measure survival and reproduction of t-bearing and non-t-
bearing mice over the approximate span of a generation,
providing rough estimates of lifetime reproductive success
for these animals.
In contrast to the 58% increase of thaplotypes during
two generations of caged breedings (in the absence of com-
petition), we found that within the context of competitive
populations, t-complex haplotypes declined dramatically.
In a single generation, eight out of ten populations experi-
enced a decline intfrequencies, with an overall reduction of
34.3% (Carroll et al. 2004). This result is even more strik-
ing with respect to the biased transmission rate of thaplo-
types from t-bearing males in the enclosures (0.86), which
was nearly equivalent to the 0.88 distortion rate measured
from t-bearing males breeding in the laboratory. Observed
tfrequencies were 48.5% lower than predictions based on
these biased transmission patterns. In analyzing the com-
ponents of this t-haplotype decline, we found that inherit-
ing a single t-bearing chromosome depressed reproductive
success for both sexes, and significantly increased mortality
rates for t-bearing females and dominant t-bearing males.
However, the strongest component of the tdecline emerg-
ing from our study was a significant reduction in the abil-
ity of /tmales to gain territories — only 32% of /tmales
gained territories, whereas 67% of /males gained terri-
tories (Carroll et al. 2004). Since female mice overwhelm-
ingly prefer to breed with dominant males, t-associated
impairment of male dominance helps explain why the t
complex was found at lower-than-expected levels among
enclosure pups. These data collectively suggest that selec-
tion against t-bearing heterozygotes in natural populations
balances the opposing force of meiotic drive, and that male-
male competition is the predominant form of this selection.
Though its effects are striking, the physiological mecha-
nisms by which the tcomplex impairs heterozygote adults
are unclear. Leamy et al. (2001) examined morphometric
skeletal features in 109 male and female littermates of our
enclosure founders (74 /, 31 /t,and 4 t/t), and found
increased fluctuating asymmetry (FA) in t-bearing mice but
not in inbred mice. FA is the deviation from bilateral sym-
metry of paired morphometric characters. Presumably, per-
fect symmetry correlates with a high level of developmental
stability and can therefore be disrupted by a variety of ge-
netic and environmental perturbations, the tcomplex and66 Chapter Five
Table 5.1 Summary of tested factors that may influence frequencies of the tcomplex in nature
Factors serving to Factors serving to
Type of study increase tfrequency Reference decrease tfrequency Reference
Models or simulations Moderate to high migration Lewontin and Dunn 1960; Drift Lewontin 1962
rates Levin et al. 1969 Homozygote and /or heterozygote Dunn et al. 1958
disadvantage
Laboratory studies or staged trials Gamete effects: Silver 1989
Up to 100% transmission of t
gametes through males
Fertility: Dunn and Suckling 1955 Fertility: Johnston and Brown
Increased fertility of /tmales Decreased fertility of /tmales 1969
Approximately 20% decrease in Lenington et al. 1994
litter size of /tmales and females
Estrus conditionb Lenington and
Heisler 1991
Survivorship: Dunn et al. 1958
Increased survival of /tpups
to sexual maturity
Behavior: Lenington et al. 1996 Behavior: Lenington 1991
Increased aggression of /t Male and female odor preferences
males for /animals
Seminatural enclosure studies Mortality: Lenington et al. 1996 Mortality: Carroll et al. 2004
Higher survival of /tmalesa Decreased survival of /tfemales
and /tdominant males
Higher survival only found in Carroll and Potts, Behavior: Carroll et al. 2004
subordinate /tmales unpublished data Reduced dominance of /tmales
Reproduction: Carroll et al. 2004
Decreased reproductive success
of /tmales and females
aIt is not known whether there was a difference in survival between dominant vs. subordinate t-bearing males in this study.
bTransmission distortion appears to decrease in litters conceived during postpartum vs. cycling estrus.