Importance of Male Sterility and Self Incompatibility in Crop Improvement

A Term Paper On The Importance of Male Sterility and self incompatibility in crop improvement By Idu Paul Odey CONTENT 0. Introduction 1. Male Sterility (MS) and Self-Incompatibility 1. Male Sterility (MS) 2. Self-Incompatibility (SI) 2. Types of Male Sterility and Self-Incompatibility Systems 1. Types of Male Sterility 2. Types of Self-Incompatibility 3. How MS and SI affect Reproduction in plant and correction of problems 1. Effect of MS in Reproduction 2. Effect of SI in Reproduction 4. MS and SI Effect/Use in Crop Breeding Exercise 1.

Male Sterility in Plant Breeding 2. Self-Incompatibility in Crop Breeding Reference 1. Introduction Self-incompatibility (SI) and Male Sterility (MS) are widespread phenomena in flowering plants. It has however been suggested that in classifying plants according to why they may fail to set seed, that, it is desirable to distinguish between incompatibility and sterility. With Self-incompatibility there is restriction of breeding within populations and it seems to contribute through the establishment of unilateral barriers between self-incompatibility and self compatible populations.

Male sterility on the other hand is characterized by non-functional gametes; caused by chromosomal aberration, genes action, etc Two phenomena plays important roles in plant breeding; as plant breeders are often interested in modifying the breeding system in crop plants in order to substitute ether permanently or temporarily some mating system that serves their purpose better than the natural system of the species In respect to this the purpose of this term paper is to review the importance of Male sterility and Self0incompatibility in crop improvement.

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This term paper shall however consider the various types of MS and SI; how they affect reproduction in plants; how the problem can be corrected and finally how the corrections affect a crop improvement or plant breeding exercise 1. Male Sterility (MS) and Self-Incompatibility (SI) 1. 1. 1Male Sterility (MS) Lasa and Bosemark (1993) saw Male Sterility in plant as an inability to produce or to release functional pollen and is the result of failure of formation or development of functional stamen, microspores or gametes.

Another author, Burojevic (1990) stated that male sterility is a phenomenon when plans do not produce viable pollen grains, which is due either to the stunting of male sexual organs (stamens) or to mitotic irregularities in apparently normal stamens leading to the development of abnormal microspores and nonfunctional or sterile pollen grains. The definition so considered, stressed on the ability of the male gametes or organs to render functional gametes or pollen.

This may be generally controlled as ascertained by Borojevic (1990) or as a result of the effects of mutant genes or cytoplasmic factors of the combined efforts of these two factors. 1. 1. 2 Self-Incompatibility (SI) Among hermaphroditic plant species there are several genetically controlled system for enforcing cross-pollination that operates through the incompatibility of pollen and style. Incompatibility is the inability or failure of the pollen to germinate on the stigma or the situation where there is a slow pollen tube growth down the style.

In the case of incompatibility the pollen and ovules are functional, and unfruitfulness results from some physiological hindrance to fertilization. However, it is believed that, the incompatibility reaction appears to be biochemical process under rather simple genetic control. Borojevic (1990) mentioned some of the several causes of incompatibility to be i) Protandry – a condition in which stamens mature before the stigma in the same flower preventing pollen grains from pollinating the stigma ii) Protogymy – a condition in which the stigma matures before stamens with the same result as above. ii) Hercogamy – psychical arrangement of male and female organs on the same plant preventing self pollination in the absence of an insect visit. 1. 2 Types of Male Sterility and Self-Incompatibility Systems 1. 2. 1 Types of Male Sterility As a consequence male sterility is sometimes divided into: a) Pollen sterility: in which male sterile individuals differs from normal only in the absence or extreme scarcity of functional pollen grains. b) Structural or staminal male sterility: in which male flowers or stamens are malformed and nonfunctional or completely absent. ) Functional male sterility: in which perfectly good and viable pollen is trapped in indehiscent anthers and thus prevented from functioning. ” Of the three types of male sterility, pollen sterility is by far the most common and only one that has played a major role in plant breeding and hybrid seed production” ( Lasa and Bosemark 1993) Male sterility can further be divided into three types according to the way they are controlled genetically; based on its inheritance or origin (Borojevic 1990; Lasa and Bosemark 1993).

