BREEDING OF FRUIT CROPS - Lecture.5
The barrier between pollination and fertilization in angiosperms is because of the self-incompatibility, a genetically controlled phenomenon. Self incompatibility is the inability of functional male and female gametes of the hermaphrodite flowers to set seeds on self pollination.
Genetic control of self incompatibility
Incompatibility is generally controlled by a special gene at S-locus represented by multiple allelic series in the population. Each of these alleles control the formation of a specific substance that determines the incompatibility reactions, both in the pistil and pollen. Identical substances specified by identical genes in pollen and pistil favour to prevent fertilization. Based on the timing and mode of S-gene activity, the incompatibility reaction among homomorphic angiosperm is categorized into two groups.
- Gametophytic control of pollen
- Sporophytic control of pollen
A. Gametophytic self incompatibility
In this type of incompatibility, pollen is binucleate and pollen behaviour is determined by the S allele present in each pollen and stigma is wet type. It means the incompatibility reaction of pollen is determined by its own genotypes, and not by the genotype of the plant on which it is produced. Generally, incompatibility reaction is determined by a single gene having multiple alleles. Sometimes, polyploidy may lead to the loss of incompatibility due to a competition between the two S alleles present in diploid pollen. Important examples are pineapple, loquat, apple, pear, plum, cherry, almond, apricot, some citrus and members of Solanaceae family.
B. Sporophytic incompatibility
The incompatibility reaction of pollen is governed by the genotype of plant on which the pollen is produced and not by the genotype of the pollen. It means the incompatibility is imposed by the maternal genotype, due to that all the pollen grains from a given plant behave similarly. Incompatibility occurs at the stigmatic surface resulting in the inhibition of pollen germination. Pollens are trinucleate and the stigmatic surface is dry e.g. Mangifera indica.
Mechanism of self incompatibility
Based on the various phenomenon observed during pollination and fertilization it can be grouped into three:
- Pollen stigma interaction
- Pollen tube style interaction
- Pollen tube ovule interaction
1) Pollen-stigma interaction
This interaction occurs just after the pollen grains reach the stigma and generally it prevents pollen germination. In the gametophytic system, stigma surface is plumose having elongated receptive cells and is commonly known as wet stigma. Incompatibility reaction occurs at a later stage. There are clear cut serological differences among the pollen grains with different S genotypes and such differences have not been observed in sporophytic system.
In sporophytic system, stigma is papillate and dry covered with a hydrated layer of proteins known as pellicle. There is evidence that the pellicle is involved in incompatibility reaction. There are striking differences in the stigma antigens related to the S allele composition. Within few minutes of reaching the stigmatic surface, the pollen releases exine exudates which are either protein or glycoprotein in nature. This exudate induces immediate callose formation in papillae (which are in direct contact with the pollen) of incompatible stigma. Often callose is also deposited on the young protruding pollen tubes preventing any further germination of the pollen. Thus, in the sporophytic system, stigma is the site of incompatibility reaction. The incompatibility reaction of pollen is probably due to the deposition of some compounds from anther tapetum on to the pollen exine.
2) Pollen tube – style interaction
In most of the gametophytic system, pollen grains germinate and pollen tubes penetrate the stigmatic surface. But, in the incompatible combinations, the growth of pollen tube is retarded within the stigma.
3) Pollen tube – ovule interaction
In some cases, pollen tube reaches the ovule and affects fertilization. However, in incompatible combinations, embryo degenerates at early stage of development.
Methods of overcoming self incompatibility
One of the following methods can be used for bringing partial fertility by temporarily suppressing the incompatibility reaction:
> Bud pollination – Application of mature pollens to immature non-receptive stigma i.e. 1-2 days prior to anthesis.
> Surgical technique – Removal of stigmatic surface.
> High temperature – Exposure of pistils to temperature up to 60oC
> Irradiation – With x rays or g rays for single locus gametophytic incompatibility.
> Double pollination – Incompatible pollen is applied as mixture with compatible pollen.
> Pollination at the end of season – Arora and Singh (1988) observed that in low chilling plum and peach cultivars, methanol killed the mentor pollen and not helpful in overcoming incompatibility barriers, however, frozen and thawed mentor pollen (one which, if alive, would be fully compatible with style receiving it) improved fruit set in both intra and inter specific incompatibility.
