PB Ch 5. Modes of Pollination

Introduction

The way a plant reproduces dictates its entire genetic makeup, which in turn dictates how a plant breeder must handle it. At the most basic level, pollination is simply the transfer of pollen (male) to the stigma (female).

Based on where that pollen comes from, crop plants are classified into three major reproductive categories:

• Self-Pollinated Crops
• Cross-Pollinated Crops
• Often Cross-Pollinated Crops

5.1 Self-Pollination

• In self-pollinating species, a flower is fertilized by its very own pollen. Evolutionarily, scientists believe self-pollinated plants originally evolved from cross-pollinated ancestors.
• For a plant to self-pollinate, it must have hermaphrodite (bisexual) flowers—meaning the male and female organs are housed together in the exact same flower.
• However, nature is rarely absolute; in most "strictly" self-pollinated crops, a tiny amount of accidental cross-pollination (up to 5%) can still occur depending on the specific variety, local temperature, humidity, and the number of insects buzzing around.

A. Mechanisms Promoting Self-Pollination

Plants have evolved several highly efficient physical mechanisms to guarantee that their own pollen reaches their stigma before any foreign pollen can interfere.

• Cleistogamy (Closed Flowers): This is the most extreme and strict form of self-pollination. The flowers simply never open. Because the petals remain permanently sealed shut, no foreign pollen can possibly enter.

  • Examples: Common in certain varieties of wheat, oats, barley, and several other grasses.

• Flowers Open AFTER Pollination: In this mechanism, the flower eventually opens to the outside world, but only after pollination has already happened inside the tightly closed bud.

  • Examples: Many major cereals use this, including wheat, barley, rice, and oats.

• Anther Position (The Stamen Cone): The male anthers physically surround the female stigma in a tight protective ring. As soon as the flower opens, the pollen from the surrounding anthers falls directly inward onto the enclosed stigma.

  • Examples: Tomato and brinjal (eggplant). The "stamen cone" of the tomato is the classic textbook example of this mechanism.

• Keel Flowers (Floral Enclosures): The flower petals are specially shaped to lock the reproductive organs away. The stamens and stigma are tightly enclosed by two fused petals called a "keel" (or carina). The stamens mature and burst open, shedding pollen entirely within this little petal-vault before the flower fully blooms.

  • Examples: Several legumes, including Pea (Pisum sativum), Mung (Vigna radiata), Urd (Vigna mungo), Soybean (Glycine max), and Gram (Cicer arietinum).

• Stigmas Elongate Through Staminal Column: The female stigma starts below the male stamens. As it becomes receptive, it grows and physically pushes its way up straight through the column of male anthers, scraping against them and gathering its own pollen on the way out.

B. Genetic Consequences of Self-Pollination

Because the plant is constantly mating with itself, it rapidly loses genetic variation. This leads to a massive increase in homozygosity (having identical pairs of genes).

• The Math: After n generations of continuous selfing, the proportion of heterozygous (mixed) genes shrinks to (1/2)^n. It literally halves with every single generation.
• Rapid Stabilization: After just 6 to 7 generations of selfing, the plant line becomes essentially 100% homozygous.
• No Inbreeding Depression: Unlike animals or cross-pollinated plants, self-pollinated crops do not suffer from inbreeding depression (weakness from inbreeding). They have evolved to be perfectly healthy while highly inbred.
• Hidden Heterosis: Even though they naturally live as pure lines, if a breeder artificially crosses two different pure lines, the resulting F1 hybrid can show massive "heterosis" (hybrid vigor).
• Breeding Aim: Because they naturally want to be pure, the primary goal for breeders working with self-pollinated crops is to develop completely homozygous varieties (pure lines).

4.2 Cross-Pollination

In cross-pollinating species, pollen is transferred from one plant to the stigma of an entirely different plant. This transfer is usually handled by the wind (anemophily) or by insects like bees (entomophily). Just like self-pollinators aren't perfect, natural cross-pollinators might accidentally self-pollinate a tiny bit (usually up to 5-10%).

