Abstract
Hybrids between species are often infertile and extremely rare among mammals. Mules, i.e. crossing between the horse and the donkey, on the other hand are very common in agricultural and leisure practices due to their enhanced post-natal physical characteristics that is believed to occur for outbreeding or hybrid vigor. Since no reports are availableon the effects of hybrid vigor during early development, this study focused on characterizing the intrauterine development of mule conceptuses during critical embryo-to-fetus transition period. Nine embryos and fetuses of early gestation, obtained after artificial insemination and transcervical flushing, were evaluated by means of gross anatomy and histology and compared to data available for the equine. We found that some events, such as C-shape turning, apearence of branchial archs, limb and tail buds, formation of primary and secondary brain vesicles, heart compartmentalization, and development of somites, occurred slightly earlier in the mule. Nonetheless, no major differences were observed in other developmental features, suggesting similarities between the mule and the horse development. In conclusion, these data suggest that the effect of hybrid vigor is present during intrauterine development in the mule, at least with regard to its maternal parent.
Highlights
Hybrids as crossbreeds of strains, varieties, or species occur in both plants and animals
The thalamus occurred in the diencephalon, a mesencephalic aqueduct reached the fourth ventricle and the cerebellum occurred dorsal to bridge and the conoid medulla oblongata (Fig. 4A-C)
The forelimb and tail buds were present earlier than limb buds that were observed at day 25, the same age at which the somites were present
Summary
Hybrids as crossbreeds of strains, varieties, or species occur in both plants and animals. Forelimb and tail buds were present early in development whereas the hind limb buds were first observed only at day 25 (Fig. 2A and B).
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