According to Lasa and Bosemark (1993), the types are: i) Nuclear Male Sterility (NMS) also called genetic male sterility ii) Cytoplasmic Male Sterility (CMS) iii) Non-Genetic, chemically induced male sterility 1. 2. 1a Nuclear Male Sterility (Genetic) This type of male sterility is mostly controlled by single recessive gene (sorghum, beans), but can also be controlled by several genes (barly, tomato) (Borojevic 1990). Driseoll (1986) however reported that, nuclear male sterility has less frequently been found to be controlled by more than one recessive gene. . 2. 1b Cytoplasmic Male Sterility This type of male sterility depends on cytoplasmic factors. Borojevic (1990)reported that it is controlled by extra-chromosomal factors or plasmagenes. Lasa and Bosemark(1993) reported that, base on origin, CMS may be divided into autoplasmic CMS and alloplasmic CMS. Autoplasmic CMS refers to those cases where CMS has arisen within a species as a result of spontaneous mutational change in the cytoplasm, most likely in the mitochondrial genome.

Alloplasmic CMS, on the other hand, refers to the case where CMS has arisen from inter-generic, inter-specific or occasionally inter-specific crosses and where the male sterility can be interpreted as being due to incompatibility or poor cooperation between the nuclear genome of another. 1. 2. 1c Chemically Induced Male Sterility Gametocide or chemical hybridizing agents such as Ethral (2-chlorethanphosphoric acid), Mendok (2,3-dichloroisobutyrate) and gibberellic acid (a growth stimulator); when sprayed on certain plants at certain stage defore flowering render the plants male sterile.

A successful chemical hybridizing agent should produce completely male sterile plants without affecting female fertility. (Borojevic 1990; Lasa and Bosemark 1993) 1. 2. 2 Types of Self-Incompatibility Incompatibility can be divided into two parts: i) Heteromorphic incompatibility ii) Homomorphic Incompatibilityu, which include gametophytiic and sporophytic incompatibility. (Lewis 1954) 1. 2. 2a Heteromorphic Incompatibility This system of incompatibility is characterized by differences in morphology of the flowers, differences in length between stamen and style (heterostyly).

The simplest and best known of the heteromorphic systems is found in any species Primula, in which one type of plant, thrum, has a short style and highly placed anthers, and the other, pin, has a long style and short anthers, The only pollinations that are compatible are those between anthers and stigmas of the same height, that is, between pin and thrum and thrum and pin. 1. 2. 2b Homomorphic Incompatibility In this system differences in floral morphology do not accompany the incompatibility. Homomorphic incompatibility includes gametophytic and sporophytic incompatibility.

Gametophytic System Borojevic (19190) reported that this type of incompatibility was discovered in 1925 by East and Mangelscorf in Nicotiana sanderae. The ability of pollen to fertilize depends on a series of alleles in as a single locus. The incompatibility is controlled by a single gene, S. Pollen-tube growth is usually very slow in style that contains the same allele of S; consequently plants are virtually always heterozygous at this locus. The gametophytic system gives rise to three main types of pollination. a) Fully incompatibility (S1S2 X S1S2), in which both alleles are common. ) Half the pollen is compatible (S1S2 X S1S3), in which one allele is different; and c) All the pollen is compatible (S1S2 X S3S4), in which both alleles differ. Sporophytic System This system is also conditioned by a series of alleles in a single locus and, unlike in the gametophytic type, the functionality of pollen is determined by the genetic constitution of the plant producing it. It has been reported that this system was first discovered in 1950 by Hughes and Babcock, working with Crepis foetida, and Gerstal, working on Parthenium argentatum (Guayule). . 3 How Ms and SI Affect Reproduction in Plant and Correction of The Problems. 1. 3. 1 Effect of MS in Reproduction. Generally male sterility affects reproduction in plant as it does not permit the development of functional stamens or gametes; as such reproduction cannot be effected. Nuclear male sterility for instance does not permit the population of a uniformly male sterile population this seriously limits its use in hybrid seed production. Seed set is likely to be low on male sterile plants, due to poor pollen dispersal.

Since safe classification of fertile versus male sterile plants cannot be made until shortly before flowering, such rouging becomes far too difficult and costly in most crops to permit economic production of large quantities of hybrid seed. When a cytoplasmic male sterile (CMS) plant is crossed with a fertile plant, male sterile progeny is obtained. CMS cannot be used in breeding heterotic hybrids because crosses between CMS line and fertile lines would result in sterile F1 hybrids.

