Assessment and disease comparisons of hybrid developmental defects. 
Rodents of the genus Peromyscus are among the most common North American mammals.  Crosses between natural populations of two of these species, P. maniculatus (BW) and P. polionotus (PO), produce parent-of-origin effects on growth and development.  BW females mated to PO males produce growth-retarded offspring.  In contrast, PO females mated to BW males produce overgrown but dysmorphic conceptuses.  Variation in imprinted loci and control of genomic imprinting appear to underlie the hybrid effects.  Prior morphological and genetic analyses have focused on placental and post-natal growth.  Here, we assess the frequency and scope of embryonic defects.  The most frequent outcome of the PO x BW cross is death prior to embryonic day 13.  Conceptuses lacking an embryo proper are also observed as in gestational trophoblast disease.  Among the common embryonic phenotypes described and tabulated are edema, blood vessel enlargement/hemorrhaging, macroglossia, retention of nucleated erythrocytes, placentomegaly.  We investigate expression of loci known to be mis-regulated in human growth/placental disorders and/or mouse knockouts with similar phenotypes.  These loci are Igf2, Cdkn1c, Grb10, Gpc3, Phlda2 and Rb1.  All exhibited significant differences in either placental or embryonic expression levels at one or more of the three timepoints examined.  The data underscore the importance of placental gene expression on embryonic defects.  We suggest that the hybrid defects offer a novel system to understand how natural allelic combinations interact to produce disease phenotypes.  We propose that such interactions and their resulting epimutations may similarly underlie the phenotypic and causal heterogeneity seen in many human diseases.