over long periods of
time, divergences between species that no longer exchange germ plasm will continue
to expand and will obviously not be limited to pollen pistil relationships; they will
extend to all phenotypic traits submitted to natural selection. If the circulation of
information is maintained between the parent and its daughter species, leaky prezygotic and post-zygotic incongruity barriers may be specifically consolidated through
the strength, if any, of selection pressure.
Crossing barriers occur frequently when intra- or interspecific crosses are
attempted. These barriers are the result of incompatibility and incongruity. Sexual
barriers preventing interspecific hybridization have been distinguished into pre- and
post fertilization barriers. The nature of the barrier governs the method to be used to
overcome the specific barrier. A range of techniques, such as bud pollination, stump
pollination, use of mentor pollen, and grafting of the style, have been applied
successfully to overcome pre fertilization barriers. In vitro methods in the form of
ovary, ovary-slice, ovule, and embryo culture are being used to overcome post
fertilization barriers that cause endosperm and embryo abortion. An integrated method
of in vitro pollination and fertilization followed by embryo rescue has been applied in
many crosses. Vital hybrid plants may display lack of flowering or male and female
sterility, resulting in failure of sexual reproduction. If sterility is caused by a lack of
chromosome pairing during meiosis, fertility may be restored by polyploidization,
enabling pairing of homologous chromosomes in the allopolyploid hybrid. Integration
of these techniques into the breeding programs would enable the breeder to introgress
genes across the species barriers. (Van Tuyl, J., and De Jeu, M. 1997).
Practical Implication
To realize interspecific crosses incongruity will have to be neutralized. In certain
cases this is probably possible by environmental influence on one or both partners.
One may think of inactivation of a certain barrier gene, for which in the pollen of the
other species the corresponding penetration gene or gene complex is lacking, e.g. by
high temperature or other physical treatment (if the gene inhibits pollen tube growth in
the style), or of offsetting the effect of such a barrier gene by application of chemicals
(if the gene inhibits germination or forms an other stigmatic barrier). The solution is
simple if the incongruity can be solved by removing the barrier, e.g. by removing stylar
parts. Other possibilities may lie in the substitution of an absent promotion or in
making up a shortage in penetration capacity. In the literature interesting examples
can be found of succesful treatments (Gardella, 1950; Swaminathan & Murty, 1959 ;
Knox et al., 1972). The discussion on the evolution of self-compatible species implicitly
indicates a possibility of breaking incongruity between species. Differences in barrier
and penetration capacities may be solved by inbreeding in cross-fertilizers. An
interesting matter is how incongruity might be exploited for practical purposes. Two
possibilities are mentioned.
1. In hermaphrodites incongruity may be applied to prevent self-fertilization. This
possibility is interesting to plant breeders with a view to hybrid seed production.
To prevent self-fertilization, self-incompatibility and male sterility are used.
Incongruity is an interesting third mechanism: by creating a shortage of
penetration capacity in the female parent, self-fertilization is made impossible. It
may be done by introduction from a related species of an extra barrier gene into
the female parent. The corresponding penetration gene(s) must be introduced into
the male. Another possibility is artificial mutation of an essential penetration gene.
2. Incongruity may also be applied for the eradication of species. In host-parasite
relationships one might think of an extra barrier in the host which cannot be
overcome by the parasite. For plant breeders this is the ideal resistance,
examples of which have been mentioned earlier. In unisexuals one may think of
bringing together incongruent gametes in cases where incongruity is only
expressed just before or during fertilization. Gametes are thus wasted, without
producing individuals. This technique has actually been practised in insect control
(Laven, 1967). It may perhaps also be a possibility for eradication of other
parasites, such as fungi or bacteria. These possibilities for genetic control by
application of incongruity require further research.
Conclusions
The intimate relationship between pistil and pollen is based on matching genic
systems in pistil and pollen, resulting from co-evolution. Non-functioning of such
intimate relationships may be caused by incompatibility or by incongruity. These two
independent mechanisms are different with respect to their nature, evolution, genetics
and action. Whereas incompatibility is based on a similar type of reaction in many
species, incongruity may in each case be expressed as a different phenomenon. In
general incompatibility between partners will be an exception and compatibility the
rule, whereas incongruity will be the rule and congruity an exception. As incongruity
was not distinguished as a separate mechanism its importance has been
underestimated. For the same reason the influence of the S gene has been
overestimated; many phenomena supposed to be governed by it are based on other
genes and principles. Incompatibility mainly or exclusively governs non-functioning
within species, whereas incongruity mainly does this between species. A better
understanding of the basis of incongruity enables a more deliberate investigation into
possibilities to break it and exploit it. Cooperation between the research on hostparasite relationships and that on sexual partner relationships is important.
References
Ascher, P. D. & Peloquin, S. J. 1968 Pollen tube growth and incompatibility following
intra- and interspecific pollinations in Lilium Am. J . Rot. 55, 1230-1234.
Abdalla, M. M. F. 1970 Inbreeding, heterosis, fertility, plasmon differentiation and
phthora resistance in Solanum verrucosum Schlechtd., and some interspecific
crosses in Solanum. Thesis, Wageningen, Agricultural University. Agric. Res.
Rep. 748.
Bateman, A. J. 1943 Specific differences in Petunia. II. Pollen growth. J. Genet. 45,
236-242.
Dobzhansky, T. (ed.) 1947 Genetics and the origin of species, 2nd ed. New York:
Columbia University Press.
