A maternal genetic effect on the composition of mouse aggregation chimaeras.

Two series of 12 1/2 day mouse chimaeric conceptuses were produced by aggregating (C57BL x CBA)F2 strain preimplantation embryos with embryos that differed at the Gpi-1s locus that encodes glucose phosphate isomerase, GPI-1. The composition of individual issues was evaluated by quantitative electrophoresis to estimate the % GPI-1A in the chimaeric tissue containing GPI-1A and GPI-1B. In one series of chimaeras, the GPI-1A cells were derived from a backcross between inbred BALB/c strain females and (BC x BALB/c)F1 males, where BC is the partly congenic strain C57BL/Ola.AKR-Gpi-lsa,c/Ws. In the other series of chimaeras, the GPI-1A cells were derived from the reciprocal backcross between (BC x BALB/c)F1 females and inbred BALB/c strain males. The [(BC x BALB/c)F1 female x BALB/c male]<==>(C57BL x CBA)F2 series of chimaeras was reasonably balanced so that GPI-1A and GPI-1B cells were fairly equally represented in the foetuses, placentas and extraembryonic membranes (tissue means: 37-51% GPI-1A). This series did not differ significantly in composition from an earlier series of (BC x BALB/c)F2<==>(C57BL x CBA)F2 chimaeras. However, the [BALB/c female x (BC x BALB/c)F1 male]<==>(C57BL x CBA)F2 series of chimaeras was unbalanced, with mean tissue compositions (28-33% GPI-1A) that were intermediate between the above two balanced series and the unbalanced (BALB/c x BALB/c)<==>(C57BL x CBA)F2 series (tissue means: 14-22% GPI-1A), that was studied previously. Thus, both (BALB/c x BALB/c) and [BALB/c x (BC x BALB/c)F1] embryos contributed less to the tissues of chimaeric conceptuses than either (BC x BALB/c)F2 or [BC x BALB/c)F1 x BALB/c] embryos. This implies that embryos from BALB/c mothers contributed less to the tissues of chimaeric conceptuses than embryos from (BC x BALB/c)F1 mothers. We, therefore, conclude that a maternal genetic effect is responsible for some of the differences in composition among the four groups of chimaeras. This maternal effect must act before the 8-cell stage but it is not yet known whether it is mediated via cytoplasmic inheritance, genomic imprinting or by the reproductive tract. Evidence that a maternal effect retards preimplantation development of embryos from BALB/c females is reviewed and the possibility that this might cause them to contribute poorly to chimaeric conceptuses when aggregated with more precociously developing embryos is discussed.