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.