Spotlight on Ploïdy
Diploid and Triploid oysters
Ploidy is a term in biology used to refer to the number of chromosomes contained in the nucleus of a living cell; these chromosomes are themselves made up of thousands of genes (which determine characteristics such as shell colour, shape, etc.), which are passed on during reproduction. Living organisms have a specific number of sets of chromosomes, this number varying depending on the species in question. Generally speaking though, they are present in pairs (2n).
One set is of male origin and the other of female origin, i.e. one from each of the two parents. In humans, there are 23+23 (making a total of 46), whilst in oysters there are 10+10 (making 20 in total). For these examples, scientists use the notation n=23 and n=10 respectively. Organisms with 2n chromosomes are referred to as diploid.
But there are some organisms that have a higher number of chromosome sets, for example 3n (known as Triploid), 4n (Tetraploid), etc. These are generically known as polyploid organisms.
Polyploidy is frequently encountered in the plant world and has played a crucial role in evolution (wheat, potato, strawberry, banana, citrus fruits, etc.).
Diploid oysters can reproduce in their natural environment. The animal accumulates reserves to prepare for reproduction. These milky oysters (rich in fatty acids, more difficult to keep fresh) are not generally eaten in months without an "R" - May to August - since most consumers do not really enjoy them during this period.
These oysters contain two sets of chromosomes (2n): one set from the male and the other set from the female, i.e. from both parents.
Available in France since 2000, the triploid oyster is plump and not very milky during months without an "R". Being sterile, reproduction activities are significantly reduced throughout this period, with much of the metabolic effort devoted to growth and fattening. Its growth cycle is thus shorter. Moreover, oyster lovers can enjoy them all year round.
These oysters contain three sets of chromosomes (3n): one set from the female (diploid parent) and two sets from the male (tetraploid parent).
Triploid oysters ARE NOT GMOs (Genetically Modified Organisms)
The technique employed to produce GMOs involves the introduction of a gene from another species into the cells of a living being. Triploid oysters are not GMOs since no "foreign" gene has been introduced into the oyster's genetic material. They are only produced in the hatchery environment.
Oyster triploidisation consists of increasing the number of chromosome sets (from 2 to 3) passed on by the parents to their offspring. Unlike the case for GMOs, no new genes from another species are introduced.
This process is an "innovation" of nature that has been "harnessed" - rather than created - by man to use specific associated characteristics, such as sterility in the case of triploids (resulting in non-milky oysters, pip-free fruit, etc.).
Furthermore, all international bioethical regulations confirm that triploids are not genetically modified organisms.
How are triploid oysters obtained?
The triploid oyster is the result of crossing a diploid female (2n) with a tetraploid male (4n) supplied by IFREMER.
The majority of triploid oysters are produced by the IFREMER Laboratory in La Tremblade. One of the laboratory's objectives was to improve French oyster strains with a view to making them more disease-resistant. This led them to focus on the genome of the oyster.