Reproductive failure and maternal-fetal relationship in a Peromyscus species cross

Mating, fertilization, implantation, prenatal mortality, fetal and placental size, and placental ultrastructure were studied in intraspecific and interspecific crosses involving Peromyscus maniculatus and P. polionotus. Failure to mate was a major factor in interspecific crosses and was much more pronounced in crosses between P. polionotus females and P. maniculatus males than in the reciprocal cross. Failure of implantation following mating, however, was more pronounced in crosses between P. maniculatus females and P. polionotus males. Failure of implanted embryos to survive to term was a factor in crosses between P. polionotus females and P. maniculatus males. Comparison of the placental labyrinth of conceptuses from intraspecific and interspecific crosses revealed no differences at the ultrastructural level. The relationship of these observations to the evolution of isolating mechanisms in mammals and to physiological aspects of the developing maternal-fetal relationship are discussed. A model of placental and fetal size inheritance is presented.