In case of sporophytic incompatibility system, the breakdown is comparatively easy because the incompatibility reaction takes place between stigmatic surface and pollen wall in comparison to gametophytic incompatibility in which reaction starts when the pollen tubes have already travelled ⅓ to ½ length of styler tissue (Arora, 1993).
Advantages of self incompatibility
- Where male sterility is non existent, self incompatibility can alternatively facilitate the production of F1
- Self fertility can be induced temporarily or permanently by mutation of S alleles to S1 through artificial irradiation in clonally propagated orchard species like cherry and apple
- Seedless varieties, such as in pineapple, grape etc. can be evolved if self incompatibility is present.
> Variations in seed set due to poor fertility.
> Poor preservation of genetic purity of improved varieties since cross pollination is non-restricted.
> Difficulties in development and maintenance of homozygous lines (inbreds) which can be utilized for hybridization.
> Uneven quality of fruits because of mixed planting of different varieties based on their cross compatibility.
Pollination pattern and incompatibility
Self-incompatible fruit cultivars/species need cross pollination for seed/fruit set which includes pollen hydration and germination, pollen tube growth into the style to the ovary, entry into the ovule and embryo sac and release of sperms. Pollination failures may, thus, create barrenness in the tress which is otherwise completely normal in health and free from diseases and insect pests. During cross pollination, the sensitive discriminations have to be made between pollen grain of different genotypes for which identity of each pollen is needed. The germination of pollen grain and its penetration into the stylar tissue to reach the embryo sac depends upon acceptability by the pistil which is selective in nature.
In aonla, male flowers appear in clusters in the axil of leaf all over the branchlet while female flowers are on the upper end of a few of these branches. Bajpai (1968) reported male to female ratio of 307.9:1 and 197:1 in two successive years indicating marked variation in the expression of sex. The maximum number of male flowers opens between 6 and 7 PM and dehiscence of anthers starts soon thereafter. The female flowers open in stages and take 72 hours to open completely and the stigma becomes receptive on the third day of anthesis. Bajpai (1968) reported that aonla pollen are light and thus the pollination occurs through wind. There is no self-incompatibility in aonla. The cause of poor fruit set may be attributed to a high percentage of staminate flowers.
Lal et al., (1972) found 9 apple cultivars completely and 4 partially self- incompatible. For Early Shanburry cultivar, Fanny (54-5%), Winter Banana (60.4%) and Rome Beauty (54.25%) were better pollinizers. In Red Delicious, highest fruit set occurred with Jonathan (87.5%) cultivars Mclntosh, Rymer, Jonathan and Rome Beauty set satisfactory crop with self pollen.
The majority of flowers are borne axillary on current season growth in clusters. The time of flowering varies in different parts of India. Godara (1981) found that cultivars Banarsi, Karaka, Mundia, Murhara, Reshmi, Sandhura, Narnaul, Safeda, Umran, Ilaichi and kakrola were self- incompatible and Umran was found to be the best pollen recipient as well as pollen donor. Being sticky, the pollen is transferred mainly by honey bees. Many flowers do not get pollinated at critical stages of gynoecium receptivity and drop off because of a short receptivity period.
Pollen development is normal in citrus except in a few cultivars like Navel oranges, Satsuma mandarin and lime which have no viable pollen. In cultivars with abundant pollen, self-pollination occurs but in mixed plantings of different cultivars, cross pollination is not uncommon. The stigma remains receptive for 6-8 days. Honey bees are the known pollinating agents. Self-incompatibility has been reported in pummelo, sweet lime and lemon.
It is a gynodioecious species. The Capri fig is monoecious while common fig is pistillate. The figs commonly grown in India are parthenocarpic and do not require pollination. In other countries, generally Capri figs (wild figs) are planted as pollinizers with the commercial cultivars. The cultivars Pune, Black Ischia and Brown Turkey were reported to be Parthenocarpic from Kodur while Turkish White failed to set fruits without caprification.
Most vinifera cultivars have perfect flowers that have both functional pistil and stamens. Some species of grapes (V.rotundifolia) are dioecious. Berry set results from pollination, fertilization and seed development. Some cultivars like Black Corianth set by stimulative parthenocarpy and in others like Perlette, Beauty Seedless, Pusa Seedless, Delight, and Thompson Seedless stenospermocarpy occurs. Self pollination is the rule in vinifera grapes. However, cross pollination is also possible and is desirable under certain conditions.
