Importance of Pollination in Forest Genetics

Sudip Raj Regmi
M.Sc. Forestry, IOF, Pokhara Campus, TU, Nepal.
Email: sudipregmi7717@gmail.com

Introduction to Forest Genetics

Forest Genetics is a subdiscipline of genetics concerned with genetic variation and inheritance in forest trees. It is the basis for conservation, maintenance, and management of healthy forest ecosystems and development of programs that breed high yielding varieties of commercially important species. It also describes techniques that are generally useful in tree improvement work including individual tree selection and breeding, provenance testing, species and racial hybridization and introduction of exotics. It then discusses concepts from basic genetics, including chromosome structure and function; DNA and RNA; nongenetic inheritance; and genotype versus phenotype. Workers in this area are commonly engaged in the translation of theory and experiments from the central field of genetics to forestry. Forest Genetics include activities that are restricted to genetic studies of forest trees. The objectives are to determine the genetic relationship between trees and species. It is an attempt to determine crossability patterns among species within a genus. The cross is made to determine the relationship, but otherwise, they have no special breeding objectives.

Pollination is the transfer of pollen from a male part of a plant to a female part of a plant, later enabling fertilization and the production of seeds, most often by an animal or by wind (Barrows, 2011). When pollination occurs between species it can produce hybrid offspring in nature and can contribute to Plant breeding work. The pollination process as an interaction between flower and pollen vector was first addressed in the 18th century by Christian Konrad Sprengel. This is an essential process for the fertilization of the flowers and for the development of seeds and fruits. The dependability that reproduction of most fruit crops has from pollination makes it of paramount importance in agricultural production. Knowledge of reproductive systems i.e. breeding systems is so clearly fundamental to plant and tree breeding that discussion of them must precede any consideration of breeding methods themselves. Based on tree breeding purposes all plants are divided into two broad groups. They are:

  1. Self-pollinating plants and
  2. Cross-pollinating plants

The division of plants into these groups is of great importance because the methods of breeding applicable to the self-pollinating group are for the most part different from those that apply to the cross-pollinated species. The important difference between the two groups is related to the influence of inbreeding vs outbreeding on the genetic structure of populations. All plants in populations of outcrossing/cross-pollinating species are highly heterozygous, while in-breeding/self-pollinating species often give rise to closely related homozygous lines. 
In cross-pollinating species, there is a high degree of heterozygosity. If selfing is imposed there will be in-breeding depression i.e. reduction or loss of vigor. Inbreeding – occurs when crossed are made between close relatives. The extreme case is selfing, or the crossing of a tree with itself.
In self-pollinating species (i.e autogamous species) each member of the population is a vigorous homozygote. These self-pollinating species reproduce independently of other plants in the population. In self-pollinating plants, there will be no inbreeding depression if the plants should be selfed. If there will be any it will be of insignificant effect. Plant reproduction is also important (Hargreaves and Eckert 2014) and is related to pollinators and seed dispersers (Vázquez et al.,2005). The variety of seed dispersal and pollination syndromes range from very generalist (e.g., wind, water, gravity) to very specific (e.g., particular species of fauna) (Waser et al., 1996). 
The success of each strategy is variable, but specialization depends highly on other organisms, which means that the pollinator or disperser can be absent or specific to a particular habitat. The presence or absence of pollinators and dispersers can be affected by anthropogenic activities such as deforestation and forest fragmentation (Setsuko et al.,2013), by unusual climatic conditions, or by specific habitat characteristics such as seasonal flooding or drought conditions. (Setsuko et al.,2013). The sexual systems (e.g. monoecy and dioecy), pollination syndromes, and the characteristics of flowers and fruits can influence the forest dynamics and successional processes (Machado et al;,2006, Ibarra-Manríquez et al;,1992).
Therefore, plant pollination and seed dispersal strategies can determine the plant community composition in a particular site (Du, Y., and Ma, K. 2009). Furthermore, the plant community can also influence the presence or absence of pollinators and seed dispersers, as well as the presence and abundance of certain plant species and breeding systems. Additionally, vegetation cover, related to different forest successional stages, can affect pollinators’ and dispersers’ presence, abundance, and composition (Murali and Sukumar 1994 and van Schaik et al; 1993). Most tree crops are cross-pollinated (Sedgley 1990). Cross-pollination favors heterozygosis, which is advantageous for seedling competitiveness, probably by reducing inbreeding. Several mechanisms can prevent self-fertilization and favor cross-fertilization, being the most outstanding dioecy, monoecy, dichogamy, and genetic self-incompatibility (Liedl and Anderson 1993). Pollination methods for mass-producing full-sib seedlings used successfully in other pine regeneration programs around the world (Balocchi 1997, Carson 1996, Walker and others 1996).