This is not a problem for crops like onion and other plant species whose F1 generation is grown for vegetative parts, but it is a serious problem for sunflower, maize and other plant species grown for seed. To correct this problem the male line or pollinator must contain genes for male fertility restoration (Rf genes) to ensure the development of seeds in F1 plants. Thus the production of hybrids by CMS lines proceeds as follows: a) Introduction of the hybrids by CMS line to be used as the female. ) Maintenance of the CMS line (“A” lines) and its fertility maintainer (“B” lines) c) Introduction of the Rf genes in the pollinator (“R” line), and d) Hybrid seed production by growing “A” line and “R” line on alternate rows. The problem with chemically induced male sterility is that the chemical gametocides tend to cause damage in female sexual organs. Another limiting factor has been the growth stage interval in which the chemical hybridizing agents must be applied to be effective.

Thus gametocides have not found a wide application yet but efforts are invested in developing better gametocides and improving application method (Lasa and Bosemark, 1993; borojevic 1990). 1. 3. 2 Effect of SI in Reproduction Self-incompatibility affects reproduction in that fertilization is hindered. The pollen and ovules are however, functional but are rather incompatible. It restricts breeding with a population. The corrective measures of heteromophic incompatibility have already been dealt with in 1. 2. 2a.

However, worthy of note here is that most problems of self-incompatibility create room for out-crossing in order to effect fertilization. Pollination may take place if the allele in the pollen grain differs from those of the stigma and the ovary, in the case of homomorhpic incompatibility. Self-incompatibility can be broken down through the pollination of buda which do not express the incompatibility phenotype of the mature flower or by means of inhibitors (high temperature, radiation, chemical preventing RNA or protein syntheses) applied before pollination to the site of the reaction (stigma or style).

Induced mutation is another way of overcoming self-incompatibility it was reported that x-irradiation of the anthers of the self-incompatible Nicotiana glauca enabled the pollen of this species to be accepted by the otherwise incompatible pistil of N. forgetiana. (de Nettancourt 1993; Pandey 1974). 1. 4 MS and SI Effect/Use in Crop Breeding Exercise These two pheromone which happens to confer problems associated with crop production, have turned out to be a mechanism on which plant breeders turn on in modifying breeding systems in crop plants. 1. 4. 1 Male Sterility in Plant Breeding

Male sterility plays important role in plant breeding, firstly in the production of hybrid seed and secondly as a plant breeding tool facilitating population improvement, back-crossing, inter-specific hybridization and other intermediate breeding procedures. Hybrid breeding methods have been successfully employed in many plants (maize, sorghum, sugar beet, sunflower and tomatoes etc. ) Cytoplasm male sterility is the most important system used in hybrid seed production and so far, with few expectations, the only one by which hybrid seed can be produced both effectively and economically.

It has real advantages in certain ornamental species, as the non-fruitful plants tend to bloom longer than their seeded counterparts, and flowers remain fresh. Cytoplasmic male sterility is also useful in producing single or double cross hybrids in crop species in which some vegetative parts of the plant is the commercial product. Nuclear male sterility may be used to make recurrent selection also available to breeders of self-pollinated crops. 1. 4. 2 Self-Incompatibility in Plant Breeding Self-incompatibility has also been adapted to hybrid varietal production.

The physiological inhibition of self-incompatibility is also important for plant breeding because it can be extensively exploited for the creation of S-homozygous line needed for F1 hybrid seed production (be Nettancourt. D 1993). This system has resulted in the modification of breeding systems in crop plants. Mutation of gene by irradiation has been found to produce self fertile plant. Reference Borojevic S. (1990), Principles and methods of plant breeding. Elsevier. Sci. Publishers de Nattanncourt D. (1993), Self and Cross-incompatibility system. In Plant Breeding Principles and Prospects Hayward M.

D. , Bosemark N. O. , Romagosa I. (eds) Campman & Hall, London. Discoll C. J. (1986) Nuclear male sterility system in seed production of hybrid varieties C. R. C Critcal Rev. Plant Sci. , 3, 227-256. Lasa J. M & Bosemark N. O. (1993), Male sterility. In Plant Breeding Principles and Prospects. Hayward M. D. , Bosemark N. O. , Romagosa I. (eds) Campman & Hall, London. Pandey K. K (1994), Elimination of heterozygosity and efficiency of genetic system. Theor. Appl. Genet. 44, 199-205. Simmonds N. W. (1979), Principles of crop improvement, Longman, England.