Bhavyasree R.K.,Incongruity in plants, TNAU, Coimbatore Slideshare.com
Bhatacharjee Indranil, Self Incompatibility, Sam Higginbottom University of Agriculture,
Technology & Sciences Allahabad-211007, Slideshare.com
Chmtelewski , T.,(1962). Cytogenetical and taxonomical studies on a new tomato
form. Part I. Genet, pol. 3:253-264.
Chmtelewski , T., (1968). Cytogenetical and taxonomical studies on a new tomato
form. Part II. Genet, pol. 9:97-124.
Gardella , C, (1950). Overcoming barriers to crossability due to style length. Am. J.
Bot. 37:219-224.
Grun, P. & Aubertin, M. 1966 The inheritance and expression of unilateral
incompatibility in Solanum.Heredity 21, 131—138.
Hermsen JGT, Sawicka E (1979) Incompatibility and incongruity in tuber-bearing
Solanum species. In: Hawkes JG, Lester RN, Skelding AD (eds) The biology
and taxonomy of the Solanaceae, series 7. Linnean Society Symposium,
London. Academic Press, London, pp 445-454
Hogenboom NG (1972a), Breaking breeding barriers in Lycopersicon. 4. Breakdown
of unilateral incompatibility between L. peruvianum (1.) Mill. and L.
esculentum Mill.Euphytica 21:397-404
Hogenboom NG (1972b), Breaking breeding barriers in Lycopersicon. 5. The
inheritance of the unilateral incompatibility between L. peruvianum (L.) Mill.
and L. esculentum Mill. and the genetics of its breakdown. Euphytica 21:405414
Hogenboom, N. G. (1973) :Incongruity and incompatibility in intimate partner
relationships : A Model For Incongruity In Intimate Partner Relationships
,Euphytica 22 : 219-233
Hogenboom, N. G. (1975) Incompatibility and incongruity: two different mechanisms
for the non-functioning of intimate partner relationships Proc. R. Soc. Lond. B.
188, 361-375 Printed in Great Britain
Knox, R. B., R. R. Willing & A. E. Ashford, (1972). Role of pollen-wall proteins as
recognition substances in interspecific incompatibility in poplars. Nature
237:381-383.
Laven, H., (1967). Eradication of Culex pipiens fatigans through cytoplasmic
incompatibility. Nature 216:383-384.
Lewis, D. 1954 Incompatibility in relation to physiology, genetics and evolutionary
taxonomy. 8e Congr. int.Bot. (Paris), 124-132.
Lewis, D. 1955 Sexual incompatibility. Sci. Progr. 172, 593-605.
Lewis, D. & L. K. CROWE, (1958). Unilateral interspecific Incompatibility in flowering
plants. Heredity 12:233-256
Lewis, D. 1965 A protein dimer hypothesis on incompatibility. Genet. Today 3, 657663.
Mather, K. 1943 Specific differences in Petunia. I. Incompatibility. J. Genet. 45, 215235.
McGuire, D. C. & Rick, C. M. 1954 Self-incompatibility in species of Lycopersicon sect.
Eriopersicon and hybrids with L. esculentum. Hilgardia 23, 101-124.
Nettancourt, Dreux De (2001) Incompatibility and Incongruity in Wild and Cultivated
Plants Springer SPIN 10695077 31/3136 543210
Newton, D. L., Kendall, W. A. & Taylor, 1ST. L. 1970 Hybridization of some Trifolium
species through stylar temperature treatments. Theor. appl. Genet. 40, 59-62.
Pandey, K. K., (1968). Compatibility relationships in flowering plants: role of the Sgene complex. Am. Nat. 102:475^189.
Pandey, K. K., (1969). Elements of the S-gene complex. V. Interspecific crosscompatibility relationships and theory of the evolution of the S complex.
Genetica 40:447-474.
Pandey, K. K. 1957 A self-compatible hybrid from a cross between two selfincompatible species in Trifolium. J . Hered. 48, 278-281.
Pandey, K. K. 1962 Interspecific incompatibility in Solanum species. Am. J. Bot. 49,
874-882.
Pandey, K. K. 1964 Elements of the S-gene complex. Genet. Res. 5, 397-409.
Pandey, K. K. 1967a Origin of genetic variability: Combinations of peroxidase
isozymes determine multiple allelism of the S-gene. Nature, Lond. 213, 669672.
Pandey, K. K. 19676 S-gene polymorphism in Nicotiana. Genet. Res. 10, 251-259.
Pandey, K. K. 1968 Compatibility relationships in flowering plants: role of the S-gene
complex. Am. Nat. 102, 475-489.
Pandey, K. K. 1969a Elements of the S-gene complex. IV. S-allele polymorphism in
Nicotiana species. Heredity 24, 601-619.
Stout, A. B. 1952 Reproduction in Petunia. Mem. Torrey bot. Club 20, 1-202.
Swamitnathan , M. S. & B. R. Murty , (1959). Effect of X-radiation on pollen tube
growth and seed setting in crosses between Nicotiana tabacum and N.
rustica. Z. Vererblehre 90:393-399.
Van Tuyl, J., and De Jeu, M. (1997). Methods for overcoming interspecific crossing
barriers. In R. Knox (Author) & K. Shivanna & V. Sawhney (Eds.), Pollen
Biotechnology for Crop Production and Improvement (pp. 273-292).
Cambridge: Cambridge University Press
Williams Elizabeth G. , Clarke,A.E. and Knox R. B. (1994) Genetic control of selfincompatibility and reproductive development in flowering plants, Library of
Congress Cataloging-in-Publication Data ISBN 978-90-481-4340-5 pp 164188