Types of Pollination:

There are mainly two types of Pollination. They are Self Pollination and Cross-Pollination. They are explained below:

(1) Self-Pollination: 

It occurs when pollen from one flower pollinates the same flower or other flowers of the same individual (Cronk and Fennessy 2001). It is generally independent of agents of pollination.
Importance of Pollination in Forest Genetics-- Self-pollination
Self-pollination: Transfer of pollen grain from the anther of the same
flower of different flowers of the same plant into the stigma.

Types of self-pollination:

(a) Autogamy or xenogamy:
Self-pollination may include autogamy, where pollen is transferred to the female part of the bisexual flower (New Living Science: Biology for Class 9). It is possible only when anther and stigma are close together. Both stamens and carpels mature at the same time.
(b) Homogamy:
The anthers and stigmas of open flowers are brought together by growth, bending or folding. In Catharanthus, the growth of style brings stigma in contact with ripe anthers present on the mouth of corolla tube. In Mirabilis, the bending of filaments brings the ripe anthers in contact with the stigma.
(c) Cleistogamy: 
The flowers remain closed causing self-pollination. The flowers are bisexual. It occurs late in the flowering season in some plants. Commenlia, Oxalis, Viola. The anthers dehisce inside close flowers. The growth of style brings pollen grains in contact with the stigma. Pollinators are not required in this method of pollination. The phenomenon of having both open and closed flowers is called chasmocleistogamy.
(d) Geitonogamy: 
Pollen grains of one flower is transferred to the stigma of another flower belonging to either the same plant or genetically similar plant. It may require an external agency like insects or wind.

(2) Cross-Pollination: 

It is also called allogamy, which occurs when pollen is delivered from the stamen of one flower to the stigma of a flower on another plant of the same species (Campbell and Reece 2002). It is of different types on the basis of external agencies:
  • Anemophily- By Wind
  • Hydrophily- By Water
  • Zoophily- By different types of animals
  • Between 100,000 and 200,000 species of animals act as pollinators of the world's 250,000 species of flowering plants (Abrol 2012).
  • Entomophily- By Insects
  • Ornithophily- By Birds
  • Chiropterophily- By Bats
  • Malacophily- By Snails



Anemophily: It is a mode of cross-pollination or transfer of pollen grains from a mature anther to the stigma of a pistil which is accomplished through the agency of wind, e.g., Coconut Palm, Date Palm, Maize, many kinds of grass, Cannabis.
Anemophilous Flowers:
  • Flowers small and inconspicuous.
  • Colourless, odorless and nectarless
  • The male flowers are more abundant
  • Stamens are numerous, light and small pollen grains, winged or dusty
  • Flowers are produced above the foliage, before the appearance of new foliage
  • Anthers burst suddenly to throw the pollen grains (a gun-powder mechanism).
  • Pollen grains are dry smooth, nonsticky and unwettable.
  • Stigma is hairy, feathery or branched to catch the wind-borne pollen grains. The large thread-like stigmas.

Hydrophily:
It is the mode of pollination or transfer of pollen grains from the mature anther of a flower to the stigma of another flower which is accomplished through the agency of water. 
Hydrophilous Flowers:
  • Flowers are small and inconspicuous,
  • Perianth and other floral parts are unwettable, boat-shaped taples
  • Nectar and odor are absent
  • Pollen grains - light and unwettable, long or filamentous (chances to touch stigma)
  • Stigma is long, sticky but unwettable
  • The mature anthers dehisce on the surface.
  • Large number of male flowers.
  • The mature female flowers are brought to the surface of water having large sticky trifid stigmas.