A. Mechanisms Promoting Cross-Pollination

To force cross-pollination, a plant must actively prevent its own pollen from fertilizing its own egg. They do this through several fascinating structural and temporal barriers:

• Dicliny (Unisexual Flowers): The most foolproof method. The flowers are strictly male (staminate) or strictly female (pistillate). Because a single flower doesn't have both parts, it cannot self-pollinate.

  • Monoecious species: Male and female flowers live on the same plant, but in different locations. Examples: Maize, Castor, Mango, Coconut, Chestnut, Strawberry, Rubber, Grapes, Cassava.

  • Dioecious species: Male and female flowers live on entirely separate plants. This enforces 100% cross-pollination. Examples: Papaya, Date palm, Hemp, Asparagus, Spinach, Pistachio.
• Dichogamy (Temporal Separation): The flower is hermaphrodite, but the male and female parts mature on different days. They are simply never "awake" at the same time.

  • Protogyny (Females first): The stigma matures and becomes receptive before its own pollen is ready. By the time its pollen sheds, the egg has already been fertilized by a neighbor. Examples: Bajra (pearl millet) and cherimoya. (Because the feathery stigmas emerge first in bajra, breeders don't even need to emasculate it for controlled crosses!).

  • Protandry (Males first): The stamens mature and shed all their pollen before their own stigma becomes receptive. Examples: Maize, Sugar beet, Carrot, Sunflower.
• Herkogamy (Spatial Separation / Physical Barriers): The male and female parts are awake at the same time, but a physical barrier blocks them.

  • The Lucerne Tripping Mechanism: In Lucerne (alfalfa, Medicago sativa), the stigma is covered by a waxy film that prevents its own pollen from germinating. When a heavy honeybee lands on the flower, its weight "trips" the flower, breaking the waxy membrane and forcefully dusting the stigma with foreign pollen carried by the bee. This is why bee colonies are mandatory near alfalfa seed farms.

• Heterostyly: Flowers on different plants have mismatched physical lengths.

  • Distyly (Primula): Some plants have "Pin" flowers (long style, short anther) and others have "Thrum" flowers (short style, long anther). A Pin can only mate with a Thrum.

  • Tristyly (Lythrum): The plant has three distinct style lengths.
• Self-Incompatibility (SI): A biochemical defense system. The flower's pollen is perfectly healthy, but the stigma chemically recognizes it as "self" and destroys it before it can fertilize the egg. Common in Brassica (crucifers), Nicotiana, Radish, Rye, and grasses.
• Male Sterility: The plant produces absolutely no functional pollen. While rare in wild nature, breeders love this trait because it makes producing commercial hybrid seeds incredibly easy.
• Combination of Mechanisms (The Maize Example): Some plants double up their defenses. Maize is the ultimate example: it is Monoecious (the male tassel is at the top, the female cob is lower down) AND it is Protandrous (the male tassel sheds its pollen days before the female silks emerge). This double-layer of security makes maize incredibly efficient at cross-pollinating.

B. Genetic Consequences of Cross-Pollination

Constant cross-pollination keeps the population's genetics thoroughly mixed up (highly heterozygous).

• Inbreeding Depression: Because their genetics are constantly mixing, these plants hide a lot of weak, recessive genes. If you force a cross-pollinated crop to self-pollinate, those weak genes pair up, causing severe inbreeding depression (the plants become stunted and sickly).
• Breeding Aim: Breeders try to improve the crop without reducing this natural heterozygosity. Therefore, the ultimate goal in cross-pollinated crops is to create Hybrid or Synthetic varieties.

4.3 Often Cross-Pollinated Species — A Third Category

Nature isn't always black and white. There is a middle ground consisting of crops where natural cross-pollination routinely exceeds 5%, but usually tops out around 30%. These are known as Often Cross-Pollinated Species.

• The Big Four Examples: Jowar (Sorghum bicolor), American Cotton (Gossypium hirsutum), Arhar / Pigeonpea (Cajanus cajan), and Safflower (Carthamus tinctorius).
• Genetic Architecture: Their genetic makeup is an intermediate mix. A single field will contain a blend of both homozygous (pure) and heterozygous (mixed) individual plants.
• Breeding Implication: Because they sit in the middle, breeders can successfully apply methods meant for both self-pollinated crops (like pure line selection) and cross-pollinated crops. However, commercial hybrid varieties are usually vastly superior and dominate the market (especially in cotton and sorghum).

