Fruits without seeds are common in grocery stores. Biologically, a fruit is the mature ovary of a flowering plant, typically protecting the seeds that develop from fertilized ovules. Since seeds are the plant’s primary method of reproduction, seedless fruits are a biological exception. This deviation results from distinct genetic processes and human intervention.
The Biological Mechanism of Seedlessness
The development of a fruit without viable seeds occurs through two primary biological pathways. The first is parthenocarpy, the formation of fruit without fertilization. This process bypasses successful pollination and seed development, triggering the ovary’s growth into mature fruit.
Parthenocarpy is categorized as either vegetative or stimulative. Vegetative parthenocarpy means the fruit develops spontaneously without any external trigger. Stimulative parthenocarpy requires a non-fertilizing stimulus, such as the growth of a sterile pollen tube. In both cases, the lack of developing seeds often results in a higher proportion of edible flesh.
The second mechanism is stenospermocarpy, where fertilization does occur, but the resulting embryo and endosperm abort early in development. This initial signal triggers fruit growth, resulting in a functionally seedless fruit. The small, soft, underdeveloped remnants sometimes found in these fruits are aborted ovules, often referred to as seed traces.
Both mechanisms are fundamentally driven by the manipulation of plant hormones, primarily auxins and gibberellins. Normally, the developing seeds are the source of these hormones, which sustain and promote the growth of the surrounding ovary tissue. In seedless varieties, the plant tissue itself produces sufficient levels of these growth regulators to trigger fruit set and enlargement.
Common Seedless Fruits and Their Origins
Seedless bananas are a widely consumed example of naturally occurring seedlessness. Modern cultivated varieties, such as Cavendish, resulted from a genetic accident leading to vegetative parthenocarpy. They are sterile triploids, possessing three sets of chromosomes instead of two, which prevents functional seed formation.
Seedless grapes are a classic example of stenospermocarpy, preferred for table consumption. Pollination and fertilization occur, but a genetic mutation causes the developing seed to abort shortly after. The resulting fruit contains only minute, soft seed traces. This trait is often linked to a mutation in the VviAGL11 gene.
The Navel orange originated from a spontaneous somatic mutation in a single branch of a tree in Brazil in the early 1800s. This mutation caused the fruit to be completely seedless. It also led to the formation of a small, secondary fruit structure at the blossom end, giving it the characteristic “navel.” Since the original mutation occurred in a non-reproductive cell line, it can only be perpetuated through asexual propagation.
Seedless watermelons are a commercially created hybrid functioning through genetic sterility. They are triploids, created by crossing a standard diploid watermelon with a chemically-induced tetraploid plant. The tetraploid has double the usual number of chromosomes. The resulting triploid plant is sterile because its three chromosome sets cannot divide evenly during meiosis.
Commercial Production and Cultivation
Commercial production relies on techniques to maintain and propagate sterile traits on a large scale. For fruits like the Navel orange and the banana, the seedless trait is maintained exclusively through vegetative propagation. Growers use grafting, cuttings, or tissue culture to create genetically identical clones.
The process for seedless watermelons requires a yearly cycle of triploid breeding. Seed companies first create a tetraploid female parent by treating diploid seeds with chemicals like colchicine to double the chromosome number. This tetraploid is then crossed with a normal diploid male parent to produce the sterile triploid seeds that growers plant to yield seedless fruit.
In some crops, such as grapes, seedlessness is induced by the external application of plant hormones. Growers apply gibberellin, a plant growth regulator, to young grape clusters. This application encourages seedless berry development and increases fruit size.