Graduate Thesis Or Dissertation
 

Selection for hybrid female reproductive performance in the mouse

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  • 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 been effective. 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 classes. 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 genetic variation. 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.
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