[NHCOLL-L:2854] Re: Preserving specimens for DNA extraction

[Doug Yanega]

Brad Hubley asked:

>Given that we may want to extract DNA from other taxa within these 
>collections at some future date, what is the best way to preserve 
>these samples over the long-term?  Should they remain in 95% or can 
>they be moved into 80%?  Our DNA lab techs say we should continue to 
>preserve in 95% and place the specimens into an ultra-cold freezer 
>-- but then we run the risk of damage to the actual specimens.
>
>So we feel we are faced with a dilemma, preserve for DNA 
>extraction, preserve for morphology, or can we preserve for both?

Controlled (published) experiments on DNA recovery have showed that 
the recovery process works best with higher concentrations of 
alcohol. HOWEVER: the actual degree of difference between a high 
concentration and a lower concentration was *very* small - only 
barely detectable statistically (e.g., a 97% success rate versus 
93%). Likewise, the difference between ethanol and isopropanol was 
very slight at equal concentrations.

The low temperature effect, on the other hand, is extremely 
significant - I think you'd do better with specimens kept in a 
freezer (even an ordinary freezer) in 70% ethanol for 5 years than 
specimens kept in 95% at room temperature for a month. After all, a 
single day that allows specimens to overheat is all it takes to make 
an entire sample almost worthless.

So, yes, you can preserve DNA at any ethanol concentration from 70% 
up (the higher, the better - but not by much), but you really do want 
to keep samples cold until and unless they're no longer needed for 
DNA.

We acquired two freezers for our collection for exactly this reason 
(unfortunately, they're both full and we have no room for a third - 
we've given a little thought to the frozen tissue depository at the 
AMNH, but bulk trap samples aren't exactly suitable). Just another 
example of the changing needs of biological collections that render 
our older facilities and practices obsolete. What we need in the 
future is a custom-designed (or easily customizable) freezer design 
that would allow truly organized storage and retrieval of ethanol 
vials - i.e., something with precisely adjustable shelf height, 
integral pull-out drawers that can be custom widths, etc. It's just 
so dang hard to maintain a well-organized freezer, without leaving 
too much wasted space.

Peace,
-- 

Doug Yanega        Dept. of Entomology         Entomology Research Museum
Univ. of California - Riverside, Riverside, CA 92521-0314
phone: (951) 827-4315 (standard disclaimer: opinions are mine, not UCR's)
              http://cache.ucr.edu/~heraty/yanega.html
   "There are some enterprises in which a careful disorderliness
         is the true method" - Herman Melville, Moby Dick, Chap. 82