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in genetic background. Here, we define genetic background as the summa-
tive effect of loci whose independent effects are too small to identify and
quantify statistically. Within the genetic background of the domesticated
stock, which has undergone strong artificial selection for paedomorphosis,
the penetrance of
met1
is greater, and this results in a higher proportion
of paedomorphic individuals in hybrid crosses (
Voss & Smith, 2005
).
8. THE LINK BETWEEN met1 AND TH REGULATION OF
METAMORPHOSIS
The link between
met1
and THwas recently established by combining
quantitative trait locus (QTL) analysis with TH induction. Recognizing that
paedomorphic species show variation in time to complete metamorphosis
when treated with TH,
Voss et al. (2012)
crossed two paedomorphic species
(
A. mexicanum
and
A. andersoni
) and then performed a backcross to segregate
TH response alleles among second generation offspring. At 120 days
postfertilization (dpf), backcross offspring were treated with 2.5 nM T4
and then scored for time to complete metamorphosis. Essentially, all off-
spring initiated and completed metamorphosis over an
160 day interval
of time. A QTL screen identified the genomic position of
met1
and two
additional QTL (
met2
,
met3
), each explaining approximately 10% of the
variation in metamorphic timing; the effects of all alleles were additive with
A. andersoni
alleles decreasing the time to metamorphosis. On average,
individuals that inherited
met1-3
alleles from
A. andersoni
metamorphosed
earlier, just as did hybrid
A. mexicanum
that inherited
met1
alleles frommeta-
morphic
A. t. tigrinum
(
Voss & Smith, 2005
)
.
The decrease in metamorphic
timing was approximately 19 days for
A. andersoni met1
versus 36 days for
A. t. tigrinum met1
. These results demonstrate that variation in metamorphic
timing is determined in part by alleles that segregate at TH-responsive QTL,
and alleles from paedomorphic species may vary in effect.
In natural populations of amphibians, variation in metamorphic timing
can affect the values of other life history traits. Life history theories predict
that individuals should metamorphose early if conditions for larval growth
are poor, but delay metamorphosis and attain large larval body sizes if growth
conditions are optimal (
Gould, 1977; Wilbur & Collins, 1973
). This is be-
cause larger body sizes at metamorphosis translate into increased reproduc-
tive success in the adult phase (
Semlitsch et al., 1988
). From a physiological
perspective, metamorphic timing and body size are expected to covary
as a function of resource allocation. From an evolutionary perspective,
