High female rate

christian lawrence clawrence at rics.bwh.harvard.edu
Wed Apr 6 08:12:00 EST 2005


Hi Julian,

In general, we see more balanced sex ratios when our larval rearing
densities are between 30-40 fish per liter.  At <20 fish/L we tend to
start seeing a female bias, and there tends to be a male bias at densities
>40 fish/L.   Your densities are considerably higher than ours, so
presumably if you were rearing fish under the same conditions that we are,
you should having a problem with an abundance of males.  Of course it is
unlikely that the conditions in your facility are the same as ours, and so
the numbers above mean little to your situation.

I should clarify that when I refer to "density" being an environmental
factor that can skew sex ratios, what I really mean is growth rate.
Assuming that everyone feeds generally the same diet, administers it in
the same manner, and maintains water chemistry parameters within the same
range, it is density that can mediate growth rates because, ultimately,
fish reared at lower densities get more food per fish and grow faster as a
result and vice versa.  Now obviously the rearing conditions vary widely
across the research community, but in general, under conditions that are
conducive to a high rate of growth during a sensitive period, one tends to
see an increase in the number of females in a brood.  Why this happens is
unclear; and it may be that it is relative proximity to other individuals
(and whatever they are releasing into the water), and not growth rates,
that is having the effect on sex ratio.

Assuming that it is growth rates that are having the effect on sex ratios,
a better way to compare the conditions from one facility to another would
be to look at how long it takes for your fish to develop from swim-up
stage to juvenile (when they look like a miniature adult).  It takes our
fish about 4 weeks to fully metamorph into juveniles at a density of 30-40
fish/L. That said, you still should look at the genotype of the fish
you're working with.  If you work with one wt strain and are not careful
in maintaining it or any lines derived from it from one generation to the
next (excessive bottlenecking, etc.), it is likely that it may begin to
become biased in one direction or another.  This is presumably because one
or more of the multiple loci involved in sex determination becomes fixed
with successive rounds of inbreeding.    Backcrossing to original f0
generation or crossing against a different wt strain usually alleviates
this, but this will almost always happen.    Maintaining as much variation
as possible within wt strains is a good way to avoid this problem.
Getting a handle on sex ratios in zebrafish (and many other fish) is a
tricky problem because of the dual and often conflicting influences of
genotype and environment on the expression of phenotypic sex.   In order
to manage your lab populations effectively, you've got to be cognizant of
this fact and adjust your rearing protocols accordingly.

Christian Lawrence
Brigham and Women's Hospital
Karp Family Research Laboratories 06-004B
One Blackfan Circle
Boston, Massachusetts 02115
Tel: 617.355.9041
Fax: 617.355.9064


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