Zoophily:
It is pollination through the agency of animals. The most common type of animal pollinators is insects. Others are birds, bats, snails, human beings, etc. Zoophilous flowers are often adapted to be pollinated by a particular type of animal. Bees and butterflies pollinate the maximum number of flowering plants Asteraceae and Lamiaceae (= Labiatae).

Entomophilous flowers:
  • Flowers are coloured for attracting pollinating insects.
  • Moths visit whitish flowers, butterflies and wasps reddish flowers, bees are attracted towards blue, purple-violet and yellow flowers
  • Where petals are not conspicuous, other parts become showy, e.g., bracts in Bougainvillea leaves in Euphorbia pulcherrima
  • Honey or nectar guides (e.g., Viola) – nectar glands
  • Some flowers produce an odor which may be pleasant (e.g., Jasmine) or foul (e.g., Aristolochia)
  • Edible pollens - Rosa, Clematis, Magnolia, etc.
  • Pollen grains – spiny with an oily sticky substance
  • Stigmas - sticky.
  • Some flowers provide a safe place to insects for laying eggs, e.g., Yucca, Amorphophallus

Ornithophily
Few types of birds are specialized for ornithophily. They usually have small size and long beaks - sunbirds and hummingbirds. Other pollinating birds are Crow, Parrot, and Mynah.
Plants: Bombax, Erythrina, Callistemon (Bottle Brush), Butea monosperma, Bignonia, Agave, etc.
Characteristics of bird pollinated flowers
  • Flowers secrete abundant watery nectar or have edible parts
  • Sugary nectar - hummingbird may suck nectar in a single day in such quantity as to have sugar equivalent to half of its body weight
  • Flowers are usually brightly colored— red, orange, yellow or blue, Floral parts are commonly leathery - in some cases, the corolla is funnel-shaped.

Chiropterophily
Pollination performed by bats. Bats are nocturnal flying mammals that can transport pollen over a long distance, sometimes over 30 km.
Chiropterophilous flowers
  • Flowers with strong fermenting or fruity odor
  • Abundant nectar and pollen grains.
  • Flowers are large and stout
  • Sausage Tree, Adansonia, Anthocephalus (Kadam Tree) and Bauhinia megalandra

Advantages and Disadvantages of Self Pollination:

Advantages:
  • Preserves Parental characters indefinitely.
  • Ensures seed production.
  • Does not need to produce a large number of pollen grains. So more economical than cross-pollination.
  • External agencies not required.
  • Can occur even the flower is closed.
  • Maintains the parental characters or purity of the race indefinitely.
  • Used to maintain pure lines for hybridization.
  • Flowers do not develop devices for attracting insect pollinators.

Disadvantages:
  • Does not cause variability. Hence plants become less adapted to changes in the environment.
  • Immunity towards diseases decreases with time.
  • Vigor and vitality of race decrease with time.
  • New useful characters are seldom introduced.
  • The undesirable or defect characters do not get eliminated from the plant.
  • Continuous self-pollination can lead to the death of the species.

Advantages and Disadvantages of Cross-Pollination:

Advantages:
  • New and useful varieties are produced.
  • Increases the adaptability of varieties towards the new environment.
  • Disease resistant potential can be increased.
  • Defective characters can be eliminated and replaced by better characters.
  • Undesirable or defective characters are eliminated and replaced by the desired characters.
  • The seeds produced are usually larger and more vigorous. A phenomenon of hybrid vigor can also be observed by this method.
  • Higher yield if enough pollinators like bees.
  • Seeds produced are larger and the offspring have characters better than the parents

Disadvantages:
  • Occurs only when flowers are open.
  • External agencies required.
  • It requires a large number of pollen grains. So less economical than self-pollination.
  • Does not preserve parental characters.
  • Wasteful because plants have to produce a larger number of pollen grains and other accessory structures in order to suit the various pollinating agencies. A factor of chance is always involved in cross-pollination.
  • Some very good characters of a variety are likely to be spoiled.

Tree improvement and Pollination:

Tree improvement refers to the application of forest genetics principles within a given silvicultural system for the purpose of improving the genetic quality of the forest. Tree improvement programs improve the genetic value of the population while maintaining genetic diversity (Namkoong et al. 1988). Meeting this goal means that genetic improvement is aimed at the population level, rather than the improvement of breeds or inbred lines. Tree improvement programs provide a known source of seed, seedlings or propagules for forest establishment.

