HYBRIDIZATION IN LIVING ORGANISMS

HYBRIDIZATION

1. INTRODUCTION

Every day, the need to incorporate different traits in one organism increases. This is because there is hardly one organism that has all the desired traits. This calls for a high need for researchers to improve on deciphering a means of having desirable traits in one organism. Among many techniques, hybridization has been reported to be highly effective.

Hybridization is a process of producing a hybrid through crosses between or among organisms of unrelated genetic constitutions. Those crosses are made on organisms with traits of interest, to have those desired traits combined in one organism. The crosses could range from crosses between organisms of the same variety (intra-varietal) but different genomes to crosses between organisms of different genera (inter-generic). An example of inter-generic crosses is seen in crosses between Sugarcane X Bamboo, although, such crosses rarely produce viable offspring due to none-relatedness of their genome. Other crosses could be between plants of different varieties (Inter-varietal), as well as crosses between two species (inter-specific).

It is also pertinent to state that hybridization may not give direct result expectations. By that, one can carry out hybridization multiple times before or without getting the expected result. It has long been reported that crossing two plant species or genotypes can create an improved hybrid with a faster growth rate, more biomass at maturity, and/or greater reproductive output than its parents.

2. ESSENTIAL INFORMATION WHICH AIDS CROSSING.                                                                         

a. Good details of both male and female parents: Here, the parents with desirable traits are chosen. The distribution of the traits across the parents is also considered. With that, all or a very high percentage of the desired traits are not captured in one parent. Also, parent organisms should be healthy, vigorous, and grown in the same season and location. They should also have similar maturity periods (both male and female plants flower simultaneously), to avoid the challenges of preserving pollens.

b. Sexual properties: Whether plants are unisexual or bisexual is very important to know about the parents. As this will help while planning their crosses. In bisexual, the plant can produce both female and male gamete. This implies that isolating the plant alone during reproduction will not be enough since it can pollinate itself, but enhanced preparation is needed to ensure the pollens do not pollinate its egg. While for unisexual plants, crosses are easy because the plant has one gamete. Isolation can go a long way in ensuring the right pollen pollinate the egg.

c. Time of anthesis (flowering period). Knowledge about the time of flowering is very relevant. Being that hybridization can be done across species, there is the possibility that the plants do not have similar flowering seasons. In this scenario, the breeder will have to adjust their planting periods to ensure that both plants fruit at a similar time.

 Source: Click on the link

d. Harvesting period of plants. These periods are highly essential. Most researchers fail to take proper note of this period. This actually allows the output to be wasted or mixed with other non-research products.

3. PROCEDURES OF HYBRIDIZATION                                                                                                               

i. Selection of desired parents: Here, parents who are healthy and vigorous with desired traits are selected. The selection is such that all the important features are kept in mind and the parents are to mature at the same time.

ii. Ensure the parents are homozygous for the traits of interest: They should be grown in isolation and inbred across seasons. For bisexual flowers, cover the reproductive organ with transparent bags to ensure that light still enters the flower and that no external pollen pollinates the female organ. At anthesis, its pollen will pollinate its stigma. When the plant is cross-pollinated, ensure you bag the reproductive organs separately before anthesis, this ensures that no foreign agent engages in reproduction. At anthesis, pollen grains are collected and dusted on the stigma. This artificial hand pollination is repeated until the parent becomes homozygous or pure for the trait of interest.

iii. Plant the pure lines generated: The pure lines generated through selfing or inbreeding are then planted as parents. They are regarded as pure lines because they have homozygous alleles for traits of interest.

iv. Emasculation: In female plants that are bisexual, emasculation is essential to prevent self-pollination. Stamens are removed from the female parent before they burst and dust pollens on the stigma.                                                                                                               

          source: Click on the Link

There are different methods through which emasculation can be achieved;

a. Use of sterile forceps and needles or scissors to cut the bud open and remove the anther without causing harm to other flower parts.

b. By dipping panicles in hot water of 45^oC to 50^oC over a short period.

c. Dipping inflorescence in alcohol or cold water for a short period.

d. Inducing male sterility by spraying chemicals like 2,4-D, NAA, and maleic hydrazide on immature flower buds.

v. Bagging: Before anthesis, flower buds are bagged in transparent nylon or paper bags to ensure that no foreign reproductive agent contaminates the flowers. The size of the flower bud is considered based on allowance while producing bags, and each sex is bagged separately.

vi. Cross-pollination: Here, pollen grains are assembled in Petri dishes from male flowers after dehiscence. And then dusted on the stigma of the female flower after temporarily opening the flower. Ensure that stigma is receptive, evident by the presence of fluid on its surface. Bag the female organ to ensure that no foreign pollen contaminates it.

vii) Tagging: This is a very intricate exercise as it carries information that will help the breeder trace all events. The organisms in the field should be tagged to show if they are emasculated and crossed. The tagging should be done either on the bag itself or on separate labels. The label should bear the following details; reference number, date of emasculation, date of crossing, and other relevant details of the male and female parent.                                                                                         

4. CHALLENGES FACED BY HYBRIDIZATION                                                                                               

a. Identification of ideal parents: In carrying out hybridization, it is a very difficult task to identify an ideal inbred to use as a parent. This is because the inbred should be homozygous for the trait of interest and this is achieved through seasons of serious field trial, record keeping, and observation.

b. Varied maturity time: Most times, it is difficult to have plants flower at the same time. Meaning that pollens are harvested and stored when such a scenario occurs. But eggs cannot be harvested to wait for pollen except an unnatural application is to be used to develop the fertilized egg. To solve this variation in maturity, varied planting time is employed, with this, the male and female organs will mature simultaneously.

c. Outbreeding depression: Being that crosses are mostly done between organisms that are genetically dissimilar, there is a high chance of reduction in fitness arising from established genetic differences.

d. Genetic defects: Due to genetic unrelatedness, some hybrid organisms usually suffer genetic-related complications such as repression of flowering, premature death, sterility, and hybrid breakage.

e. Incompatibility: This is commonly experienced in crosses between species and crosses between genera. This incompatibility may cause the fertilization to fail, hence, leading to a fruitless hybridization exercise.                                                                                                                                                                           

f. Damage of flower during emasculation: Flowers, being sexual organs, are delicate. Hence, can easily damage during handling. During emasculation procedures, the flowers can experience injuries which may not favour hybridization.