Cross pollination is the rule in guava. However, Singh Sehgal (1968) found that self pollination was also predominant and that the possibility of open pollination cannot be ruled out. Under open pollination, Allahabad Safeda had the highest fruit set of 85.5 per cent in spring and 84.4 per cent in rainy seasons, while cultivar Sardar reorded 83.3 and 82.2 per cent fruit set respectively. Under self pollination, Allahabad Safeda recorded 67.7 per cent fruit set in spring and 66.6 per cent in rainy seasons.
In the tropics, flowering and fruiting are continuous throughout the year in the terminal leaf axil of leader and lateral shoots. There appears to be no regular sequence in the incidence of male and female inflorescences. Although they are similar during early development, the female is later distinguished by a thicker peduncle and a large annular disc at the anthesis, but later emerged males are smaller. Sharma (1964) reported a high degree of sterility with some fruits having 12,000 flowers producing only five fully developed segments surrounded by 448 aborted flowers. They also noted partial seed development, suggesting that some might have occurred after fertilization.
The panicles bear male and perfect flowers and the cross pollination is mainly done by the house fly. The number of perfect flowers per panicle varies between 1000 and 6000. Uniform cross pollination of cultivars Dashehari, Langra and Bombay Green with the pollen of Totapari and of Bombay Green with that of Langra and Chausa, Dashehari and Totapari and of Bombay Green indicated that in nature about 50 per cent of perfect flowers remain unpollinated, stigma remains receptive from one day prior to anthesis with a maximum on the day of anthesis and that fruit set is generally improved by mixed pollination.
Male sterility is characterized by non-functional pollen grains, while female gametes are functional. Male sterility can be classified into three groups viz., genetic male sterility, cytoplasmic male sterility, and cytoplasmic genetic male sterility.
(a) Genetic male sterility
Like any other morphological traits, particularly mono and oligogenic, this type of male sterility occurs in plant due to mutation of the fertility locus, situated on chromosomes within the nucleus. In this case, cytoplasm is not involved in bringing the sterility. There could be three possible genotypes for this locus and only one of them is male sterile.
By crossing AxB lines, sterile and fertile progenies are produced in equal proportions. For the maintenance of sterile line, the fertile plants need to be quickly removed before the shedding of the pollen grains. The fertile plants can be removed in early stage of plant growth by using marker gene.
Fertile lines can be obtained by crossing A-line with R-line. It can be used in hybrid seed production and genetical studies or for the preservation of variability.
(b) Cytoplasmic male sterility
It occurs due to the mutation of mitochondria or to due to some other cytoplasmic factors outside the nucleus, resulting in the transformation of the fertile cytoplasm into a sterile one. Nuclear genes are not involved. Further, with two types of cytoplasm i.e. sterile and fertile, at the most, only two kinds of genotypes are possible, one of them is sterile and another fertile. The fertile cytoplasm is denoted by (F – B Line) and sterile cytoplasm is denoted by (f – A line).
Due to two different types of genotypes, cytoplasmic sterility can be maintained as under:
Since there is no third type of genotype which can act as R-line, as such restoration of fertility is not feasible. However, this does not exhaust all the possibilities of use of cytoplasmic sterile lines.
As restoration is not possible, this type of sterility is useful only in crops where the seed is not the desired end product. This is important for horticultural crops where vegetative parts are of economic value.
(c) Cytoplasmic-geneic male sterility
Such sterility arises from the interaction of nuclear gene(s) and conditioning sterility with sterile cytoplasm. The cytoplasmi-geneic sterility is essentially a cytoplasmic sterility with a provision for restoration of fertility. The fertility is restored by (R) gene present in the nucleus. The combination of both nuclear gene(s) and cytoplasmic factors determine the fertility or sterility in such plants. Based on these combinations, there can be maximum of six types of genotypes and only one of them is sterile
As visualized by their genetic composition and cytoplasm, only [(rr) f] genotype can maintain the sterility of A-line.
This is achieved by suitable restorer lines which can give rise to all fertile progenies on crossing with A-line. Among the possible six genotypes, only [(RR) F] and [(RR) r] are such restorer or R-line. They produce all fertile progenies.
Cytoplasmic-genetic male sterile lines are of immense importance in exploitation of hybrid vigour in crops where seed is the desired end product.