Diploidy versus Triploidy

Listen the podcast : Diploidy versus Triploidy (in French)

What is Ploidy ?

Ploidy refers to the number of sets of chromosomes present in a cell. Organisms are diploid if they have two sets of chromosomes (such as humans: one from the mother and one from the father), triploid if they have three, tetraploid if four, and so on. Many species are diploid, but it is common for the number of chromosome sets to be higher. Polyploidy (more than two sets of chromosomes) is prevalent in plants (wheat, potato, strawberry, banana, citrus fruits, etc.) but also in some animal species (marbled crayfish, sturgeon, etc.).

Differences in ploidy can impact various characteristics such as growth, size, morphology, reproduction, disease resistance, and other traits of organisms. While oysters are primarily diploid, triploid individuals can also occur naturally.

Diploid Oysters

Diploid oysters can reproduce in the wild. The animal accumulates reserves for reproductive readiness. These milky oysters (rich in fatty acids, with more challenging preservation) are generally not consumed during months without the letter "R." They possess two sets of chromosomes: one set from the male and the other from the female.

Triploid Oysters

Triploid oysters have three sets of chromosomes. Being sterile, their reproductive effort is significantly reduced during this period, and their metabolism is directed towards growth and fattening. Oyster enthusiasts can enjoy them throughout the year. These oysters have three sets of chromosomes: one set from the female (diploid parent) and two sets from the male (tetraploid parent).

Similar to diploid oysters, taste is primarily linked to natural conditions in oyster farming (ocean currents, freshwater influx, abundance and diversity of phytoplankton, etc.) and choices made by the oyster farmer: merroir, importance of tides, whether or not oysters are placed in claires, etc. Unlike diploid oysters, whose taste evolves throughout the year based on their sexual maturity, triploid oysters (sterile or non-milky) have a more consistent taste.

Tetraploid Oysters

Triploid oysters are obtained by crossing diploid female oysters with tetraploid male oysters.

In the controlled production process, a fertilized egg is placed in an environment that promotes polyploidy (during meiosis). By replicating this step twice, a tetraploid oyster is obtained.

Are triploid oysters GMOs ?

No, creating a triploid oyster involves increasing the number of sets of chromosomes (from 2 to 3) during fertilization. This occurs naturally in a controlled environment by crossing a diploid female oyster with a tetraploid male oyster, both raised by the teams at France Naissain. There is no introduction of new genes from another species, as is the case with GMOs.

It's a 'nature-based' innovation that humans have not created but only 'controlled' to utilize specific associated characteristics, such as sterility in the case of triploids (resulting in non-milky oysters or seedless fruits).

What is a GMO ?

The acronym GMO stands for 'Genetically Modified Organism.'

There isn't a single definition, but the scientific community generally agrees that a GMO is an organism (animal, plant, bacterium) that has been altered using genetic engineering techniques. These techniques, conducted in laboratories, can add new genes, delete, or modify existing genes within the organism to impart new traits. A genetically modified organism or GMO is, therefore, a living organism whose genetic material has been modified by human intervention.

Triploid oysters are not GMOs since no "foreign" genes are introduced into the oyster's genetic material.

From a regulatory standpoint, European laws (particularly Directive 2001/18/EC) define a GMO as :