Worldwide, tree improvement programs are linked to a range of silvicultural systems but they are most commonly integrated with plantation silviculture (Zobel and Talbert 1984). As such, each tree improvement program must be designed to fit not only the life history and natural range of the species but also the organization’s planting schedule, annual budget, and harvest goals. Most tree improvement programs follow recurrent selection schemes, consisting of multiple populations, including base populations where initial phenotypic selections are made, breeding populations where crosses among the selected individuals are created and tested and deployment populations (e.g. seed orchards) harboring elite genotypes for seed production (Namkoong et al.,1988).

Most forest tree species chosen for tree improvement programs have high levels of inherent genetic variation. Observed characteristics for a single tree is defined as the phenotype. An individual phenotype is determined both by its genetic constitution or genotype and its environment. In Breeding, many programs, controlled-pollinations are made among the select group of parents. Control-pollination starts with isolating female reproductive structures (female flowers for angiosperms and female strobili for conifers and other gymnosperms) with bags. Pollen from the selected paternal parent is injected into the bag.

The choice of the paternal parent depends on the type of mating design. At the end of the pollination season, the bag is removed and the seeds mature. The resulting seeds from a controlled-pollination have a pedigree complete with a known mother and a known father. This is the start of the breeding population.

A planted stand suitable for conversion to a seed production area should have the following specifications:
  • The trees should have a proven capacity to produce flowers and seeds in the area: To avoid failure, a detailed survey of the candidate species’ ability to flower and seed in a particular environment should be a prerequisite to establishing seed production area. This is especially important for exotic species where flowering may fail or seeds are not produced due to incompatibility with the site. Non-favorable environments, such as drought-prone areas, may be unsuitable for the seed production area. The availability of pollinators can also be important.
  • For provenance trial plots and many seed production areas, pollination is very important. without which no fertilization occurs in the trees. Tree improvement work is incomplete without pollination. However, an isolation zone or pollen dilution zone surrounding the seed production area can reduce it. The dilution zone may be an open area of some 200 m. If trees are to be grown in the dilution zone they must be of a species that does not hybridize with the species of interest for seed production.

The sequence of tree improvement steps is as follows:
  1. Species and Provenance Trials 
  2. Mass Selection 
  3. Progeny Testing 
  4. Advance Generation Breeding and 
  5. Genetic

Engineering Progeny testing is advantageous in the case where most of the variation is environmental. One can make substantial gains by separating the total variation in environmental and genetic components through progeny testing. Progeny trials are either half-sibs or full sibs. In half-sib progeny trials, open-pollinated seeds are collected from selected parents to layout trials for assessment of General Combining Ability (GCA). In full sib progeny trials, controlled cross-pollinated seeds are collected to layout trials for the assessment of Specific Combining Ability (SCA).

Type of progeny Test
Advantages
Disadvantages
Open-pollinated (half-sibs)
Can be established easily
Lack of precision

cheap
Limited usefulness as a source of second-generation selection


No information on SCA
Controlled crosses using pollen mixture
Slightly       cheaper        than
individual crosses
No information on SCA


Inbreeding will result if used for the second generation
selections
Controlled       crosses             using male parents
High Precision
Costly

Gives information on SCA
Gives the narrow genetic base for second-generation selections due to common parents
Controlled          crosses, using
small diallesis
High precision
Costly

Excellent        for         second
generation selections


Gives maximum information on SCA





Table: Comparison of advantages and disadvantages of different methods of progeny testing.

Some examples of Cross-Pollination in Forestry Sector:
  • By Wind (Anemophily)- Conifers, Betula, Utis, Okhar
  • By Birds (Zoophily)- Simal, Palash, Phaledo
  • By Bats (Chiropterophily)- Kadam, Oroxylum
  • By Insects (Entomophily)- Sal, Teak, Sisso

Conclusion:

Forest genetics mainly deals with the study of forest trees at the genetic level with many constraints and opportunities. Pollination is one of the factors to be considered in tree improvement without which fertilization does not occur. The fertilization of male and female gametes is important for producing new offsprings. Pollination also helps in progeny testing which is described above. There are mainly two types of pollination namely self and cross with their own advantages and disadvantages. Cross-pollination has various agencies (wind, bats, insects, etc).


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