INTERSPECIFIC AND EXTRASPECIFIC PREGNANCIES IN EQUIDS - ANYTHING GOES

Equids possess the unusual ability to interbreed freely among the phen otypically and karyotypically diverse member species of the genus to p roduce viable, but usually infertile, offspring. The mule (female hors e x male donkey) was humanity's first successful attempt at genetic en gineering acid its clear expression of both parental phenotypes has co ntributed much to our understanding of genetic inheritance over the ce nturies, Even more surprising, mares and donkeys have been shown to be capable of carrying to term a range of true, xenogeneic extraspecies pregnancies created by embryo transfer, including Przewalski's horse ( Equus prezwalskii; 2n = 66)-in-horse, (E. caballus; 2n = 64), and Gran t's zebra (E. burchelli 2n = 44)-in-horse pregnancies, Fetal genotypes has a marked influence on placental development in equids, expecially on the width and general development of the annulate chorionic girdle , progenitor tissue of the gonadotrophin (eCG)-secreting endometrial c ups, However, transfer of intact and bisected demi-mule embryos (E. mu lus; 2n = 63) to Jenny donkeys (E, asinus; 2n = 62) showed convincingl y that maternal uterine environment, probably mediated by intrauterine growth factor production, can exert an overriding influence on chorio nic girdle development and its invasion of the maternal endometrium, T ransfer of donkey embryos (2n 62) to horse mares (2n = 64) results in the development of an exceptionally small chorionic girdle that comple tely fails to invade the endometrium to form endometrial cups, Around 70% of these donkey-in-horse preganancies are aborted between days 80 and 85 of gestation in conjuction with delayed and abnormal placental attachment combined with a vigorous maternal cell-mediated reaction ag ainst the xenogeneic donkey trophoblast. This model of pregnancy loss shows strong evidence of immune memory and the rate of fetal death is reduced by immunization of the surrogate mare against donkey lymphocyt es. The findings suggest an important role for the invasive trophoblas t cells of the equine placenta in initiating and driving attachment an d interdigitation of the non-invasive placenta for fetal sustenance, a nd in modulating materno-fetal immunological interaction to enable sur vival of the antigenetically foreign fetus in the uterus.