Tilapia, like some other species, has the ability to change sex under certain conditions during the development phase after hatching. Temperature is clearly related with this phenomena. Depending on the tilapia species as well as the strain, an increased water temperature after hatching can generate relatively more males or more females.
Til-Aqua has used this ability of its tilapia to develop the Til-Aqua YY-line. Our strains are sensible for high temperature (> 36°C) and under these circumstances they change from male into females within 10 days after hatching.
This sex-change is what we call a change in phenotype: the physical sexual characteristics change externally and internally; i.e. the gonads. However, from the moment that the egg cell is fertilized by the sperm cell, the genetic make-up has been already determined by the DNA. This complete collection of genes is called the genotype. Typically, the genetic make-up of a male consists of an X and a Y chromosome while the genetic make-up of a female consists of two X chromosomes.
This combination of a flexible phenotype and a solid genotype allows the exceptional case to happen that a fish with female genetic make-up (XX) produces male gametes (sperm). The other way around is also possible which means that a fish with male genetic make-up (XY) produces female gametes (eggs). This second option is the case for the Til-Aqua lines.
Using these egg-producing males, a breeding program was set up to develop the Til-Aqua YY-lines:
The males from this Til-Aqua YY-line are matched with the females from the Til-Aqua Mixed Sex line to produce our Natural Male Tilapia offspring with 99% male ratio. The two lines are genetically completely separated, causing a so-called 'heterosis effect' which means the offspring perform better than could be expected when adding up both parents; 1+1=3! In practice, this results in well performing and robust NMT fry.
Production of Til-Aqua's YY-males.
Each YY-male for the mass production of YY-males for the industry is tested against a female (XX) for offspring testing.
Only YY-males producing 100% male offspring are kept for further YY-production.
Production of Til-Aqua's YY-females.
For the mass production of YY-males we need also YY-females. Each YY-female produced is tested against a normal male (XY).
Only YY-females producing 100% male offspring are kept for further YY-production.
After years of this genetic selection we achieve nowadays a male ratio of 99% and even with females of other known Tilapia stains (from Egypt, Philippines, Kenya, etc.) we obtain > 96%!
It is not only the Y-chromosome that determines if a tilapia is male or not. Also genes on other chromosomes can play a role.
After years of this genetic selection we achieve nowadays a male ratio of 99%.
So far, several other female-lines were already tested with the Til-Aqua YY-male. A Kenyan strain, a GIFT strain, a strain from Lake Manzala (Egypt) and a strain derived from a gene-pool (Phillipines) were found all to have a male ratio >96% with excellent performance!
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