[Zbrafish] ratio males/females

Lawrence, Christian via zbrafish%40net.bio.net (by Christian.Lawrence from childrens.harvard.edu)
Wed May 13 12:14:35 EST 2009

There are a number of factors that influence sexual differentiation in zebrafish.

Temperature is most often implicated but is unlikely to be the environmental factor at work in fish facilities, since published data indicates that masculinizing temperatures are in the mid to low 30s.

Food consumption, mediated by densities, is more likely to affect lab stocks, although if there is a bias caused by this, it is more likely to be a female one, since in practice it is much easier to overfeed and the scenario (female bias) is more in line with the potential underlying mechanism (estrogen content in feeds).

The other well known environmental factor known to impact sexual differentiation is dissolved oxygen.  Very low do2 (hypoxic conditions) can exert masculinizing effects.  Certainly this is conceivable on nursery racks especially but given reports on this effect in the literature, one would also expect to see high mortality, with a strong male bias in survivors.  This doesn't seem to be your pattern, either.

It may not be environmental.  Sex ratios can be heritable in zebrafish.  Certain crosses will sometimes yield a higher proportion of one sex over the other. This reasons for this are not well understood but this seems to tend to crop up in facilities where stocks are maintained in small, closed populations for extended number of generations.   The sex ratio effect seems to be most pronounced when fish from one closed population are crossed to another.  Again, in the literature, and in our hands, the bias seems to typically be female, but it may work in the other direction.

Quite conveniently, it may also be a combination of the above, or it could even be some synthetic masculining agent leaching into the water.

Such are the joys of working with a teleost fish with a yet to be determined and likely polygenic sex determining system.  No easy answers!

Christian Lawrence
Aquatic Resources Program
Children's Hospital Boston
320 Longwood Avenue
Boston, MA 02115
617.919.2738 (office)
617.730.0836 (fax)
christian.lawrence from childrens.harvard.edu

On 5/12/09 4:00 AM, "Sophie Louwette" <sophielouwette from hotmail.com> wrote:

Dear all,

Lately we are having problems with raising our zebrafish. The survival rate is fine (70%-80%) but the majority of them are males. We keep our fish on 26°C and not too crowded. I was wondering if there are correlations between temperature and density and the ratio of females/males. Maybe you can give us some advice on this issue.

Best regards,

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