|Abstract or Summary
- The design of this study was suggested by the following observations:
(1) the successful application of recurrent selection
techniques in obtaining improvement of hybrid performance in corn
and Drosophila, (2) the increased production realized by rotational
crossbreeding, and (3) the expression of a relatively large degree
of hybrid vigor in traits of reproductive performance. On the
basis of these observations, a rotational recurrent selection scheme
was proposed as a means of improving the reproductive performance
of females in a rotational crossbreeding program.
In this study recurrent selection was conducted in three
strains of mice for hybrid female reproductive performance (10-day litter weight). Selection in each strain was based upon the
average litter weight at 10 days of a sire's interstrain hybrid
daughters. Hybrid daughters were obtained from a rotational
pattern of interstrain top-cross matings in which males of strain
#20 were mated to strain BCX females, BCX males were mated to
females of strain BCL and BCL males were mated to #20 females.
The traits of litter size at one and 10 days were also examined.
A comparison of the hybrid and purebred reproductive performance
indicated hybrid females were superior in performance
to purebred sibs and dams in generation I. A change from harems
to the colony groups method of mating caused a decline in the litter
size at one-day of generation II hybrids, but the 10-day weight of
their litters was still comparable to the 10-day litter weights of
their purebred dams.
Because the change in environment was correlated with generations
the evaluation of selection was based upon a comparison
of selected and realized divergence between high and low classes
of two-way selection. The response to selection was positive in
strains #20 and BCL but slightly negative in BCX. The pooled response
was positive. On the basis of the positive response in two
strains and in the pooled data it was concluded that selection had
Predictions of response, based upon the assumption that
differences between sire families were additive, were made from
components of variance derived from sib analyses. The ratio of
expected response and realized response for litter weight at 10
days in the pooled data was 0.835. When the comparisons were made by strains, only strain #20 showed a positive correspondence
between predicted and realized divergence between high and low
An examination of the data by strains suggested that the following
factors should be considered in evaluating the response to
selection: (1) the genetic foundation of each sire strain, (2) the
genetic foundation of the dam's strain, (3) the presence and influence
of maternal effects by the dams of hybrid daughters, and (4)
the relative degree of sampling error which influenced selections
and predictions of response in generation I.
In strain BCL the realized response was positive but the expected
response was zero. The lack of sire variability and consequently
the estimated response of zero was attributed to sampling
error and the inbred foundation of BCL. The realized divergence
between high and low classes in generation II of BCL hybrid
daughters was ascribed to the effectiveness of high and low grouping
of the dams from strain #20.
Two possible reasons for the negative response in BCX were
discussed. First, a higher degree of inbreeding occurred in the
intrastrain matings of generation I than in either #20 or BCL.
Second, a misclassification of the BCL dams of generation II BCX
hybrids may have occurred. The second factor was suggested by
the negative relationship between generation I hybrid and purebred
performance in the high and low classification.
On the basis of the correspondence between hybrid and purebred
selection differences in strains #20 and BCX, the response
to selection in strain #20 and the agreement between the expected
and realized divergence in the pooled data it was concluded that
selection for 10-day litter weight had acted primarily upon additive
As a consequence of repeated interstrain matings, an additional
component of variation between litters in dams was included
in the sib analyses of generation II. In strain #20 hybrids, the
relative size of the component of variance due to litters indicated
that the influence of inbred BCX dams was more variable between
successive litters than the influence of less inbred dams of #20
and BCL. Repeatability estimates of litter size performance
further indicated that BCX dams were more variable in performance
than were purebred dams of #20 and BCL.
The implication that selection had acted largely on additive
genetic variation suggested that females within a rotational crossbreeding
program could be utilized as dams to obtain hybrid progeny
for sire evaluation. The greater variability of dam influence
of inbred BCX dams indicated that the use of crossbred dams
would lead to a more uniform dam influence in a sires progeny.