Comprehensive Crop Pollination Data Table

Memorize this table for quick referencing during exams.

Crop Name	% Cross-Pollination	Breeding Implication & Key Features
Rice (Oryza sativa)	< 1-5%	Strictly self-pollinated; pure line varieties; hybrid rice now also commercial.
Wheat (Triticum aestivum)	1-5%	Mostly self-pollinated; pure line varieties dominate.
Barley (Hordeum vulgare)	< 2%	Strictly self-pollinated; cleistogamy is common.
Groundnut (Arachis hypogaea)	< 2%	Strictly self-pollinated; pure line varieties.
Soybean (Glycine max)	< 1%	Strictly self-pollinated; uses the keel flower mechanism.
Chickpea (Cicer arietinum)	< 1%	Strictly self-pollinated; uses the keel flower mechanism.
Tomato (Solanum lycopersicum)	< 2-5%	Mostly self-pollinated; stamen cone mechanism; F1 hybrids are commercially important.
Sorghum (Sorghum bicolor)	5-10%	Often cross-pollinated; CMS hybrids are commercially dominant.
Cotton (G. hirsutum)	5-20%	Often cross-pollinated; hand emasculation used for hybrids; Bt hybrids dominate.
Pigeonpea (Cajanus cajan)	5-20%	Often cross-pollinated; CMS hybrids are currently being developed.
Safflower (Carthamus tinctorius)	5-30%	Often cross-pollinated; GMS system is used for hybrid seed production.
Maize (Zea mays)	95-100%	Strictly cross-pollinated; uses monoecy + protandry; hybrids dominate.
Pearl millet / Bajra (Pennisetum glaucum)	> 90%	Strictly cross-pollinated; highly protogynous; CMS hybrids dominate.
Sunflower (Helianthus annuus)	> 90%	Strictly cross-pollinated; heavily insect (bee) pollinated; CMS hybrids dominate.
Sugarcane (Saccharum)	> 90%	Strictly cross-pollinated; wind-pollinated; primarily propagated clonally (vegetatively).
Rye (Secale cereale)	> 95%	Strictly cross-pollinated; wind-pollinated (anemophilous); driven by Self-Incompatibility.
Lucerne / Alfalfa (Medicago sativa)	> 95%	Strictly cross-pollinated; requires bee-pollination to trigger the tripping mechanism; synthetic varieties used.

Exam Angle: How to Structure Your Answers

1. Distinguishing between Self and Cross-Pollinated Crops (IFoS 2018)

Do not write a block of text. Always construct a comparison table to contrast them sharply across multiple parameters.

• Parameter 1 (% Cross-Pollination): Self (<5%) vs Cross (>90%).
• Parameter 2 (Genetic Constitution): Self (Highly Homozygous) vs Cross (Highly Heterozygous).
• Parameter 3 (Inbreeding Depression): Self (None/Absent) vs Cross (High/Severe).
• Parameter 4 (Floral Mechanisms): Self (Cleistogamy, Keel, Stamen cone) vs Cross (Dicliny, Dichogamy, SI, Male Sterility).
• Parameter 5 (Breeding Objective): Self (Pure lines) vs Cross (Hybrids and Synthetics).
• Examples: Always list at least 4-5 crops per column (e.g., Self = Wheat, Rice, Barley, Pea, Groundnut. Cross = Maize, Sunflower, Bajra, Rye, Sugarcane).

2. Explaining Mechanisms that Promote Cross-Pollination (IFoS 2021)

To get full marks (8M/10M), you must explicitly list all 6 distinct mechanisms. Use clear headings for:

• Dicliny (Define monoecious vs dioecious).
• Dichogamy (Define protogyny vs protandry).
• Herkogamy (Must mention Lucerne's waxy film/tripping by bees).
• Heterostyly (Mention Primula pin/thrum).
• Self-Incompatibility (Chemical rejection).
• Male Sterility (Lack of functional pollen).

  Crucial step: You must attach a specific crop example to every single mechanism you list. No example = lost marks