be identical nucleotide-for-nucleotide to the same strain 50 years
ago. It will nevertheless be far more similar to its own ancestor
in 1950 than to substrains separated early in the history of a
major strain, for example C57BL兾10, C57L, and C58 (51), or to
another substrain separated around 1950.
A ll of the phenot ypes examined here rank as highly complex
traits. Quantitative trait locus analyses have indicated influences
of multiple loci for brain weight (16) and for the behavioral traits
studied (53). A minor change in one or two of many pertinent
genes should not shift the mean score of a specific strain
appreciably when it is tested several decades later. There does
not seem to be any tendency in the dat a reviewed here for strains
to differ more substantially when the studies were conducted
many years apart. For example, the ethanol preference data of
A.B. were more similar to those of Rodgers (22) from 40 years
earlier than those of Belknap et al. (23) just 10 years earlier.
A lthough the Thompson (24) activit y dat a were not highly
c orrelated with the data from Liu and Gershenfeld (26), they
were very similar to more recent data from the Edmonton
The importance of many fine det ails of husbandry and testing
is recognized on the basis of carefully c ontrolled experiments
c onducted within a single laboratory. Phytoestrogens in the diet
(54), cage enrichment (4), cage position in the colony room (55),
size of the drinking spout orifice (56), shipping before testing
(57), and even the specific experimenter administering the test
(4, 58) have been found to af fect laboratory mouse physiology
and behavior. Nevertheless, just as a single gene usually ac counts
for little variance on its own, it is highly unlikely that a multi-
dimensional laboratory difference in behavioral dat a can be
traced to a single environmental variant. A small environmental
ef fect might be evident within a carefully controlled ex periment,
whereas the same effect might not be audible amidst the
cac ophony of multiple laboratory-specific parameters in rearing
The sample of phenotypes in this systematic comparison of
recent and classical data sets is not sufficient to warrant strong
c onclusions about what kinds of behavior should be most and
least robust across laboratories. Very tentatively, we suggest that
things more closely associated with sensory input and motor
output will tend to be less affected by minor variants in the
laboratory environment, whereas behaviors related to emotional
and social processes will be more labile. A similar classification
regarding degree of genetic influence has been proposed on
theoretical grounds by Lipp (59).
Animals and Laboratories. Data on brain weight, open field activ-
it y, and elevated plus maze were collected simultaneously in the
J.C.C. laboratory in Portland and the D.W. laboratory in Edm-
onton, whereas data on ethanol preference were collected in the
A.B. and D.A.F. laboratories as separate studies done w ithin a
few months of each other. We all obtained the animals from The
Jackson Laboratory at 4–6 weeks of age. The D.W. and J.C.C.
laboratories evaluated the same 21 inbred strains, c onsisting of
priorit y lists A and B of the Mouse Phenome Project (129S1 兾
SvImJ, A兾J, A KR兾J, BALB兾cByJ, C3H兾HeJ, C57BL兾6J,
C57L兾J, C58兾J, CAST兾EiJ, DBA兾2J, FVB兾NJ, MOLF兾EiJ,
NOD兾LtJ, NZB兾BlNJ, PER A兾EiJ, PL兾J, SJL兾J, SM兾J,
SPRET兾EiJ, and SWR兾J) plus the strain BTBR T⫹ tf兾J from
list D (www.jax.org兾phenome), which has an interesting but
viable loss of forebrain commissures (15). The A.B. laboratory
studied 28 strains, and the D.A.F. laboratory studied 22 strains
(Table 3), 10 of which were common to at least t wo studies but
were in addition to those studied by the J.C.C. and D.W.
laboratories (129P3兾J, BALB兾cJ, BUB兾BnJ, C57BL兾KsJ,
CBA兾J, CE兾J, I兾LnJ, LP兾J, RIIIS兾J, and SEA兾GnJ). No at-
tempt was made to equate the housing conditions in the four
laboratories, but conditions were nevertheless quite similar, as
described in Table 7, which is published as supporting informa-
tion on the PNAS web site.
Data Analysis. There currently is no st andard in the field for what
size a strain correlation denotes a bona fide replication of results.
Instead, we rely strongly on close inspection of the data by those
having extensive experience with a particular kind of test. Strictly
speak ing, replication of results in two laboratories requires a
large strain difference but no strain-by-laboratory interaction
ef fect. The magn itude of a strain difference can be ex pressed as
, the proportion of variance attributable to the differ-
ences among strain means when laboratory differences are
removed f rom the equation. The partial
can also be estimated
for an interaction effect (1). If
for an interaction effect is
c onsiderably less than
for a strain main effect, then we
c onsider the interaction to be relatively small, even if its statis-
tical sign ificance (P value) is beyond reproach. Likewise, to be
c onsidered large or substantial in a strain-by-laboratories study,
the interaction ef fect size should be at least half the strain main
ef fect size when dat a are compared across two laboratories.
We thank Naomi Yoneyama, Andrea Wetzel, Maria Theodorides, Sue
Burkhart-Kasch, Janet D. Dorow, Jason R. Sibert, Jason P. Schlumbohm,
Charlotte D. Wenger, Chia-Hua Yu, Pamela Metten, Brandie Moisan,
Sean F. Cooper, Tera Mosher, Tim Frigon, and Elizabeth Munn for
assistance in collecting the data. This work was supported in part by
National Institutes of Health grants to D.W. (Grant AA12714), A.B.
(Grant AA11028), J.C.C. (Grant AA10270 and Integrative Neuro-
science Initiative on Alc oholism Consortium Grant AA13519), and
D.A.F. (Integrative Neuroscience Initiative on Alcoholism Consortium
Grants AA134785, AA10760, AA12439, and AA13478); Department of
Veterans Affairs grants (to J.C.C. and D.A.F.); and Natural Sciences and
Engineering Research Council Grant 45825 (to D.W.).
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